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Thread: LM7815 power supply question

  1. #36
    Tubewreck jmaf's Avatar
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    Quote Originally Posted by g1 View Post
    Thanks for the clarification. I appreciate your sharing your practical experience.
    I agree with not using a 1A regulator in a circuit that needs 1A most of the time. (In this case we are off on a bit of a tangent as the circuit requirement was stated as 500mA, but tangents are par for the course around here anyway. )
    For a circuit that needed constant 1A, would you be comfortable with a 1.5A reg. or go with 2A ?
    It depends. Would it be mass produced and low cost device? If so, then I'd go the extra mile to make it work with a 1.5 amp regulator to save costs. If it were a boutique/custom or personal hobby project with just one unit, then I'd use 2A or more, or whatever I found in the drawer in the latter case. How will it be used? Is it critical? Will it run 24x7? Does someone die if it fails?

    For guitar amps I used to use the LM350 (3 amp) @ 1 Amp draw for preamp tube heaters.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by jmaf View Post
    I've tested hundreds of LM regulators in real production settings and they're not reliable at 1A.
    If you chop your post down to the useful part, which is what I left above, Karma will be better for everybody

    FWIW I implied something similar, based on roughly similar reasons, but didnīt turn it into a pissing contest, just restricted it to the OP apparent needs

    self quoting:
    The board manual only says "10 - 15V DC power supply, 500 mA min."
    Then it can be fed by a 7815.

    You need at least 3V more than +15V feeding the 7815, including ripple, so aim at some 20/22V DC RAW to account both for ripple and drop under load.

    If you can get a 15/16VAC transformer, rated 1 to 1.5A, it will be perfect, so you donīt need more than, say, 22/25VA .

    All of this is very conservative, the board may not even take 500mA by a large margin, if a "500mA supply" was suggested.
    Didnīt even mention 1A but was happy that it would take 500mA or less

    And on the other side suggested reducing Raw Voltage input to decrease dissipation .

    On the other side, understand JMAFīs position very well, very often walk away in disgust from other Forums (wonīt mention names) when TV couch "designers" steer some poor OP straight into the quicksand pit and boo boo my efforts to save him ... oh well.

    Just not here

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    It's interesting to see how components in mass-produced, commercial designs are run at or close to their maximum rating - including voltage regulators. So we see 15v supply rails with 16v caps, power amps with transistors run right to the edge of their SOA, 300v tubes run at over 400v, 3w resistors run at 2.7w. In many cases, this is all down to cost and the component manufacturer's device specification. Why install components in (say) a set-top box to ensure 25-year reliability when it only needs to last maybe 3 years? There also factors into this the predicted failure rate and warranty costs. Fitting higher spec components may give increased reliability but the cost may outweigh fitting cheaper components and taking a hit on the increased failures.

    Think of all the Fender amps with cooked boards where the 16v zeners and dropper resistors are fitted. Fender have made them with that failure area from day one and they started to fail after about three years. They have to know after a short while that almost every Deville/Hotrod Deluxe would fail in that area. They just kept building them like that, year-in, year-out. They never relocated the resistors and stuck a couple of aluminium-clad 15W devices on a piece of angle bolted to the chassis. They never uprated the zeners and mounted them off the board surface, they never installed 'proper' regulators. They saved money instead. Perhaps they only need to last the warranty period - maybe 5 years design life but no more.

    When it comes to 78xx regulators, I find these to be pretty robust. They're short-circuit and thermally protected. So what's the worst that can happen if it's over-current or thermally compromised? It just shuts down. I see them running so hot they'll sizzle spit and they run for years like that on tiny (or no) heatsinks. They must be right on the edge. Often, if the load is too high, they won't conduct in the first place. A ready-made 500mA board does not need over-thinking.

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    Tubewreck jmaf's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    It's interesting to see how components in mass-produced, commercial designs are run at or close to their maximum rating - including voltage regulators. So we see 15v supply rails with 16v caps, power amps with transistors run right to the edge of their SOA, 300v tubes run at over 400v, 3w resistors run at 2.7w. In many cases, this is all down to cost and the component manufacturer's device specification. Why install components in (say) a set-top box to ensure 25-year reliability when it only needs to last maybe 3 years? There also factors into this the predicted failure rate and warranty costs. Fitting higher spec components may give increased reliability but the cost may outweigh fitting cheaper components and taking a hit on the increased failures.

    Think of all the Fender amps with cooked boards where the 16v zeners and dropper resistors are fitted. Fender have made them with that failure area from day one and they started to fail after about three years. They have to know after a short while that almost every Deville/Hotrod Deluxe would fail in that area. They just kept building them like that, year-in, year-out. They never relocated the resistors and stuck a couple of aluminium-clad 15W devices on a piece of angle bolted to the chassis. They never uprated the zeners and mounted them off the board surface, they never installed 'proper' regulators. They saved money instead. Perhaps they only need to last the warranty period - maybe 5 years design life but no more.

    When it comes to 78xx regulators, I find these to be pretty robust. They're short-circuit and thermally protected. So what's the worst that can happen if it's over-current or thermally compromised? It just shuts down. I see them running so hot they'll sizzle spit and they run for years like that on tiny (or no) heatsinks. They must be right on the edge. Often, if the load is too high, they won't conduct in the first place. A ready-made 500mA board does not need over-thinking.
    You're right about everything working on the limit. It's all about planned obsolescence and it moves our markets ever since Edison and friends came up with the idea. But that's a market thing, not technical good practice, and it creates mountains of electronic junk in Africa.

    Quoting myself:

    Quote Originally Posted by jmaf View Post
    Would it be mass produced and low cost device? If so, then I'd go the extra mile to make it work with a 1.5 amp regulator to save costs. If it were a boutique/custom or personal hobby project with just one unit, then I'd use 2A or more, or whatever I found in the drawer in the latter case. How will it be used? Is it critical? Will it run 24x7? Does someone die if it fails?
    If a fellow member is building a circuit from zero, why give advice to work on the limit when they can buy regulators for 3X, 5X the rated current cheaply? What's the point of running a toaster in a circuit?

    Was it really bad advice to tell OP to not use 78xx's @ 1A?

    Fender tube abuse is part of Leo's magic. But tubes are different beasts. Silicon fails sharply, vacuum doesn't. Abused tubes can last for 5000 hours which is a lot for a musician's instrument. 5000 hours is not a lot for a set top box or a PC USB port. They're different applications. Leo didn't have the cheap readily available components we have today, he had to adapt. It's bad electronic design, but it's what he had back then. He ran tubes like that because he had to adapt but then it became part of his tone recipe. Today you have 100's of different regulator IC's you cab buy by clicking a mouse, why use anything at 100% rated current if you're building one piece for yourself?



    Quote Originally Posted by J M Fahey View Post
    Didnīt even mention 1A but was happy that it would take 500mA or less
    If anyone is curious, test a LM7812 to regulate 3 x 12AX7 150mA heaters. 450mA total, right? Safely under 50% Imax? Then take a thermometer gun and measure temperature after 30 minutes. You'd need a truck sized heatsink. Can't even fit a heatsink that big into most projects which demand 500mA. (PS. I also picked the useful part of your reply .)

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    Last edited by jmaf; 06-21-2018 at 01:21 PM.

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    Quote Originally Posted by jmaf View Post
    If a fellow member is building a circuit from zero, why give advice to work on the limit when they can buy regulators for 3X, 5X the rated current cheaply? What's the point of running a toaster in a circuit?
    1. He isn't building a circuit from zero.
    2. I have never, anywhere, advised him to work on the limit - those are your words, not mine. In fact, I advised him on sizing for a transformer that will reduce thermal dissipation.
    3. The regulators are already fitted.
    4. He was asking for advice on transformer sizing for the boards he already has

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    Tubewreck jmaf's Avatar
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    Quote Originally Posted by Mick Bailey View Post
    1. He isn't building a circuit from zero.
    This is unnecessary pedantry. My point was clear, we're electronic hobbyists, you can employ components any way you want to.

    Quote Originally Posted by Mick Bailey View Post
    2. I have never, anywhere, advised him to work on the limit - those are your words, not mine. In fact, I advised him on sizing for a transformer that will reduce thermal dissipation.
    You wrote a long post about how everything uses components on the limit. Pardon me if I understood your implication wrong. What was your point then?


    Quote Originally Posted by Mick Bailey View Post
    3. The regulators are already fitted.
    I assume he has a soldering iron.

    Quote Originally Posted by Mick Bailey View Post
    4. He was asking for advice on transformer sizing for the boards he already has
    Which he's going to use somewhere on his own projects. Right?

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    Quote Originally Posted by jmaf View Post
    You wrote a long post about how everything uses components on the limit. Pardon me if I understood your implication wrong. What was your point then?
    Just illustrating the commercial approach and wishing I'd never bothered to post anything on the subject.

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  8. #43
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Thanks Nick. After some toil, I found these: https://www.mouser.com/ProductDetail/546-187E16 16v 56va, 3.5a, a little bigger than needed, I think, for a 1a PS. Allows for some voltage drop and still stay above 17v min input.
    For some reason Hammond haven't given the regulation for those. At 56VA I'll guess 15%.

    With a 3300uF and taking the the worst case line voltage of 115V less 7.5% at the input to the regulator would be about 17.1V, only just enough. Given your wall voltage is higher and the current is 500mA you should be fine. The power dissipation at max line volts of 115 + 7.5% and 1A is 7.4W so you'd need a 7.8C/W heatsink or better. If you want to limit to 500mA then 20C/W should do. The cooler you keep it the more reliable it should be.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by nickb View Post
    For some reason Hammond haven't given the regulation for those. At 56VA I'll guess 15%.
    Take manufacturerīs regulation with a pinch of salt.

    The "number" they quote is for AC regulation into resistive loads (and maybe it holds for choke input supplies) , but capacitor input ones get charged by narrow pulses with quite high peak current and narrow duty cycle.

    Since loss is proportional to I *squared*, 1A 50% of the time requires 2A 25% of the time just to keep *average* current the same, but RMS current will be twice as high, with matching doubled loss.
    And even worse with narrower pulses.

    My point being that rectified voltage drop is higher than expected.
    I have used overbuilt 5% regulation transformers where DC under full load dropped some 15%

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    Quote Originally Posted by J M Fahey View Post
    Take manufacturerīs regulation with a pinch of salt.

    The "number" they quote is for AC regulation into resistive loads (and maybe it holds for choke input supplies) , but capacitor input ones get charged by narrow pulses with quite high peak current and narrow duty cycle.

    Since loss is proportional to I *squared*, 1A 50% of the time requires 2A 25% of the time just to keep *average* current the same, but RMS current will be twice as high, with matching doubled loss.
    And even worse with narrower pulses.

    My point being that rectified voltage drop is higher than expected.
    I have used overbuilt 5% regulation transformers where DC under full load dropped some 15%
    Wow, didn't think it would drop 15%. Is that a smooth drop off, or a hockey stick thing that goes down quickly near the limit?
    EDIT: Oops, sorry, you wrote clearly above "loss is proportional to I squared".

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    Last edited by mikepukmel; 06-22-2018 at 03:57 AM.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Wow, didn't think it would drop 15%. Is that a smooth drop off, or a hockey stick thing that goes down quickly near the limit?
    EDIT: Oops, sorry, you wrote clearly above "loss is proportional to I squared".
    Yes, itīs a smooth drop.
    Commercial Amp designers know this of course, since forever; Booteeq ones or homemakers often do not and either write crap about the transformers they get OR overrate specs 2x or 3x above whatīs actually needed.

    Most ampmkanufacturers donīt write supply or rail voltages any more, just check it yourself ... except +/- 15V which is regulated, of course.

    Tube amps often do not change that much, but only because at idle, no signal, they are *already* pulling a lot: filaments and too much idle current through power tubes, plus low efficiency all over the place, but SS amps which pull feeble current at idle, drop rails a lot when actually played loud.

    VERY FEW Commercial amps tell the whole story on schematics, (which I find VERY annoying and unprofessional) , only Roland (and maybe scant others) tell all.

    Just check this Roland KB100 posted here a couple days ago:
    Click image for larger version. 

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    a) rail voltage: +47 to +54V , meaning: +54V at idle, dropping to +47V at full power.

    b) something which causes MUCH confusion, it took me a long time to decode, and had to check a ton of Roland schematics to self confirm it was so after all: transformer primary rating is AC (mains) **BUT** in Rolandspeak secondary voltage is NOT RATED IN VAC but in "DC rectified voltage achieved with that transformer" .

    A normal tech and even more a somewhat advanced end user will see that schematic and, if neded, will buy a 47VAC+47VAC transformer ... and blow everything in its path.

    That label must be read as: "transformer whose rectified secondary gives you +/-49 Vdc under a 1.7A load" which "will swing from 54V at idle down to 47V under full load and still be under spec".

    Which looks complex but in fact is a very accurate way to fully describe the transformer which will fit there.

    LAB Series amps, "by Engineers for Engineers", usually stated transformer secondary VAC but stated two rail voltages, for example: "+54 (+49)V" meaning: "+54 at idle, +49 full load"

    Others just post nominal rail voltage and "forget" to mention it will drop a lot.

    And as I said before, MANY plain do not post any rail voltages, period.

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    Quote Originally Posted by J M Fahey View Post
    Take manufacturerīs regulation with a pinch of salt.

    The "number" they quote is for AC regulation into resistive loads (and maybe it holds for choke input supplies) , but capacitor input ones get charged by narrow pulses with quite high peak current and narrow duty cycle.

    Since loss is proportional to I *squared*, 1A 50% of the time requires 2A 25% of the time just to keep *average* current the same, but RMS current will be twice as high, with matching doubled loss.
    And even worse with narrower pulses.

    My point being that rectified voltage drop is higher than expected.
    I have used overbuilt 5% regulation transformers where DC under full load dropped some 15%
    The point of knowing the regulation (and it's such a basic and important parameter that I can't believe they didn't include it) is I can calculate the effective resistance and from that I can accurately allow for the high current pulses and determine the min output voltage. It's a bit more complicated than that, but I think we said that way back...

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    As an aside, how do you test a power supply: i.e. OK I know how to test no load, just ... don't hook it up to any load. But is there a reasonable way to test various current draws, then check the voltage? Some kind of resistor network?

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    Quote Originally Posted by mikepukmel View Post
    As an aside, how do you test a power supply: i.e. OK I know how to test no load, just ... don't hook it up to any load. But is there a reasonable way to test various current draws, then check the voltage? Some kind of resistor network?
    Enzo was telling me he used to have a big (I think 1 ohm resistor) in his drawer and he would use that to figure current draw. Kinda of like the 1 ohm resistor people will use on power tubes.

    nosaj

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    This will probably be shocking to you all, and I hope I didn't ruin anyone's day/evening, but these little regulator board kits I got for 1.60 USD each, have the + and - outputs labelled reversed. And the heat sink looks, rather small. Will post photos. The sinks are about the height of the installed regulator.

    I got these kits mainly for the boards, thinking some of the components might need to be updated.

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    Lifetime Member Enzo's Avatar
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    I used to repair piles of SMPS for the arcade video game industry. Black boxes with +5, +12, and -5 outputs +12 at an amp or two, +5 at anaything from 5A to 20A. -5 was usually an amp.

    First test it unloaded, then indeed I had some resistors for loads. They were mostly ceramic cylindrical ones. I had a 5 ohm one - to draw 1A from 5v. I had a 1 ohm one to draw 5A from 5v. I had a 12 ohm one for.. oh you can guess.

    Plus I had a whole selection of power resistors in my stock, I could grab.

    Put an appropriate resistor across the output and see if the voltage holds up. If I were in a lab setting I could use manay load values and chart the complete performance, but that is rarely necessary here. If the thing works at essentially full out, and at a gentle load, chances are real good it will also work at points between.

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    Quote Originally Posted by Enzo View Post
    I used to repair piles of SMPS for the arcade video game industry. Black boxes with +5, +12, and -5 outputs +12 at an amp or two, +5 at anaything from 5A to 20A. -5 was usually an amp.

    First test it unloaded, then indeed I had some resistors for loads. They were mostly ceramic cylindrical ones. I had a 5 ohm one - to draw 1A from 5v. I had a 1 ohm one to draw 5A from 5v. I had a 12 ohm one for.. oh you can guess.

    Plus I had a whole selection of power resistors in my stock, I could grab.

    Put an appropriate resistor across the output and see if the voltage holds up. If I were in a lab setting I could use manay load values and chart the complete performance, but that is rarely necessary here. If the thing works at essentially full out, and at a gentle load, chances are real good it will also work at points between.
    Cool, thanks Enzo. I will give these cheapies a run at 1/2a and 3/4a before going full bore with the programming board.

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    Tubewreck jmaf's Avatar
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    Quote Originally Posted by mikepukmel View Post
    As an aside, how do you test a power supply: i.e. OK I know how to test no load, just ... don't hook it up to any load. But is there a reasonable way to test various current draws, then check the voltage? Some kind of resistor network?
    Adding to what Enzo already replied, this guy has a cool Youtube channel and he's done a test on Chinese batteries: http://www.instructables.com/id/DIY-...-current-load/

    Similar gadget can be used to test the power supply, just place it where the battery would be and build the test rig to match your supply voltage (15V?).

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    Quote Originally Posted by jmaf View Post
    Adding to what Enzo already replied, this guy has a cool Youtube channel and he's done a test on Chinese batteries: http://www.instructables.com/id/DIY-...-current-load/

    Similar gadget can be used to test the power supply, just place it where the battery would be and build the test rig to match your supply voltage (15V?).
    Thanks for the link Jmaf! (Just read the page, cool project, I could build that!)

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    Best Epoxy to glue heat sink to circuit board

    Hi All!

    So I got these way cheap kits (very cheap), came with all the parts. The one on the left is almost done, but has the smallest heat sink I've ever seen. I dug around on Mouser, and found better ones that I thought would fit ok.


    Click image for larger version. 

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    The pins on these bigger heat sinks don't line up with the holes that the board came with. No surprise there. An hour with my hand drill and I got the bigger sinks into the board reasonably well for stone knives and bear skin tools I have. One fits reasonably tight. One fits a little loose. Either way, Id like to put some epoxy on the pins under the board.

    I started reading about epoxy, found some high rolling stuff, but the links only have "Get a quote" so they want me to buy gallons of the stuff. The opposite end of the spectrum is the cr@p available at the big box stores. <frown>.

    Can anyone suggest some kind of epoxy I can use: put a drop on the pins on the bottom of the board, that won't emit something corrosive like the home fixit epoxy and silicone?

    Its not going into a high vibration environment, but I will bolt the board and transformer into a box and move it around, so I'd hate to have the thing loosen up the solder connections on the LMXX.

    Thanks!

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  21. #56
    Tubewreck jmaf's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Hi All!

    So I got these way cheap kits (very cheap), came with all the parts. The one on the left is almost done, but has the smallest heat sink I've ever seen. I dug around on Mouser, and found better ones that I thought would fit ok.


    Click image for larger version. 

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ID:	49514

    The pins on these bigger heat sinks don't line up with the holes that the board came with. No surprise there. An hour with my hand drill and I got the bigger sinks into the board reasonably well for stone knives and bear skin tools I have. One fits reasonably tight. One fits a little loose. Either way, Id like to put some epoxy on the pins under the board.

    I started reading about epoxy, found some high rolling stuff, but the links only have "Get a quote" so they want me to buy gallons of the stuff. The opposite end of the spectrum is the cr@p available at the big box stores. <frown>.

    Can anyone suggest some kind of epoxy I can use: put a drop on the pins on the bottom of the board, that won't emit something corrosive like the home fixit epoxy and silicone?

    Its not going into a high vibration environment, but I will bolt the board and transformer into a box and move it around, so I'd hate to have the thing loosen up the solder connections on the LMXX.

    Thanks!
    IMO the best solution would not be epoxy or any kind of filler.

    Maybe you could bore a hole into the heatsink and affix it with a screw or pop rivet through the board.

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  22. #57
    Lifetime Member Enzo's Avatar
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    Amen, I agree.

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    Well, I got holes in the board that let the bigger pins come through. I gota few extra heat sinks, could pull off the pins from one and try to put a screw in the same place. The pins look like they're friction fit into the fins, so a similar size screw could work.

    Thanks! !! I'll try that! And I was dreading fooling around with messy epoxy.

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    Quote Originally Posted by mikepukmel View Post
    Well, I got holes in the board that let the bigger pins come through. I gota few extra heat sinks, could pull off the pins from one and try to put a screw in the same place. The pins look like they're friction fit into the fins, so a similar size screw could work.

    Thanks! !! I'll try that! And I was dreading fooling around with messy epoxy.
    Those heatsinks are aluminum and the pins are tin, so they can't be soldered together. They're usually rivetted. To remove the pins just drill a hole into the pop rivet.

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    Thanks Jmaf. I finally got some time today, and the pins are press fit into the bottom of the sink. its not a closed hole, but more than "U" shape, so that it fits around the screw or pin slightly so it won't slide out. So I could pill the pin out with a vice grip, (its got a little knurling to make it stick) and use a screw in place. I'll try to get a photo, its tough to focus in there. Pretty cool design, I think. Have to trek out to the hardware store tomorrow to see what screws I can find. Hope to get at least a couple of these running tomorrow.

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    Good Lord. got the board soldered up except for the regulator. Got the screws into the het sinks, and heat sinks screwed onto the boards. I wanted to measure the resistance between the heat sink and each of the 3 pin pads to make sure I didn't create a short. One board measured fine (0 ohms) the other one kept reading 3 to 28 megohms. back and forth took apart, reassembled. FINGERS have 3 -30 megohm resistance. Cleaned the board off, used a piece of clean plastic to hold the board, 0 ohms. No load (no regulator) reading about 23.5 volts dc (that would go into the regulator). Next step, wrist band, put the regulators in, solder, and ... smoke test. (I love that phrase).

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    Another hopefully not too annoying question for you all: I have both 12vdc and 5vdc rectifier/regulator boards built and tested. Managed not to smoke anything. The particular transformer I could find is 18vac secondary, but 3.5 amps, way larger than needed for one 1 amp or 1.5amp power supply. I did a lot of searching, but didn't find both the primary and secondary voltages and the current rating, could find 2 both not all 3. And this transformer was way less expensive than one closer to the specs for the power supply. So, I get a little extra unused current.

    Anyway, if I use one 3.5A rated transformer to power both rectifier/regulator boards, (1 x 1.5a 5vdc, and 1 x 1A 12VDC) is this OK to use like this? Is there some reason I should use one transformer for each board?

    The 12vdc will power the microncontroller dev/programming board (manual says 500ma min), and the 5v will be for some other stuff, e.g. one of those little 16 char 2 line LCD display, not sure what else.

    Thanks.
    Mike

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  28. #63
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by mikepukmel View Post
    I'll try to get a photo, its tough to focus in there.
    You can use a standard reading glass in front of cellphone camera as a close up lens.
    A "+4" one lets you focus between 20/25cm (8/10") with very good sharpness.
    Just remember to keep it parallel with cellphone front.

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    Juan Manuel Fahey

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    Thanks Juan.

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    The heat sink has fins. The fins have slots that hold a screw or pin in place. Can't describe it well, but its not a 180 degree slot, its a little more, I ground the screw into the end of teh sink, and it holds pretty well, I put some glue in there as well with a broken Q tip.

    The transformer and heat sinks don't quite fit exactly into the little cheap plastic boxes I got. A few microns off.

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    Anyway, I have two rectifier/regulator boards (1 x 12v 1A, and 1 x 5 v 1.5A) hooked up to the same 18v sec 3.5A transformer. Fusing is the only issue I don't quite understand yet. I have a 1A on the primary. But, I think I should also fuse the secondary as well. Or maybe split it up into 2 separate power supplies (I did get another transformer).

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    Hey, those look GOOD!!!

    Just curious: is that some kind of double cabinet or you krazy glued 2 side by side?

    In any case, if you are going to close those boxes with some kind of cover, I suggest you drill some ventilation holes.

    PD: and your pictures came out very sharp

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    Juan Manuel Fahey

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    Lifetime Member Enzo's Avatar
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    Juan,, I think I see two screws holding the two boxes together.

    A dab of Loctite on those nuts will keep them from coming loose. Or if you are cheap like I am, a bottle of cheap nail polish daubed on them will work too.

    For cooling, remember, for warm air to leave there must be a way for cool air to enter. This applies to fan cooled as well as simple convection. So vent holes in the lid would be happy with some more vent holes in the bottom or around the bottom sides.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by mikepukmel View Post
    Another hopefully not too annoying question for you all: I have both 12vdc and 5vdc rectifier/regulator boards built and tested. Managed not to smoke anything. The particular transformer I could find is 18vac secondary, but 3.5 amps, way larger than needed for one 1 amp or 1.5amp power supply. I did a lot of searching, but didn't find both the primary and secondary voltages and the current rating, could find 2 both not all 3. And this transformer was way less expensive than one closer to the specs for the power supply. So, I get a little extra unused current.

    Anyway, if I use one 3.5A rated transformer to power both rectifier/regulator boards, (1 x 1.5a 5vdc, and 1 x 1A 12VDC) is this OK to use like this? Is there some reason I should use one transformer for each board?

    The 12vdc will power the microncontroller dev/programming board (manual says 500ma min), and the 5v will be for some other stuff, e.g. one of those little 16 char 2 line LCD display, not sure what else.

    Thanks.
    Mike
    With 0.5A on 12V and 1.5A on 5V the RMS current in the transformer secondary will be about 3.5A ,just at it's limit. A bigger problem will be the 5V regulator will be dissipating 20W. That requires a massive heatsink of about 4C/W.

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    Experience is something you get, just after you really needed it.

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    Quote Originally Posted by J M Fahey View Post
    Hey, those look GOOD!!!

    Just curious: is that some kind of double cabinet or you krazy glued 2 side by side?

    In any case, if you are going to close those boxes with some kind of cover, I suggest you drill some ventilation holes.

    PD: and your pictures came out very sharp
    Thanks Juan. its a "junker" project, i.e. put something I can use together for the lowest cost. Thanks for the tip on vents, but the top won't fit. The heat sinks and transformer stick up about 1 1/2" above the edges of those 2.00 boxes.

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    Quote Originally Posted by Enzo View Post
    Juan,, I think I see two screws holding the two boxes together.

    A dab of Loctite on those nuts will keep them from coming loose. Or if you are cheap like I am, a bottle of cheap nail polish daubed on them will work too.

    For cooling, remember, for warm air to leave there must be a way for cool air to enter. This applies to fan cooled as well as simple convection. So vent holes in the lid would be happy with some more vent holes in the bottom or around the bottom sides.
    Thanks for the tips, yes cheap here too, like those experiment project dollars to go far. I don't think I'l be able to get the top on the boxes, but I will put them on a shelf out of fingers way.

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