LM7815 power supply question

[mikepukmel]

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).

[mikepukmel]

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

[J M Fahey]

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.

[mikepukmel]

Thanks Juan.

[mikepukmel]

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.








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).

[J M Fahey]

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

[Enzo]

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.

[nickb]

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.

[mikepukmel]

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.

[mikepukmel]

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.

[mikepukmel]

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.

Thanks Nick. I don't think I'll come anywhere close to 1.5a on the 5v. I can't quite understand the data sheet for the LCD display (16 x 2 OSEPP) cheapie. The data sheet has "input current low/input current high" columns +2ua -100ua, didn't understand waht the sign meant since they have separate output current section. I think its very low. Will be that, plus a few LED's and switches.

20W ouch, that's a good size guitar amp power. The regulator is a 1.5A unit, but I didnt plan to put more stuff on it than 1/3rd that or so. No where near 1a.

Reading what 4C/W heat sinks look like...

[mikepukmel]

Wow that's a big heat sink. I don't know what the rating is on the ones I got, but they look to be about 1/3rd the mass.

[mikepukmel]

Getting there. Got the jack wired up, worked up the nerve and started up the board. Powers up fine, everything seems to work. The LED's on the board do the right thing. And the kit comes with a MCU preprogrammed with a cheesey program that makes moving light patterns on LED's by button presses. So, at least I know the inputs and outputs work, the power supply on the board is OK. and the power supply I made at least works.

When I have the board powered for a long time (hour?) the heat sink does get hotter than I expected. Not too hot to touch, but ... hot. The manual said 10 to 15v, so I used a 12v regulator.

Still can't get the AVR studio to see the board. I got a few really cheap USB-To-RS232 cable/converters, that came with drivers. Ive been installing and uninstalling drivers for a few days now, and I think Windows XP sees the cable, but either the cable/converter is no good, or maybe the RS232 port on the board is burned out (I REALLY hope that's not the case)

The last stretch, I opened a RS232 communications program I downloaded, and could send characters in, and when I hit return I got some garbage back. So, I think that means data is going all the way into the cable and to the board, and the board is responding although the comm's might be all screwed up. When I disconnect the board end of the cable, I dont get any response when I send garbage char's in.

So far, haven't found any way to test the RS232 on the board other than get a good working connection. so stuck for now.

[The Dude]

You lost me, Mike. What is it you're building? I must have missed something. I see a thread about regulation and then the last post about RS232, etc. Curiosity is getting the best of me. Is there maybe an associated thread I missed?

[Enzo]

Without review, what I recalled was he wanted to build a basic power supply in a box for his bench. He wanted it to power small circuits he wanted to play with. I think those circuits are the USB and all we hear of now. I could be wrong.