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Thread: LM7815 power supply question

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    LM7815 power supply question

    Hi All!

    I got some cheap regulated power supply boards: has 4 diodes LM7815, heat sink, and a couple of electrolytic capacitors.

    I think I screwed up ordering the transformers.

    The LM7815 data sheet specifies absolute max input 35v. I got a couple of Triad F229X 115V primary, 24v secondary. After rectification, about 24 * 1.414 = 33.94v almost right on the limit.

    I hate to waste usable current, but should I put a dropping resistor to get the input to the LM7815 down 15% or so to give the LM7815 some breathing room?

    Thanks
    Mike

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    Supporting Member Jazz P Bass's Avatar
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    Either way you have to 'dump' some 10 volts.

    One or the other must do it.
    The resistor or the device itself.

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    Thanks Jazz. Wondering if I should exchange the transformers.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    It's complicated. Hard to believe for so few components, I know

    1) You mustn't have too many volts in i.e < 35V
    2) You mustn't overheat it i.e voltage drop x current and is your heatsink big enough?
    3) Oh.. but wait, you can't just lower the voltage as can't have too few either i.e >18V

    It gets worse...
    Under no load conditions expect about 10% more volts out of the secondary. Now add in say 5% for higher than usual line voltage.
    Under load, the output will drop due to winding resistance. Naturally the drop will be more than you expect as the current flows in (very) big gulps to recharge the smoothing caps.
    To add to it all, the voltage on the caps is not constant but a ripple that depends of the size of the cap and the current draw. You must allow for that too.
    Finally allow for 5% line voltage drop.

    All this is why we have simulation tools. Go and get Duncan's power supply designer and work it all out using that. Define your inputs i.e. min and max volts and output min & max volts and current.

    You might want to consider sending the transformers back before it's too late. Dropping volts just wastes power.

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    Last edited by nickb; 06-17-2018 at 08:46 AM.
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    Thanks Nick. The calculation I should have done before (one of them anyway) but screwed up:

    (35-17)/2 = about 26 -> 26/1.414 -> about 18.4ac. They sell a 20vac sec, similar price.

    Thanks, I got a copy of Duncan's software, looking at it now...

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    Lifetime Member Enzo's Avatar
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    If that is a transformer rated for 24v, that will be at its rated current. Unloaded or lightly loaded, that 24v can rise too.

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    Oh man, you flick that power switch on, and stuff really starts to happen before it settles down.

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    Quote Originally Posted by Enzo View Post
    If that is a transformer rated for 24v, that will be at its rated current. Unloaded or lightly loaded, that 24v can rise too.
    Thanks Enzo. So, its important to have the post rectifier voltage right in the middle of the regulators min/max input range.

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    Quote Originally Posted by nickb View Post
    It gets worse...
    And then it gets even worser!
    In addition to the items nick mentioned, the data sheet says 115V primary. Do they mean it? How about at 120V, or 124V as I have?

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    Quote Originally Posted by g1 View Post
    And then it gets even worser!
    In addition to the items nick mentioned, the data sheet says 115V primary. Do they mean it? How about at 120V, or 124V as I have?
    Got me thinking, maybe I should give up this hobby and take up whittling. Ain't quite as fun, tho.

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    At least I'm not the only one a-fussin' and a-frettin' over power trannys!

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    Quote Originally Posted by g1 View Post
    And then it gets even worser!
    In addition to the items nick mentioned, the data sheet says 115V primary. Do they mean it? How about at 120V, or 124V as I have?
    Guess I better break out the meter and measure that as well . . . Well, my 115vac is 122.4 to 122.6 at least at 8:36pm on Monday evening.

    The secondary on these 24v transformers measures 27.4vac. Didn't hook up to rectifiers, but should be 38.8v or so. unloaded. Not the worstest, but definitely worser for the LM7815.

    2 x 16vac sec transformers on the way. Also, bought some beefier heat sinks. Should I put that white heat sink grease on the connection?

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    Last edited by mikepukmel; 06-19-2018 at 02:38 AM.

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    Quote Originally Posted by mikepukmel View Post
    Guess I better break out the meter and measure that as well . . . Well, my 115vac is 122.4 to 122.6 at least at 8:36pm on Monday evening.

    The secondary on these 24v transformers measures 27.4vac. Didn't hook up to rectifiers, but should be 38.8v or so. unloaded. Not the worstest, but definitely worser for the LM7815.

    2 x 16vac sec transformers on the way. Also, bought some beefier heat sinks. Should I put that white heat sink grease on the connection?
    My gut tells be that might be too low in general. But we'll see. I can't answer the heatsink question w/o knowing a bunch of other things.
    The first question is what is your required max load current?
    Who is your preferred transformer vendor (so I can look up the specs)?

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    When I'm sizing up for small PSUs using 78XX regulators I get the closest VA rating (higher rather than lower) to my maximum load and the closest AC voltage to my output voltage. Assuming 10% regulation this usually works out fine for dropout voltage. So for a 15V regulator I'd go for a 16v transformer. Usually the rectified output is a little higher than calculated due to primaries being wound for 2x115v or 1x230v and the mains being higher (mine is 248v today).

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    The secondary current will be much higher (about 1.7x) than the output current plus the secondary voltage is higher than the output meaning the VA rating needs to be at least twice the output power (as a starting guess). For such a transformer the regulation is be around 25%.

    Here are my estimates for a 16V 32VA transformer.

    Click image for larger version. 

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    Plugging the numbers into a simulation to be more precise I get,
    Vo min = 18.3V this is above the 17V 7815 requirement, OK
    Vo max = 28.2V this is below the 35V 7815 requirement, OK
    Pd = 7.9 Watts dissipated in 7815
    => Heatsink better than 7.8 C/W, so yes best use thermal compound.
    Secondary current= 1.64A rms
    Min VA rating - 1.64 x 21.5 = 35.3 VA ( very worst case of continuous max load and max input voltage).

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    Why produce heat? Instead, add a switcher between the 24VAC output and the rectifier:

    Click image for larger version. 

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    Adjust it to switch at around 20 VAC peak.

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    Quote Originally Posted by nickb View Post
    My gut tells be that might be too low in general. But we'll see. I can't answer the heatsink question w/o knowing a bunch of other things.
    The first question is what is your required max load current?
    Who is your preferred transformer vendor (so I can look up the specs)?
    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.

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    Quote Originally Posted by Mick Bailey View Post
    When I'm sizing up for small PSUs using 78XX regulators I get the closest VA rating (higher rather than lower) to my maximum load and the closest AC voltage to my output voltage. Assuming 10% regulation this usually works out fine for dropout voltage. So for a 15V regulator I'd go for a 16v transformer. Usually the rectified output is a little higher than calculated due to primaries being wound for 2x115v or 1x230v and the mains being higher (mine is 248v today).

    Thanks Mick, that's what I did (more or less by accident or trial and error), 16v and a little higher VA than needed 56va. Mains are a little higher here but Im not quite sure what that means all the way at the input of the 7815, Id have to measure.

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    Quote Originally Posted by nickb View Post
    The secondary current will be much higher (about 1.7x) than the output current plus the secondary voltage is higher than the output meaning the VA rating needs to be at least twice the output power (as a starting guess). For such a transformer the regulation is be around 25%.

    Here are my estimates for a 16V 32VA transformer.

    Click image for larger version. 

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    Plugging the numbers into a simulation to be more precise I get,
    Vo min = 18.3V this is above the 17V 7815 requirement, OK
    Vo max = 28.2V this is below the 35V 7815 requirement, OK
    Pd = 7.9 Watts dissipated in 7815
    => Heatsink better than 7.8 C/W, so yes best use thermal compound.
    Secondary current= 1.64A rms
    Min VA rating - 1.64 x 21.5 = 35.3 VA ( very worst case of continuous max load and max input voltage).
    Cool, thanks for the calc's. As far as I can tell: the transformer should never go to full load, its a 3.5a rated, and the board should not ever draw more than about 1a. So, I don't think the supply will ever get to that point. The board manual only says "10 - 15V DC power supply, 500 mA min." I believe that is to program and run one chip, without having to drive any additional hardware, i.e. only a few LED's on board., outside the boards power supply.

    I got the duncan software, but spending some effort to figure out how to use it.

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    Quote Originally Posted by jmaf View Post
    Why produce heat? Instead, add a switcher between the 24VAC output and the rectifier:

    Click image for larger version. 

Name:	Basic-circuit-and-the-associated-waveforms.jpg 
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    Adjust it to switch at around 20 VAC peak.
    Thanks JMAF! Reading up . . .

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    Quote Originally Posted by nickb View Post
    My gut tells be that might be too low in general. But we'll see. I can't answer the heatsink question w/o knowing a bunch of other things.
    The first question is what is your required max load current?
    Who is your preferred transformer vendor (so I can look up the specs)?
    Nick, sorry, missed your question on first read: The board manual says 10-15v DC min 500ma. I think it doesn't state a max since you could be programming a microcontroller chip to drives other stuff, so I think they assume the user knows something and will calculate that.

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    A more general question about these small regulated power supplies: I searched and found a bunch of different but very similar circuits for LM7815 regulated power supplies. It looks like, or at least what I found, most have an electrolytic cap on either side of the regulator, one on the input side and one on the output side. Say 470uf on the input side, 100uf on the output side. A few had a much bigger cap on the input side, like 2200uf and none on the output side.

    Do both of these caps serve the same purpose: if the current draw increases rapidly by the load, then the cap will make up the difference until either the LM7815 or the transformer + rectifiers, have time to compensate?

    If we know the max output is, say 1a, it seems like we also need to know the rate at which the load could change current draw to know how big capacitors to use?

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    Quote Originally Posted by mikepukmel View Post
    A more general question about these small regulated power supplies: I searched and found a bunch of different but very similar circuits for LM7815 regulated power supplies. It looks like, or at least what I found, most have an electrolytic cap on either side of the regulator, one on the input side and one on the output side. Say 470uf on the input side, 100uf on the output side. A few had a much bigger cap on the input side, like 2200uf and none on the output side.

    Do both of these caps serve the same purpose: if the current draw increases rapidly by the load, then the cap will make up the difference until either the LM7815 or the transformer + rectifiers, have time to compensate?

    If we know the max output is, say 1a, it seems like we also need to know the rate at which the load could change current draw to know how big capacitors to use?
    The regulators can oscillate in some circuits. These caps are mostly meant to clamp oscillation, in precision circuits you usually place the cap very near the regulator and very near the load, with short copper tracks from the cap to the regulator and to the load.

    Your concern with the max current is not the caps, forget the capacitors, the regulator needs no capacitor to maintain constant voltage, ripple rejection is absurd as long as your source can keep a voltage above 15V RMS + a offset (see the datasheet).

    The problem is power dissipation, this is a passive regulator, it turns excess power into heat.

    The 7815 can source max 1A but not continuously, all the LM78xx's fry if you do 1A 100% duty cycle.

    If you have too large a voltage offset like you do in your project you may be better off with the switching version of the LM regulators. You didn't say what your application is, so gotta observe that the switching ones are noisy and may require special shielding and extra filtering.

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    Quote Originally Posted by jmaf View Post
    Why produce heat? Instead, add a switcher between the 24VAC output and the rectifier:

    Click image for larger version. 

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    Adjust it to switch at around 20 VAC peak.
    I may be wrong, but I would have though that an audio circuit powered this way would be subject to switching noise - like having a light dimmer in the same enclosure.

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    Quote Originally Posted by jmaf View Post
    Why produce heat? Instead, add a switcher between the 24VAC output and the rectifier:

    Click image for larger version. 

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    Adjust it to switch at around 20 VAC peak.
    The OP might need a switcher but *definitely* not that one, which can only handle *resistive* loads (itīs the classic cheap lamp dimmer) or universal series wound motors, both of which integrate that horrible output waveform (because of thermal or mechanical mass) into something useful.
    --------------------------------
    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.

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    Quote Originally Posted by J M Fahey View Post
    The OP might need a switcher but *definitely* not that one, which can only handle *resistive* loads (itīs the classic cheap lamp dimmer) or universal series wound motors, both of which integrate that horrible output waveform (because of thermal or mechanical mass) into something useful.
    I wouldn't feed the output of that to motors or other inductors. It can definitely be rectified and regulated, no problem.

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    Quote Originally Posted by jmaf View Post
    The 7815 can source max 1A but not continuously, all the LM78xx's fry if you do 1A 100% duty cycle.

    If you have too large
    I really don't think that is right. If the thermal issues are managed properly i.e. you don't hit the internal thermal or safe are operating limits limit then it will be fine. Some versions will even do 1.5A. The data sheet I'm looking at specifies a max operating junction temperature of 150C and a thermal resistance of 5C/W junction to case. In my calcs I used 125C as max and determined the heatsink size using that for continuous 1A. BTW I see I used 3.5C/W for chip plus interface - that should be 5.5C/W making the heatsink 7.2C/W.

    All that will be conservative if the max load is 500mA anyway.

    Mikep, the best resource for reference circuits is the data sheet. The National LM7815 shows 0.1uf on the output and 0.33uF on the input. It states the input one is only required if the regulator is located an "appreciable distance" (whatever that is) from the main filter cap and the the output one improves stability and transient response. Just use both and locate close to the regulator.

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    Quote Originally Posted by nickb View Post
    I really don't think that is right. If the thermal issues are managed properly i.e. you don't hit the internal thermal or safe are operating limits limit then it will be fine. Some versions will even do 1.5A. The data sheet I'm looking at specifies a max operating junction temperature of 150C and a thermal resistance of 5C/W junction to case. In my calcs I used 125C as max and determined the heatsink size using that for continuous 1A. BTW I see I used 3.5C/W for chip plus interface - that should be 5.5C/W making the heatsink 7.2C/W.
    I'm not giving advice based on datasheets. In fact I haven't opened a single PDF today.

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    You may have experienced a non-representative sample at some point.

    We can test the hypothesis easily enough. Here we have have a L7815CV with 25V in and 1A out. It's been running happily for hours.

    I don't have an LM7815 here but I'm sure you'll agree it would dissipate the same amount of power so we should expect the same result.

    I do have an LM7805 if you'd like me to test that.


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    Quote Originally Posted by nickb View Post
    Here we have have a L7815CV with 25V in and 1A out. It's been running happily for hours.

    I don't have an LM7815 here but I'm sure you'll agree it would dissipate the same amount of power so we should expect the same result.
    Since you're choosing more powerful components, why not go with one of these just to shoot the shit?

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    I'm sure you'll agree it would dissipate the same amount of power so we should expect the same result.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by jmaf View Post
    Since you're choosing more powerful components, why not go with one of these just to shoot the shit?

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    I'm sure you'll agree it would dissipate the same amount of power so we should expect the same result.
    Still don't believe me huh? OK, here's a LM7805, 15V in well over 1A out. Also quite happy.

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    Quote Originally Posted by nickb View Post
    Still don't believe me huh? OK, here's a LM7805, 15V in well over 1A out. Also quite happy.

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    I've tested hundreds of LM regulators in real production settings and they're not reliable at 1A.

    Not much else I can say except that it's getting really pedantic to try and share some experience here and be confronted by random experimenters all the time.

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    jmaf, I have great respect for your posts, but nickb is hardly someone I would call a 'random experimenter'. Never mind the implication that the forum is full of them. That seems somewhat of a low blow to be honest.

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  34. #34
    Tubewreck jmaf's Avatar
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    Quote Originally Posted by g1 View Post
    jmaf, I have great respect for your posts, but nickb is hardly someone I would call a 'random experimenter'. Never mind the implication that the forum is full of them. That seems somewhat of a low blow to be honest.
    I didn't imply the forum is full of them. I might have implied that I've been rebuffed by this kind of reply more than once where one thing is cherry picked out of a general post where you're trying to help.

    nickb's entire point here is that a component survived some time running at max rated current on his rig. That's all. But if he ever builds anything like that professionally, it'll be back for repair in less than a month. My advice to the original poster is to not do that professionally, because I've seen these components break, for years, since they don't really stand their max rated current in continuous operation. I hope it helps the member who asked originally, that was the only point of me posting here, trying to help.

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    don't forget the joker g1's Avatar
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    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 ?

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