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I used a L7805 to power a Raspberry PI and it worked for about 10 seconds then started to smoke and die. L7805 is rated for 1.5A and 35V DC input. Power supply was 24V DC (was actually two plugin 12V DC 1A adapters wired in series) that was metering 30V DC without a load (will obviously drop close to 24V DC with a load). 24V DC is industrial controls standard. I didn't put the caps on the input and output which I now see in the datasheets. Would omitting these caps cause the meltdown?

Because the input is 24V DC and output is 5V DC there would be a lot of heat generated. Would it be better to use a switching power circuit? Datasheet says heatsink is needed for anything over 1A. The PI uses less than 1A without any devices connected which I didn't have any connected.

What else could I do to make the 7805 more robust (other than adding the caps of course)?

Thanks

Peter Quiring
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    You are dropping 19V at 1A. That is 19W of dissipated power that must go somewhere, in your case it went up with the smoke. You need a significant heatsink, if you want to use 7805. Small capacitors close to the regulator are required, otherwise the regulator can oscillate. Generally a switched-mode power supply is the preferred solution here. – filo Jun 15 '17 at 11:34
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    Don't LM7805s contain thermal overload protection? Shouldn't have blown the regulator, unless it was oscillating and the thermal protection never got a chance to kick in (because the current went low by the time it could, triggering a reset). – Joren Vaes Jun 15 '17 at 11:47
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    Schematic? TWO 12V transformers wired in SERIES? Why? And did you rectify the resultant 24 VAC ? or just feed it to the regulator? – R Drast Jun 15 '17 at 11:56
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    Two 12VAC output transformers connected in series and rectified/filtered would probably give you around 40VDC without load. This is above the absolute maximum input of the 7805. At 35V in you can draw only a few tens of mA safely without a heatsink. – Spehro Pefhany Jun 15 '17 at 12:11
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    on top of that, if the OP is really just using transformers without filtering or rectification, they are lucky they didn't destroy more than just the 7805 – Joren Vaes Jun 15 '17 at 12:15
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    @JorenVaes In which case, the plucky little 7805 gave up its life to save the Pi. A moment of silence, please. – Spehro Pefhany Jun 15 '17 at 12:26
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    Age shall not weary it, nor the years condemn. – Ian Bland Jun 15 '17 at 12:36
  • @RDrast - They are plugin DC output wall adapters. I just call them transformers (which of course have AC to DC rectifiers, etc.) – Peter Quiring Jun 15 '17 at 13:34
  • Ok, I removed the word "transformer" since that was confusing everyone. What else do you call those plugin adapters? – Peter Quiring Jun 15 '17 at 13:43
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    Power supplies, normally, with some kind of specification. Like "a plus 5 volt, 100 milliamp, DC power supply" or "plus/minus 12 volt, 1 Amp, DC power supply". You'd normally say whether it's switched mode or not as well. – Ian Bland Jun 15 '17 at 13:48

3 Answers3

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Linear regulators (like the 7805 and similar parts) have a very simple characteristic.

Power dissipated = \$ (V_{in}-V_{out}) \cdot I_{out} \$ (plus \$ V_{in} \cdot I_{q} \$ where \$ I_{q} \$ is about 5 mA for the 7805).

That means that even if we have a 10 V average input (about the lowest that is practical if unregulated power is used) the dissipation at 1 A out will be 5.05 W, which requires a fairly large heatsink or a smaller heatsink and a fan.

You must satisfy all constraints on the datasheet simultaneously, not just the ones that happen to attract your attention. The absolute maximum input voltage is 35 V, and you should make sure never to even get close to that. There is a maximum output current, and there is a maximum power dissipation. If you dissipate too much power for the heatsink etc. the chip gets too hot and the lifetime is compromised, sometimes dramatically.

The current version of Raspberry Pi 3 uses a lot of current, as much as 730 mA plus whatever is plugged into those USB ports. That's why we generally use a 2.5 A wall wart.

TL;DR: The L7805 is totally unsuited for this application. If you have an industrial application (and still want to use a Pi) you can buy a DIN rail-mounted supply.

enter image description here

However, the Pi is not a hardened industrial computer, so you may have other fascinating discoveries to come.

Edit:

There are drop-in replacements for the L7805 which use a switching regulator. They would be suitable for 24 VDC input (not 35 VDC, not 24 VAC rectified/filtered or not). They do not require additional heat sinking and some may be adequate for your output current.

Here is one from Murata capable of 1.5 A. They are actually quite inexpensive- probably less expensive than a 7805 + heatsink:

enter image description here

ilkkachu
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Spehro Pefhany
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  • I've actually had to buy some of those DIN rail 24 to 5V regulators, but I want to build something into my PCB. Thanks – Peter Quiring Jun 15 '17 at 13:39
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    @PeterQuiring There are plenty of board mounted DC-DC converters for sale. Take a glimpse [in the catalogs](https://www.digikey.nl/products/en/power-supplies-board-mount/dc-dc-converters/922). Maybe even an AC-DC converter. – Jeroen3 Jun 15 '17 at 14:12
  • If you have 24VDC you could use a buck converter such as the [LM2596](http://www.ti.com/lit/ds/symlink/lm2596.pdf) or a module based on that, but the modules are from Chinese makers and may not be the same from one purchase to the next, and also usually use counterfeit chips. – Spehro Pefhany Jun 15 '17 at 14:31
  • @PeterQuiring See my edit above, for drop-in replacement if you already have a PCB designed. – Spehro Pefhany Jun 15 '17 at 14:41
  • Is something like that murata available on digikey? – Peter Quiring Jun 15 '17 at 17:19
  • @PeterQuiring Yes it is, 18,000+ of the vertical type and 22,000+ of the horizontal type currently in stock. 15 weeks if you need more than that. ;-) – Spehro Pefhany Jun 15 '17 at 18:08
  • Why wouldn't this part be suitable for 35VDC input (datasheet says up to 36VDC input) or rectified/filtered 24VAC (which is just DC if the filtering is done correctly)? – Matti Virkkunen Jun 15 '17 at 21:12
  • @MattiVirkkunen Too close to abs max. 24VAC with 20% regulation and filtering is more like 40VDC. Bzzzzzzt & acrid smoke. – Spehro Pefhany Jun 15 '17 at 21:23
  • @SpehroPefhany I've got a bunch of Chinese DC-DC switching converters from eBay and other such places. They all seem to work fine and meet the specs in the datasheet. I've had issues with counterfeit MOSFETs (usually logic-level ones) but never power chips. Do you have any information about counterfeit DC-DC converter chips and how one might identify them? – heypete Jun 16 '17 at 00:44
  • @heypete Anything with NS markings is probably counterfeit as TI bought them years ago (and changed the markings to say TI). Doesn't mean they are *bad* of course, but they don't seem to be very proud of them. There is at least one Chinese company that makes an improved version good for more current and it seems okay, at least without pushing the limits- but they have a real datasheet and all. – Spehro Pefhany Jun 16 '17 at 00:47
  • @SpehroPefhany Interesting, thanks. After I responded to your first comment, I searched for some stuff on Google. Turns out these are all LM2596 and LM2577-labeled (though latter are actually mislabeled LM2576, or clones) chips. They work, to the point where I've never had any issues and the output voltage is nice and smooth, and I (usually) don't push them too hard. Still, it appears the ones I have are fakes that have a different switching frequency than the datasheet advertises. – heypete Jun 16 '17 at 01:25
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Your 7805 has to drop say 19 V if your effective input is 24VDC .Even if you took 100mA you would be burning 1.9watts and your finger if you touched it .You must use a big heatsink because your input /output differential is large .This is why Buck convertors are so popular.

Autistic
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In my opinion you are doing it wrong in 3 ways.

  • You have two 12V transformers in series, since this does not sound like a streamlined / series production you may as well sacrifice some pennies and put a bridge + capacitor(s) at the 12V junction. You'll have a third wire that provides a much more efficient (less heat dissipated) and apt voltage to feed to the 7805.

  • Second: even with 12V you could still have heat dissipation issues. There are TO-3 versions to help with that.

7805 in TO-3 package

  • Third, even a TO-3 7805 sounds quite insufficient for a fully expanded (USB and other stuff) Raspberry. You might want to check alternatives out. In example: LM323 (TO-3, 5V, max 3A). Here is a link to a LM323 datasheet.

Or, you could skip the issue altogether and use a switching DC-DC converter.

Dario Fumagalli
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  • My understanding is that the OP is using two 12 V DC PSUs in series to simulate a industrial standard 24 V supply. – Andrew Morton Jun 15 '17 at 21:14
  • Correct, my two 12V supplies is all I have at home. At work I have industrial 24V supplies ;) – Peter Quiring Jun 15 '17 at 23:04
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    Is Tesla still around? I remember they made TVs back in the old days. And components. Yugoslavian, right? – Spehro Pefhany Jun 16 '17 at 01:20
  • @PeterQuiring in this case you might want to see if something like these could help you: https://www.youtube.com/watch?v=QR8qsygs1Kw and http://www.instructables.com/id/DC-to-DC-Converter/ – Dario Fumagalli Jun 17 '17 at 08:24
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    @SpehroPefhany I wanted to post a 7805 in TO-3 package (regardless of brand), so I picked a random picture from Google Images. That's it :) – Dario Fumagalli Jun 17 '17 at 08:29