Feeding 15 vac to a 7805 regulator

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

Hep!
Before I toast my Paia mid2cv8 - the manual says to get a 12 vac transformer, but I can only get 15 vac. It goes through a 7805 regulator, and if I read the datasheet correctly, it should work fine with 15 vac.

But...should I heatsink the regulator? The manual says the circuit could draw around 200 mA, but I think I read somewhere to heatsink 7805's when drawing more than 100 mA, and won't it have to dissipate more power with a 15v supply, than with 12V?

C

Blue Hell · Posted: Mon Jan 15, 2007 3:32 pm Post subject:

ok, here we go.

15 Volt, allow for 10% regulation of the transformer (it's output voltage being too high when no load is present). This results in 16.5 V. Allow for the mains voltage being 10% high, which results in 18.15 V. Which means the paek voltage out is 1.414 * 18.15 -> 25.7 Volt. (1.414 being the square root of 2)

This is smaller than the allowed 40 V in for the 7805, so that's good.

Then for the power dissipiation. Power is Voltage times Current. Over the regulator we have 25,7 - 5 = 22.7 V. The current is 0,2 A, so the dissipated power will be a bit under 5 W(att).

This is smaller than the allowed dissipation of 20.8 W (at 25 deg Celcius environment temperature, which is "room temperature"), so that's good as well, and it allows for some higher env. temps.

However the maximum dissipation in free air (no heatsink) is just 2 W, so you will need cooling, you'll even want good cooling -> a somewhat larger heatsink.

The good news is that the regulator has thermal self protection, when it gets too hot it will shut down it's output and not fry your circuit. So when your circuit stops working on a hot day it's a sign that a better heatsink is needed.

As a rule of thumb, when you touch the regulator with a wet finger the water should not start to boil off immediately.

Dragon's Lair · Posted: Mon Jan 15, 2007 10:37 pm Post subject:

I have a PAiA MidiCV8 in my dotcom system, I am running it off the dotcom +/-15v power supply, with no problems. Even after it's been on for a few hours the regulator on the midiCV8 is not even warm, which is good, since there is not a lot of room around the 7805 to put a heatsink on it.

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

My question answered both in theory and in practice! Cool

Jan, I have to say you've cleared up some things for me that have been puzzling me in the past, so thanks for that. And, apparently, you do a much better job of reading a datasheet than I do - I never got that much info out of it...

Also, it's good to hear from someone actually doing this in practice, with the same circuit, so thank you, Dave.

Great place, this...

C

Yorky · Joined: Feb 14, 2005 Posts: 244 Location: Boston, UK

fluxmonkey · Joined: Jun 24, 2005 Posts: 686 Location: cleve

Blue Hell · Posted: Tue Jan 16, 2007 9:36 am Post subject:

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

Hmm, I've got a couple of heatsinks - not that any of them fit in there, but still: How big a heatsink would I need to dissipate 5W?

Would something like this be enough?
http://www.referenceaudiomods.com/images/products/ram/hs5w.jpg

C

Dragon's Lair · Posted: Tue Jan 16, 2007 3:20 pm Post subject:

One of those should work fine, besides, it's probably the only type that will fit in there, but before you go all out on heatsinks, hook up the power and see if it actually gets hot enough to warrant it.

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

Hi Dave, will do that.
Nice site you have there, btw... Cool

C

Dragon's Lair · Posted: Tue Jan 16, 2007 11:46 pm Post subject:

Thanks, I'm working on some updates as we speak, I'm also looking at doing some DIY pages, but I have to get a better camera first, so I can take some close up pictures.