Two LM7805 in one circuit

Good day!

Is it possible to use two LM7805 in one circuit connecting their grounds together and powering parts of the project. I need more that 1A (for powering LEDs) and have a lot LM7805 lying around. Will using LM317 will be more efficient?

Don't forget the decoupling capacitors on each regulator on the input and output as close to the regulator as you can get.

[quote author=Richard Crowley link=topic=50075.msg356887#msg356887 date=1296125095] Yes, if we understand your verbal description correctly, you can use as many regulators as you wish, as long as all the grounds are tied together. LM317 is a linear regulator just like 7895. They are equivalent in efficiency. Switching regulators are more efficient, but somewhat more difficult to use. [/quote] Yes, you understand my faulty description right :) Some more questions:

  1. It seems LM317 can handle 1.5A. Which is right if I need about 1.2A -- use one LM317 (with appropriate support circuit as described in datasheet) or two LM7805?
  2. Another related question -- is this appropriate to connect LM7805 in parallel (connect together not only grounds but Vout too?) to get more than 1A

[quote author=Richard Crowley link=topic=50075.msg356887#msg356887 date=1296125095] Don't forget the decoupling capacitors on each regulator on the input and output as close to the regulator as you can get. [/quote] Thanks, it seems datasheet says that they are needed on LM7805.

Nice thread about connecting LM7805 in parallel (and why one shouldn't do this): http://www.mp3car.com/vbulletin/power-supplies/65225-2-lm7805s-parallel.html

For the sake of completeness, the max current (power) available from a regulator is dependent on a good heat sink. It does not actually matter what the Maximum current rating is if you do not have a heat sink capable of keeping the regulator cool.

So 3A regulator anyway will need heatsink even if running on 1,5A and minimal Vin Vout difference?

So 3A regulator anyway will need heatsink even if running on 1,5A and minimal Vin Vout difference?

Not necessarily I think. You have to look at the package being used and it's ability to dissipate heat, this will be shown as deg-per-watt figure in the spec sheet for the different packages. Also the max die temp allowed and the way the package is mounted on the PCB and the size of any copper pour under the device (which is affectively a heat sink) and if it's in free air, encapsulated etc. Then look at the expected ambient temps.


Rob

Thanks!

I need to narrow down my regulator options and make power calculations (Crumpy Mike has nice examples).

artvolk: Is it possible to use two LM7805 in one circuit connecting their grounds together and powering parts of the project. I need more that 1A (for powering LEDs) and have a lot LM7805 lying around. Will using LM317 will be more efficient?

You could hook to lm7805 devices in parallel to provide more current, rather than purchasing a +5v regulator rated at more than 1Amp. ill post a schematic if you like. you want to still include the decoupling caps in such a design. it would be the lm7805 share VIN and share GND. Then put decoupling caps from Vout to ground on both Vout1 & Vout2. Then... IDK, you might want to connect a fuse from Vout1 & Vout2 to the Vregout. I guess thats optional, an inductor might work, or a 0.5ohm resistor, or what ever. basically you want to make it so that if one regulator fails, it doesn't take out all your regulators, you blow a fuse or something. But you might decide you have numerous regulators to spare.

if you have a stable Vin, within the specs of an lm7805, save for the current, I wouldnt bother with an lm317. The efficiency is probably negligible. I would guess that a simpler circuit has greater efficiency. lm317 requires a voltage divider, so.. that's where I'm making the efficiency guess. In actuality, thy are vary similar regulators. i would guess that the lm7805 just has a fixed resistor bridge inside the device. data sheets may provide efficiency curves you can look at.

how much current do you need?

It's 2013, linear regulators are so last century when wasting energy and making waste heat was still in fashion. Make the break to switching regulators whenever possible.

http://www.ebay.com/itm/251066005460?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Lefty

If you Must deal with the heat and other issues… And stay in the last decade… Try an LM338K Steel, It’s a variable regulator 5 A voltage regulator in a TO-3 case (Steel) The TO-220’s (LM338T) are good for 3A I think… With suitable ballast resistors they ‘can’ be paralleled But what a waste especially since 2A buck mode (step down) switchers can be had in Ebay for a dollar or two… Certainly two for less than the cost of a high current wasteful linear regulator. Lefty was WAY Right. IMO

Bob

artvolk:
So 3A regulator anyway will need heatsink even if running on 1,5A and minimal Vin Vout difference?

Most 7805 style regs have a drop-out spec of 2V, so minimal Vin Vout difference is 2V - which
means 3W, which means a decent heatsink required.