5V / 3A with 2xLM7805?

Hello,

I have a project that requires powering a device through a USB connector. The device is a micro wifi router, which I've found does not like any PS under 1 amp continuous. Anything below that, it starts rebooting and acting strange. I've tried it with a cell charger (rated nominal 800 mA), an mp3 charger (rated nominal 500 mA), and with both it bombed. Then I plugged in one of those apple 1.2 Amp chargers... it's been going for almost 2 weeks without a hitch.

So that got me thinking about building something with an LM7805, which is rated at around 1.5 amps. And then I thought of something, that sparked this post.

Let's say I want to power something heavier off a USB type interface. Say I have a couple of those micro wifi's running off a single supply, and need to draw 2 amps or more.

Assuming I have a DC source with enough current capacity (say I build a rectifier circuit and end up with 7V or so with a 5 amp rating)... could I wire in several LM7805's in parallel, and join the output pins together, to get a single large current 5VDC source?

It will probably work if you're careful about decoupling the outputs on each one.

OTOH if this is something which is meant to be permanently switched on then using voltage regulators are a very inefficient way to do it. A switching power supply will save you a lot of money in the long term.

fungus: It will probably work if you're careful about decoupling the outputs on each one.

OTOH if this is something which is meant to be permanently switched on then using voltage regulators are a very inefficient way to do it. A switching power supply will save you a lot of money in the long term.

Agreed, in this day and age there is just no reason someone should build a 5v 3amp regulator using linear regulators, unless you already have all the parts on hand and don't care about efficiency and wasted heat. Switching regulators are very inexpensive these days: http://www.ebay.com/itm/251066005460?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Lefty

it's also not the best conceived idea to run linear 3 terminal regulators in parallel because there is just no nice way to guarantee that they share load equally.

I would go for a switching regulator too.

But if you want to use a 7805, then you can add a pnp power transistor+heatsink

It is still possible to put 2 (or more ;) ) 7805 in // , but as the output is not exactly the same for each, you need to add a diode at the output of each 7805, and you'll drop the output by about 0,6V . To make the ouput 5V again, you'll have to add another diode between the 0V pins of the 7805s (soldered together) and the GND ..... not really a good idea ;)

I agree with retrolefty. Use switching regulators. They are cheap, use three then, one for each board, if you want isolation. Caps are probably not needed, but if you have a few laying around, would not hurt to solder them on to.

Balancing the 7805's load levels is a tricky one....

Switching regulators (2-3amp) start around a buck on ebay....

If you really want to use a linear regulator, you can still get the 78H05, which is a 5V 5A linear regulator In a TO3 case. But I agree with the others, these days it makes far more sense to use a switching regulator, unless you need extremely low output noise/ripple.

pwillard: it's also not the best conceived idea to run linear 3 terminal regulators in parallel because there is just no nice way to guarantee that they share load equally.

Agree. In fact, it's virtually guaranteed that they won't. As others have said, switching regulators have many advantages, but if you're set on using a linear regulator, check the LM78xx datasheet, there are a couple circuits there that use an external pass transistor along with an LM78xx for higher current capability. Don't forget heat sinks :D

pwillard:
it’s also not the best conceived idea to run linear 3 terminal regulators in parallel because there is just no nice way to guarantee that they share load equally.

You can’t do this, they completely fight each other. Each will have a slightly different idea of what 5.00V is and one will take all
the current.

With some LDO regulators I’ve seen this cause severe over-voltage at the output (high frequency oscillation),
6 chips fried…

MarkT:

pwillard: it's also not the best conceived idea to run linear 3 terminal regulators in parallel because there is just no nice way to guarantee that they share load equally.

You can't do this, they completely fight each other. Each will have a slightly different idea of what 5.00V is and one will take all the current.

With some LDO regulators I've seen this cause severe over-voltage at the output (high frequency oscillation), 6 chips fried...

Mmmmm fried chips ....

You'd be better off feeding the 7805 to pass transistor as stated using a big assed power transistor and getting the current you need...

Try a 7805 for each board.

The 7805 idea, as you mention, is because I have more than a couple on hand. But then again, I had no idea switching regulators had gotten so cheap. Makes me want to leave those 7805's in the drawer and order the SR's!