secrets about voltage regulator

manjish · February 20, 2007, 03:26:36 AM

guys anyone can explain how current is limited in a volt regulator like 7805.

it says it gives 1.2-1.3 ampers of current.but my equipment only needs 300 ma of current.
so can i use it for my device safely.i heard that these regulators only provide current that is demanded by the device .
is it true??guys help really wanted

cheers

zyxwyvu · February 20, 2007, 04:28:33 AM

You are correct, the voltage regulator will only supply the current that is needed. All you need to do is connect the in, out, and ground terminals correctly, and you should get 5V (for a 7805) on the out end. Just make sure you have a few volts higher on the in end so the regulator can work properly.

Sir H C · February 20, 2007, 06:26:59 AM

A voltage regulator tries to keep the output voltage constant regardless of the current load demanded of it. The problem is that if you short the output then you can get more current than the regulator can handle and it would blow up. To keep this from happening, they put current limiting and thermal protection on most if not all regulators. So, so long as the regulator is good for more current than you need (with the heatsink if needed), you are safe.

RaceDriver205 · February 20, 2007, 08:04:06 AM

= You can safely use a 7805 for your equipment.

manjish · February 22, 2007, 03:07:40 AM

thanks guys actually i was planning to built a ac adaptor for my creative zen micro which needs about 2 ampere of current to charge at 5 volts
so i am planning to use two 7805 regulators in parallel to supply the power needed as 1 7805 can only handle about 1.3 amps of current
is that the thing i had to do ?am i correct with my little knowledge?

help guys

Paul Perry (Frostwave) · February 22, 2007, 06:45:14 AM

Sorry, you can't simply parallel regulators. They don't play nicely that way.

Sir H C · February 22, 2007, 04:53:13 PM

Linear regulators will smoke if you try to run them in parallel. First, there desired output voltages will be slightly different and so one will be working much harder than the other to supply the current.

They make TO3 can 7805 regulators that can handle 3 amps and more. Be sure to properly heatsink the device too, as the regulator is only as good as the heatsink it is attached to.

manjish · February 23, 2007, 03:29:56 AM

thanks guys.you saved my money.can anyone say what am i supposed to do now then

gez · February 23, 2007, 03:35:55 AM

IIR there was a little snippet in EPE once that showed how to parallel regulators. I think it used small value resistors at the outputs and linked them, so that the resistors drop any slight voltage differences. Don't quote me on that though (I'd need to find the article)!!

There are better ways to get more current from a regulator. Some techniques are outlined in R Penfold's Practical Power Supply Projects.

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secrets about voltage regulator