noobie question re: voltage regulators

As often happens in engineering, the answer is "it depends".

If it's a linear regulator, the maximum load current will be 750mA minus whatever the regulator uses for its own purposes (eg. 5mA for a

If it's a switching regulator, the maximum load current could be more like 1.5A, depending on the efficiency of the regulator and how much input voltage tolerance you need.

Best regards, Spehro Pefhany

On Sat, 22 Oct 2005 22:05:22 +0800, "Rheilly Phoull" wroth:

The Ohm's law that I'm familar with goes something like this: I=E/R. So, are you're trying to say that if I added a one Ohm load to the combination in question, that I should expect 5/1 or 5 Amps of current?

Jim

I'll read more in your question: that you argued with someone on this topic and decided to ask the "experts." Reading the answers so far, I'd say you're more confused than before asking.

That's partly because you're question was ambiguous and engineers don't want to be wrong. I'll go on a limb here and rephrase your question so it reflects what you want to know and removes ambiguousness: "can a voltage regulator put out more current that goes in?"

The answer is yes -- if it's a switching voltage regulator. Those try to "burn" as little power as possible in the process, so what comes out is almost all of what goes in -- almost all of a maximum 9W in your case, so the 5V current is no more than 9W/5V = 1.8A. (A good number is

If it's not a switching regulator, it's a linear regulator, and the answer is no. The linear regulator is like a variable resistance in series with the load, changed by a feed-back loop so the voltage on the load stays just so (5V); everything being in series (12V souce, regulator, load), they all get the same current.

Two main points (I'm assuming an unregulated, "analog" supply - not a "switcher" like a PC supply):

1. You would get 5 VDC, regulated, as long as: a) the supply voltage to the regulator exceeded "5 and a bit more" volts, (check the regulator specs for "the bit more"), b) you stay wthin the current rating of the regulator, e.g 1 amp, or whatever, and c) the current drawn multiplied by the voltage drop across the regulator (the "5 and a bit" volts minus 5 volts") does not exceed its power rating.
2. The original power supply is presumably 12 volts (nominal) at around 750 mA. Its voltage/current characteristics (internal resistance - read up on Thevenin as well as Ohm's law!) will determine what current it will give before it goes below the "5 and a bit" volts that I mentioned above. Anyway, at that point it would get very hot and burn out quickly! There's not enough data to go on, so which limits first (reg. or P/S) would be pure speculation.

Hope this helps. Be careful in everything you do... always. Cheers, Roger

The output voltage would be around 5volts, the current would be decided by the load you apply to it. You should perhaps revise your knowledge of ohms law etc.

Hello Rheilly,

Wasn't there a guy recently who wanted to have that law declared unconstitutional?

Regards, Joerg

Was it a 'free speech' issue ? ;-)

Graham

We've got 'ome invasion laws over here, is that similar?

Ken

At a maximum of 750 mA, assuming you're talking about a series regulator, like a 7805. A series regulator is _not_ a transformer, only a self- adjustable series resistor, in effect.

Now, with a switching power supply, you _can_ get more than 750 mA out, because instead of just throwing power away, it sort of transforms it - Somebody who knows more about it than I do will have to take up the explanation of switcher theory here. ;-)

Hope This Helps! Rich

Yes, and an ohmmeter is a British termite.

Thanks, Rich