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Author Topic: Current Draw of A Voltage Regulator  (Read 4629 times)

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Offline airman00Topic starter

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Current Draw of A Voltage Regulator
« on: October 03, 2008, 09:48:22 AM »
Hello,

Lets say a voltage regulator provides 5V @500mA output . Does that mean that the regulator is always drawing 500mA from the power source or is the current draw of the regulator dependent of the circuit draw on the output of the regulator?

So for example if I were to connect a 25 mA draw to the regulator, would the regulator be drawing the full 500mA from the source or would it be drawing only the 25mA ?

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Offline Trumpkin

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Re: Current Draw of A Voltage Regulator
« Reply #1 on: October 03, 2008, 10:20:50 AM »
I think the regulator can draw up to 500ma but I don't think it would draw 500ma all the time.
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Offline izua

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Re: Current Draw of A Voltage Regulator
« Reply #2 on: October 03, 2008, 10:34:52 AM »
No, it will always draw more. It depends on the input voltage, too. Talking, of course, about the regular type (with a voltage reference and an unsaturated junction acting as a power output)
There are also the low dropout and the switchmode regulators, which reach way higher efficiencies.
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Offline airman00Topic starter

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Re: Current Draw of A Voltage Regulator
« Reply #3 on: October 03, 2008, 10:45:59 AM »
so it would draw the full 500mA all the time?
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Offline izua

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Re: Current Draw of A Voltage Regulator
« Reply #4 on: October 03, 2008, 10:51:06 AM »
in my experience, linear voltage regulators sink a bit more on the input then the current sourced on their output.
so, if you will draw 500mA all the time from a LVR's output, it will draw at least 500mA on its input.
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Offline ArcMan

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Re: Current Draw of A Voltage Regulator
« Reply #5 on: October 03, 2008, 10:53:06 AM »
No.  In your example, the regulator would draw approximately 25 mA - about the same as your load.  The thing to remember about linear VRs is that (Vin-Vout)*I = power dissipated by the chip.

Offline dunk

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Re: Current Draw of A Voltage Regulator
« Reply #6 on: October 03, 2008, 11:34:48 AM »
ArcMan beat me to it while i was writing but posting anyway...

Quote
Lets say a voltage regulator provides 5V @500mA output .
sorry to be picky but there is a problem with your opening statement.
the voltage regulator you are talking about can provide up to 500mA @ 5V.
this regulator will always provide 5V.
how much current is drawn from it depends what you are connecting to it.
the maximum load of the regulator is 500mA but it is perfectly acceptable to draw less from it.
it's important to understand this bit if you are going to understand the next bit.

Quote
No, it will always draw more. It depends on the input voltage, too.
nope. not true.
in a typical linear regulator the current draw of the regulator is around the same as the current draw of the components drawing current from it.
i say "almost". the regulators Datasheet will tell you what the additional quiescent current loss is. it is safe to ignore this small amount for the sake of this discussion.

so, in answer to Airman's original question,
if you are using a linear regulator you can presume that the current drawn is the same all the way through.
for your example where you draw 25 mA from a regulator, it will draw 25 mA from it's source.
if you connect nothing to the output of your regulator it will draw nothing from your battery. (apart from the afore mentioned quiescent current...)

it's worth noting that if you connect a 5V regulator to a 10V supply and draw 25 mA from the regulator the regulator is providing 0.125Watts of power but it is using 0.250Watts of power so it is only being 50% efficient.
(Power = Volts * Amps)

obviously for other kinds of regulators things are different.


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Offline airman00Topic starter

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Re: Current Draw of A Voltage Regulator
« Reply #7 on: October 03, 2008, 11:38:48 AM »
OK I understand now

thanks everyone
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