# limiting amperage

Discussion in 'Electronic Basics' started by John Whittaker, Dec 3, 2006.

1. ### John WhittakerGuest

Hello all.

I have an electronic device that runs on twelve volt, and I would like to
limit the amount of amperage it can draw to approximately one amp, but
maintain the voltage at twelve. How do you do that?

Thank you in advance, John Whittaker, Portland Oregon

2. ### Tom BiasiGuest

Well, first does you device need more than one amp.?
Devices take the current that they need at their rated voltage.
You can limit the current delivered but the device may not work.

Tom

3. ### BaronGuest

You will need to use a current limiting series regulator ! But you will
also need to account for the overhead in voltage required by it !

4. ### Costas VlachosGuest

Check the "Typical Applications" section of National Semiconductor's
data sheet for the LM317 adjustable regulator. There are a few current
limiter circuits in there that can do what you want.

Note that if your electronic device requires a current that is larger
than the chosen threshold (1A in your case), then the current limiter
will reduce its output voltage automatically to limit the current. So
you'll either have V = 12V when I <= 1A, or V < 12V when I = 1A.

--
Regards,
Costas
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5. ### Costas VlachosGuest

Correction: V = 12V when I < 1A, or V <= 12V when I = 1A.

--
Regards,
Costas
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6. ### GarethGuest

Voltage and current cannot be controlled separately, so if your device
is drawing more than 1 Amp any current limiting device that I'm aware of
would reduce the voltage to reduce the current.

It may help us to help you if you could tell us why you want to limit
the current and what the device is.

Gareth.

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7. ### John WhittakerGuest

Yes, it will draw up to probably or seven total. It is actually an
electric locker for the rear end of my jeep, and the more amperage it draws,
the more it locks, so I would like to limit it to around one amp with one
switch position which should offer a strong limited slip, and then total
amps in another.

Thanks, John

8. ### feeboGuest

what does it take already? Have you measured the current it takes?

you can't limit the current and expect no side-effects.

very basically, if your device draws 2 amps at 12 v, thet is 24Watts.
If you put a regulatror (or even a resistor) in place to limit the
current to 1 amp, the device will still try to draw 2 and the voltage
will fall in an attempt to provide that current - whichever... it is
unlikely the device will work propperly on reduced current or reduced
voltage.

What is the application? Put more meat ont ehbone for us.

9. ### JamieGuest

That is kind of a twisted question. if the device attempts
to draw more than 1 amp, lowering the voltage to prevent over
load is the only thing you can do! thus, there goes your voltage!
Maybe if you could tell us what the device is? I think you would
need to instruct the device it self to not request any more when it
reaches it's 1 amp, instead of just choking it off which might just
make it malfunction.

10. ### Rich GriseGuest

I'd use a 1A fuse, or maybe 1.5A, if the device is actually rated 1A.

Good Luck!
Rich