Limiting current without effect on voltage?

[MathGeek]

Say I have a 9V source. Also, assume that I have a component that requires 9V and 50mA(constant). 50mA must not be exceeded. So I need to limit the current.

So, if I use a resistor in series with the component mentioned, the resistor would limit the current accordingly, but it would also eat away the voltage. (So the supplied voltage to the component would be less than 9v, depending on the value of resistance.)

So, how do I limit the current without affecting the voltage?

 

[akg]

using a series pass transistor , but the o/p will be slighlty less

 

[Nigel Goodwin]

Say I have a 9V source. Also, assume that I have a component that requires 9V and 50mA(constant). 50mA must not be exceeded. So I need to limit the current.

So, if I use a resistor in series with the component mentioned, the resistor would limit the current accordingly, but it would also eat away the voltage. (So the supplied voltage to the component would be less than 9v, depending on the value of resistance.)

So, how do I limit the current without affecting the voltage?

You need to specify EXACTLY what you're trying to do, it doesn't make any sense without knowing what's going on.

 

[oldtimer]

There are lots of ways to do it but we need to know what you have in mind since a fuse would do it but do you want to keep changing the fuse. A series tansistor would do it but you would have less than 9V (slightly) You say 50Ma but how much tolerance are you allowed.

 

[Dr.EM]

A series MOSFET though, running with 50ma through it can be made to have virtually no drop.

 

[Hero999]

You can either have a constant current or constant voltage source, you can't have both at the same time!

For examplle; voltage regulator might be a constant voltage source up to 1A then slip into constant current mode when the load tries to draw more than 1A. Constant current sources limit the current by reducing the voltage, this is why you can't do both at the same time.

 

[berniedd]

You could also step up the 9V to a slightly higher voltage via a DC to DC inverter and use a series pass transistor and resistor to bring it back down to 9V. The transistor would have to be wired with a clever foldback current limiting feature with the "knee" built at 50 ma. The foldback limiter is designed to reduce the output current should an overload occur; it automatically goes back to normal when the overload condition disappears. A clumsy design, but doable.

 

[jong143]

any one know this plz...curent limiter schematic diagram

can anyone help me plz....current limiter power supply 30V 6A output.. plz...a schematic diagram will do...plzz

 

[ericgibbs]

hi jong,

You are hijacking mathgeek's thread, not considered good manners.

Please start your own thread.

EricG

 

[ericgibbs]

hi mathgeek,
As requested, tell us what you trying to do with this 9V and 50mA.

Is the 9v source a battery???

EricG

 

[AllVol]

So, if I use a resistor in series with the component mentioned, the resistor would limit the current accordingly, but it would also eat away the voltage. (So the supplied voltage to the component would be less than 9v, depending on the value of resistance.)

So, how do I limit the current without affecting the voltage?

Live up to your forum name, MathGeek, and apply Ohm's Law to your situation. You will find that a constant 9 volts will supply a constant 50 mA through a 180 ohm resistance all day long, as long as v and load remain constant.

 

[JimB]

Eric and All

This is an eight month old thread which has been resurected by an "I want a circuit" request.

I dont think there is anything to be gained from replying to the original post.

JimB

 

[ericgibbs]

many thanks JimB.
Eric

 

[AllVol]

Good eye, JimB. Seems there's been a lot resurfacing lately.

 

[Nigel Goodwin]

OK - thread closed!

Moderator.