Adjustable Current Output

[moctod]

Trying to build an adjustable power supply that will maintain constant voltage output but will vary available current output based on signal voltage. 12-24VDC input for power, 0 to 12 VDC variable input signal voltage, 0 to 9 amps variable output current based on signal input voltage. Tolerence within +- 10%. The signal voltage will be from a different power source than than the 12-24 VDC power supply. Anyone know of such a circuit?

 

[audioguru]

If the output of a power supply is maintained at a constant voltage then the load's resistance determines the output current. It is Ohm's Law.

You can also make a variable current power supply. It can maintain a constant current and the resistance of the load determines the output voltage. It is also Ohm's Law. Do you want your signal to vary the amount of constant current?

 

[Russlk]

The usual bench power supply has adjustable constant voltage and adjustable current limit. Is that what you are trying to do? Some power supplies fold back to a low current and voltage when the current limit is exceeded, but it appears that you want the output to be constant current when the limit is reached. Is that the case?

 

[moctod]

Adjustable Power Supply - Reply

Resistance of load is essentially fixed but very low. At the rated forward voltage the load will essentially draw as much current as allowed (or possible) until it self destructs. I am attempting to restrict the amount of current that the load can draw (while still being turned) on based on a voltage signal input. The load may best be compared to an avalanche or zener diode that once the Vf is attained will conduct. The current that the device is allowed to conduct is to be controlled by a separate voltage signal. The current draw for the voltage signal must be very low. I am working with an LM117 voltage regulator driving two MOSFETs and attempting to control the current in that manner but have been unsuccessful thus far.
I do want the signal to vary the amount of constant current within the limits of the load. I also want to maintain a constant voltage within reason. I understand that there will be voltage drop based on the amount of current drawn, but the power supply should attempt to compensate for the voltage drop due to increased current flow.

 

[Sceadwian]

What you're asking for is impossible moctod. If you want to adjust the current you have to change the voltage, you can't have a static voltage and a variable current the load itself determines where that curve is. You can either have a constant voltage, which by the sounds of your device won't work unless the voltage regulation is absurdly high. Or what you really want is just a variable current source that's controlled linearly by a voltage source. What exactly is the load? Couldn't you just design the power circuit to handle the maximum possible load all the time and the PWM it using a voltage to duty cycle converter or micro controller?

 

[Russlk]

This is one way to have a constant current controllable by 0 to 12 volts. You could put an op amp buffer in front of the control if the source cannot drive 11K.

 

[moctod]

Thanks! I was considering something like this. It may be the best solution.

 

[moctod]

This is one way to have a constant current controllable by 0 to 12 volts. You could put an op amp buffer in front of the control if the source cannot drive 11K.

Thanks! This may be the best solution. I had considered this but wanted to investigate other posible solutions.

 

[moctod]

What you're asking for is impossible moctod. If you want to adjust the current you have to change the voltage, you can't have a static voltage and a variable current the load itself determines where that curve is. You can either have a constant voltage, which by the sounds of your device won't work unless the voltage regulation is absurdly high. Or what you really want is just a variable current source that's controlled linearly by a voltage source. What exactly is the load? Couldn't you just design the power circuit to handle the maximum possible load all the time and the PWM it using a voltage to duty cycle converter or micro controller?

Maybe! Yes it can be done using PWM but was attempting to keep circuitry as simple as possible. I have seen several power supplies using PWM for this purpose and considered it. Thanks!