Current Source where the source generation and load are isolated

I have a circuit I am treating as a "black box" but it involves a switching regulator. I am trying to "inject current" into the feedback loop of the regulator in order to change the output voltage, but I find that when I do so, it throws both the circuit involving the regulator, and the current source I have created into an unstable state. The current source I used is basically the howland current source (with no zener diode because current must flow in both directions) **broken link removed**

I find that when I greatly increase the output resistance of the current source, then the circuit with the regulator doesnt break down, but the power dissipation of the current source is just way too high (in the range of over 100W). What I need is a design of a current source where the source generation and the load are ISOLATED from one another, so that the feedback loop of the regulator circuit will not affect the current source in any way!


The reason I want to do this is because just using eepots to change resistor values is not stable / accurate enough... the current source i need has to be in the range of -20 to 20 mA, so stability / low noise is a MUST.

Also, any good information about switching regulators, and IF I CAN actually change the output voltage of the switching regulator by injecting current would be awesome!

Any help would be greatly appreciated!

Re: Current Source

If you need a 20mA clean current source, why not build it with and opamp?
Why do you need the switching regulator?
Perhaps I do not fully understand the application. Can you be more specific?

A good source of info for switching regulators are the websites of TI, National, Linear Tech, Maxim.

Re: Current Source

VVV said:

If you need a 20mA clean current source, why not build it with and opamp?
Why do you need the switching regulator?
Perhaps I do not fully understand the application. Can you be more specific?

A good source of info for switching regulators are the websites of TI, National, Linear Tech, Maxim.

Click to expand...

Basically I have 2 separate circuits... The switching regulator is on the circuit I am trying to modify and The current source gets connected to the circuit with the switching regulator in order to modify the output voltage of the switching regulator.

There is a voltage divider in the feedback loop of the switching regulator, and I want to use the current source to inject current into the voltage divider to change the output voltage of the switching regulator. I can NOT modify the circuit with the switching regulator besides the resistors in the feedback loop, but I thought to make things easier, just soldering in 1 point (current source) to the feedback loop would be ideal!

The output resistance of the current source is not high enough, so the regulator is messing around with my current source... for this reason, I am looking for a current source with a theoretical infinite output resistance, or something where the load can be electrically isolated from the source.

Hope that clears up what I am trying to do!

Re: Current Source

Hi,

Part of the problem *may* be that you are upsetting the control loop of the regulator, so that becomes unstable. If you want to change the output, can you not take the feedback signal from the reg output and modify that for instance with a variable gain amp driven by a DC voltage? That would keep the control loop intact but modifies the loop gain. That would have a better chance to work IMHO.

Jan Didden