# Variable Current Source for mixed signals - Need help

Discussion in 'The Projects Forum' started by pindnet, Jan 18, 2011.

1. ### pindnet Thread Starter New Member

Nov 24, 2010
2
0
Hello all, recently I started working on a function generator that gives ouput of different waveforms, such as: sine, sine half wave, sine full rectified, square, triangle, saw tooth, biphasic square, etc. with a constant output of 30Vpp. My question is about how dow I need to handle this signal in order to vary its ouput current in a range of 1mA up to 150mA with a variable Rload in a range of 500Ω up to 10KΩ so the output current must stay "constant" within this range. I need to be able of change the output current digitally, maybe using a DAC, Variable gain opamp...

I have been looking and some theory and circuits about Voltage to Current Converter with grounded and floated load, Current Mirror. But I still don't fully understand how do I need to "connect" the constructed signal into the current block prior to connect it to the RLoad. Since all the examples I found in books, google, connect the respective outputs to a fixed source such as V+ instead of a variable one (maybe AC).

So any idea, further information, example is highly appreciated. Thanks in advance.

2. ### wayneh Expert

Sep 9, 2010
13,427
4,265
You mean the time-weighted-average current must remain constant? If so, I think at the least you'll need an integrator to convert your waveform to its average value. What does your load look like - purely resistive? That would help, because in that case the voltage waveform is the same as the current waveform. Inductive and capacitive loads can change that, so that current and voltage are out of phase.

3. ### SgtWookie Expert

Jul 17, 2007
22,194
1,761
If your load is a fixed resistance, to keep a constant current flow through the load implies a constant voltage, as I=E/R (current = voltage/resistance).

In order to get 150mA current through a 10k Ohm load, you would need .15*10k = 1,500 Volts on the output, and your load would be dissipating 1500*.15 = 225 Watts of power. The high voltage, high power aspect makes this a very dangerous project for a hobbyist to attempt.

So, your request for help really doesn't make much sense at this point.

What is it that you really want to do?

4. ### pindnet Thread Starter New Member

Nov 24, 2010
2
0
Thanks for your replies. Finally I understand and accept that Ohm is the law. I decided to increase the voltage to handle few mA and with a current mirror and a "digital" control using an array of transistors I make some resistors divisions and a finally get a acceptable current control. I took the idea of the transistors from the datasheet of the L200 voltage regulator and adapted to my needs. Maybe is not a very smooth control but I can achieve 2mA steps from "0" up to 36mA. Thanks for your assistance.