I need to build a variable current source that operates off a 36V supply, if possible using a single op-amp and a mosfet. I need to control current between around 10uA - 0.5mA ie very low current. This is for some iontophoresis research I'm doing.
The design I envisage is attached. The zener should give a ref voltage of 2.5V and R3 should give variable current based on I=2.5/R. That's my theory anyway. Anyone able to recommend R values, Mosfet, IC choices? My power source must be 36V.
This circuit would feed into an H-bridge, the output of which would go to an iontophoresis cell which is just two electrodes that connect to (usually) porcine skin (ears), inside a sealed chamber (like a Franz Diffusion cell). Typical resistance of pigs ears is 10k but sometimes we use skin obtained from surgical procedures or cadavers, in which case the resistance is approx 100k. My aim is to test various active molecules in terms of transdermal penetration, so it is quite important to have reasonably stable current for reproducibility. This is more important than accuracy. 36V is a convenient voltage for iontophoresis but in some cases lower voltages are used. For example, to treat hyperhidrosis (excess sweating) of the hands, it is possible to build a simple iontophoresis setup with a 12V battery. However I am dealing with quite large molecules that may require higher voltages.