AC constant-current source design

I want to provide a fairly constant current (say 10mA RMS, peak 20mA, of 60Hz AC, using a 120V supply) to a load of highly variable resistance. It doesn't have to be super-clean or precise, but should be able to adjust within a few cycles and never stray more than 100% from set current level.

The contemplated load is an electrolytic chemical reactor. It'll be a lot easier to tell once I can feed some current through actual reagents, but best guess right now is that resistance can vary from single-digit to thousands of ohms depending on all sorts of things (temperature, reagent phase, etc.). So I'll want to pick a current and be able to hold that relatively constant as all the other internal and external parameters vary.

What components or circuits can accomplish this?

Answer

The simplest way to build an active AC constant-current source takes only 4 parts:

• A suitably rated bridge rectifier (600PIV, 1A works)


• A suitable resistor (you'll have to try several values)


• A HV depletion MOSFET such as the IXTH20N50D


• And a bit of heatsinking -- the FET dissipates a fair bit of power

Theory of operation: This is your standard JFET constant current source, just bigger thanks to the power depletion MOSFET. AC operation is provided by connecting it to the DC terminals of a bridge rectifier. (RL is a sample load -- whatever load you wish just connects in series, the circuit is insensitive to load position and polarity.)

^{simulate this circuit – Schematic created using CircuitLab}