AC constant-current source design

Question

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 1

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}

Answer 2

A DC constant current source

This answer is based on a simple op-amp DC constant-current configuration.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. DC constant-current amplifier.

• Set the wiper on VR1 to give 2 V.
• OA1 output will quickly rise, turning on Q1, causing current to flow through the load, Q1 and R1.
• When the voltage on R1 rises to 2 V the circuit will stabilise.
• At this point the current through R1 = 2 / 100 = 20 mA so we have made a variable current source and the control voltage gives 10 mA/V on the output.
• In practice R1 is measuring both the load current and the base current so there is a little error.

An AC constant current source

10 mA at 120 V is only 1.2 W. An audio amplifer should be able to drive a transformer to achieve what you require.

^{simulate this circuit}

Figure 2. Constant current inverter.

This is only a rough sketch but may get you on your way.

• XFMR1 gives a 6 Vrms sine-wave to the power amplifier chip wired as a unity gain buffer.
• The power amplifier output drives a step-up transformer.
• R3 in the output completes the amplifier feedback circuit. With 600R shunt 6 Vrms feedback will be generated at 10 mA.
• It would probably be a good idea to add overvoltage / spike protection on the feedback circuit in case of transients from the transformer. A 1k resistor and a pair of diodes to each power rail might do the trick.
• R2 is intended to limit the current into the amplifier in the event of XFMR1 powering up before the amp.

Comments on stability, etc., are welcome.

Full circuit

^{simulate this circuit}

Figure 3. Full (untested) circuit.

• XFMR1 upgraded to 12 V to power rest of circuit using D1, C1, D2, C2. No voltage regulation is required as we are not too worried about noise.
• R2, VR1 form an adjustable voltage reference between 0 and 6 V rms. C3 helps take out any high-frequency noise from XFMR1 as this may cause problems with XFMR2 and the feedback loop.
• Note the 'dot' convention on the output transformer. You won't have dots on your transformer so you may have to invert polarity.

Search for a high-powered op-amp to do the job.

Low-voltage AC constant-current source

This section is to aid understanding of the feedback circuit.

^{simulate this circuit}

Figure 4. Simple, transformer-less AC constant-current source.

The circuit shown in Figure 4 leaves out all but the essential components for the variable AC CC supply.