LM334 Constant Current Source

[Gerry Burne]

I am feeling very stupid because I cannot see where I am going wrong with the simplest of circuits. I want to build a TENS device which passes a square wave current through the skin adjustable between 10 to 600 microAmps. The LM334 device looks suitable. Its datasheet shows it giving a current determined by a resistor between its OUT terminal and its ADJ terminal but I am getting an unexpected Output current.

I am feeding its INPUT from +15Vdc through an ammeter. With a 1K resistor the current is 1.9mA, with a 20K I get 1.7mA
and with 100K I meas ure 1,8mA. Can anyone suggest what might be the reason for these unexpected results? Thanks.

 

[OBW0549]

LM334's are about as absolutely foolproof as you can get; your circuit on the right, with IC1 and R1, should give you an output current of approximately 194 μA at room temperature. What I can't figure out is what in the world you are doing in the circuit on the left, with P1; what's up with that?

By the way, you're not going to have any luck using an LM334 to generate a usable output for a TENS device: it takes a LOT more voltage to force 600 μA through typical skin resistance than what an LM334 can tolerate, and with only 15 volts powering the circuit the maximum current you'll get is insignificant.

 

[Gerry Burne]

Thank you so much for responding. The machine that I wish to emulate can be viewed on www,precisiondistributing.com . Its specification provides two outputs isolated from each other whose frequencies are independently set in the range 0.1Hz to 999.9Hz, The maximum output voltage is 30. The current outputs are not independently adjustable and are 10, 20, 40, 80, 100, 150, 200, 400, 500 and 600μA.

Sorry about the confusion; I found a picture to illustrate what I am doing, My circuit is the one with the 330Ω resistor. My Vo is 15V, It passes through my Ammeter to +V and as I said in my post, I measure mA's rather than μA's for values of R1 between 1K and 100K.

If I hold the probes on my multimeter with moistened fingers then I measure about 100K across my body which would give 300μA from 30V so maybe 600μA might be possible with proper electrodes and a shorter skin path?

 

[OBW0549]

My circuit is the one with the 330Ω resistor. My Vo is 15V, It passes through my Ammeter to +V and as I said in my post, I measure mA's rather than μA's for values of R1 between 1K and 100K.

If you're measuring mA instead of μA, either your LM334 is defective or you've wired something wrong; those are the only two possibilities.

 

[Gerry Burne]

Thanks once again. I took it all apart and put it back together again and I can now get 9.6μA with a 10MΩ resistor so it must have been finger trouble on my part. Best wishes.