Tag Archive for isolated power supplies

Voltage, it’s relative

So yeah, playing about with my new transformers, everything working fine. Until I realized one tiny hiccup. When I switched the control voltage board to it’s own transformer for the sake of the meters, I thought it a good idea to keep the circuit as is and have independent references for Vset and Iset. Though in theory this sounds like a good idea, I had one of those face-palm epiphanies. With two isolated power supplies, they have no common voltage reference. If I try to inject a voltage on the set pin of an LM317 and use the power supply ground as a ground references I get: 0V. It’s interesting to think about what this means. With no common reference, the potential is, effectively, zero. Hilarious! Also, utterly useless for my application.

Fixing this minor issue is quite easy. I have two possible options:

  • connect Vset and Iset to the set pin of the LM317 and the input on the current comparator. I am not sure at all this will work. In fact, I’m pretty sure it won’t.
  • move the 0-24V Vset and 0-3V Iset back to the main power board so they share a common reference. This seems more likely to work.

What a learning experience! When I simulated it up in CircuitLab, it was assumed that ground is ground. Whenever I define a ground reference it’s universal. Sadly not true for reality. Voltage, by definition, is a relative measurement – the difference in potential between any two points in the circuit. With no common reference, it’s meaningless.

What kind of sucks is now my control voltage board has much less functionality. The two transformers I bought, sadly, not for this project. No matter, their values are common enough that they will find definite use elsewhere. Especially since they are both centre-tapped.

So, what’s left? If I subtract the Vset and Iset from the cv board I am left with the relay control circuits and the power for the meters. Not bad that. I can use another, smaller, transformer rated for 12V. As for the voltage references for the current and voltage setting, I can use either fixed linear regulators or zener diodes (probably the former). Not what I had in mind, but not the end of the world either. At least it stopped me from moving things between the two boards, unsure of where they should be.