Scoposcopy: PSU Meter Reads Voltage From FG
Can a regulated power supply (PSU) indicate a _higher_ voltage when the circuit is connected, and a _lower_ voltage when the circuit is disconnected? Can the voltage from the Function Generator (FG) that is sending a signal to a powered circuit actually show up on the PSU’s meter?
The answer is yes, to both questions.
This is a demonstration of a phenomenon that seems to have fooled some people who have encountered it in their own laboratories.
When the FG is simply hooked directly to the output of the PSU, its 10 volt amplitude square wave signal goes into the final output circuit of the PSU, which in this case consists of a small electrolytic cap and a reverse-biased Schottky diode across the output. Further in are smoothing caps and bleeder resistors. This accounts for the voltage collapse in the square wave output of the FG. The meter on the PSU reads the average voltage of the collapsed, nearly constant voltage, as shown by looking at the scope.
When the voltage of the PSU is cranked up, it then supplies power to the FG’s output impedance resistors, which in the high-power F43 are a bank of 3-watt carbon resistors in a series-parallel stack. The PSU is now dissipating power inside the FG, but the PSU meter still will show the greater of the two voltages. When the FG (or external circuit) is disconnected from the PSU the meter then of course shows the power supply’s setpoint voltage.
