I’ve now created a few posts about the little split power supply that is used to power the SCELBI Oscilloscope Analog board. Here is another, and most likely, not the last.
After wiring the chassis and powering it up I found the positive side of this supply was 4 or 5 volts low with both digital and analog boards installed. After spending a few evenings fooling around with it, I decided that the main part of the problem probably is the relatively high zener impedance of 45 ohms of the BZX79C18 zener that I had installed in the circuit. I believe that the zener’s impediance is dominating the voltage regulation, not the zener effect. I’ve decided to try an alternate part, the 1N5355, with an impredance of 2.5 ohms. Hopefully it will work better for this application.
Wouldn’t zener impedance give a too high voltage rather than a too low voltage?
My understanding is that the zener impedance acts like an impedance in series with a perfect theoretical zener diode.
Or are you thinking about lowering the series resistance in the psu aswell? If so I think you are on the right track 🙂
What is actually happening when the analog board is connected to the digital board with the BZX diode, is that the plus voltage really drops, but at the same time, the minus voltage goes up well above the zener pass voltage. It’s like the zeners are not acting like zeners at all, but just series resistors. I haven’t done any calcuations, but I think the zener resistance is dominating the circuit, not the zener pass voltage.