Over voltage "spikes" have been seen from Thurlby-Thandar power supplies on turn on. This has led to components being damaged. To document the output of such a supply before and after implementing "fixes" recommended by the manufacturer, the following tests were performed.

Waveforms taken from a Thurlby-Thandar PL310QMD power supply, serial number 011885, QA 10 passed.

The power supply was loaded with a 220R wire wound variable resistor, 25W.
See here for details of the load, Vishay.


The experiment was set up as in the following image.
The scope probe was connected directly to the power supply output.



The following images show the output of the supply on switch on under different load conditions before modification.
Plots 0 to 23.

The Left Hand supply then had a new switch fitted.
Plots 24 to 32.
The voltage can still overshoot. Are there more signs of "fast" spikes?

The Right Hand supply then had a new switch fitted and another modification, removing a capacitor and replacing a resistor with a diode.
Plots 33 to 52.
The long term overshoots have now gone but the short term spikes can still be seen. These do not seem to have the same magnitude and certainly do not have the same duration and energy as the other overshoot issue. Capacitors on the output of the supply look to help this condition and also remove some of the switch bounce.

Writen by Jason A Tandy 5/4/01