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Power Supply: Output On/Off


How is the output on/off perform in a good power supply ?

Is is opening a connection ? Like opening a relay or any other element ?

Changing the voltage to zero ?

The transition should be smooth ? or it does not matter ?



On my AIM-TTi QL355T Power supply (Great supply!), the output voltage is set to zero to effectively turn the output off. There are no relays etc.

Cheers, Roger...

Usually the whole idea is to make the output perform better than being operated just by mains switch and/or physical output wire (dis)connectiion.

The desired effects are:
1. In case of emergency, be able to turn off the power quickly, without residual capacitor charge but without any inductive spikes, too. The best way to do that is to set output to zero smoothly in fractions of second. Not at instant, not with a relay since the device under test connected to the PSU may have a choke at its input.
2. When mains switch is just turned on, do not turn on output power and allow to check output settings first. Any kind of switching is Ok to do that.
3. When output is turned on, the output voltage and current should grow to desired values but without overruns. This is done by turning on the output slower than the output regulator works.
4. On multi-output PSU, turn all outputs on/off at once (excatly at the same time). Relay is not good (random delays between diferent contact groups).

Depending on the construction of the regulator itself, different methods may be used. The simpliest one is to set reference voltage to zero smoothly in fractioons of second (0.05 sec should be Ok). In some cases additional output switching (shortening?) may be used to guarantee 0.000V output.

It does not matter if you short the output or let it float unless you have any special requirements. Both are useful in some cases. I personally prefer shortening to discharge capacitors in the device under test (if any).


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