Electronics > Power & Renewable Energy

Isolated secondaries of Offline SMPS's need 1MEG discharge resistors?

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Faringdon:
Hi, We all know that with an isolated offline SMPS, the isolated secondary ground  will generally be at earth potential or thereabouts (even if not directly connected to earth). -And this being due to  the stray high impedances across the transformer/ the Y caps,  etc  etc.
However,  at least in theory,  such an isolated secondary may  float up to mains peak in voltage.
As such, its quite common to see a 1MEG resistor being used  across the isolation barrier of the offline SMPS,   in  order to bleed off any charge on secondary ground  that would cause voltages well  above earth potential that would otherwise build up.

So what regulations cover this use of an isolation barrier resistor?

(I remember once working at a TV power supply company. It was quite common for an engineer to go off for coffee break…and while absent, his mates would snip off the 1MEG isolation resistor on his test board…..  then on return from coffee break, the engineer would get a zap from touching the  isolated secondary side circuitry….and hoots  of laughter would follow from his pals.)

Do you mandate use of such 1MEG resistors?

Simon:
It's your design, you are to make it safe. If the primary has a ground and this "charge" comes from the primary how des it end up at such a voltage?

dmills:
Secondaries on most isolated switchers are in my experience seldom anywhere near ground, they usually seem to wind up at about half mains (High impedance, but still)!

Besides, on an isolated double insulated switching supply, how the hell do you know which input pin is closer to ground?

1 meg across the input, sure, but that is to discharge the EMC filter, but I don't think I have ever seen much done at the output.

It is your design and only you know to which safety standard you are working.

Simon:
I don't actually understand the problem. If the secondary is left floating then why was it isolated in the first place? Isolation is for when you want to change the ground reference, if you don't have another earth reference then earth the secondary with the primary or use a non isolated converter.

Faringdon:
Yes i suppose in a suitably sealed enclosure,  a non isolated offline SMPS would be a good prospect.
A non isolated offline Flyback is certainly less EMC noisy than an isolated one.
As long as the service engineers were up with it, and kept safe.

..Got me thinking now...got to think about the comms wires that might (might not) be needing to be connected from outside the enclosure, and they might be earthed....if so, we would need to earth an isolated  secondary.

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