Author Topic: Isolated secondaries of Offline SMPS's need 1MEG discharge resistors?  (Read 674 times)

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Offline Faringdon

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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?
 

Offline Simon

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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?
 
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Offline dmills

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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.
 
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Offline Simon

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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.
 
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Offline Faringdon

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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.
« Last Edit: July 18, 2021, 01:59:21 pm by Faringdon »
 

Offline Simon

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If your secondary does not have a reference give it one. If it's a mains powered converter make that earth/chassis. I suppose a high value resistor to neutral could also be added but I'm not experienced enough to advise and do not know the regs. Assuming you have to account for the converter being wired up wrong you need something like a 1M resistor.
 
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Offline PKTKS

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The answer depends on some assumptions.

The first one is that your device will have "EARTH".

Well at least here the "EARTH" pin was just about recently
(kinda half dozen years) introduced as a mains standard.

I have done repair for more than decades in the 70s 80 90s...
a period where the EARTH pin was 100% non existent.

And even today the vast majority of installations have no EARTH pin
ready although the plug and wall socket are (by legislation) 3 pins.

So.. does it matter? short answer NO.

I have dealt with CRT TV set for more than decades.
FLYBACKs designs vary widely across the timeline but NONE ZERO ZIP NADA
none of them have (or had) the secondary by any means EARTH referenced.

Even the primary with a 2 pin plug was never ever EARTH (ed)

It seems today hairy heads are too spoiled by these regulations of
claimed safety... I have never seen a problem or any sort of problem
by not having secondary referenced -  usually with voltages above 400V
although TV sets are low energy.. compared to today SMPS.

As of today .. I am more concerned with these sub-milimeter SMD
devices which allow ARCs orders of magnitude easier that THT.

Even though nobody cares marking these F*K*  devices properly.
And yet still they require such bullshit earth safety.

No - rest in peace that no secondary EARTH will never be more or less
safe than  a non referenced one. As long as your design is properly sized.

As usual just a high freq KV rated ceramic CAP is always there to
reference the secondary..

Paul
 
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Offline ejeffrey

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In isolated supplies the output tends to float at 1/2 the input voltage due to EMI capacitors.  In my experience it is not common to use anything like 1M bleeder resistor in such a configuration although I am sure it exists.  If you are connected to a grounded power outlet and you want ground referenced output you would typically just connect the output to ground, not with a resistor.  I have seen examples that use a lower value resistor (like 1K) to try to break ground loops while preserving the earth reference.  In the US, most home AV gear uses floating supplies with 2-pin power input so that your entire home audio system has a local circuit ground that is isolated from the electrical wiring ground.  Since the US is 120 VAC, the mid-rail voltage is 60 VAC, and when coupled with correctly sized EMI caps this doesn't pose any problem.
 
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