Smps failure

rejoe.koshy · Aug 15, 2020

Hey Guys,
I have had a recent experience of case of SMPS failure. It was a SITOP (siemens) unit which abruptly failed and was replaced by another one. But unfortunately issue recurred within 2 hours of its functioning. The input voltage readings recorded are as under:

Phase - Neutral = 230 V

Phase - Earth =110 V

Neutral - Earth = 120 V

The 230 V is fed from a isolation transformer which was removed from the circuit and the readings returned to normal. Now this is two part query:

1. How do I technically justify the above readings? How can I get phase - earth as 110 V and if N-E is 120 V , how come P-N is 230V?

2. What effect does this have on the health of the SMPS?

Please help me to understand this case, which will facilitate in preventing future failures

gclshortt · Aug 15, 2020

https://diyaudioheaven.wordpress.com/tutorials/power-supplies/why-smps-fail/

https://youtu.be/Xp-9F1jBq6E

The above links may help you out.
Regards,
Garry

rejoe.koshy · Aug 21, 2020

Any ideas folks?

parky · Aug 21, 2020

Is this a centre tapped transformer? i.e. 110/110 also if the main incomer is earthed but not the secondary then effectively you have a floating neutral on the secondary, also you have not stated if the primary is 240 or 415. So depending on the transformer & how it is connected then there are a number of possibilities, The transformer is centre tapped, the transformer has a winding fault, or the earthing is incorrect. Why are they using an isolation transformer? , there could be many reasons for the voltages you describe & personally I never use isolation transformers for 240v supply unless there is no neutral and 415/240v is required, the only time I have used 240 to 240 isolation is by using a line voltage conditioner, also came across a system that used one where the PLC had failed it turned out that a retro-fit conditioner was fitted where the plc was set at 110v, the replacement conditioner was 240v but it was fed from 110v, turned out that the conditioner although fed from 110v, supplied almost 200 + volts on the output blowing the PLC.

rejoe.koshy · Aug 24, 2020

parky said:
Is this a centre tapped transformer? i.e. 110/110 also if the main incomer is earthed but not the secondary then effectively you have a floating neutral on the secondary, also you have not stated if the primary is 240 or 415. So depending on the transformer & how it is connected then there are a number of possibilities, The transformer is centre tapped, the transformer has a winding fault, or the earthing is incorrect. Why are they using an isolation transformer? , there could be many reasons for the voltages you describe & personally I never use isolation transformers for 240v supply unless there is no neutral and 415/240v is required, the only time I have used 240 to 240 isolation is by using a line voltage conditioner, also came across a system that used one where the PLC had failed it turned out that a retro-fit conditioner was fitted where the plc was set at 110v, the replacement conditioner was 240v but it was fed from 110v, turned out that the conditioner although fed from 110v, supplied almost 200 + volts on the output blowing the PLC.

Thank you so much for that clarification. It is quite probable that the earthing of the secondary was disturbed. Its a 230V - 230V transformer , installed to for suppression of propagation of noise signals from primary to secondary.(at least thats what the OEM said).

Well, even if the lack of earthing is the issue why would the SMPS unit fail? The SMPS had a 110/ 220 V input toggle switch, which was flipped towards 220 V.
Another SMPS which was on the same circuit had a 110/220 V input voltage rating , which I guess would changeover automatically depending on input voltage remained unaffected.

Does this fact lead us somewhere??

CLanford · Aug 24, 2020

Since it appears you've worked through your mains readings, maybe you just had a bad spare power supply? I know it's a long shot, but it might be worth looking into.

One thing you can do is take a look at how the new power supply failed vs the old one. Most (if not all) of them have an internal fuse on the mains side. Sometimes just by seeing if or how the fuse failed, that can point you in the right direction.

Maybe your new one had something wrong with one of the internal components of the unit? Most of the ones I've seen it's the capacitors that blow out. This could be especially true if it's a typical electrolytic cap and it's been sitting on a shelf for years. In this case they usually fail immediately, but you said it did fail within a couple hours so it's still a possibility.

Just my 2 cents

Smps failure

rejoe.koshy

Member

gclshortt

Member

rejoe.koshy

Member

parky

Member

rejoe.koshy

Member

CLanford

Member

Similar Topics