Transformer in Power Supply affects devices on same ring main - back EMF?

Thread Starter

Standisher

Joined Jan 16, 2015
125
Strange situation yesterday. I powered up my DIY bench PSU but the mains (240VAC) plug into the unit does not always make full contact and it therefore shut down abruptly. When this happened the auto fan on our gas cooker (which normally comes on when the oven door has been opened) turned on. The radio also turned itself on! Both these items are running off the same ring main as the Power Supply. Rightly or wrongly I'm believing that this may be a result of back EMF in the PSU transformer when the field collapsed suddenly?

If this is the likely cause of events I understand that diodes or MOVs are generally used as back EMF suppression. I would appreciate your thoughts/advice on:
a) Am I right in thinking back EMF is likely to be the cause of random triggering of circuits on the same ring main?
b) if so, what would be the best form of suppression to prevent this happening?

On the other hand, perhaps I'm completely wrong in my theory and you can put me straight :)
 

ericgibbs

Joined Jan 29, 2010
9,525
hi,
Your thinking is correct.:)
I had a similar problem with my home security system, it was sometimes triggered when other devices were powered On/Off.
Add mains and signal filtering to the security system, no repeat problems.
E
 

Thread Starter

Standisher

Joined Jan 16, 2015
125
Hi, thanks for the quick response Eric. Nice to know that I may not be too far off beam :D I don't think there is anyway of providing filtering on the cooker's fan circuit, so I'm looking for a solution on the bench power supply end. Would a MOV across the transformers Primary be appropriate and, if so, would I go with one rated at 250VAC or 275VAC (my, supposedly 230VAC, domestic supply is generally reading 242VAC on a power meter but I have measured it at 249VAC!)
 

Thread Starter

Standisher

Joined Jan 16, 2015
125
Better to fix the poor contact from all points of view.
I am going to look at fixing the connection. It's a case of a very tight fitting iec plug which (on a few occasions) I have failed to push fully home into the socket. Just interested though, in view of what you say, whether a MOV would be detrimental/totally ineffective?
 

Thread Starter

Standisher

Joined Jan 16, 2015
125
Thanks for the replies guys. Quite a bit of work involved in changing to have it wired straight into the PSU @Dodgydave (love the avatar by the way - my lifelong club) as it would leave a gaping hole where the all-in-one socket/fuse-holder/on-off switch is currently sited. But I take your point and will probably do it that way on future builds. I have 'eased' the plug, marginally, to make it a snug but not 'requiring force' fit and will put a 250V across the primary.
 
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