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Capacitor across transformer primary

a.palfreyman

a.palfreyman

pfm Member
Going to replace the transformer in a Quad 606 as the original c-core is toast. This transformer has X2 0.33uF fitted across the primary winding.
Should I replace this with 100nF X2, or just fit the existing 0.33uF X2? New traffo will be 400VA.
 
Providing the original is good, i.e. not a transparent Wima based on resin-encapsulated paper - I'd re-use the original (or replace with same value or next larger that will fit.

(NB - X-caps tend to diminish in capacitance value with age, from mains transients, even well-below 'sparky'. That's what they are designed to do. I am not a fan of smallish X2 caps like 100nF alone - you can end up with more mains noise apparent than without, from resonant effects. I do like these, tho - proper snubber rated for such use:

LCR FE-SP-CR 100N 100R 250V 100nF ±10% 250VAC Radial RC suppression Network

LCR - FE-SP-CR 100N 100R 250V - LCR FE-SP-CR 100N 100R 250V 100nF ±10% 250VAC Radial RC suppression Network - <p>This RC network contact suppressor, 0.1uF
www.rapidonline.com www.rapidonline.com

- Rapid used to supply a 220nF+100R version with insulated flying leads. I may have a spare if you fancy it - PM me an address, yours gratis.)
 
:)
Superb advice as usual @martin clark.
PM sent.
Edit: are there any 'rules' on selection of value WRT VA rating? Can't seem to find anything on the net...maybe just using the wrong search terms.
 
No, not really: you can try to model the interaction (i did) - but even with a few bits of data and a very wide read-around: it's still assumptions - pretty much all the way down... for each instance/location.
 
Although it is absolutely against the flat earth philosophy, I favour putting a varistor across the transformer primary, rated to ignore mains peaks, on the transformer side of the mains switch. What it helps with is the voltage transient at switch off, when the switch is trying to interrupt the current in the primary. And I suppose it might snip the top off spikes coming in from upstream, but that is secondary. On my amps, it reduces the bang at power amp turn off.

But the Linn / Naim purists disapprove of such things, so forget I ever said anything.
 
FME?
(That's back EMF )
;)
Interestingly enough, on the subject of varistors, a few years ago now, a train operator had a door switch set fire to itself. I did the visual inspection on the circuit board and narrowed the site of ignition to a particular component. With the help of the instrumentation guys and a schematic, we identified the component as a varistor. Reckon that over time, its threshold voltage must have slowly got lower and lower (with the head generated from 'spikes' as it was 110V transformed from the 25kV o/head) until it became 'conducting' at line voltage - burn baby, burn...
 
In industrial surge protection, varistors are fused for this reason. Domestic equipment varistors are normally very reliable.
 
a.palfreyman said:
a train
Click to expand...

a.palfreyman said:
voltage must have slowly got lower and lower (with the head generated from 'spikes' as it was 110V transformed from the 25kV o/head)
Click to expand...
Every time I get on the electrified GWR for a day in That London, I do ponder how electrically noisy the whole mess must be!
- the catenary with 25kV ac/50Hz is 4+ meters away from the current return (the track), for miles at a time - no 'closely-twisted pair there;
- the occasional arcs&sparks as the pickup rides the tensioned overhead cable (though the overlaps in catenary sections, and the way the sliding contact is engineered to apparantly run side-side over a considerable width of the pickup, not merely machine a groove in it, must help.)

... but drawing 2-3MW through that lot (or returning it, under regen braking) must a EMC nightmare!
 
martin clark said:
Every time I get on the electrified GWR for a day in That London, I do ponder how electrically noisy the whole mess must be!
- the catenary with 25kV ac/50Hz is 4+ meters away from the current return (the track), for miles at a time - no 'closely-twisted pair there;
- the occasional arcs&sparks as the pickup rides the tensioned overhead cable (though the overlaps in catenary sections, and the way the sliding contact is engineered to apparantly run side-side over a considerable width of the pickup, not merely machine a groove in it, must help.)

... but drawing 2-3MW through that lot (or returning it, under regen braking) must a EMC nightmare!
Click to expand...
It is and why we have EN 50121-2 and EN 50121-3-1 putting some upper limits on what is allowed to be radiated.
 
martin clark said:
Every time I get on the electrified GWR for a day in That London, I do ponder how electrically noisy the whole mess must be!
- the catenary with 25kV ac/50Hz is 4+ meters away from the current return (the track), for miles at a time - no 'closely-twisted pair there;
- the occasional arcs&sparks as the pickup rides the tensioned overhead cable (though the overlaps in catenary sections, and the way the sliding contact is engineered to apparantly run side-side over a considerable width of the pickup, not merely machine a groove in it, must help.)

... but drawing 2-3MW through that lot (or returning it, under regen braking) must a EMC nightmare!
Click to expand...
Long ago (*) I used to visit locations in Fleet Street to work on typesetters. I learnt never to put my bag down when travelling on the tube. There was enough electromagnetism down there to damage the software on floppy discs! Fortunately the inverse square law is such that simply keeping my bag at waist height was a sufficient remedy.

BugBear


(*) and oh so far away
 
Magnetic fields from the 750V traction and the motors is hard to shield against. Fortunately the field cancels at a distance, very quickly on 4th rail systems like London, a bit slower on 3rd rail
 
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