ef37a wrote: ↑Thu Feb 02, 2023 3:41 pm
Back in the bad old days of 'domestic' valve gear caps were often just rated for the 'working voltage' and not the peak that a fast heating rectifier slammed on them (got even worse with Selenium rects!) Nonetheless, capacitors lasted decades in most bits of kit, the exception being TVs when Silicon diodes arrived and caps blew about every 3 or 4 years.
Dave.
I have grim memories of the weird multi cap assemblies - often festooned with an unholy array of add-on ones, with some of the original terminals cut off (S/C cap) while others were just hanging across dried out ones. Then there were the copper oxide, or selenium rectifiers bodged with a silicon one bypassing them (so just one tag retained as a support, sometimes with a 10W resistor in series in an attempt to limit the surge.
Yes, there were a lot of 'cowboys' about then Will. The old B&W TVs were a special case. The main caps were rated at 275V when peak mains is nearer 340V, the cheap buggers discovered that if you stress an electrolytic several tens of volts over its rating it draws a much greater leakage current and acts as a 'free' surge limiter. The caps did not take kindly to this and failed after just a few years. Then you could get an O/C heater chain so the set never drew any HT current...BIG bang and a big mess.
Well that transformer will 'do' but surely, if you have to buy a new one buy better specc? The mouser traff has a slightly lower VA rating than mine and nominally a volt more output and mine runs quite warm. A toroid will be more efficient, especially since it will be run constantly at close to VA limit. You are not subject to the cost restraints of production.
CPC & Rapid have a reasonable selection of 18-0-18V transformers.
The Rapid site is a bit more searchable/filterable. They also do enclosures [some plug top style].
Extra care is required mounting torroidals to make sure the bolt through the middle doesnt become a 1 turn coil !
N i g e l wrote: ↑Fri Feb 03, 2023 12:12 pmExtra care is required mounting torroidals to make sure the bolt through the middle doesnt become a 1 turn coil !
Sorry, Nigel, for the ignorant amongst us, i.e. me, can you elaborate on that please?
The fixing bolt passes through the centre of the toroid and needs to be insulated from forming a complete loop with the chassis, i.e. a secondary winding with one turn. If it does a very large current would flow through the bolt and the chassis. Have a look at those instructibles on making a spot welder with the transformer out of a broken microwave, the secondary is 3 turns and the current it can produce welds metal.
The mounting bolt that goes through the middle of the torrid is subjected to the magnetic field and acts like a 1 turn seconday coil, with a potential at either end.
The bolt should only connect to a metal chassis at one end and be an "open circuit".
If both the top and bottom connect to the chassis, a circuit is formed and current will flow [possibly quite a lot ].
A torrodial transformer probably comes with a mounting kit, like this one [transformer shown mounted on top here rather than on a base]
