P-Series Valves, what to do with them?

[ppppenguin]

Perhaps I'm being thick but how does a cap dropper act to stop EB91s etc lighting up brightly?

Unless you're seeking utmost originality, if it's feasible to use a cap dropper or a diode dropper then it's a good idea. Keeps down the heat, sometimes by a lot. A diode dropper in a 300mA chain saves over 20W of heat. You can't sensibly use both cap and diode droppers together in the same heater chain. A simple way to look at a diode dropper is to say that it reduces the mains voltage to 71% of its original value (SQRT(2)/2). Capacitor droppers are a little harder to calculate though it's no more than pythaogorus. Spreadsheets etc have been published to help you do this.

[Murphyv310]

Probably as the load resistance is low when cold and the available current is limited by the cap value. Never seem to have these surges with a cap.

[ppppenguin]

Probably as the load resistance is low when cold and the available current is limited by the cap value. Never seem to have these surges with a cap.

Still don't quite get it. Whether it's resistance from a dropper or reactance from a capacitor there's still a surge current. With a cap there could also be a very short term surge depending on when the switch is closed in the AC cycle. I don't think this matters. We're not talking about connecting a large cap direct to the mains where you could possibly weld the switch contacts with the surge.

[Murphyv310]

I'll pop up a video of the TV22 at switch on. There is no thermistor in the chain.
I know David Boynes has done the same and reports no surges too.

[ppppenguin]

Trevor, I totally beleive you, don't need to see a video. What I'm trying to find out is what's actually happening. Ultimately I suppose it needs a digital scope measuring V and I at switch-on to prove the point.

[radioman]

I've simulated a capacitor dropper circuit with computer software and you do get a short initial surge but it's for less than a quarter of a cycle - the current is determined by the capacitor value and is more or less constant (as read by a meter) over a range of resistive loads.
Hence there is no surge due to the lower heater resistance at switch on.

Andy

[Murphyv310]

That's probably why that changing the incoming volts within reason or chopping off say 20 volts of heaters the current remains reasonably constant.

[ppppenguin]

Andy, thanks for doing the simulation. Any chance of doing it with a resistor whose resistance is comparable to the reactance of the cap?

I don't see how a capacitor can act as an approximation to a constant current source. Unless its reactance is substantially greater than the load resistance. If the reactance of the cap (or resistance of the dropper) is fairly large compared to the heater chain then it will indeed approach being a current source. But that isn't what's normally happening in a real life heater chain.

[Murphyv310]

Hi.
Monitoring the current on a 300ma chain after a 1 second switch on is 320ma on a cap fed chain, it's 440 ma on a resistor fed chain, both less a thermistor. I reconnected the dropper temporarily and let the chain cool for 15 minutes. Cap is a 4uf.

[radioman]

Andy, thanks for doing the simulation. Any chance of doing it with a resistor whose resistance is comparable to the reactance of the cap?

I don't see how a capacitor can act as an approximation to a constant current source. Unless its reactance is substantially greater than the load resistance. If the reactance of the cap (or resistance of the dropper) is fairly large compared to the heater chain then it will indeed approach being a current source. But that isn't what's normally happening in a real life heater chain.

Quite right Jeffrey - see the attached screen-grab : It's a simulated circuit with six parallel paths having the same value capacitor but with progressively smaller load resistances.
   
I must admit that when I was experimenting with a dropper circuit, the capacitive reactance was much greater than the load resistance giving the apparent results I described.

Andy