Bush Dac 90 Capacitive Dropper

[ppppenguin]

Remember that a diode dropper halves the power, not the voltage. Hence for 240V mains the effective voltage is about 170V, not 120V. The dropper resistor can then be calculated using ohm's law in the usual way. If the heater chain is 112V and the current is 200mA this will be 240R and will dissipate 9.6W.

Modern recitifiers are probably more reliable than a BY126 etc. The crowbar method is simple and effective providing the fusing is right and you put the diode before the dropper resistor. 2 diodes in series is another cheap and simple belt and braces approach.

Calculating a capacitor dropper is a little more complicated but not difficult once you have realised that the reactance and resistance add pythagorus style. If I'm doing it without a spreadsheet here's a simple way:

Total mains voltage/total current = required impedance
Chain voltage/chain current = chain resistance
Cap reactance = SQRT (Impedance ^2 - Resistance ^2)
Cap value = 1/(2 * PI * f * reactance) = 1/(314 * reactance)

[Joe]

Isn't 170V half the RMS voltage of the mains?

- Joe

[Refugee]

The actual RMS when measured on a true RMS meter would be 240/2-0.6V. There is a tiny drop across the diode.
The diode drop means nothing to us working at mains voltage but it would matter if you were working with 3V RMS from a small transformer or making a doubler from the heater supply on a valve amplifier.

[Joe]

That's me writing when half-asleep — I meant to say "peak to peak" not "RMS." (240÷0·7071)÷2=169·7072.

- Joe

[Refugee]

Whatever figure you use it will be divided by two.

[Aerodyne]

Jeffrey points out:

If the heater chain is 112V and the current is 200mA this will be 240R and will dissipate 9.6W.

Just to clarify things for anyone contemplating this approach - a perfectly acceptable one, IMHO - a resistor of 9.6W can physically be made up from more than one resistor, in a series chain. In the case of Jeffrey's example, you could use two 6W 120R in series to give you a measure of reserve capability and it might well be easier to source such sizes than a single large wattage resistor.

Tony

[tim.54]

Thanks for everyones input ,certainly some food for thought there. I am inclined to go down the motor run capacitor route using a 2.5uf capacitor with a o.33 uf
x2 capacitor in parallel. with a ,2w 50 ohm surge limiter resistor and a 100k 1w discharge resistor ,this in theory would give me 72.6v @ 0.2amps.which goes to pin 7 on v5, the start of the heater chain . but this is where i`m slightly puzzled , on the original dropper there are 2 taps ,the top one for the heaters and another feed to the 150 ohm (r19) surge limiter and then on to the rectifier valve supply,
as this isn`t the full 240v how do i reconcile this or am i missing something ?
all advice appreciated ,
regards, tim

[pwdrive]

To keep things simple I would feed the rectifier anode via a separate dropper from the mains input and just use the capacitor to drop the heater volts otherwise it might get complicated.

Lawrence

[Aerodyne]

Here's my take on this: forget the existing dropper connections. Feed the rectifier anode as Lawrence says, using something in the region of a 300 ohm resistor in series. Fit the output of your capacitive dropper plus the series surge limiter and connect to V5 heater. The use of the discharge resistor is probably a good idea but I'd see it as optional. 100k is likely to be at the low end of a value I'd use but should be OK. I'm assuming you've worked your dropper value using a mains input of 240V.
I would also rewire the on-off switch to work on the live input. Replace the mains RF bypass capacitor with an X rated high voltage component or remove it entirely, refitting only if essential.
Tony

[pwdrive]

I based my post #28 on reading this:

http://www.vintage-radio.net/forum/showt...hp?t=92867

I've never done the capacitive dropper in practice but from reading through the posts in the above link it would appear that dropping the HT feed via the same capacitor used to drop the heaters or via a separate capacitor is a bit more involved.

Lawrence.

EDIT: Fixed link (by mods)