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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 1st Mar 2005, 12:43 am   #1
igranic
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Default DAC90A Dropper Capacitor configurations?

Would anybody here like to share their experience of installing dropper capacitors in a DAC90A?

Bearing in mind that the resistive dropper (R17 on Trader sheet) supplying full-wave AC current to the heater chain, also acts as a potential divider supplying half-wave current to the the anode of the UY41 rectifier via R15, then there are multiple possibilities in terms of capacitor/resistor configurations/combinations.

What works best in practice?
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Old 1st Mar 2005, 9:55 am   #2
Ed_Dinning
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Default Re: DAC90A Dropper Capacitor configurations?

Hi In theory it all gets horribly complicated as the DC component for the HT should not pass through the capacitor. In practice it works well as the heater AC is much greater than the Ht DC. In actual use you will not get the actual value of capacitor calculated, you should go for a higher value (lets more current through) then pad it down with a low value series resistor. This series resistor is best found experimentally as the capacitor and resistor volt drops are vector sums, not "straight" sums.
You should always use Class X caps in this position as they are specially designed for mains use.

Ed
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Old 1st Mar 2005, 1:08 pm   #3
Duke_Nukem
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Default Re: DAC90A Dropper Capacitor configurations?

I'd be tempted to use a capacitor in the heater line but continue with a seperate resistor for the HT side of things. That way you'll be able to set the heater volts accurately (important) seperatly from the HT (less important).

True there is still a resistor to generate heat, but 100mA heater plus lets say 50mA HT would still mean the wasted heat would be reduced to 1/3rd.

TTFN,
Jon
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