Thread: HV regulation.
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Old 28th Sep 2019, 5:05 pm   #19
Diabolical Artificer
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Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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Default Re: HV regulation.

"If your transformer is rated at 60mA, the DC current limit should be at most 40mA (the max. DC current is a factor sqrt(2) below the max. RMS current)." Thanks, didn't know that. The power tfmr HT winding is really weedy, unloaded and rectified etc gives me 470v, depending on how the RC filters are configured. Any sniff of a load, the tfmr lifts it's skirts and runs off crying, EG HT drops dramatically, IE by 100v or more.

Anyhoo... tried a "ripple gobbler" AKA capacitive multiplier today, and whilst not a regulator it's supposed to nobble ripple, see att 1. Whilst it does reduce ripple as advertised I'm having an issue,see pic 2, I think with VHF oscillation. Looking at the trace in more detail, with the scope cranked up on it's fastest time setting - 2uS/div, four peaks fit in 1 div, I calculated this as 2Mhz ish.

I tidied up the layout, shortened leads and popped a ferrite bead on the lead from the MJE340 emitter to BU208 base, this improved things a tad, but the problem was still there. Also tried a 0.047u across the 820u cap, as well as a 1p and 10r "snubber". I suspected the choke, but there is no oscillation at the junction of L and the two 22u caps, IE first LC filter. However note the trace at the two collectors of the Darlington pair - pic 3, oscillation is on the pos and neg part of the cycle.

Andy.
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Name:	Osc OP Cap Multi 150mV P-P 2.jpg
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Name:	Osc 1st LC filter, C MJE340 BU208.JPG
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