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Old 23rd Jan 2018, 1:06 am   #41
GrimJosef
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Default Re: Wanted: Circuit for HV PSU

There are a few complications when it comes to estimating the voltage at TP1. These include

1. When the transformer is off-load it will supply more than 300-0-300 on the HT secondary

2. There will be some forward voltage drop in the rectifier (it may be pretty small when the current is low)

3. There will always be some current drawn - you'll have a few milliamps flowing through the 56k and the 85A2 and you'll have some more (maybe only 1-2 mA ?) flowing through Rtop+47k+Rbot.

Cheers,

GJ
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Old 28th Jan 2018, 11:36 am   #42
eddie_ce
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Default Re: Wanted: Circuit for HV PSU

Hi Guys,

I have uploaded a revised circuit for my simple PSU project. Again, I would welcome any comments, tips etc.

I have included a cathode follower after the 85V gas stabiliser as suggested.

Would it be better to strap the two EL86s as triodes?

I intend to start by building the left hand side of the circuit and to measure the voltage at TP1 for varying loads which should make calculations for the right hand side of the circuit easier and/or more realistic.

Many thanks and regards
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Old 29th Jan 2018, 2:56 am   #43
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Default Re: Wanted: Circuit for HV PSU

Hi Eddie, a current limit circuit may be useful (better with semiconds), but not essential.

Ed
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Old 29th Jan 2018, 4:47 pm   #44
Skywave
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Arrow Re: Wanted: Circuit for HV PSU

Suggestions:

1. Add suitably-rated mains I/P fuse and DPDT switch.
2. Add fuses in series with each rectifier anode or one in the cathode of the rectifier. (You have a choke I/P filter, so the surge current to the filter cap. is via that choke. That should prevent that from fuse from 'switch-on blowing', which couldl occur with a pi-filter).
3. Metering the O/P: add voltmeter and ammeter.
4. On / off switch for O/P and indicator bulb / light / neon. Fit indicator on load side of switch; fit O/P voltmeter on source side. A glance at the voltmeter will tell you how many volts are 'available', prior to connecting the load via the switch. The indicator will tell you when the volts are actually applied to the load.
5. Fit fuse - or a suitable current-controlling circuit - to the O/P.

Al.
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Old 29th Jan 2018, 5:16 pm   #45
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Default Re: Wanted: Circuit for HV PSU

The attached circuit is of my ancient Brandenburg Stabpak A50. It seems to break some of the rules already raised, particularly the high anode voltage for the EL86. But I post it as an example of a commercial unit. It does work fine but has a limited variable voltage range.
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Old 29th Jan 2018, 6:53 pm   #46
Skywave
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Exclamation Re: Wanted: Circuit for HV PSU

Perhaps I've over-looked a few things or maybe my arithmetic is in error, but I don't like the look of that design.

1. The rectifier reservoir cap. is rated at 500 v. Yet the cct. diag. states that it has 490-v. across it - presumably at no load. (My inference is obvious. )
2. That 490-v. will drop a bit when current is drawn by the load: O.K. Let's assume that at a load current of 25 mA and a Vout of 200-v, that voltage drops to 450-v. There will be 250-v. dropped across the EL86 and it will be passing the 25 mA. So the EL86 anode will be dissipating 6.25-watts.
For same 450-v. as above, but for 35 mA O/P at 200-v, that dissipation will be 9-watts.
The EL86 has a max. anode dissipation of 5.6-watts.
The O/P fuse is 50 mA. At that lowest voltage setting of 200-v., by the time the load current has got anywhere near that 50 mA, the EL86 will have fried. The EL86 will protect the fuse!

Al.
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