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HV DC Filtering

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LAN_403

Sulaiman

Wed Aug 10 2011, 12:28AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Could you use a cathode-follower voltage regulator for the eht?

radiotech

Wed Aug 10 2011, 03:01AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

You might consider using another MOT as as series iron core inductor. Just
mount it on an insulated base. Use the HV secondary as a series connected
choke after your rectifier. Then you could follow it with lower capacity filters.
(other MOT capacitors.) You can tweek the series indoctance with resistors
across the 120 VAC winding, or even lightbulbs.

This would be a good junk parts project for liittle money.

Arcstarter

Wed Aug 10 2011, 04:21AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

A good filtering capacitor for a MOT isn't hard to find or expensive, just make a series-parallel arrangement of some 400v or so small electrolytics. The amount of capacitance you need depends on the load. I would highly recommend a fullwave rectifier, it would lessen the ripple current on the caps and be easier on the diodes. 16 1n4007's costs almost nothing, and would make a bridge rectifier that should last a long time. That would be a 4kv 1 amp bridge rectifier.

10's of uf sounds about right, and is easy to achieve. I used 14uf in the first sparks here:

And as you can see there is still quite a bit of ripple current (you can hear the 120hz hum rather clearly), but a spark is a huge load and the voltage probably dropped quite a lot.

You could put all of those in series for a 4kv 10uf capacitor, which should be fine for what you need them for. MAKE SURE you use bleeder resistors across each one to make sure you don't get shocked, and to (try to) make sure the capacitors charge evenly. Around 220K ohms should suffice, but they will need to be around .5-1 watt ideally. Assuming the capacitors are all charged to 400v (they will be somewhat less), the resistors would dissipate .7w.

Proud Mary

Wed Aug 10 2011, 11:33AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

A well-known good approximation for reservoir capacitor value for full-wave circuits is given by

C = ( IL / Vpp) * k * 1000 Î¼F ... where

IL = Load current
Vpp = peak-peak ripple voltage
k = 7 for 100Hz ripple frequency and 6 for 120Hz in the Great Elsewhere

If we say that 2Vpp is the maximum ripple acceptable for good quality audio, then it will be seen that for an MOT with an output current of 100 mA, a reservoir capacitor of 350Î¼F is required in full-wave smoothing (i.e. by folk less concerned that I over the safety outlook for un-Earthing the 'cold' end of the secondary.)

cduma

Wed Aug 10 2011, 12:54PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300

I think that this

would do very well in my application. I think I would have to place the primaries in series on two MOTs. This would also give me a bit of ballast.

I would like to do this with as little capacitance as possible. I like to avoid HV capacitors since I have had a few shocks

Arcstarter

Wed Aug 10 2011, 08:38PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

cduma wrote ...

I think that this

Well you are playing with MOTs, so im not sure why you don't want HV capacitors. If you want to filter HV, of course you will need HV caps.

Why would you use a voltage doubler? Do you need that much voltage for the vttc? You didn't mention that in the first post. Why would you use 2 MOTs for a fullwave doubler when you can use one?

cduma

Thu Aug 11 2011, 11:44AM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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Posts: 300

I thought that if you place the primaries in series that it halves the output? I also liked the idea of having some additional ballast.

Would

be a good choice? I would prefer the cheaper one but and I dont think it would exceed the specs. I would probably run it with 10 in series.

Forty

Thu Aug 11 2011, 04:38PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

well if you're gonna use a bunch of 1000v 1A diodes, wouldn't the common 1n4007's be even cheaper?

To halve the output, you'd need the secondaries in parallel. but if the transformers are even slightly different, then that might cause some problems, so it's probably not a good idea.
If you series the primaries but left the secondaries separate, then you'd just have a mot with a big inductor in series with it's primary wouldn't you?

It might be easier to get smoothed out hvdc by starting with a lower voltage transformer (or no transformer at all) and using a multiplier with big lytics. the lytics would also act as your smoothing capacitors.

cduma

Thu Aug 11 2011, 06:24PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
Location:
Posts: 300

Is it practical to take 120VAC to 2KV+ at 500mA with multipliers?

Forty

Thu Aug 11 2011, 07:00PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

not really. but I think it's more practical than using a >2kv 350uF capacitor to smooth a MOT's output. And if it happens to work then you will have saved a lot of money, and if it doesn't, you will not have lost a lot.
10+ 200v 300+uF (have to all be the same capacitance) electrolytics would cost very little, especially since you already have or could salvage a few of them of equal value. put them in an enclosure with a fuse in case one explodes and goes short circuit.

I think it's at least worth a shot.

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