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Thomas

Tue Sept 28 2010, 12:52AM

Registered Member #120 Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83

Hi all.

I thinking of making a current limited and voltage regulated power supply for capacitor charging.

I'm thinking of making a PWM which controls a MOT.

I'd use a full-bridge topology, that drives the MOT's primary windings. The output of the MOT will be full-wave rectified.
Now I should be able to get a nice closed loop, fixed voltage power supply out of this by changing the duty cycle. I will add a current shunt or some sort of current regulation later. However, I don't think any over current conditions can occur because its an RC circuit. Though sweet shorts could occur.

I'm not trying to make this Energy Star as I know PWM's aren't too brilliant.

I'm just curious if this is a good route to take to get a 20V to 1805V power supply.

-Tom

803

Tue Sept 28 2010, 02:14AM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

I think MOTs arn't ment for voltages other than 120/240. I thought if you put in any other very different voltage the voltages would sway/go crazy.

ScotchTapeLord

Tue Sept 28 2010, 02:47AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

I'd recommend just running it off mains with a variac, with a ballast if you need current limiting. This is much easier and MOTs are meant to be run off 60Hz. Not to mention fewer losses/spikes associated with the square wave switching...
I have used MOTs below their rated voltage with this exact method to charge capacitors. A variac will let you control the input voltage from 0-110 (because you're in 110 land), making the secondary's output voltage variable from 0V to ~2kV, which is close to what you're looking for.

Mains>variac>MOT>rectifier>cap

This is easier than making a SMPS!

quicksilver

Tue Sept 28 2010, 05:53PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Is using a Variac even possible to achieve that effect with such a transformer? If a dip below a given point were reached would that not allow the functionality to be maintained?

Mads Barnkob

Tue Sept 28 2010, 05:56PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

quicksilver wrote ...

Is using a Variac even possible with such a transformer? If a dip below a given point were reached would that not allow the functionality to be maintained?

Its just a regular iron core transformer with a core that is greatly under dimensioned to its power rating in its original form as a MOT.

It follows the standard rules for transformers with primary:secondary ratio to the input voltage, a MOT is generally 1:10 in EU and must be 1:20 in US?

Patrick

Tue Sept 28 2010, 07:40PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Mads Barnkob wrote ...

...generally 1:10 in EU and must be 1:20 in US?

is this because every socket in the EU is 240vac? and we americans use 120 or 240?

Adam Munich

Tue Sept 28 2010, 09:02PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Yes. In the USA the MOT's are 1:20.

HazzWold 1993

Tue Sept 28 2010, 10:39PM

Registered Member #2563 Joined: Mon Dec 21 2009, 10:17AM
Location: Australia
Posts: 280

You can run a MOT with a light dimmer switch the way you would an ignition coil -

with a 1uf microwave oven cap in series i got 1cm blue sparks, with a 14uf cap i got 1cm starting arcs not as big as a normally run MOT but the voltage appeared higher..

Patrick

Wed Sept 29 2010, 01:41AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

HazzWold Labs wrote ...

You can run a MOT with a light dimmer switch the way you would an ignition coil -

with a 1uf microwave oven cap in series i got 1cm blue sparks, with a 14uf cap i got 1cm starting arcs not as big as a normally run MOT but the voltage appeared higher..

yikes! i wonder if there is enuff line to line insulation, and i wonder what the line quality looks like for your house circuits, RFI too. using a SCR dimmer must be rough on a MOT.presumably you would aalso get much less power through the primary too, though the higher secondary volts makes sense as you are forcibly chopping the curretn faster then the normal 60Hz sine.

Thomas

Wed Sept 29 2010, 10:22PM

Registered Member #120 Joined: Fri Feb 10 2006, 07:07AM
Location: Westchester New York
Posts: 83

I sort of want switching only because then I can have full control digitally.

Would I run into problems if I run it at 60Hz switching? I'm not too sure what the cut off frequency of iron is. I assume if its too low it'd knock off the frequency components of my square wave and give me a rubbish output on the secondary.
Thats really the only thing I'm worried about. Also I should not have problems with 0 - 50% duty cycle at 60Hz being feed into the primary via full bridge topology right?

Thanks basically my only area of concern.

-Tom

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