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Old 10th September 2006, 07:00 PM   #1
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Default Another cheap fix - MOT as plate choke.

Someone on the forum suggested that MOT's (Microwave Oven Transformers) can be used as plate chokes.
I have just finished converting my RH807 parafeed amp from a CCS load to a MOT choke load. So in the specific case of my setup, ie 50mA DC current 330V plate load and a low output impedence output stage, - it works a treat. Response is good down to 10hz and up to 35khz where it starts to roll off.

All I did was disconnect the stud mount where the HT secondary is connected to the earthed core of the transformer. Take this connection and wire it in phase and in series with the primary winding. I also tried removing the primary winding (aluminium wire - and very difficult) and the shunts, but it made no noticable difference.

So if you fancy trying your hand at a parafeed design but are scared off by the price of plate load chokes - give it a go. What have you got to lose but about £200 for some plate chokes.

Shoog
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Old 10th September 2006, 08:49 PM   #2
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Hi Shoog

Nice to read this works! As always I would like to try, someday I will have the time (university, job, house and girlfriend take lots of my time). But just a question...

Quote:
All I did was disconnect the stud mount where the HT secondary is connected to the earthed core of the transformer. Take this connection and wire it in phase and in series with the primary winding. I also tried removing the primary winding (aluminium wire - and very difficult) and the shunts, but it made no noticable difference.
I don't understand exactly. Why must the secondary be wired in series with the primary? And a sentence later you say you took the primary out, so I assume you are using just the secondary as choke? And what do you mean with shunts?

Erik
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Old 10th September 2006, 08:49 PM   #3
Klimon is offline Klimon  Belgium
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Another thing we can start to disembowel
The real difficulty I think is to find a pair of identical trafo's, unless ofcourse there are only a handfull of models manufactured by a few companies for the whole microwave industry... Where did you get your pair, shoog?

Simon
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Old 10th September 2006, 09:13 PM   #4
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I removed the primary from one of the MOT's just to see if it helped. It didn't so the other has the primary in series with the secondary. Its not absolutely necissary to wire them in series, but I hate to leave a coil unterminated just in case of ringing.

I got mine from the local council recycling center. When I went there were about 6 of the same model of microwave. They tend to come in like this because some shop does a special offer on a particular model, everyone buys them and then they all fail at about the same time. I'am not certain absolute matching is critical. They all tend to come in 800W or 900W models and will all be fairly similar.


Shoog
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Old 10th September 2006, 09:16 PM   #5
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Wow, got a pic? I've always been fascinated by those 3" thick lams in the big ones.
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Old 10th September 2006, 09:27 PM   #6
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Thanks for the reply Shoog!

Erik
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Old 10th September 2006, 11:23 PM   #7
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2 or 3 Flouroescent Light Balasts in series also work a treat. They are usually around 3H each.

Cheers,
Ian
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Old 13th September 2006, 01:35 AM   #8
bufalo is offline bufalo  Viet Nam
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Hi there,

There are so many TF transformers, chokes, tubes resembling from military machines in my place.
Sometime, I bought some transformers and chokes, esepecially I like chokes much.
But with the things I bought I didn't know how many henrry they are. So does anyone here know how to measure the henrry of a specific choke?
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Old 13th September 2006, 04:54 AM   #9
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Default Re: Another cheap fix - MOT as plate choke.

Quote:
Originally posted by Shoog
Someone on the forum suggested that MOT's (Microwave Oven Transformers) can be used as plate chokes.

Shoog

I resemble that remark.
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Old 13th September 2006, 07:37 AM   #10
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Quote:
I resemble that remark.
Thanks for that one rcavictim.

Quote:
But with the things I bought I didn't know how many henrry they are. So does anyone here know how to measure the henrry of a specific choke?
You will need a signal generator and a scope to derive the inductance.

If you put a fixed capacitor(say 0.1-1uf) in parallel with the choke and then feed in a range of frequencies. One frequency should produce a resonant hump in the response. There is then a formula that allows you to feed in the frequency of the resonance, the value of the cap and pull out the henries of the choke. This will give you the static inductance. If you want the real world inductance you will have to pass the expected current which the inductor will be required to pass. Inductance drops with increased current.

Here is the formula:

F= 1/(2*Pi*x) x=the square root of (L*C)

F= resonance hz
L= Inductance Henries
C= Capacitance farads

You will need to rearrange the formula to get the inductance, but I don't trust my maths.

Shoog
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