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Old 9th Feb 2018, 4:27 pm   #1
johnyjf
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Question HV PSU Question

I am trying to build a 5KV power supply to fire up some Geisler tubes. I anticipate the current requirement will be very low, 2-3m/a at most. I have a Gardners GR118229 transformer which claims to output 4KV @ 10 m/a. It is installed in a light chassis box and when I power it up from the AC mains (no sec connections) it makes a loud hum resulting in my powering it off immediately. I have used a number of vintage transformers in the past and not had this experience. Can anyone tell me if it is normal? I took some resistance readings from the primary terminals, there are 5 of them in the standard 10-0-200-220-240 config. I get 4.2Ohms, 4.3 Ohms, 40.3Ohms and 2.3 Ohms respectively. When I measure the overall primary resistance I get 50.6 Ohms. These readings are consistent when taken by 2 digital test meters. The power is applied across the outside 2 terminals. Any comments gratefully received.
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Old 9th Feb 2018, 7:07 pm   #2
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Default Re: HV PSU Question

It seems the only worry that you have is the hum. Judging whether the hum is a sign of a fault or not depends on us knowing in some absolute sense how loud it is. Is it as loud as a fridge when that's running ? Or as loud as a microwave oven ? Or as loud as a washing machine on full spin ? Or as loud as an electric drill ?

Can you measure the current flowing in the primary circuit ? If not then what's the lowest value fuse that survives in that circuit - you could at least try a 3A one in the mains plug, although that might well survive quite a serious fault in the transformer ?

Cheers,

GJ
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Old 9th Feb 2018, 7:34 pm   #3
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Default Re: HV PSU Question

Have you tried it with a lamp limiter in the primary circuit? With no load on the secondaries, the current should be low enough not to cause (say) a 60W bulb to glow (or if it does glow, only a very dull red on the fillament wires). If there's a shorted turn somewhere (and it could be in any of the windings) then the lamp limiter will light up.
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Old 10th Feb 2018, 1:18 am   #4
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Default Re: HV PSU Question

Could just be loose laminations. When I have a mains transformer that I think is suspect, my first test is to place in the bench, power it up on no-load and periodically test the temperature of the laminations. If they are more than 'just warm', most likely there will be a shorted turn or two - probably in the primary winding.

Al.
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Old 10th Feb 2018, 6:13 am   #5
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Default Re: HV PSU Question

Hi Jonny, the tests suggested above should prove the transformer. If it is faulty there are a number of HV modules available that run from a 12v supply and can be used to produce a variable EHT output at the low currents you need. They may give a nasty bite but are not lethal as is a mains transformer.


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Old 10th Feb 2018, 11:53 am   #6
johnyjf
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Default Re: HV PSU Question

The hum is quite loud, certainly more than I am prepared to live with in an oil filled transformer. Its about as noisy as a small battery power drill. I do have a Variac somewhere and I will hook that up to it with my Avo to measure the AC current. Will try it this afternoon. To be honest even if it is not a fault I'm not prepared to use it if it makes that level of noise. Will let you now how I get on. Thanks for the responses.
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Old 10th Feb 2018, 12:25 pm   #7
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Quote:
Originally Posted by johnyjf View Post
... Its about as noisy as a small battery power drill ...
You're right, that is worryingly loud. Oil is a much better coupling medium for sound than air, so although we might expect any noise level to go down if we seal our transformer in a can, if the can is full of oil the sound level won't go down. All the same, if an open-air transformer was making that much noise I'd switch it off very quickly too.

If the no-load primary current turns out to be high then I'm afraid there is a short-circuit somewhere. If it's not so high then I fear there's something very seriously wrong with the lamination assembly. But Ed_Dinning will know a lot more about this than I do.

Cheers,

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Old 11th Feb 2018, 4:26 pm   #8
johnyjf
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Default Re: HV PSU Question

OK thanks, Not sure I can get to it before Monday now but will do so then for sure.
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Old 11th Feb 2018, 4:58 pm   #9
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Default Re: HV PSU Question

Just a thought. Is it designed for 50c/s operation?
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Old 11th Feb 2018, 11:20 pm   #10
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Default Re: HV PSU Question

Primary winding resistance sounds a little low, I would be expecting something like 100 Ohms. Strangely enough, my valve amplifier suddenly went off and I found the supply fuse blown. There was nothing untoward on the secondary circuits but a lamp limiter glowed full brilliance. The primary winding measures just 14 Ohms so I suspect a shorted turn somewhere, probably shorting multiple layers. Maybe your transformer has suffered the same fate, try it on a lamp limiter.
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Old 11th Feb 2018, 11:32 pm   #11
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Default Re: HV PSU Question

Once the transformer has "charged up" to maximum flux the current should be very low, magnetising current only in fact. The transformer is 40 watts, but say its inefficient we can call it 50 watts. Magnetising current should be so small the lamp limiter ( depending on bulb wattage) shouldnt glow. I would " expect" maybe 3 or 4 watts for magnetising current only.
As far as the hum goes, it may be that the case its mounted in is making the noise ??
The last is just a suggestion.

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Old 12th Feb 2018, 12:28 am   #12
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Default Re: HV PSU Question

Actually, resistances sound about right!

2.3 ohms for the 10V portion, multiply by 20 to get approximately the resistance for the 200V portion (because it's going to be wound with the same wire gauge) gives 46 ohms, and that's roughly what you've got. There may still be a shorted turn, of course.

4kV at 10mA is 40W, and at 240V you'll draw 167mA ideally. So with your total primary resistance of about 50 ohms, you will lose 8.3V, about 3.5%, which is about right for a reasonably efficient transformer. If the secondary is designed to have the same percentage losses, you'll lose a total of 7% of input power. Much more resistance and you'd lose even more, which is not elegant unless the transformer is specifically designed to have an output which droops highly with load.
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Old 12th Feb 2018, 11:45 am   #13
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Arrow Re: HV PSU Question

Quote:
Originally Posted by johnyjf View Post
It [this transformer] is installed in a light chassis box and when I power it up from the AC mains (no sec. connections) it makes a loud hum.
Try powering it up outside that box and with the transformer sitting on a block of wood. That box may be acting as a resonating chamber, substantially amplifying the normal residual hum, possibly coming from loose laminations.

Al.
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Old 13th Feb 2018, 11:41 am   #14
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Default Re: HV PSU Question

I have powered it up outside the box and it sounds the same, hoping to have a long play this Thursday, real life keeps interfering with my plans. Thanks for all the help everybody!
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Old 13th Feb 2018, 12:33 pm   #15
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Default Re: HV PSU Question

Thanks for testing that idea: at least it's another 'possible' out of the way.

To my mind, there are just two things left: either badly loose lams (which I strongly suspect) or a defective transformer. But, saying that, have I over-looked something? The size of the no-load primary current will be indicative.

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Old 14th Feb 2018, 4:49 pm   #16
johnyjf
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Default Re: HV PSU Question

Hello Again, my what jolly japes. I found 10 mins or so to put it on a variac. The results with no load were hum starts at about 70% of input voltage and is far to loud for comfort at 90%. The ratio of OPv to %age IPv seems about right. I then hooked a load comprising a 470K and 220k HV resistors in series across the output tags with much the same results up to 3,225v, couldn't go further the resistors began to smell. Didn't have time to place the avo in series with the primary this time but my best guess on results to date is that I have a lamination problem rather than a winding one. The secondary current works out at about4.7 m/a which is half power. I reckon the transformer is useless but am wondering if stripping it down would yield any useful parts such as the secondary wire which I could then use to make an old fashioned shocking coil like we used to play with at school. Stripping it down could merely make a hell of a mess with oil everywhere. Have also read that the oil used in those transformers is poisonous and best kept well clear of your skin. Does anyone know?
John
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Old 15th Feb 2018, 1:57 am   #17
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Arrow Re: HV PSU Question

Quote:
Originally Posted by johnyjf View Post
Didn't have time to place the AVO in series with the primary this time but my best guess on results to date is that I have a lamination problem rather than a winding one.
Lamination problem: I suspect that you're correct. But you said "didn't have time, etc." Presumably, that was because of the over-heating resistors on the secondary winding. However, what we (or certainly myself) would like to know is what is the primary current (at full input voltage to the primary) when there is no load on the secondary.

Sometimes, loose lams. can be fixed by adjusting the bolts that hold the xformer together and similar mechanical considerations. Perhaps some additional clamps? May be worth spending some time investigating that line before condemning that xfromer permanently.

Al.
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Old 15th Feb 2018, 4:49 am   #18
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Default Re: HV PSU Question

Al, apologies lf you are aware, but it is oil filled!
Rob
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Old 15th Feb 2018, 6:04 am   #19
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Default Re: HV PSU Question

Hi Jonny, the dangerous oil is called PCB and is normally semisolid at room temps. I would think it unlikely this type was used in your transformer.
Not normally a good plan to try and re-use the thinner gauges of wire again, scientific wire company supply new, double coated (Grade2) wire at reasonable prices.

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Old 16th Feb 2018, 11:32 am   #20
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Arrow Re: HV PSU Question

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Al, apologies lf you are aware, but it is oil filled!
Ah! I see! I wasn't aware of that: perhaps I missed something. Mind you, a photograph of this xformer would have been a useful adjunct to this thread.

Al.
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