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Switching 800VDC with a MOSFET




 
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Author Topic: Switching 800VDC with a MOSFET  (Read 1003 times)
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KK4YY
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« Reply #25 on: May 08, 2019, 05:12:45 PM »

Nico,

I don't know technically why there's an issue with floating a center tap. I know it's been mentioned here and there. The only reference I can provide is from an old UTC catalog. Image attached.


Don


* UTC W.png (122.6 KB, 738x279 - viewed 43 times.)
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W2JBL
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« Reply #26 on: May 08, 2019, 06:58:55 PM »

If that was my transmitter I would have installed a relay to emulate the function of the high voltage toggle switch and had the thing on the air by now. Step start for a DX100? Yeah, right.   
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w8khk
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« Reply #27 on: May 08, 2019, 07:30:29 PM »

I have personally experienced the insulation failure mode regarding floating the center tap of a high voltage plate transformer.  Back in the '60s, I was using a UTC S-46 plate transformer in a bridge configuration, with two oil caps in series, floating the center tap between the two capacitors.   I actually trashed two of the new S-46 transformers.

I later used a UTC S-48 transformer, in the same circuit configuration, and never had a problem.  (Even though it was not recommended, the S-48 probably survived due to its higher voltage rating.)

I had the opportunity to speak with a support rep at UTC in NYC, and, as I recall, he explained the problem regarding the insulation between the winding layers.  The primary winding is applied to the core first, followed by some insulation.  Then the start of the secondary is applied, two windings that each begin at the center tap.  Subsequent layers of windings are added, with interleaving insulation, to arrive at the desired end-to-end voltage at the outside of the windings.  He explained that the voltage between the individual layers close to the primary was not as high as the voltage at the outer end, therefore the insulation between the beginning of the secondary (actually the center) and the primary did not need to withstand the full voltage of the secondary.  His advice:  If it is desired to use a circuit where the center tap is elevated from ground, then select from the CG (Commercial Grade) devices that are specified for the additional insulation voltage tolerance.  

There was actually a requirement that for the S series transformers, the CT should not be disconnected when in standby mode.  I copy an excerpt of the catalog to this post, from page 49 in this link:

http://www.junkbox.com/electronics/utc_transformer_catalog_1963.pdf

You may be able to determine whether the overall insulation rating of the Heathkit DX-100 plate transformer is sufficient for your planned use by performing a hi-pot test.  This test will show leakage at a voltage where the insulation breaks down, but since it is a DC test, it will not be possible to determine where in the transformer the breakdown occurs. This should not be an issue, so long as when using the transformer, the overall combined AC and DC potentials to not come dangerously close to the hi-pot test voltage.  If you find this test voltage to be significantly in excess of the voltage you expect at the CT, then that specific transformer may be safe to standby the CT.  If the hi-pot test indicates a lower safe voltage than the working voltage of the transformer, I would suspect an insulation deficiency between center tap and core or primary windings.  

It is also important to note that floating a plate transformer is not analogous to floating a filter or modulation reactor.  Even if the core of a plate transformer is floated, remember that there is a low impedance path from the primary winding to ground, so floating the core does not avoid the potential for insulation breakdown.

Personally, I believe it would be simpler, and more reliable, to bias the final and modulator to cutoff if it is desired to leave the plate voltage on continuously.  Rather than decrease bleeder resistance to eliminate soaring, or add series resistance to the primary with a relay, you might consider using filter capacitors that can withstand the unloaded output of the power supply.  With choke input, you still should not need any soft-start circuitry.

By the way, I have been told by several folks that the BC-610 plate iron has the same restriction, no bridge rectifiers, and don't float the CT.  This apparently is a cost-saving design consideration on some transformers....

Good luck on the modifications.


* CT.JPG (37.87 KB, 441x223 - viewed 36 times.)
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Rick / W8KHK  ex WB2HKX, WB4GNR
KK4YY
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« Reply #28 on: May 08, 2019, 08:03:30 PM »

Thank you, Rick, for your well thought out and informative response. I think we were looking at the very same catalog - I should have read along another 2 pages!

Unfortunately, I don't have access to a hi-pot tester as I don't work in the electronics field anymore.

My purpose isn't simply to add t/r keying to a DX-100. I could do that as fast as anyone else with a no-brainer relay. But, I am enjoying exploring alternative ways of doing things that require thought and experimentation.

Thanks again, Rick, for your advice and for taking my work seriously.


Don
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KD6VXI
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« Reply #29 on: May 08, 2019, 10:05:58 PM »

I have a Henry 2k xformer here that has a secondary to case short.

It's been painted red and I used a white paint pen to put a warning on it.  It's mounted in 2 inch glastic insulators and has been working fb in a gs35b amp for a few years

Although not ideal, it's saved about 4 hundred bucks.

If you're building for yourself, and put warnings inside I don't see a problem with it.

WTF would you be doing with your hands inside with power applied anyway 😋. (tongue in cheek, of course).

Yeah, I don't trust Millen connectors any longer.  I was bitten by an amp with the covers ON and supposedly completed, remember?  Not by something I was servicing.  Big difference, and the way HV was handled.



I do agree, it's not ideal.  But it can be pressed into service.


--Shane
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KK4YY
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« Reply #30 on: May 09, 2019, 07:38:05 AM »

Shane,

I think we all have a "this part is safe to touch" and "that part isn't safe to touch" thing going on in our heads when servicing equipment. And when working on a 12 volt powered unit, it's "all safe to touch".

I think that painting a transformer red and writing a warning on it clearly put in the mental "do not touch" category. But a transformer is large and easy to inadvertently contact. That makes me worry a little. The fact that it's not mounted that way to prevent the case from becoming live, but that it's known to be live, makes me worry a little bit more.

A plexiglass cover would make me worry a lot less.


Don
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