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General Vintage Technology Discussions For general discussions about vintage radio and other vintage electronics etc. |
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7th Apr 2014, 11:56 am | #1 |
Nonode
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,130
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Half wave rectification and mains supplies
As is well known, many later valve TV sets used a series heater chain with a silicon power diode and a dropper in order to run the heaters correctly with less heat production than just a resistance.
Many valve radio sets, especialy AC/DC types obtained the HT by half wave rectification of the mains. Either technique results in DC current being drawn from AC mains, this is generaly accepted to be a bad idea since DC current passing through the electricty company transformer results in potential magnetic saturation of the core and possibly overheating or extra losses. I doubt that 0.3A or less would noticeably affect a substation transformer of hundreds of KVA capacity. But what about a large housing estate with dozens of sets on one substation ? Or even a couple of sets on a small pole mounted transormer ? There would seem to be a potential problem, but I never heard that it was an actual problem in practice. |
7th Apr 2014, 12:18 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,005
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Re: Half wave rectification and mains supplies
In days past there was sometimes a significant DC component noticeable on AC supplies.
2 or 3 TVs might add what, 500mA each of DC - which is going to be trivial compared to the load of a couple of electric fires which the transformer will be capable of handling. Perhaps if the DC component were to become a significant component of the maximum-rated-capacoty of the transformer it might be an issue? Remember that the DC, as well as potentialy flowing through the distribution-transformer, also "sees" all other loads on the circuit as being presented in parallel with the transformer so these loads will sink some of the DC. I haven't done the maths but I suspect that in the case of a 3-phase system the magnetising currents of the DC components on the three phases may well cancel each other out. |
7th Apr 2014, 12:27 pm | #3 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,397
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Re: Half wave rectification and mains supplies
I'd wondered about this, too- as you say, it's one of those cumulative things and I'm sure that some national jurisdictions frowned on the practice. I have a feeling that some of our antipodean and continental members may know more. I think that at least some of the TVs that used half wave HT rectification and also used "heatless dropper" diodes for the heater chain had this diode negative-going, which would have at least slightly balanced the input current characteristic- though it would still have been asymmetric and ragged with a positive peak. It would have tested even the deliberately high h-k insulation of P-series valves, too- I recall that the boost diode tended to be at the most negative point in the chain, taking advantage of its exceptionally highly insulating heater.
I gather that the Philips G8, with a simple single thyristor direct input PSU was notorious for chopping holes in the supply when present en masse! |
7th Apr 2014, 12:45 pm | #4 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,087
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Re: Half wave rectification and mains supplies
I'd never heard of it being an actual problem in practice, though I agree with you it's a potential problem.
DC on the mains is bad news - forget about the supply distribution transformer, have you considered the effect on other transformers in equipment you plug in yourself? With one half-cycle marginally clipped, the average voltage will no longer be zero and you'll have a small DC voltage biasing your own transformers one way or the other. Luckily, again, it doesn't seem to be a significant problem in practice. |
7th Apr 2014, 5:05 pm | #5 |
Retired Dormant Member
Join Date: Mar 2014
Location: Wolverhampton, West Midlands, UK.
Posts: 153
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Re: Half wave rectification and mains supplies
Its probably a bigger problem now than years ago.
Harmonic distortion linked to DC drives etc. is a major problem. Robotics/inverter motor control and DC drives were not so common in the past. http://www.danfoss.com/NR/rdonlyres/...CPowerLine.pdf In the past Ward Leonard with regeneration was probably more in vogue. Then there is, we poor DIY types having to suffer mechanically noisy toroid's. http://www.lcaudio.com/index.php?page=316 I guess its another fun build DC blocking filter..LOL Tubeglow. Last edited by Tubeglow; 7th Apr 2014 at 5:22 pm. |
7th Apr 2014, 6:17 pm | #6 |
Octode
Join Date: Jul 2009
Location: Carmel, Llannerchymedd, Anglesey, UK.
Posts: 1,509
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Re: Half wave rectification and mains supplies
The worst example of mains degradation I encountered was in Kent, where a plating shop was using triac control and actually caused a negative-going spike at the peak of the positive 1/2 cycle. This sometimes caused mayhem with a motor controller in another unit nearby and I got the blame for it, having designed it. When we located the culprit, a large choke was all that was required.
Les. Les. |