Half wave rectification and mains supplies - UK Vintage Radio Repair and Restoration Discussion Forum

broadgage · 7th Apr 2014, 11:56 am

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.

G6Tanuki · 7th Apr 2014, 12:18 pm

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.

turretslug · 7th Apr 2014, 12:27 pm

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!

kalee20 · 7th Apr 2014, 12:45 pm

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.

Tubeglow · 7th Apr 2014, 5:05 pm

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.

lesmw0sec · 7th Apr 2014, 6:17 pm

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.

7th Apr 2014, 11:56 am	#1
broadgage Nonode Join Date: Jan 2010 Location: North Somerset, UK. Posts: 2,130	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
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,005	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
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,397	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
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	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
Tubeglow Retired Dormant Member Join Date: Mar 2014 Location: Wolverhampton, West Midlands, UK. Posts: 153	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
lesmw0sec Octode Join Date: Jul 2009 Location: Carmel, Llannerchymedd, Anglesey, UK. Posts: 1,509	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.