UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 3rd Oct 2021, 6:34 pm   #1
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
Default Rectifier surge-limiter resistors rethought.

The "classic" bi-phase-full-wave power supply, using a centre-tapped transformer to feed the two anodes of a valve rectifier like an EZ81, normally includes a pair of resistors wired in series between the ends of the transformer windings and the anodes, in order to limit the peak currents seen by the rectifier.

My 'production accountant' sense sees this as wrong - it needs two components, meaning unnecessary soldered-joints and possibly an extra tagstrip.

A few designs made more sense, wiring the single surge-limiter between the rectifier's cathode and the first smoothing-capacitor. Fewer soldered-joints and only one component - that's an engineered saving.

[Another consideration, from a reliability perspective - the two-resistors approach always had the problem that if one resistor went O/C things would probaly struggle-on using half-wave rectification, it then being a game-of-chance whether the DC-saturation would drive the transformer to meltdown or the one-half-of-the-rectifier would melt under the load].

In recent times I have reworked a few communications-receivers/amplifiers/transmitters to have _no_ resistors on the anodes/cathodes of the rectifiers, and instead moved the surge-limiter resistor to sit between the centre-tap of the transformer and earth. It performs the same function, and has the benefit that the you can use a nice metal-clad resistor and bolt it to the chassis as a heatsink so things run cooler (and there are fewer parts of the radio that are running at HT so you're less likely to get bitten).
__________________
I'm the Operator of my Pocket Calculator. -Kraftwerk.
G6Tanuki is offline  
Old 3rd Oct 2021, 8:14 pm   #2
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Rectifier surge-limiter resistors rethought.

It does make sense, what you suggest.

Though, having separate resistors would allow slightly different values, to compensate for the slightly different winding resistances (with the same number of turns, the half-secondary that's wound first will have a lower resistance than the one wound on top, because the turn length is slightly less).

However, I have never seen this done in practice.

Me, when I design a transformer, if the winding resistance is less than the recommended minimum limiting resistance, I just reduce the wire gauge. No resistors added, thinner wire, the accountants would be double-happy!
kalee20 is offline  
Old 3rd Oct 2021, 9:08 pm   #3
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,172
Default Re: Rectifier surge-limiter resistors rethought.

Well said Gents. this is what engineering is meant to be about

Ed
Ed_Dinning is offline  
Old 3rd Oct 2021, 11:54 pm   #4
Silicon
Nonode
 
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,152
Default Re: Rectifier surge-limiter resistors rethought.

I have a radio with a charred mains transformer.

I think this happened because the filament fractured and shorted one anode to the other and effectively shorted the HT windings.

Alternatively it could have shorted an anode to the cathode and fed AC into the reservoir capacitor.

A resistors on each anode could have made a difference.
Silicon is online now  
Old 4th Oct 2021, 2:20 am   #5
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: Rectifier surge-limiter resistors rethought.

Also, a rectifier losing vacuum can go into an anode-to-anode arc mode. If the individual anode resistors are fusible and can break that current/voltage without arcing indefinitely themselves, they could save your transformer.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 5th Oct 2021, 8:00 pm   #6
Leon Crampin
Octode
 
Join Date: Nov 2005
Location: Surrey, UK.
Posts: 1,869
Default Re: Rectifier surge-limiter resistors rethought.

When restoring a pre-war set with a fragile and ancient rectifier such as the UU3 in my Murphy A30C, I wire a 1N4007 in series with each valve rectifier anode. Together with a 1A fuse to BS1362 in the plug, this provides some protection to the mains transformer in the event of a valve developing a short. This does not change the operation of the set in normal running.

Having said that, I've yet to see a UU3 failure, but its construction is a bit flaky - including a nut and bolt securing the electrode assembly to the glass pinch pedestal. Mazda produced some incredibly long-lived valves at that time.

Leon.
Leon Crampin is offline  
Old 5th Oct 2021, 11:57 pm   #7
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Rectifier surge-limiter resistors rethought.

The use of two resistors was likely the more practical approach back in vintage days, as lower wattage resistors would have been more common.

Relying on those winding arm resistors, or a single resistor in the cathode or CT leads, as a fuse would be very hit and miss imho. Even for a fuse to be effective in those locations requires technical assessment of the max operational current and an awareness of the tolerance of the fuse's operational characteristic, and an even bigger assumption about the time/overload capability of the PT secondary winding. And as an aside, a fuse in the cathode to filter lead isn't going to save the transformer from an arcing rectifier valve.

With respect to any valve diode, whether an EZ81 or UU3, I'd agree that proactive protection against internal arcing faults by installing 1N4007's has got to be top priority.
trobbins is offline  
Old 6th Oct 2021, 12:07 pm   #8
Leon Crampin
Octode
 
Join Date: Nov 2005
Location: Surrey, UK.
Posts: 1,869
Default Re: Rectifier surge-limiter resistors rethought.

I guess PT = power transformer.

Murphy, Philips and some others introduced thermal circuit breakers into the mains transformers of their later post-war sets. This probably provided the best protection at a modest additional cost. I've yet to see a failure of one of these later transformers.

Likewise, I've never ever seen an EZ80/1 fail with a short a-a or h-k.

Leon.
Leon Crampin is offline  
Old 6th Oct 2021, 2:58 pm   #9
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
Default Re: Rectifier surge-limiter resistors rethought.

I too haven't seen a modern rectifier go short. I do remember a directly heated one where one of the flat tape heater cathodes broke and sprung across to touch the anode. That cost me an AR88 mains transformer.

The thing about two resistors of different values to compensate for the different lengths of secondary windings, I have never seen it done, to me it's overkill, surely a bifilar winding of the secondary would be the best solution.

OK it would increase demands on the winding insulation....
__________________
I'm the Operator of my Pocket Calculator. -Kraftwerk.
G6Tanuki is offline  
Old 6th Oct 2021, 5:50 pm   #10
Leon Crampin
Octode
 
Join Date: Nov 2005
Location: Surrey, UK.
Posts: 1,869
Default Re: Rectifier surge-limiter resistors rethought.

It's impossible to bifilar wind an HT secondary with about 700V rms between adjacent enamelled wires. If it's really necessary to have matched windings, stacked bobbins is the only way - used by Murphy amongst others in the '30s.

Leon.
Leon Crampin is offline  
Old 6th Oct 2021, 11:41 pm   #11
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Rectifier surge-limiter resistors rethought.

Quote:
Originally Posted by G6Tanuki View Post
The thing about two resistors of different values to compensate for the different lengths of secondary windings, I have never seen it done, to me it's overkill.
Measuring winding resistance is easy, and so customising the anode buffer resistance values is also relatively easy to do, but that does assume the rectifier diodes are balanced if the outcome is to be of any tangible benefit. Measuring the actual winding/anode current to confirm each arm is conducting the same level of current is a little more onerous, and probably best done with a current sense resistor in the CT leg, and measuring the peak level rather than the rms level.

Of the secondary winding DCR's I've measured on vintage power transformers, its a rarity to get more than a 10% difference, and many of those secondaries would not need additional buffer resistance to meet a rectifier's minimum resistance requirement.
trobbins is offline  
Old 8th Oct 2021, 3:55 pm   #12
broadgage
Nonode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
Default Re: Rectifier surge-limiter resistors rethought.

HT transformers did indeed often a slightly different resistance for each half of the winding. However a few extra turns were often added to the higher resistance winding to compensate for this.

The design aim was that at say two thirds load that each half of the winding would have the same on load voltage.
broadgage is offline  
Old 8th Oct 2021, 5:53 pm   #13
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
Default Re: Rectifier surge-limiter resistors rethought.

As regards fusing, I remember some circuits included a MES filament-bulb in the lead from the transformer's centre-tap to ground; this served both as a pilot-light [it was sized to glow dimly under normal load] and as a fuse in the event that something downstream caused excess current to flow.

The kit I recall using it had one big flaw: if there was a _brief_ power-outage [some roadie tripped-over an extension-lead in the dark] when the power came back on you had the situation where the smoothing-capapcitors had been drained to emptiness but the heaters of both the amplifier and the rectifier were still pretty much fully-hot.

A load eager to drag 200mA from the supply, entirely-depleted smoothing-capacitors, and a rectifier whose cathode is still hot-enough to be close to full emission.

Exit stage left one 12V 2.2W bulb. Fortunately it wasn't hard to source replacements, they were used in loads of 60s/70s cars so you only needed to make an appeal to the audience - "does anyone here drive a Mini?" and the gig could go on.
__________________
I'm the Operator of my Pocket Calculator. -Kraftwerk.
G6Tanuki is offline  
Old 11th Oct 2021, 9:44 am   #14
stuarth
Heptode
 
Join Date: Feb 2010
Location: Heysham, Lancashire, UK.
Posts: 665
Default Re: Rectifier surge-limiter resistors rethought.

The two diodes are not on at the same time, they take it in turn to conduct on each half cycle (should that be “each half Hertz-second”?) of the incoming mains. So the situation is not the same as, for example, parallel transistors in a power supply, where forced current sharing is essential to stop one - the hottest transistor - hogging all the current.

Serious imbalance in either the transformer or the diodes should be visible if you look at the ripple waveform on a ‘scope.

On fusing, I’ve always thought fuses between the hot ends of the transformer and each diode to be a good thing, and essential with semiconductor diodes. Even valve diodes can go short circuit and take out the expensive/irreplaceable mains transformer, as has been mentioned above. The fuses can be slow blow types, they’re there to protect the transformer, not the diodes.

Stuart
stuarth is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 1:35 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.