MR diode - half symbol

[ScottBouch]

In certain 50's/60's circuit diagrams (mainly aircraft / air ministry), I have seen MR used as the identifier for what I would call diodes and would identify them with a D.

Can anyone tell me what the M and R stand for? (I guess R is probably rectifier, but I am stuck on the M).

Also these component identifications are accompanied by a half of a diode symbol. Is there any relevance of this symbol to the MR identification, or is it just an older way of drawing a diode?

See attached example image form a Plessey PS112 power supply, regarding MR1 and MR2.

Thanks, Scott, (yes a child of the 1980s....)

[paulsherwin]

Metal Rectifier, usually selenium.

I'm not sure why they drew the half symbol like that.

[dazzlevision]

Originally Posted by ScottBouch

In certain 50's/60's circuit diagrams (mainly aircraft / air ministry), I have seen MR used as the identifier for what I would call diodes and would identify them with a D.

Can anyone tell me what the M and R stand for? (I guess R is probably rectifier, but I am stuck on the M).

Also these component identifications are accompanied by a half of a diode symbol. Is there any relevance of this symbol to the MR identification, or is it just an older way of drawing a diode?

See attached example image form a Plessey PS112 power supply, regarding MR1 and MR2.

Thanks, Scott, (yes a child of the 1980s....)

I'd say it stands for Metal Rectifier. Quite a common reference in those days, before germanium and silicon rectifiers.

[TonyDuell]

I've also seen diodes given 'CR' references, presumably 'Crystal Rectifier'

[ScottBouch]

Thanks for this!

Could a metal rectifier be built in such a way that one "package" contains two rectifiers such as the half-wave rectifiers shown in the diagram? ie; could this explain the half symbol as it's one half of the overall rectifier. Just thinking like modern full wave bridge rectifier packages with 4x diodes.

Cheers, Scott.

[paulsherwin]

You can certainly have two metal rectifiers in a single case, it was actually quite common. I don't think your circuit fragment shows that though.

[Radio Wrangler]

THere were even packages with four stacks of metal rectifier plates configured as bridges.

Each junction ony had a small breakdown voltage, so high voltage rectifiers were stacks of many junctions. The damned things were lossy, so interleaved in the stack of junction discs were large, thin metal plates as heat sinks. A through bolt holding everything together. You could do two rectifier assemblies on the same through bolt with the DC output common cathode in the centre, and the two AC connections at the ends.

Overheated selenium made an awful stink. Ingested selenium is teratogenic - causes birth defects. So the industry was pleased to see them go.

There are also small metal rectifiers, low voltage, low current, used as signal detectors and for AC ranges on multimeters. These were usually copper-(copper oxide) junctions.

David

[ms660]

Originally Posted by ScottBouch

Just thinking like modern full wave bridge rectifier packages with 4x diodes

Metal rectifiers were made with 4 diodes encapsulated in metal case, the Westinghouse EC1 being an example.

Lawrence.

[Boulevardier]

“Also these component identifications are accompanied by a half of a diode symbol. Is there any relevance of this symbol to the MR identification, or is it just an older way of drawing a diode?”

Difficult to know now. I wonder if it’s an earlier, rather fanciful, attempt to reflect the fact that a diode only passes half the waveform – i.e. the part "above the line" (horizontal axis) in a graph of the ac voltage…?


Mike

[ScottBouch]

Originally Posted by paulsherwin

You can certainly have two metal rectifiers in a single case, it was actually quite common. I don't think your circuit fragment shows that though.

I have a PS112 here, I'll take a look inside soon and see physically if its two separate MR's or one with three terminals.

Cheers, Scott.

[ScottBouch]

Originally Posted by Boulevardier

I wonder if it’s an earlier, rather fanciful, attempt to reflect the fact that a diode only passes half the waveform – i.e. the part "above the line" (horizontal axis) in a graph of the ac voltage…?

A quaint thought! Could just be that!

[G6Tanuki]

Yes, MR = Metal Rectifier.

In MoD diagrams you also come across CR = Crystal Rectifier.

And the odd "VT" - used by both the MoD and the Post-Office and also Government departments like AERE Harwell - which I've always interpreted as "Vave, Transistor" !!

I've got several small [about the diameter of the old half-crown] two-Selenium-diodes-back-to-back-with-a-centre-tap here, I think they came from a 1950s R4187 aircraft receiver where they were used across relay-coils to 'clamp' switch-off back-EMF spikes.

[paulsherwin]

There was also CT, Crystal Triode (transistor).

[G6Tanuki]

Looking at the R4187 schematic, the diodes I mentioned are actually marked as "W"

The only semiconductor I can think of which would be expected to have a W as its designator is the Westector, and these are definitely rather more high-power than a Westector ever was.

[ScottBouch]

Interesting to note they also are shown with the half symbol too!

Maybe it was an Air Ministry thing? Was the R4187 mady by Plessey, if so, maybe it was a Plessey thing?

[emeritus]

The half diode symbol was often used on drawings in my time with Plessey in the early 1970's. It might have been a BS standard but I'm away from home at present so can't check my old books.

I remember one of our early TVs breaking down and the repair man fixing it by soldering a "top hat"-cased wire-ended diode across the terminals of one of those large finned rectifiers, which I suppose must have gone open circuit. Dad was amazed that something that small could do the work of something that big.

[Craig Sawyers]

Originally Posted by Radio Wrangler

THere were even packages with four stacks of metal rectifier plates configured as bridges.

Each junction ony had a small breakdown voltage, so high voltage rectifiers were stacks of many junctions. The damned things were lossy, so interleaved in the stack of junction discs were large, thin metal plates as heat sinks. A through bolt holding everything together. You could do two rectifier assemblies on the same through bolt with the DC output common cathode in the centre, and the two AC connections at the ends.

Overheated selenium made an awful stink. Ingested selenium is teratogenic - causes birth defects. So the industry was pleased to see them go.

There are also small metal rectifiers, low voltage, low current, used as signal detectors and for AC ranges on multimeters. These were usually copper-(copper oxide) junctions.

David

Selenium rectifiers are still made in the UK https://www.gdrectifiers.co.uk/produ...um-rectifiers/

They made replica selenium bridges for me when I was restoring the Bletchley SZ42 cipher machine.

Craig

[ScottBouch]

So, today I opened the PS112 power supply to take a photo of the Metal Rectifiers. The vintage smell of Shellac, and other substances of the era was the first noticeable detail!

I was expecting to find MRs with fins, but these are instead bolted to a large aluminium heat sink. The heat sink is electrically insulated from the chassis.

They are Ferranti CV5376 rectifiers. I had a quick look online, can't find a datasheet scan.

Huge gauge cables from them! I wonder what their current rating is!

the AP's I've read state these power supplies can deliver between 7 and 15A at 28V DC. But I wouldn't be surprised if they are capable of a lot more, especially in short bursts like starting rotary inverters.

I also turned it on for the first time; off load the output voltage rose to ~42V, but I'm aware that with some load these regulate well. I assume the lower limit of 7A mentioned in the APs may be to ensure accurate voltage regulation.

Cheers, Scott.

[paulsherwin]

Very meaty! You'll often find the 50s ones heatsinked to a chassis. The big air cooled ones with lots of discs are mostly earlier.

[Lucien Nunes]

The CV number for those diodes decodes to a commercial type ZR32P which I see is also printed on your heatsink. I haven't immediately been able to find data for the ZR32P though. Evidently they are not metal rectifiers but ordinary stud-mounted junction diodes.

Apparently this PSU uses saturable reactor regulation, which will have a strictly limited current range over which it can control the voltage. Saturable reactors were used as amplifiers and power controllers for all sorts of applications, including lighting dimmers where the wattage of the lamp to be controlled had to be set on the dimmer in order to obtain the best curve, again due to the limited ratio of minimum to maximum flux in the control choke.

As for the MR identifier and half-arrow symbol, I think they were popular around the same time and there is probably a correlation with the use of selenium, but I'm not aware of any rigid connection between them. I'm not near my library at the moment but it would be interesting to compare the symbols used by popular commercial publications with 'official' ones that are likely to adhere closely to the BS, and what rectifier manufacturers themselves were using over time.