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Old 29th Nov 2023, 6:11 am   #1
petervk2mlg
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Default MR91H zener diode data

I'm in the process of rejuvenating an APT TSU power supply and need a replacement zener for the installed MR91H which is physically damaged.
Can someone please point me in the direction of a suitable replacement?
I attach the schematic of the power supply.
Would I be correct in thinking that it's a 9.1 volt zener?
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Old 29th Nov 2023, 7:26 am   #2
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Default Re: MR91H zener diode data

It looks to be marked MR51(M, H or W suffix, it's not clear) Which might suggest 5.1v which is a standard value in the E24 series. 5.1v would be enough to bias on the string of three emitter followers in the main regulator series-pass arrangement.

ZD1 is the main voltage reference and is marked MR51H which seems to be clearer than the other one. This suggests 5.1v which is an excellent choice for a voltage reference. What we habitually call zener diodes, if above about 3v aren't zeners at all, they're avalanche diodes, which do the same job, but work differently down on the semiconductor physics level. Avalanche diodes have a temperature coefficient depending on their voltage which passes through zero close to 5v. So 5.1v diodes are very commonly chosen as voltage references, because their voltage is less dependent on temperature than other voltages.

So this seems to fit with reading the marking as 5.1v. ZD3 might have a different suffix marking it as possibly a lower accuracy grade or maybe a higher power variant.

I think I'd just bang a 5.1v job of the same body size in.

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Old 29th Nov 2023, 8:06 am   #3
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Default Re: MR91H zener diode data

Sorry my pic wasn't very good quality. This one might be better.
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Old 29th Nov 2023, 8:09 am   #4
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Default Re: MR91H zener diode data

And here is the relevant section of the parts list.
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Old 29th Nov 2023, 8:30 am   #5
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Default Re: MR91H zener diode data

Looks like it's a 9.1V 0.5W Zener - see the attached cross reference.
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Old 29th Nov 2023, 8:32 am   #6
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Default Re: MR91H zener diode data

Given that the parts list shows two MR91H zeners could you not measure the voltage across the other one?

Cross posted with Peter.
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Old 29th Nov 2023, 9:23 am   #7
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Default Re: MR91H zener diode data

Oh, well, it looks like they missed the temperature coefficient trick.

There are 9.1v and 6.2v 'zeners' with low tempcos, made by Motorola. They are a compound device with one or more plain Si diodes forwards and an avalanche diode in reverse. Operated at a specified current they arranged it for the temperature variation of the silicon diode to quite accurately compensate the temperature variation of the avalanche diode. They're somewhat later than this power supply looks, and it's not likely to be of a precision needing such things.

9.1v is still reasonable for the overhead supply to drive the series-pass arrangement well on.

We don't know what the transformer secondary is producing, so there's not enough info to be able to estimate the dissipation in the diode. Just have to go by body size...


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Old 29th Nov 2023, 11:25 am   #8
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Default Re: MR91H zener diode data

Quote:
Originally Posted by Station X View Post
Given that the parts list shows two MR91H zeners could you not measure the voltage across the other one?

Cross posted with Peter.
Yes, I'll be able to do that once I'm sure that the other parts of the supply are in good enough shape to power it up.
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Old 29th Nov 2023, 1:12 pm   #9
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Default Re: MR91H zener diode data

Brief data for AEI MR91H ...

Nominal Voltage = 9.1V ±5%
Max Zener current = 23mA
Max Dynamic resistance = 20 ohms
Max dissipation = 250mW

Rich
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Last edited by Marconi_MPT4; 29th Nov 2023 at 1:15 pm. Reason: incorrect symbol display for ohms
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Old 29th Nov 2023, 10:44 pm   #10
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Default Re: MR91H zener diode data

Quote:
Originally Posted by Marconi_MPT4 View Post
Brief data for AEI MR91H ...

Nominal Voltage = 9.1V ±5%
Max Zener current = 23mA
Max Dynamic resistance = 20 ohms
Max dissipation = 250mW

Rich
Excellent. Many thanks Rich and to all you gentlemen who responded to my enquiry.
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