Maha MH-C9000 zener diode identification

[TheGrave]

Hi,

One of my slots on the C9000 charger is having issues and this diode seems to be dead:

Can anyone help me with the identification? Guy troubleshooting it measured voltage around 4.96V on its healthy brothers We were having issues identifying it by the colour code (if this is possible at all).

Cheers

 

[aznsx]

Hi,

One of my slots on the C9000 charger is having issues and this diode seems to be dead:

https://i.ibb.co/GCbswy6/IMG-0f30a084cec30f60cc956c5e8d3fc542-V.jpg

Can anyone help me with the identification? Guy troubleshooting it measured voltage around 4.96V on its healthy brothers We were having issues identifying it by the colour code (if this is possible at all).

Cheers

It has been decades since I studied them (quite a bit) / worked with them/analyzed the circuits (somewhat less), so I probably shouldn't speculate from memory, but...

Isn't the measured in-circuit voltage drop across a 'good' one in a good circuit under power in fact its rating?

As an aside: The voltage drop across the zener, plus the voltage drop across the circuit's dropping resistor, should equal the applied source voltage being regulated.

That's how I remember it, but I certainly could be wrong after all these years. If so, you won't need the color code if the good one you're measuring is the same as the failed one (which they appear to be), and your meter is accurate.

 

[LEDphile]

Given that green-black is 5-0 following the standard resistor color code, and that 2-stripe zeners are usually marked in decivolts, that is likely a 5V zener. And that aligns nicely with the 4.97V measurement.

 

[aznsx]

Given that green-black is 5-0 following the standard resistor color code, and that 2-stripe zeners are usually marked in decivolts, that is likely a 5V zener. And that aligns nicely with the 4.97V measurement.

Sounds like 2 votes for '5V'. How could we both be wrong?

 

[aznsx]

Just a final thought. Because this is essentially considered T&M (test and measurement) equipment, when you shop for a replacement diode, if you have a choice of tolerance, go low. Close tolerance is an asset in such equipment. You likely know that, but just wanted to mention it, since you may have a choice. I'm not sure about the original, but lower is always better in test gear / instrumentation.

 

[Dave_H]

There is more to a zener than just the voltage, depending on how it's used. Standard 5% values are 4.7v and 5.1v. Judging by the big transistor or regulator nearby, it might be a clamp, precise voltage probably not too critical. Voltage measurement on good unit suggests 5v is correct, but...

There's a 3rd band (orange). Usually code indicates part number (part of it) not value e.g. 1N4148.

This is a now somewhat less common MELF package, might not be easy to find exact replacement. If you don't, a leaded version with leads cut back very short should work.

Dave

 

[Dave_H]

Part it's close/connected to is NIKOS P2504BDG N-channel MOSFET.

If the zener (assuming it is) is used as a clamp, and has 5v across it, this might not be the zener voltage of the part if it is not conducting at the time. This is a risk in reverse-engineering.

An example: MOSFET 20v gate (max.) circuit commonly uses 15v zener for protection although only (say) 10v is applied to the gate, which you measure; but it's not a 10v zener. Putting one in could be a problem, if it starts conducting when it shouldn't.

Further examination/test on good unit might be required, unless you can make a positive ID on the part.

Dave

 

[aznsx]

The cat has now been thrown amongst the pigeons

 

[Dave_H]

Further caveat, the zener itself may not be defective, if it is connected in parallel with another component which is damaged. Zener of that size would require a fair bit to blow it, but there's no visible sign of damage, even discolouration.

If the zener part number can't be positively identified, it may be necessary to remove one from a good unit (or another working channel) and do some testing on it; inconvenient that it is.


Dave