EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: voltampere on May 01, 2019, 07:14:42 am
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Probably easy for the experts here, but i am scratching my head too long now about this topic.
I am troubleshooting the power supply of a Fluke 5440A and have a burnt zener diode.
Before i just replace the zener with a spare i want to fully understand the function of this part of the circuit.
Here is the part of the schematic:
(https://www.eevblog.com/forum/beginners/diode-and-zener-diode-as-voltage-divider/?action=dlattach;attach=722169)
I have drawn a red line to show the relevant path.
As i see it there is no series resistance for the diode and zener combination.
So the only resistane would be the combined impedance of the zener and diode.
At 22 volts across them how can you keep these parts from glowing?
From the "List of replacable Parts":
CR 5 DIODE, SI, BV= 75.0V, IO= 150MA, 500MW 1N4448
VR 13 ZENER, UNCOMP, 5.1V, 5%, 20.0MA, 0.4W 1N751A
Thanks for helping
Joerg
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It's most likely to prevent the zener conducting in the reverse direction. Remember that if you reverse the polarity across a zener it becomes an ordinary diode. If, say, it's between two supply rails, if one gets shorted you don't want the other one to get dragged down too. The series diode ensures that you only get conduction in the 'zener' direction.
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Makes sense...
But to me the question still remains:
How can the zener ( and the diode ) survive without some current limiting resistance?
So far every explanatory circuit with a zener that i could find had a resistor in series.
regards
joerg
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Maybe the L4 inductor has enough resistance to keep the diode current within spec.
A small 5x5x2mm SMT inductor can have 2k of resistance
etc SRR4528A-391MDKR-ND (https://www.digikey.co.nz/product-detail/en/bourns-inc/SRR4528A-391M/SRR4528A-391MDKR-ND/9349846)
Also i dunno where the coil is for that relay, maybe if current flows through the diodes it pulls the relay in and breaks current through the diode? ie, self protecting.
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Okay, i did not expect the inductor to have much impact.
2k ohm would definitely help here.
But your datasheet reference gives 1910 typ. and 2380 max, but that is milliohms!
The incuctor is described as "6 turn choke".
I checked the datasheet of a 1N751A and it says max regulator current (IZM) 70 mA.
So if i calculate it then the whole "string" should have not less than 315 ohm.
Is this possible?
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May be the diode is for temperature compensation of the zener.
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It’s somewhat hard to tell with the little given because we can’t see where these pins go, but here is my S.W.A.G..
The -17 VDC is a rail and that is fixed. In the relay's de-energized state there is the series zener/rectifier for about 5.6 VDC to pin 29 (which I think is a control line to another part of the circuit), so pin 29 can be no more positive than -11.4 VDC or the zener/rectifier will clamp the voltage at that level. If pin 29 tries to go more negative, the 1N4448 will be reverse biased, the zener can not act like a regular rectifier, and the clamp is then effectively out of the circuit.
There is another zener/rectifier for about 15.6 VDC between the -24 VDC rail to pin 29 with the relay de-energized so pin 29 can't go more positive than about -8.4 VDC with the relay de-energized. This means that when the relay is de-energized, pin 29 is held between -11.4 and -8.4 volts.
When the relay is energized the 15 VDC zener is shorted out and pin 29 can't go more negative than -24.6 VDC or more positive than -8.4 VDC.
https://www.eevblog.com/forum/beginners/diode-and-zener-diode-as-voltage-divider/?action=dlattach;attach=722169 (https://www.eevblog.com/forum/beginners/diode-and-zener-diode-as-voltage-divider/?action=dlattach;attach=722169)
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The rails are referenced to different grounds (S COM != DAC COM), and S COM is connected to DAC COM via a 422k and a 10k resistor in series.
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I realize that there are multiple commons, grounds, or reference points but that doesn’t mean my explanation is incorrect. The zener/rectifier in series is used as a clamp in one direction only. Whether the reference is ‘R’ or ‘A’ or ‘FG’ may be important for operation of the reference but that is what those diodes are used for. The relay contacts show are on K2 and it is energized for negative outputs and de-energized for positive outputs.
After a search of the service manual I found this chart of voltages at various test points and the point in question above is marked as V1(FH) and is shown to have a nominal value of -11.3 VDC and a limit of -8.6 VDC which is pretty close to my guess of -11.4 to -8.4.
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Okay, i did not expect the inductor to have much impact.
2k ohm would definitely help here.
But your datasheet reference gives 1910 typ. and 2380 max, but that is milliohms!
Lol digikey are listing spec wrong.
Says 2.38kOhm
In any case, 6 turns wont have any real resistance.
It would need to be 1000s of turns