Forum: Analog Circuits I burnt my PZEM-061 AC volt ammeter


von Syed (Guest)



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Hi,
I would like to repair my AC volt ammeter PZEM-061 which got burnt due 
to connecting an AC dimmer switch to it ( I had made using Triac BTA41 ) 
but there was no load on the dimmer, I just rotated the pot to check any 
changes in voltage.

I have checked the 50ohm resistor and 1mf capacitor are good but dont 
know which component got damaged.

Please help me repair the PZEM-061 as its very useful tool for me.

Thanks in advance.

: Moved by Moderator
von Franko P. (sgssn)


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Hi, seems you're joking;
The item (resistor?) between the connector and the big capacitor looks 
burnt. Don't you see?

regards
Gerhard

von Syed (Guest)



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Hi,
Yes, It is burnt but its showing the correct 50ohms value on MM. I 
removed from pcb and checked as you too doubted it. Then I checked the 
diode M7 its also fine then I tested the zener diode which is giving 
zero ohms in both the directions.

I will remove it from pcb and test again to reconfirm. Wondering whats 
the value if it has to be replaced.

Apart from repairing the VA meter I would appreciate if someone can 
explain what went wrong in my above testing which burnt my va meter 
(PZEM-061).

Thanks Gerhard, for your response.

Regards,
Syed

von Syed (Guest)


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Hi,

I removed the Zener from above pcb and it is shorted. It is showing 
0ohms resistance in both directions.

Hope other components and chips are working.

I need the value of Zener diode for replacement.

Thanks in advance for helping me in repairing my VA meter.

Regards,
Syed

von Mario P. (Guest)


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Hi,

there is a description for the powersupply in this device. Maybe it 
helps?
Beitrag "Kompliziertes Kondensatornetzteil"

Mario P.

von Toxic (Guest)


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Syed wrote:
> Wondering whats
> the value if it has to be replaced.

According to the german thread it's a 12V zener diode.The TO of the 
german thread also replaced the 1uF X2 capacitor with a 0.33uF X2 to 
reduce the heat of the 100 Ohm resistor.He also wrote that the voltmeter 
works fine with 3.3V and just draws 1mA

Regarding your triac problem.
The 100 Ohm resistor (in series with the X2 capacitor) reduces the 
inrush current for the zener diode.Without this resistor it wouldn't 
take long to short out the zener diode.
Just imagine:You connect the voltmeter exactly at the time when the 
mains voltage reaches it's maximum value and the X2-capacitor is 
completely discharged.You would get a huge inrush current(10,20...50 
amps) to charge the X2 capacitor.This current must be reduced to an 
acceptable level.Maybe to 2 amps or less.The capacitor will charge up 
quite quickly in a very short time,so that a zener diode can cope with 
it.
I am now assuming(!) the following:
The constant sharp rise of the triac controlled voltage always hurts the 
zenerdiode constantly with - let's say - 2A in rush current but in a 
manner that is worse than it would be with a pure sine wave.

English is not my native language so it's a bit difficult for me to 
exactly explain what I actually want to say.
Anyway: With LTspice one could exactly simulate what's really going 
on,when a capacitor power supply is fed with an "unusual signal"

I think that by replacing the zener diode your voltmeter should be fine 
again.

But there is something else that makes me wonder.Why is that inrush 
current limiting resistor in such a bad condition?Even if the Diode 
fails it should not cause any damage to it.Maybe it's not properly 
designed or the Chinese are well aware of it and kept it that way to 
make the voltmeter as cheap as possible...Should your voltmeter work 
again,I would probably try to increase the value of the resistor and 
reduce the value of the X2 capacitor.Saves heat and money.

von Toxic (Guest)


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Toxic wrote:
> The constant sharp rise of the triac controlled voltage always hurts the
> zenerdiode constantly with - let's say - 2A

I might not be that wrong at all.I just simulated your circuit with 
LTspice.The first picture shows the voltmeter connected normally to the 
mains.The second picture shows it connected to a simple phase dimmer.
If you compare the 2 pictures you will see that the zener diode is 
constantly hit with a 6A pulse every 10ms if the phse dimmer is in use 
which in the end lead to the diode's demise.

I also uploaded the LTspice files so that other users can simulate the 
circuit themselves and correct me if I did something wrong or 
misinterpreted the diagrams.I am not Einstein - at least not yet 😉

von Syed (Guest)



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Hi,

I am very happy to inform that my VA meter is working now :). I didn't 
find the smd zener in my market but i got IN4742a. I kept the rest of 
the components as is because they were all ok except zener.

Hope that the meter works well with 4742a.

I am really thankful, Toxic for your help and the efforts you have put 
down to understand and resolve my issue. I must appreciate your 
professional approach and I am just a learner in electronics and keep 
making small gadgets for utility.

By the way your English is far superior and you have good command in 
expressing the technicalities. :)

Once again thanks for your help. I would surely get in touch with you if 
I face any other issues.

> I am not Einstein - at least not yet 😉
But you are no less than Einstein for me :D

Regards,
Syed

von Toxic (Guest)


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Syed wrote:
> I am very happy to inform that my VA meter is working now :).

Well done and I really mean it.
For a beginner not bad: you found the faulty part and replaced it 
yourself which can sometimes be a bit tricky.The diode you bought should 
be working fine.

Let's not forget  @Mario P. for posting the link.

Note:
Never use a simple phase dimmer as a (variable) voltage source for 
complex devices like TVs,Amplifiers etc. They usually expect a pure sine 
wave.
Simple phase dimmer can be used to dim old fashioned incandescent bulbs 
as they just contain a purely resistive wire and not complex electronic 
parts like in a modern Led bulb.

von Syed (Guest)


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Toxic wrote:
> Simple phase dimmer can be used to dim old fashioned incandescent bulbs
> as they just contain a purely resistive wire and not complex electronic
> parts like in a modern Led bulb.

Actually I made this dimmer/speed control circuit for my old drill 
machine which does not have speed control. It always operated on full 
speed. With this module I got more control in doing different jobs.

I got very good software LTspice which can really help to see how the 
circuits perform.

Thanks & regards,
Syed

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