Tonghui TH2821A LCR meter charging cct blown.

[PhilBrown]

Hello everyone, I am new to the forum, and fairly new to electronics, although I did serve my apprenticeship with British Telecom, It was a very long time ago, in the days of relays and selectors.
I have a problem with my LCR meter that I hope someone can help me with.
(You can open this image in a new tab) https://onedrive.live.com/redir?resid=b2a921a7a4f797bc!19349&authkey=!AJ12BrR23U3gEQQ&v=3&ithint=photo%2cjpg
I have a second hand Tonghui TH2821A LCR meter with a burnt out charging circuit, as you can see from the photo, D8 was fried, but also R29. I replaced D8 and this has got the unit charging again, but the 9v battery never reaches full charge, so I recon R29 has more likely gone out of spec. Looking at the data sheet for the ic MAX 1641 (switch mode rectifier) it relies on the resistors to set up the charge rate. The problem is that I don’t know the original value of R29.
So does anyone have one of these that they would be willing to open up to take a look? Or would anyone have any idea how to find out the value of this resistor. Or has anyone a circuit diagram?
I have tried to calculate the value from the data sheet of MAX1641, and I came up with 39.8 ohms, trouble is I don’t have much experience at this, so I am not very confident!!
Any help would be appreciated.
Thanks
Phil

[Smith]

Looks like its setting the charging current like you said. It looks like it stil has its value visible on the resistor. I cant read it on the photo though. Could you clean the resistor with ipa or something. Sometimes looking at it at the right angle clearly shows its printed value.

Some other thoughts, does the battery charge on another psu?  If its the old battery maybe it failed taking out the other components? Have you measured the charging current? How do you recon the battery isn't fully charging?

[Djinn]

Since I have been lurking thru the forum a while I thought my first post should be one that matters or is helpful. The resistor R29, is both marked and measures; "1R50", or at least it is on my Tonghui TH2821A 10 KHz LCR meter.

Hope this helps.

With regards

[Fraser]

Hi Philbrown,

I hope you can repair your meter but please do ask yourself the question of "cause & effect"

In thsi case you need to reverse the statement and analyse the effect that has occurred (a burn up of components) and deduce what caused such. A burn up does not occur without good reason. Try to work out how thee burnt components could possibly be in a position that leads to too much current flowing through them. It cases such as this, I reverse engineer the PCB around the area of interest and this makes the failure mode more easily identified.

Common failures are, as you likely know, Reverse polarity or over voltage through use of the incorrect charging power pack, and a failure in the battery causing excess current draw. Failure of electrolytic capacitors has also been an issue that could lead to an overload.

I will see if I can offer any more comment once I have studied the PCB picture and the data sheet.

Aurora

[Fraser]

OK I have had a look at the picture and an issue just screamed out at me..... take a look the point where the battery leads connect to the PCB. I would say that a short circuit has occurred at that point as the black wire insulation has melted. Please take a close look at this area of the PCB looking to see if a wire strand from the black cable caused a burn-up. Best to sort this out before trying t0 charge the battery again as it could potentially fry the meter all over again.

If it is not a faulty wiring issue it suggests that the battery has supplied excess current through the cables to a low impedance on the PCB. This is where the reverse engineered schematic can help. You trace the current flow path to see what potential low impedances exist, such as a shorted switching MOSFET etc.

Aurora

[Fraser]

Ok, my really quick analysis of the failure.

R9 is in series with the rechargeable battery and is fed from the Q1 MOSFET switching pair. Ignore the cooked D8 for the moment as it is likely in series with the supply rail that feeds this charging stage.

To overload R9, one of four events is likely.

1. A short of the P MOSFET caused unregulated charging of the battery and an over-current situation.
2. A short of the N MOSFET effectively connected the battery positive terminal to 0V via R9 and overloaded it.
3. A power supply of the incorrect voltage was connected to the charging circuit and caused an current overload situation that cooked R9
4. The charger was operating when the leads connecting it to the battery became shorted by loose conductors in the cable. The charger was then working into a short circuit and overloaded R9 PLUS the battery passed large currents through the shorted wires as it was effectively shorted out. This melted the black wires insulation, suggesting that this is the source of the stray wires.

Now we bring D8 back into the equation. D8 is assumed to be in series with the supply from the external plug pack. It will be overloaded in all of the above scenarios if the meter is being charged from its external power pack.

Note that scenario 4 would likely has the potential to damage the rechargeable battery through excessive current discharge. A new battery would be a good idea if this scenario applies.

A value of 1.5 Ohms for R9 appears reasonable and such a low resistance would fry if the full 9V was placed across it.

Hope this helps a little

Aurora

[Djinn]

Since I am new on the forum and I can understand I could have low credibility so I opened up the meter again, and took a picture of the stated R29.

With regards

[Fraser]

@Djinn,

This is a friendly forum and I can assure you that I took your comment as a credible and I feel sure others did as well. We don't see many trolls being silly on EEVBlog 

I took a close look at the resistor in the OPs image and I could see that 1R5 fitted what remained of the writing on it.

I hope the OP appreciates your very helpful information and photograph. 

Waiting to hear his findings after my comments as well 

Also, I note that your meter appears better manufactured than the OP's. Yours maybe a later model ?

Aurora