Did something really stupid; is my DMM a goner?

[cigmas]

I was measuring something on one of those racket-style bug zappers... while it was live. After a spark at one of the leads, the DMM's screen went blank and no longer turns on. It was in DC volt mode.

Is there any chance of repair or is the chip likely fried? Even though I was not using one of the fused ports, I checked the two fuses for continuity with a bulb. I don't see anything visibly burnt or damaged.

RIP Thsinde 19B+ ? At least it wasn't very expensive, but I really like this DMM and I'd likely get another if it can't be saved.

[Jeff L]

It probably took a hit of 1500volts--- I'd be very surprised if it survived.

[wraper]

You will need to replace main IC. That will require removing black blob and soldering new IC on the pads around that blob. The main problem is to figure out what it is

[IanB]

If the screen doesn't turn on the meter is probably toast. You would have to recycle it and get another.

Check out Joe's meter robustness tests to find a meter that is less likely to be damaged by such a mishap.

https://www.eevblog.com/forum/testgear/hear-kitty-kitty-kitty-nope-not-that-kind-of-cat/

[wraper]

Found a picture of PCB with IC in soldered package. Though it's blurry and I cannot figure out what's written on it.

[wraper]

Apparently it's DM1106EN. It seems to be a clone of HY12P66 and DTM0660L, but check yourself if they are actually compatible. https://thearduinoshed.wordpress.com/2020/04/13/uni-t-ut210e-current-clamp-hacking/
EDIT: DTM0660L can be purchased https://www.aliexpress.com/item/1005002016637380.html
However the issue is that your PCB unlike newer one has only pads for larger COB on PCB (blob IC mounted on a small adaptor circuit board which is then soldered on main PCB). Therefore to replace IC you will also need to make adaptor board or make a dead bug.

[rsjsouza]

If the screen doesn't turn on the meter is probably toast. You would have to recycle it and get another.

That is the most probable cause, unless a voltage regulator was shot.

In your first photo, I think I see a small hole on the top left of the black blob - it could be dust, though. 

[Gyro]

The material used to cover those blob top ICs is very hard. Make sure that you have a chance of completely removing it before considering buying a replacement IC.

[mikerj]

Strange that they went to the expense of fitting proper ceramic fuses but not a single MOV in sight.

[SeanB]

Strange that they went to the expense of fitting proper ceramic fuses but not a single MOV in sight.

Ceramic fuses are very cheap to make, just a clay that is blended, then fired after extrusion, and ultra cheap to make. Add the crimped ends and the thin wire inside. MOV on the other hand needs precise blending of the exotic material oxide powders, then forming, drying, then sintering, then plating on the silver electrodes, soldering the wire leads on and then dipping in epoxy and heat curing, then classifying the voltage range. A lot higher cost.

[Kleinstein]

MOVs are not absolutely needed. There are alternative ways to clamp a voltage. The MOVs may give better performance for ESD protection to the switch - which may very well have been the failure path in this case.

The ceramic fuses (especially the high current one) are usually more expensive than MOVs.  The MOVs are made in large quantities with not that exotic materials needed.
When bought in larger numbers the ceramic fuses may also be more afforable.

[MadTux]

Tinywiny protection diodes inside the chip are certainly not enough to clamp an overvoltage event, much less capable in sinking enough current to blow a fuse. So MOVs or at least big external protection diodes are needed.

Anyway, RIP, chinese piece of junk

[Kleinstein]

The MOVs don't stand the current to blow the fuses. The fuses are only for the current ranges and that current will flow through the wire type shunt for the large fuse and the 5 diodes (likely similar to 1N4007) close to the lower range fuse.

MOVs would mainly clamp the current from ESD or similar higher overvoltage. Normal voltage significantly exceeding the ranges are normally clamped by diodes at or inside the chip. The current is there limited by a large resistor (here it looke like the 4 x 2.5 M) and thus not that large (e.g. 100 µA for 1000 V at the input).

[Renate]

Just out of curiousity, did you look at the other side of the board with the selector switch to see if you could see any arcing?

[MadTux]

A nice ESD spark is quite a bit more than 1kV, more in the 10kV range, so already 1mA for the diode to clamp.
Then there is parallel capacitance on the resistors, arc-over on resistors, diode turn on time....
Everything which a MOV would easily swallow, but a tiny diode inside the chip clearly can't.
Otherwise, that thing wouldn't have failed.

I've also killed a cheapie in a similar way years ago. Thought I could simply measure the voltage on chainsaw ignition coil instead of hooking it to a spark plug. I then turned the chainsaw flywheel rather slowly and the DMM was gone and the coil confirmed as good.... Ooops.

[cigmas]

Thanks so much for all the comments! For some reason I'm not getting email notifications of replies even though I'm subscribed for notifications to my thread. (Yes, nothing in spam either.)

Would replacing the chip affect calibration validity? That's assuming I could even source the correct chip (@wraper thanks for the reasearch!) and manage to replace it at all, as that black stuff is really hard and I have no idea how to remove it without potentially destroying the board.

Are there other components I should measure first in case the failure is not the chip? I would have to revive an even cheaper multimeter to measure anything.

It might be a cheapo device for most of you, but it was the nicest DMM in this household and will be missed... all because of a stupid mistake. Poor thing.

On a side note, is there a safe way to measure the high voltage of the zapper to know what the DMM got hit with? You know, without killing anything. This zapper makes a fatter spark than I've seen from other similar devices. Though maybe it's more trouble than it's worth.

[cigmas]

And yes, the speck on the chip in the image above is just dust.

I got a little bold with the disassembly on the other side. Not much to arc, nothing under the backlight, plus the display's contacts are nothing I've seen before. Wonder if they still work now...

[IanB]

On a side note, is there a safe way to measure the high voltage of the zapper to know what the DMM got hit with? You know, without killing anything. This zapper makes a fatter spark than I've seen from other similar devices. Though maybe it's more trouble than it's worth.

In general the question is less about the voltage and more about the energy of the discharge. To measure that energy you would generally have to use an oscilloscope--you might for example discharge through a resistor, plot the voltage against time across the resistor, and measure the area under the curve. But of course you would have to really know what you are doing, or you could damage the oscilloscope the same way you damaged the meter 

The lesson to learn here is that choosing a meter with robust input protection can save you from this kind of mishap. There are meters out there that will survive trying to measure a bug zapper without being damaged.

[Renate]

Is there a safe way to measure the high voltage of the zapper?

I used to have a high voltage probe for working with the HV on color TVs which ran to 30 kV or so.
Most of them were 1000:1 resistor dividers, but you had to make sure that you DVM had the correct input resistance for that model probe.
I'm not even sure now if there was a "low" resistor in the divider or if they relied entirely on the DVM for the "low" load.
That would make the "top" resistor 10G for a 10M DVM.

https://www.fluke.com/en-us/product/accessories/probes/fluke-80k-40
Kind of pricey for my taste. I don't do many CRTs nowadays.

[fordem]

I got a little bold with the disassembly on the other side. Not much to arc, nothing under the backlight, plus the display's contacts are nothing I've seen before. Wonder if they still work now...

Google elastomeric zebra connector - they are fairly common with LCD displays - and yes, they're quite sturdy, and survive repeated disassembly & reassembly, and even the occasional cleaning with an alcohol wipe when the display has missing or faded segments.

[Kleinstein]

There are not many more parts to fail - so chances are near 100% it is the main chip that failed.

Replacing the chip would ruin the calibration - there may be some ratios still valid, but not much.

[cigmas]

In general the question is less about the voltage and more about the energy of the discharge. To measure that energy you would generally have to use an oscilloscope--you might for example discharge through a resistor, plot the voltage against time across the resistor, and measure the area under the curve. But of course you would have to really know what you are doing, or you could damage the oscilloscope the same way you damaged the meter 

That would be a nice, but also risky, analysis! I will carefully step back now. 

The lesson to learn here is that choosing a meter with robust input protection can save you from this kind of mishap. There are meters out there that will survive trying to measure a bug zapper without being damaged.

Yes but at what cost? Looking through the meter testing google doc, I could have killed meters worth an order of magnitude more.

Are there better options at a similar price point ~$35 CAD? https://www.amazon.ca/multimeter-MeasuresVoltage-Capacitance-Transistors-Temperature/dp/B075WRQYX5/

The nice thing about getting another Thsinde 19B+ is that it's a "known quantity" that I was happy with. On the other hand, there are newer "cheapo" DMMs that seem about as well reviewed (paid? ..) with updated features. eg: https://www.amazon.ca/KAIWEETS-Multimeter-Auto-Ranging-Capacitance-Temperature/dp/B07SHLS639/

This "HT118A" (one of many such clones) adds NCT, duty cycle, and some other stuff, while retaining the temperature probe (but loses the REL feature??), though I don't care for that aesthetic. Plus I haven't seen others use as beefy a 10A fuse as the Thsinde other than Fluke, though I doubt I'll be using it for high current applications.

Any preference between the above or something else priced similarly?

I used to have a high voltage probe for working with the HV on color TVs which ran to 30 kV or so.
Most of them were 1000:1 resistor dividers, but you had to make sure that you DVM had the correct input resistance for that model probe.
I'm not even sure now if there was a "low" resistor in the divider or if they relied entirely on the DVM for the "low" load.
That would make the "top" resistor 10G for a 10M DVM.

https://www.fluke.com/en-us/product/accessories/probes/fluke-80k-40
Kind of pricey for my taste. I don't do many CRTs nowadays.

Aha, my first thought was "voltage divider" too but thought it still risky to DIY if I don't know roughly what the voltage is. Yes quite pricey, but I guess that's relative to what it's attached to. 

Google elastomeric zebra connector - they are fairly common with LCD displays - and yes, they're quite sturdy, and survive repeated disassembly & reassembly, and even the occasional cleaning with an alcohol wipe when the display has missing or faded segments.

Thanks!! That is such a cool connector! At least the silver lining of this mishap is that I learn something interesting.

Replacing the chip would ruin the calibration - there may be some ratios still valid, but not much.

I was afraid of this. In that case it's not worth attempting a repair. Maybe I'll just get a second one of the same so that I can use the dead unit for spare parts, though hopefully that will not be necessary.

[james_s]

It's dead, Jim.

Those bug zapper things produce quite substantial voltage, >1kV is typical. Always be aware of what you are trying to measure, even a good quality multimeter can be damaged by trying to probe something well beyond what it is rated to tolerate.

[IanB]

Yes but at what cost? Looking through the meter testing google doc, I could have killed meters worth an order of magnitude more.

Unfortunately, $35 is in the "disposable" category of gadgets. At that price point you have to treat it as fragile and be really careful what you do with it. It will be fine for measuring batteries and low voltage electronics, but don't take it close to anything you wouldn't want to touch with your fingers.

[rsjsouza]

The Thsinde is a good meter, provided you respect its limitations, which unfortunately you found in a not-so-ideal way. If you like it, by all means get another one.

Safer meters in more or less the price bracket of up to US$50 may come in the shape of a Klein MM400 or a Fluke 101. But they are less featured.