DMM Questions: Protection, Hz Measurement, Shunt?

[bdunham7]

Yeah, actually I think the spirit of these CAT and most safety standards were to ensure no hazardous condition exists for the user. As such, absolutely the meter can be utterly destroyed. As long as there is no shock, burn, chemical, radiation or inhalation hazard created, its perfectly fine. No matter the guts are blown to oblivion.

For overcurrent, perhaps.  For the CAT overvoltage transient test, the meter needs to still be able to indicate the presence of hazardous voltages, although it need not be precise or accurate anymore.

[bdunham7]

Okay, that's  a start. Why can't a series of diodes, then, act as that 1000V protection? Diode protection was my mistake. I asked what diode protection they are using and this is what he said:

"We use 9013 or 8050 transistors. The CAT.III 1000V voltage value is the pulse withstand voltage value, which is an insulation voltage value inside the shell protection, which is different from the voltage value of the current fuse. The fuse of this multimeter can only reach 250V."

So, can those transistors offer 1000V protection?

I don't really understand the question or the answer, but if you (or they) make an extraordinary claim, the burden of proof thereof falls on you (or them), at least in most rational discussions.  We'll leave courts of law outside of that domain for now. 

How you could protect against several kiloampere overcurrents, 1000V accidental overvoltages or 8 kilovolt transients using mystery diodes or transistors with a 25 volt Vce rating all in 0805 or SOT-23 packages is beyond me.  I think that their answer is preposterous and that either they don't understand the issue or they're hoping you don't.  But I've been proven wrong before.

[DW1961]

Okay, that's  a start. Why can't a series of diodes, then, act as that 1000V protection? Diode protection was my mistake. I asked what diode protection they are using and this is what he said:

"We use 9013 or 8050 transistors. The CAT.III 1000V voltage value is the pulse withstand voltage value, which is an insulation voltage value inside the shell protection, which is different from the voltage value of the current fuse. The fuse of this multimeter can only reach 250V."

So, can those transistors offer 1000V protection?

I don't really understand the question or the answer, but if you (or they) make an extraordinary claim, the burden of proof thereof falls on you (or them), at least in most rational discussions.  We'll leave courts of law outside of that domain for now. 

How you could protect against several kiloampere overcurrents, 1000V accidental overvoltages or 8 kilovolt transients using mystery diodes or transistors with a 25 volt Vce rating all in 0805 or SOT-23 packages is beyond me.  I think that their answer is preposterous and that either they don't understand the issue or they're hoping you don't.  But I've been proven wrong before.

OK so some clarification. He actually said in one reply that they use TRIODE protection, but when I looked up "triode" it said it was old tube technology and tube oriented, although I didn't get in depth researching it. so I assumed he meant DIODE.

I just got a reply back and he said it is "Triode" protection.

So a little more research and I found this: Mosfet N-CH 900V 4A to-220 Transistor Triode Fqpf4n90c  here:
https://www.made-in-china.com/products-search/hot-china-products/Triode.html

I'm not sure what "triode" could do the voltage protection, but I'm sure someone here knows. After the image is open, right click (Windows) and choose, :View Image." Then you can get a good look at them.

So, just got a reply back with the "triodes" they are using in a picture of the PCB he gave me.
"We use 9013 or 8050 Triode. I have marked the position of the triode on the PCB with a red box on the picture. Please check the attachment."

[Wytnucls]

They probably are crowbar components (NPN transistors). Very common on multimeters.
Here is one on my Uni-T 71D multimeter.

[E-Design]

Yeah, actually I think the spirit of these CAT and most safety standards were to ensure no hazardous condition exists for the user. As such, absolutely the meter can be utterly destroyed. As long as there is no shock, burn, chemical, radiation or inhalation hazard created, its perfectly fine. No matter the guts are blown to oblivion.

For overcurrent, perhaps.  For the CAT overvoltage transient test, the meter needs to still be able to indicate the presence of hazardous voltages, although it need not be precise or accurate anymore.

although it need not be precise or accurate anymore.

whos definition of *precise* and whos definition of *accurate* -- those are subjective terms. If the standard calls out specific ways the measurements can be in error after a likely failed test result, those would need to be quantified perfectly.

[bdunham7]

They probably are crowbar components (NPN transistors). Very common on multimeters.
Here is one on my Uni-T 71D multimeter.

Where in the circuit are these placed?  I would expect those (as shown on the OPs circuit board photo) to fail short at 40-50 volts or less and to blow off the board over a few amps or so.

[bdunham7]

Yeah, actually I think the spirit of these CAT and most safety standards were to ensure no hazardous condition exists for the user. As such, absolutely the meter can be utterly destroyed. As long as there is no shock, burn, chemical, radiation or inhalation hazard created, its perfectly fine. No matter the guts are blown to oblivion.

For overcurrent, perhaps.  For the CAT overvoltage transient test, the meter needs to still be able to indicate the presence of hazardous voltages, although it need not be precise or accurate anymore.

although it need not be precise or accurate anymore.

whos definition of *precise* and whos definition of *accurate* -- those are subjective terms. If the standard calls out specific ways the measurements can be in error after a likely failed test result, those would need to be quantified perfectly.

That was't a direct quote from any standard--I don't have a copy of it to refer to.  I read it once somewhere, but I don't remember the exact details.   I believe that the standard does quantify, or at least specify, the requirements quite clearly.

[Wytnucls]

They probably are crowbar components (NPN transistors). Very common on multimeters.
Here is one on my Uni-T 71D multimeter.

Where in the circuit are these placed?  I would expect those (as shown on the OPs circuit board photo) to fail short at 40-50 volts or less and to blow off the board over a few amps or so.

Usually as protection for inputs to micro-controllers.

[TimFox]

Those NPN transistors are connected to use the Base-Emitter junctions as PN diodes.  The transistors are cheap, are not in transparent housings, and typically have lower leakage current than "regular" diodes.  (Good diodes in glass packages can easily have enough photocurrent to upset the rest of the circuit.)  The BE diode has a low breakdown voltage, but since it is in parallel with another diode in the reverse direction, it should not see breakdown.

[Wytnucls]

The Chinese rep was probably thinking of the triode region of a MOSFET, named as such because the device is dependent on the drain source voltage like a tube triode, although these parts are more likely to be BJTs.

With the base and collector connected, the transistor acts like a fast switching Zener diode.

[bdunham7]

Those NPN transistors are connected to use the Base-Emitter junctions as PN diodes.  The transistors are cheap, are not in transparent housings, and typically have lower leakage current than "regular" diodes.  (Good diodes in glass packages can easily have enough photocurrent to upset the rest of the circuit.)  The BE diode has a low breakdown voltage, but since it is in parallel with another diode in the reverse direction, it should not see breakdown.

I'm not seeing the parallel part in Wytnucls' schematic.  It looks like one or the other BE junction has to break down before the pair conducts anything at all.  The transistors in the OPs photos appear to be these:

https://www.mouser.com/datasheet/2/258/MMSS8050(SOT-23)-1626440.pdf

so that breakdown is going to happen a bit north of 5 volts. 

[DW1961]

So, just got a reply back with the "triodes" they are using in a picture of the PCB he gave me.
"We use 9013 or 8050 Triode. I have marked the position of the triode on the PCB with a red box on the picture. Please check the attachment."
(Attachment Link)

So what about what the rep said about the triode protects and the image he sent to me. Does that change anyone's mind about the CAT ratings of this meter? (see image)

[bdunham7]

So what about what the rep said about the triode protects and the image he sent to me. Does that change anyone's mind about the CAT ratings of this meter? (see image)
(Attachment Link)

NO.  But proving it would be another matter in the face of deliberate obfuscation. 

The only way your 'triodes' could help would be keeping the meter alive to meet the requirement that it still indicate hazardous voltages after the transient test. 

My exception to Wytnucls' statement about fuses was purely academic.  Looking at the board there appears to be no conceivable secondary overcurrent protection unless the PCB traces happen to vaporize in just the right way. I would call it a clear fail on that account alone.

Since there aren't any MOVs or devices physically large enough to absorb the required transients, I'd want hear their explanation as to how that is handled.  Good luck!

[TimFox]

My mistake.  The connection is two Zeners back-to-back in series on each leg, where the Zener voltage would be 5 to 6 V.

[DW1961]

So what about what the rep said about the triode protects and the image he sent to me. Does that change anyone's mind about the CAT ratings of this meter? (see image)
(Attachment Link)

NO.  But proving it would be another matter in the face of deliberate obfuscation. 

The only way your 'triodes' could help would be keeping the meter alive to meet the requirement that it still indicate hazardous voltages after the transient test. 

My exception to Wytnucls' statement about fuses was purely academic.  Looking at the board there appears to be no conceivable secondary overcurrent protection unless the PCB traces happen to vaporize in just the right way. I would call it a clear fail on that account alone.

Since there aren't any MOVs or devices physically large enough to absorb the required transients, I'd want hear their explanation as to how that is handled.  Good luck!

I sent the question. So far the rep/vendor has been very upfront about my questions, even taking the time to post a photo for me and outline the triodes. That doesn't force teh CAT ratings to be met, but it at least shows some transparency.

If I understand you correctly, your concern is how does something so small (the triodes) absorb a CATIII 1000V rating surge, up to 8000V? (I think it's 8000v, but you get my question.)

[Gyro]

Ask him if he means transistors, this whole triode thing is confusing the issue.

[floobydust]

chinese use the term "triode" for transistor. Lost in translation or too many syllables.

[E-Design]

The speculation and conjecture meter for this thread is off the scale. 

Just ask the Chinese vendor for the schematic and BOM already. He might just give it to you if you tell him you will buy 10 of them. 

[floobydust]

As far as safety compliance goes, the manufacturer/seller must be able to produce a certificate of compliance, or at least a file number with the agency used (certificates are public information) meeting the claims such as "61010". Otherwise- the product is completely untested and considered unsafe. Who wants to use this at 600V?
It's unfortunate the expression/term "CAT xxx" is not legally protected, and thoroughly abused by Asian DMM manufacturers.

There are three ways you can overload a multimeter. You could write a book about it, there's Youtube videos too.
We've looked at the current measurement function which is protected solely by the fuses, with (OP's DMM) five diodes D1-D5 to protect the mid/high R shunts R8, R9. The fuses are not suitable past their rated 250V, or for an HRC environment like Cat. III/IV.
For overload during voltage measurement, there are no MOV's or gas tubes, only the usual diode-connected transistors Q1, Q2 SS8050 giving light protection to the DMM IC for overload.
But if you are on the Ohms/Continuity/Diode-Test function and accidentally connect to say 240VAC, something has to drop the voltage down to prevent it from burning up and here it is the PTC thermistors. This DMM features two, not sure how they are wired up. You can't put them in series.

The big question is if the cute little PTC's can take rated voltage and if they are UL or agency approved parts. UL requires the safety components to be suitable (rated voltage and current) and have agency approvals. It makes sense the parts are tested. Little PTC's are rarely good to 600V with approvals. They just do a little crack and shoot a bit of stuff.
Beyond this, the pcb spacings are untested and usually the rotary switch has a bad design and any arcing occurs there.

Just because a meter has some protective elements doesn't mean they are suitable or will even work as hoped. This is why stuff is sent out to a lab for testing and certification.

[DW1961]

Ask him if he means transistors, this whole triode thing is confusing the issue.

Yes, transistors.

"We use 9013 or 8050 transistors."
I think these may be the spec sheets:
9013 spec sheet:
https://pdf1.alldatasheet.com/datasheet-pdf/view/33678/WINGS/9013.html

8050
https://pdf1.alldatasheet.com/datasheet-pdf/view/956716/FOSHAN/8050T.html

[DW1961]

As far as safety compliance goes, the manufacturer/seller must be able to produce a certificate of compliance, or at least a file number with the agency used (certificates are public information) meeting the claims such as "61010". Otherwise- the product is completely untested and considered unsafe. Who wants to use this at 600V?
It's unfortunate the expression/term "CAT xxx" is not legally protected, and thoroughly abused by Asian DMM manufacturers.

There are three ways you can overload a multimeter. You could write a book about it, there's Youtube videos too.
We've looked at the current measurement function which is protected solely by the fuses, with (OP's DMM) five diodes D1-D5 to protect the mid/high R shunts R8, R9. The fuses are not suitable past their rated 250V, or for an HRC environment like Cat. III/IV.
For overload during voltage measurement, there are no MOV's or gas tubes, only the usual diode-connected transistors Q1, Q2 SS8050 giving light protection to the DMM IC for overload.
But if you are on the Ohms/Continuity/Diode-Test function and accidentally connect to say 240VAC, something has to drop the voltage down to prevent it from burning up and here it is the PTC thermistors. This DMM features two, not sure how they are wired up. You can't put them in series.

The big question is if the cute little PTC's can take rated voltage and if they are UL or agency approved parts. UL requires the safety components to be suitable (rated voltage and current) and have agency approvals. It makes sense the parts are tested. Little PTC's are rarely good to 600V with approvals. They just do a little crack and shoot a bit of stuff.
Beyond this, the pcb spacings are untested and usually the rotary switch has a bad design and any arcing occurs there.

Just because a meter has some protective elements doesn't mean they are suitable or will even work as hoped. This is why stuff is sent out to a lab for testing and certification.

Could the transistor be slowing down  the surge before they burn out just enough to let the PTCs heat up and take over that job? Also, maybe the PTCs are small, but all they need to do is stop the surge long enough to prevent user from harm, right?

[floobydust]

The diode-connected transistors clamp the overload hard at first, then the PTC heats up and the fault current backs off.
Usually a ~1.5k ohm PTC, so at 240VAC on ohms it sees around 222mA or 75W and then heats up fast in maybe 1sec. It's just a lot of heat generated in a small part, that would love to shatter, while the transistors are taking abuse too.

In multimeters with stronger input protection like Fluke and Brymen they also include a 1k ohm surge resistor (in series with PTC) to lower the fault current so the PTC survives. Their PTC's are also pretty huge. Look at curves for YS4020 to see time and current for a multimeter large PTC.

[bdunham7]

Could the transistor be slowing down  the surge before they burn out just enough to let the PTCs heat up and take over that job? Also, maybe the PTCs are small, but all they need to do is stop the surge long enough to prevent user from harm, right?

No, because those transistors are connected to and protect an internal circuit and aren't connected to anything anywhere near the inputs.  They can't withstand even a tiny fraction of the normal maximum inputs.  50 volts would vaporize them.  And even if there was a large resistance between them and the inputs, they make the meter useless for anything over few volts.  You couldn't even measure a 9V battery if they were your input crowbar.

Also, in order to shift the load to the PTC, whatever is used as a clamp has to have low impedance after being triggered otherwise the dissipated power will be excessive. 

[DW1961]

Could the transistor be slowing down  the surge before they burn out just enough to let the PTCs heat up and take over that job? Also, maybe the PTCs are small, but all they need to do is stop the surge long enough to prevent user from harm, right?

No, because those transistors are connected to and protect an internal circuit and aren't connected to anything anywhere near the inputs.  They can't withstand even a tiny fraction of the normal maximum inputs.  50 volts would vaporize them.  And even if there was a large resistance between them and the inputs, they make the meter useless for anything over few volts.  You couldn't even measure a 9V battery if they were your input crowbar.

Also, in order to shift the load to the PTC, whatever is used as a clamp has to have low impedance after being triggered otherwise the dissipated power will be excessive.

So, what Flooby said?

[MosherIV]

So, what Flooby said?

Yes.

I thought you had watched the eevblog video on dmm input protection?
All this had been covered in that video.

Multiple techniques  are needed in order to fully protect a dmm:
Over voltage
Surge suppression
Over current
Energy dissipation
If you skip some of them, then better make sure the dmm will fail in a safe way for the user.