DMM Questions: Protection, Hz Measurement, Shunt?

[DW1961]

I have a few question, and I hope some of you can answer.

Why do some MM use 9V but some use 2 AA batteries?

I just got a new MM and it has NCV and what they call "NCV LIVE." From what I can make of the manual, the NCV LIVE function is the same as NCV, but you just use the V side (Red) lead w/o the COM lead to test for live voltage?

This MM displayed Hz with Voltage measurements. However, it has a dedicated Hz option too. The manual states for the dedicated Hz measurement option, "Voltage above 10V can't be measured ; otherwise the instrument may be damaged." I don't understand how it can measure Hz when in Voltage mode over 10V, but not in the dedicated Hz mode?

I didn't see a shunt in this unit (See image), and I was also wondering if anyone can look at the board picture and see how well it is protected? I think the image is good enough to see resisters, diodes, etc., if any.

[Kleinstein]

Old meters used 9 V as the older meter chips needed something like 5-10 V. Some newer meter us AA batteries and a step up converter or 3/4xAA. 2 x AA is usually a little cheaper.

The dedicated frequency mode may use a different input path, that is not that well protected    . When in volts mode the maximum frequency may be limited.

One of the shunts is R7 at the edge (right side on the picture) of the board. R8 and R9 may also be shunts.
Anyway the R7 looks tiny, so it may heat up quite a bit with higher currents. It looks like the board cold alternatively have a more standard bare wire shunt.

There are 5 diodes for protecting the shunts (except R7). However the connection through 2 tiny vias could be a weak point. The fuses look like 32 mm ceramic fused - so not great, but also not very bad and at least the clearance looks good. The PTCs look like a bit close to ground - so this could be an issue, when used with mains voltage.

[MosherIV]

Hi

The Hertz thing is more to do with being able to measure the freq of mains voltage and warning you NOT to try and measure mains freq in the dedicated freq measurement mode.
It is there to measure freq of low voltage circuits eg oscillator circuits.

However, looking at the protection devices in the dmm, it is NOT a good idea to measure mains. There are no MOVs or GDT (over volt protection)

[DW1961]

Hi

The Hertz thing is more to do with being able to measure the freq of mains voltage and warning you NOT to try and measure mains freq in the dedicated freq measurement mode.
It is there to measure freq of low voltage circuits eg oscillator circuits.

However, looking at the protection devices in the dmm, it is NOT a good idea to measure mains. There are no MOVs or GDT (over volt protection)

Interesting. Can you elaborate on that a little? Are you saying that the CAT rating on this unit is bogus, because that those are suppose to take into consideration over voltage spikes. Or did you mean if it does get a spike above the CAT spike rating, you're in trouble?

[Kleinstein]

The cat rating is definitely bogus. It may still meet some thing like 300 V CAT2, though not for sure.

The fuses may be good for 500 V or maybe 600 V with not that much breaking capacity - so no way to get a CAT4 rating.

The over-voltage protection (especially for the Ohms part) with only the tiny PTC is likely also a weak point, possibly even for CAT2. There is no absolute need for MOVs or visible spark gaps - but it helps. 
The limitations on the frequency mode makes me even doubt about CAT 2 with a significant voltage.

[DW1961]

Old meters used 9 V as the older meter chips needed something like 5-10 V. Some newer meter us AA batteries and a step up converter or 3/4xAA. 2 x AA is usually a little cheaper.

The dedicated frequency mode may use a different input path, that is not that well protected    . When in volts mode the maximum frequency may be limited.

One of the shunts is R7 at the edge (right side on the picture) of the board. R8 and R9 may also be shunts.
Anyway the R7 looks tiny, so it may heat up quite a bit with higher currents. It looks like the board cold alternatively have a more standard bare wire shunt.

There are 5 diodes for protecting the shunts (except R7). However the connection through 2 tiny vias could be a weak point. The fuses look like 32 mm ceramic fused - so not great, but also not very bad and at least the clearance looks good. The PTCs look like a bit close to ground - so this could be an issue, when used with mains voltage.

I have two brand new meters that use 9V. Does 9V have anything to do with testing a battery on load? Or could a MM also test a battery on load with two AAs?

[DW1961]

The cat rating is definitely bogus. It may still meet some thing like 300 V CAT2, though not for sure.

The fuses may be good for 500 V or maybe 600 V with not that much breaking capacity - so no way to get a CAT4 rating.

The over-voltage protection (especially for the Ohms part) with only the tiny PTC is likely also a weak point, possibly even for CAT2. There is no absolute need for MOVs or visible spark gaps - but it helps. 
The limitations on the frequency mode makes me even doubt about CAT 2 with a significant voltage.

Why would you need over voltage protection on Ohms? You mean the over voltage could cross over to the Ohms circuit?

Could you further explain why this meter cannot meet its CAT rating? I'll report it to Amazon. That's deadly. I mean someone could actual die. If you could give me you specific reasons why the CAT rating is a lie, I'll send that information to Amazon.

[Nusa]

Here, educate yourself on what CAT ratings are: https://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf

The rating is nice to have, but the average user should never work in the CAT4 environment -- leave that to the professionals who have better equipment, safety gear and training. Also because your homeowners authority and ownership of the wires generally stops at the electric meter.

I note the current inputs are labeled 250V max at the jacks, which is actually a pretty common restriction on the cheap meters. Besides, if you want to use the current ranges on mains voltages, you're usually not thinking straight. Not a good idea, except in controlled bench situations.

[bob91343]

There is probably a bare wire shunt for 10A range under the circuit board.

9V used to be the power supply of choice but these days it seems lithium cells are used more.  9V batteries are expensive, maybe a few dollars each, while CR2032 cells are a few pennies.  The AG13 cells are popular too, still cheaper than 9V.

I have converted some hand held units to a pair of CR2032 cells, giving about 7V and a 'low battery' warning but it works great.

[DW1961]

Here, educate yourself on what CAT ratings are: https://content.fluke.com/promotions/promo-dmm/0518-dmm-campaign/dmm/fluke_dmm-chfr/files/safetyguidelines.pdf

The rating is nice to have, but the average user should never work in the CAT4 environment -- leave that to the professionals who have better equipment, safety gear and training. Also because your homeowners authority and ownership of the wires generally stops at the electric meter.

I note the current inputs are labeled 250V max at the jacks, which is actually a pretty common restriction on the cheap meters. Besides, if you want to use the current ranges on mains voltages, you're usually not thinking straight. Not a good idea, except in controlled bench situations.

Read over what you said about current and volts and read it several times, and that is so true. "Not thinking straight" is right, or just ignorant.

[DW1961]

There is probably a bare wire shunt for 10A range under the circuit board.

9V used to be the power supply of choice but these days it seems lithium cells are used more.  9V batteries are expensive, maybe a few dollars each, while CR2032 cells are a few pennies.  The AG13 cells are popular too, still cheaper than 9V.

I have converted some hand held units to a pair of CR2032 cells, giving about 7V and a 'low battery' warning but it works great.

OK, so curiosity got the best of me, and I took the circuit board out. No shut that I can see.

[DW1961]

There is probably a bare wire shunt for 10A range under the circuit board.

9V used to be the power supply of choice but these days it seems lithium cells are used more.  9V batteries are expensive, maybe a few dollars each, while CR2032 cells are a few pennies.  The AG13 cells are popular too, still cheaper than 9V.

I have converted some hand held units to a pair of CR2032 cells, giving about 7V and a 'low battery' warning but it works great.

I get my 9V batteries at the dollar store. lol You know, because the only place I sue them is in a MM.

[DW1961]

Old meters used 9 V as the older meter chips needed something like 5-10 V. Some newer meter us AA batteries and a step up converter or 3/4xAA. 2 x AA is usually a little cheaper.

The dedicated frequency mode may use a different input path, that is not that well protected    . When in volts mode the maximum frequency may be limited.

One of the shunts is R7 at the edge (right side on the picture) of the board. R8 and R9 may also be shunts.
Anyway the R7 looks tiny, so it may heat up quite a bit with higher currents. It looks like the board cold alternatively have a more standard bare wire shunt.

There are 5 diodes for protecting the shunts (except R7). However the connection through 2 tiny vias could be a weak point. The fuses look like 32 mm ceramic fused - so not great, but also not very bad and at least the clearance looks good. The PTCs look like a bit close to ground - so this could be an issue, when used with mains voltage.

If R5 and 6 are shunts, then there seems to be 6 more top left: R1-R6
However, aren't those resistors, not shunts? They say Rx on them, and they look like resistors, not shunts.

So the2  PTCs are current limiting like a resistor, but uses increasing heat to lower current?

[Nusa]

Other way around. Shunts are resistors, but most resistors are not shunts. Shunts are usually very low value precise resistors that can handle the required current. Knowing the value of the resistor and measuring the voltage drop across it tells you what the current is.  In this case, looks like R7 (marking R005 = 0.005 Ohms) is serving that purpose. But those unpopulated holes are likely there so that a calibrated wire shunt (all wire has resistance) could be used instead.

R1-R6 look more like they're input protection on microcontroller inputs, but hard to say without a schematic.

PTCs increase their resistance rapidly when they are passing more than their rated current to protect the rest of the meter. So basically an input-protection fuse, except that they will reset when a minor overload is removed. A major overload may turn the PTC into ash, in which case they don't reset...and if you're lucky they still protected the meter.

[DW1961]

Other way around. Shunts are resistors, but most resistors are not shunts. Shunts are usually very low value precise resistors that can handle the required current. Knowing the value of the resistor and measuring the voltage drop across it tells you what the current is.  In this case, looks like R7 (marking R005 = 0.005 Ohms) is serving that purpose. But those unpopulated holes are likely there so that a calibrated wire shunt (all wire has resistance) could be used instead.

R1-R6 look more like they're input protection on microcontroller inputs, but hard to say without a schematic.

PTCs increase their resistance rapidly when they are passing more than their rated current to protect the rest of the meter. So basically an input-protection fuse, except that they will reset when a minor overload is removed. A major overload may turn the PTC into ash, in which case they don't reset...and if you're lucky they still protected the meter.

You mean if the PTC turns to ash, they won't reconstitute themselves? What a rip.

So are R8 and R9 shunts or resistors? Also, the R7 shunt, what is it protecting? Is that the current shunt?

So, in your opinion, is this meter well enough protected for up to 250V and 10A current?

[Nusa]

The fuses are the main protection for the current ranges. R7 isn't protecting anything...it's acting like a piece of wire that's being measured, and the fuse should fail before it does. R8 and R9 I don't know from a poor picture and no schematic.

The PTC's are protection for the other ranges (when the red lead is in the V input). What I don't see are any MOV's or power resistors, which is likely why the others are saying the CAT ratings are likely somewhat fictional and self-certified.

That said, for low-voltage electronics, it's safe enough. For mains voltages, not as much. But I'll let Dave lecture you on the subject:

[Wytnucls]

The meter is not legal in the US or Europe. There is no such thing as a 250V voltage limit on current input.
The fuses must have the same voltage rating as the CAT rating of the multimeter.
If the meter is rated CAT III 1000V, the HRC fuses must be rated for 1000V also.

Don't buy cheap meters with no certification to work on household mains.

The SMD current shunt looks like a 2 watt part and may suffer some damage before the fuse blows, in case of a short across a power supply.

[floobydust]

It's your typical chinese DMM with fake safety claims; it's the the norm.
Habotest advertises "Safety Rating EN61010-1,-2-030 EN61010-2-033, EN61326-1 CAT III 1000V, CAT IV 600V"

The spacings and fuse (length) are a bit better than usual but still pcb layout errors and the three current shunts should be able to clear the fuse first.  The 10A fuse- overloaded it may just first melt the pcb trace, they are a disaster coming off the jack.
It's like there's one person in china doing pcb layout on multimeters and they still don't know what they are doing 

Amazon also puts in under Festnight, Kaiweets, Taiss brandnames. Mastech also has dibs.
"A new version of this multimeter will have Bluetooth function in the future. It is under development and will be available about 6 six months later, please pay attention to our KAIWEETS Facebook page."

[DW1961]

@floobydust @Wytnucls

Who do I contact to report the illegal sales of this meter in the USA?

Does anyone have a Fluke (or other legal brand) they can show images of for a meter of this CAT category and how it should look inside?

[Kleinstein]

It is the typical low cost Chinese meter with false claimed cat ratings. This is very common can found on something like 90% of the cheap meters in the US. The fuses are a dead giveaway - proper fuses for CAT 3 1000 V are twice the size and maybe more expensive than the meter.  If you see a sub $50 meter with CAT4 rating one can be nearly sure it is fake. With a 1000 V CAT 3 rating it is highly suspicious. I don't know a proper place to report for the US, but it seems like there is no effective system to take those parts with false ratings off the market.

The fuses here are even slightly better than found in many cheap meters - so it maybe OK for a CAT2 300 V rating and thus OK to use with 110 V mains with some care.

It does not really matter if one only wants to do voltage measurements. The typical mistake / accident scenario where you need a proper fuse is to accidentally have the wires in the current terminals when you want to measure voltage.

[Wytnucls]

You could try to alert this US department:

https://www.cpsc.gov/About-CPSC/Contact-Information/Contact-Specific-Offices-and-Public-Information/Information-Center/

If unsuccessful, ask Fluke to assist you with the reporting.

[CharlotteSwiss]

but what brand of this multimeter would it be? It doesn't seem to me to have been mentioned..
 

[MosherIV]

It does not really matter if one only wants to do voltage measurements. The typical mistake / accident scenario where you need a proper fuse is to accidentally have the wires in the current terminals when you want to measure voltage.

While yes, you can do voltage measurements. Particularly on low voltage circuits, be very careful when measuring mains.
Yes, most common mistake is user leaving probes in current jacks and blowing the fuse.

The point of CAT safety rating is guarrentee the user will be safe from energy surges.
Yes, the chances of getting a high energy surge through mains is very very low.
If you do get one, the CAT rating means that the meter will protect you from explosive damage inside the dmm. You are protected - not the dmm.
Just check out eevblog video on blowing up dmms.
MJ Lorton also got a demonstration from Fluke labs on how bad dmm explode - yes this one is over exagerated but yes dmm do explode like that.

My advice is to not worry too much, just like the others are saying. The meter is ok to use on low voltage circuits.
Avoid using the dmm on live mains measurements. Be very careful if you do. Avoid holding the dmm if you do.

[Kleinstein]

The silk-screen on the PCB says  HT118A. There is a Hobotest 118A that looks similar (sightly different color). It seems to come with other "brands" too.

[MosherIV]

Does anyone have a Fluke (or other legal brand) they can show images of for a meter of this CAT category and how it should look inside? 

Did you not watch the eevblog video nusa posted?
It clearly explains the typical input protection circuits. If I remember correctly, Dave points to the insides of an old Fluke 23 to illustrate.

There are also dozens of dmm tear downs with images in the test equipment section.
Many of them have comments on how safe the input protection is for that dmm.
In general, all the name brand (Fluke, HP/Agilent/Keysignt, Hioki, Gossen, AVO, Brymen, to mention a few) will have proper safe input protection.
Anything cheap (less than 75$) is not likely to have good safe input protection. You get what you pay for.