DMM with PTC fuse

[Shrek]

This being a tech site and all, I ask for advice on

• Digital Multimeters with PTC resettable fuses

to avoid having to replace fuses.

 

[LabRat 891]

On the cheaper side, these market resettable fuse(s):
https://www.homedepot.com/p/Digital-Multimeter-MT-1280/206304344

https://www.amazon.com/Mastech-MS8268-Digital-Manual-Multimeter/dp/B000JQ4O2U

Sinometer and Mastech (on amazon) both seem to offer units w/ resettable fuses.
Seems Fluke, etc. use quick blow disposable fuses for reliable protection.

 

[Shrek]

Much, much appreciated

 

[Wirko]

PTC resistors are not necessarily superior to plain fuses. I can think of some drawbacks:
- they have a higher resistance ... probably, althogh this would have to be confirmed.
- they don't break the circuit, they just limit the current to a level that keeps them warm, and if you have your ammeter connected the wrong way in your circuit, it may damage the circuit.

I just made a quick test, I took two quick-blow 1A/250V fuses (5 x 20 mm size) and put 1 A through them, for measurement and to warm them up. The voltage drop on each was around 90 mV. I don't have any PTCs to compare.

Here's the rear panel of the Keithley DMM7510, a multimeter that costs two orders of magnitude more the ones you're looking for. Fuses? Yes! Big fuses? Yes!

 

[Shrek]

They may not be superior, but they make life so much easier.

 

[Wirko]

They may not be superior, but they make life so much easier.

Here's one issue I had. A 6 x 30 mm fuse in my multimeter (Zotek/Zoyi, a great cheap brand) blew, so I bought a few spares. Turns out they are 6 x 32 mm and don't fit. Yeah, things get lost in translation between millimetres and inches. I'll have to grind away, or maybe deform, parts of the fuse holder that prevent a longer fuse from being inserted.

 

[ty_ger]

I would rather have a stable and accurate reading all the time over various loads and outside temperatures instead of prioritizing the rare occasion that a fuse is harder to maintain.

Seems Fluke, etc. use quick blow disposable fuses for reliable protection.

And greater accuracy, I believe.

 

[Shrek]

I have a breakout board that uses PTC fuses and my meter cannot detect any resistance for the cold fuses,
but it only has a resolution of 0.1 Ω. The fuses are the 5 devices around the center.

I just made a quick test, I took two quick-blow 1A/250V fuses (5 x 20 mm size) and put 1 A through them, for measurement and to warm them up. The voltage drop on each was around 90 mV. I don't have any PTCs to compare.

Which suggests a resistance of around 0.1 Ω

 

[ty_ger]

I have a breakout board that uses PTC fuses and my meter cannot detect any resistance for the cold PTC fuses,

A resistance measurement is low current. I thought you were talking about using them in a multimeter for current measurement; that's where fuses are traditionally used. Under load (a current measurement) their resistance should be higher.
You can put a load on those PTC fuses and then measure the voltage differential (drop) from one side to the other. I think that would be a more useful measurement of their resistance while in use.
And I don't think the small resistance is the biggest concern; at least not for me. The bigger concern is that the resistance is not consistent. Although the numbers are small, changing from one small number to four times that small number is a hard thing to deal with when trying to make an accurate measurement.

 

[ShrimpBrime]

Measuring a fuse with ohms is just to see if the fuse is broken or not. Otherwise, as close to 0 ohms as possible, it is a clean and closed circuit.

 

[Shrek]

How right you both are, there will be a warm error, and a good meter should signify 'out of range' when the voltage across the fuse is significant.
Reason I sought advice here, but I imagine that cheap meters don't have this feature.

Still, I like having such a meter around for general use.

 

[S-TKUC]

The main purpose of fuses installed in DMMs is to protect the DMM when measuring current.
When measuring current, the lower the internal resistance of the measuring instrument, the more accurate the measurement.
0 Ω is ideal.

If you are replacing the fuses in a DMM you already have with PTCs, you will be making a trade-off with accuracy.
I don't know what the range of current values you want to measure is, but you will generally sacrifice accuracy.

Furthermore, when comparing a fuse and a PTC with the same rated voltage/current, the PTC will be several times larger.
Unfortunately, it seems that there are no practical PTCs when using in measurement ranges of 1A or more and a maximum of 250V or more.

*To be precise, the current flowing through the DMM will be measured reliably no matter how high the PTC's resistance value.
However, the current itself flowing through the DMM will be lower.
For example, when measuring at a measurement point where 5A should flow, only 3A will actually flow, and the actual measurement will be 3A.


Although it is a bit expensive...
A clamp-type ammeter (current clamp) does not pass current through the measuring device and is a non-contact measurement, so your DMM will not break during measurement.

There are also active current clamps that convert current to voltage and can be used in the voltage measurement mode of a DMM you already have.

---

Many current clamps on the market are designed for large currents, and are often thought to be useless for measuring small currents of up to a few amps.

If you wrap the wire carrying the current you want to measure around the clamp meter 10 times, you can measure 10 times more.
In exchange for reducing the maximum measurement range to 1/10, you can increase the accuracy by 10 times.

*Image reference source

 

[Dr. Dro]

This thread reminds me, I have a cheap multimeter that came from the factory with the fuse jumped. How on earth do they even get to sell something like that

good thing I just use it for low voltage DC, and even then I probably shouldn't

 

[S-TKUC]

The inside of a fuse is delicate and can be blown by vibrations during transport.

Devices containing fuses should be packaged and transported appropriately, and vibration tests should be performed during design and development testing.
The manufacturer or distributor of the multimeter may have neglected one or all of the above.

I have seen cases where someone installed equipment with fuses that were not designed for use in vehicles into automotive equipment, resulting in frequent blown fuses.

 

[Wirko]

Although it is a bit expensive...
A clamp-type ammeter (current clamp) does not pass current through the measuring device and is a non-contact measurement, so your DMM will not break during measurement.

There are also active current clamps that convert current to voltage and can be used in the voltage measurement mode of a DMM you already have.

---

Many current clamps on the market are designed for large currents, and are often thought to be useless for measuring small currents of up to a few amps.

If you wrap the wire carrying the current you want to measure around the clamp meter 10 times, you can measure 10 times more.
In exchange for reducing the maximum measurement range to 1/10, you can increase the accuracy by 10 times.

Indeed, a current clamp is also an option. Many of them have a 40A or 60A range with a 0.01A resolution, so they are quite good for medium-small (as I'd call it) currents. HOWEVER, many cheap ones don't measure DC currents! Also the accuracy is medium-poor, usually 2.5%.

When measuring current, the lower the internal resistance of the measuring instrument, the more accurate the measurement.
0 Ω is ideal.

*To be precise, the current flowing through the DMM will be measured reliably no matter how high the PTC's resistance value.
However, the current itself flowing through the DMM will be lower.
For example, when measuring at a measurement point where 5A should flow, only 3A will actually flow, and the actual measurement will be 3A.

Well, it depends on the circuit. Sometimes low resistance is critical, and sometimes it isn't.

I have seen cases where someone installed equipment with fuses that were not designed for use in vehicles into automotive equipment, resulting in frequent blown fuses.

That's interesting. Small fuses (5 x 20 mm) for higher current ratings (say, 1 A and above) seem very sturdy. At the other end, some fuses (for very low currents) have a coil of very thin wire inside, so they might be sensitive, just like light bulbs. I've rarely seen those, maybe they don't make them that way any longer.

 

[S-TKUC]

That's interesting. Small fuses (5 x 20 mm) for higher current ratings (say, 1 A and above) seem very sturdy. At the other end, some fuses (for very low currents) have a coil of very thin wire inside, so they might be sensitive, just like light bulbs. I've rarely seen those, maybe they don't make them that way any longer.

It was a glass tube fuse rated at about 0.1A.
It had a hair-thin wire running through it, so there was a possibility that it could break due to vibration.
When I searched for automotive fuses, I could only find blade fuses, so that must be why there was a problem.