Is it OK to replace standard multimeter fuses with PTC? - Page 1
Forum section: electronics / testgear

author: canaldin
author_rank: Newbie
timestamp: September 05, 2015, 01:57:29 pm
content: Hello,Today I blew my digital multimeter's 250ma fuse by using just the USB power.It was my first time so I opened it up and found 2 fuses inside, I checked them with an LED. The 250ma one was effectively not conducting.Since it was lunch time, a bunch of PTCs laying around looked much more attractive than the trip to the hardware store. So I soldered a 300ma one across the terminals. And tadah it worked.I was wondering is it OK to leave it there? Why didn't the manufacturer put it there in the first place, aside from cost? Or could it negatively affect the performance, safety or device life?The PTC is a RXEF300

author: mij59
author_rank: Frequent Contributor
timestamp: September 05, 2015, 02:12:21 pm
content: No, its not OK, the ptc is only rated for 72V.

author: SeanB
author_rank: Super Contributor
timestamp: September 05, 2015, 02:16:17 pm
content: The PTC has a higher resistance than a fuse, and as well it never really goes open circuit, just goes high resistance to limit the current to some value not as high to the short circuit current. As well it will not operate properly with over a certain applied voltage, and might fail as a short with excessive dissipation, or will arc over and allow unlimited current to flow. A fuse has good repeatable performance, low resistance in the working state, and it is effectively an open circuit when faulted and capable of breaking a very high fault current.Usable if the meter is never used to measure over the SELV limits of 50V and no more than 10A even into a short circuit, as otherwise the PTC can fail with destructive results.

author: andtfoot
author_rank: Unknown
timestamp: September 05, 2015, 02:20:08 pm
content: The RXEF300 looks to be a 3A (6A trip) PTC...I'm guessing PTCs wouldn't be used for at least a couple of reasons; from a brief look at a datasheet of the RXEF300: Slow time to trip (20 seconds at 15A for the RXEF300), inconsistent resistance affecting accuracy, and safety (it doesn't actually break the circuit completely, low voltage rating and can catch on fire apparently).

author: canaldin
author_rank: Newbie
timestamp: September 05, 2015, 02:37:50 pm
content: Sorry, it'RXEF030.So I'd better go get the right fuse I suppose..I am planning on measuring voltages and currents that you typically find on a breadboard circuits. My multimeter has 4 posts. The blown fuse was attached on the second from the left.

author: mos6502
author_rank: Frequent Contributor
timestamp: September 05, 2015, 03:58:27 pm
content: The biggest problems with PTCs are a) that they have a much higher resistance than a fuse. So you'd increase your burden voltage. But it shouldn't affect the accuracy. And b) that the voltage rating is much lower than that of a fuse. The RXEF030 can take a max. of 72V.Also, the relatively high resistance even in the normal state will already limit the current somewhat. I'd say they're pretty safe at low voltages. They're used in all sorts of equipment (e.g. for protecting USB ports) and I haven't even heard of one failing to protect or cause problems.There are multimeters that have PTC fuses, like some Gossen meters.

author: zapta
author_rank: Super Contributor
timestamp: September 05, 2015, 04:05:15 pm
content: Quote from: canaldin on September 05, 2015, 01:57:29 pmHow do you know that it works? Did you test it with over current and high voltage? Did you check the accuracy?

author: Neilm
author_rank: Super Contributor
timestamp: September 05, 2015, 06:18:00 pm
content: The fuse in the picture looks like a ceramic High Rupture Capacity type. These are designed to safely blow and quench an arc under very high currents. A quick check of a SIBA 20mA fuse shows a rated breaking capacity of 1.5kA with a voltage rating of 250V. I have never seen a PTC that could approach this.The result of having a PCT here would be that if you accidentally connected it to the mains in current mode rather than voltage mode the voltage would arc over and there would be an arc flash explosion. If you are lucky, this might be contained within the meter. Most likely it wouldn't.

author: nctnico
author_rank: Super Contributor
timestamp: September 05, 2015, 06:34:45 pm
content: I don't recommend using a PTC as a fuse in a multimeter. OTOH I also hate fuses in multimeters because I tend to blow them up as well. My solution was buying a >4.5 digit multimeter so the 20A range still has mA resolution.

author: mos6502
author_rank: Frequent Contributor
timestamp: September 05, 2015, 08:02:13 pm
content: Another option would be to wire an inline fuse holder into one of your test leads and insert a fast acting fuse that is rated at a slightly lower current than the one inside your meter.

author: SeanB
author_rank: Super Contributor
timestamp: September 05, 2015, 08:06:17 pm
content: I just buy a pack of 10 fuses and put the small cardboard box they come in into the meter back ( there is room inside most cheap meters for it) so I always have a few spares. That way when you are down to 5 you make a note to buy a new pack or two for when you pop the last one.

author: andybarrett1
author_rank: Regular Contributor
timestamp: September 05, 2015, 08:39:06 pm
content: Quote from: mos6502 on September 05, 2015, 08:02:13 pmWe have to use fused test leads in industry now a days……Still not worked out how to measure >>500mA current with 500mA fused leads ?

author: mos6502
author_rank: Frequent Contributor
timestamp: September 05, 2015, 09:00:32 pm
content: Quote from: andybarrett1 on September 05, 2015, 08:39:06 pmVery carefully ...

author: Jeroen3
author_rank: Super Contributor
timestamp: September 05, 2015, 09:57:12 pm
content: The fuses in multimeters have 2 purposes:1. To protect the multimeters holy internals. Because when abused, they lose accuracy and thus calibration.2. To protect the operator from multimeter self destruction in his face due to operator failure.Multimeter contain fuses made in moonlight with unicorn hairs to either:1. Not influence the accuracy, or influence in a predictable manner.2. Not explode and create an conducting arc path.Code:To prevent the forum from creating a bunch of embedded videos.Bad meter: https://www.youtube.com/watch?v=lg44zuxGmMgBad meter: https://www.youtube.com/watch?v=Ewyf9mzIfi0Good meter: https://youtu.be/uvpKP4awjDQ?t=658