Replacing Fuse in Cheap DMM - Page 1
Forum section: electronics / testgear

author: Mario87
author_rank: Frequent Contributor
timestamp: April 30, 2022, 04:44:23 pm
content: Hi all, I have a cheap (£30-ish) DMM that is about 10-12 years old from Maplins in the UK, specifically a Precision Gold PG017. Now I have since replaced this with a proper DMM (Brymen BM789), but thought it would be good to keep the proper meter for electronics work only and use this for stuff on the car that is quite basic, or around the house.However I have noticed the fuse (500mA, FF, 250V) has popped. Now I am just wondering how critical is it to get an FF fuse vs an F fuse? I know the latter won’t blow as quick, but is there any real risk in using an F rated fuse?Reason I ask is that RS want £25 for a pack of FF fuses which is more than what the meter is worth today, but I can get a pack of F rated fuses for just £2, which is worth it.These are 5mm x 20mm fuses and I am not sure what the Brymen uses, so if these fuses will only work on this DMM then I really don’t want to spend £25 on fuses for a meter that cost £30 over 10 years ago.Any thoughts?Thanks

author: BeBuLamar
author_rank: Super Contributor
timestamp: April 30, 2022, 04:55:59 pm
content: I would keep using the FF fuse because I want to use it up to the rated current without blowing but once exceed it, the fuse blow immediately. If I measure something with surge current the surge current must within the measurement range of the meter. You shouldn't blow the fuse often enough to worry about the cost. If you blow it often you do something wrong too often. That is the fuse should not blow if we don't make mistake. We would make mistake occasionally and that's what the fuse is for. We shouldn't make mistake too often.

author: HKJ
author_rank: Super Contributor
timestamp: April 30, 2022, 04:57:46 pm
content: The reason for a FF use is to avoid damage to the meter, you safety will not change when using low voltage.The Brymen uses much better fuses that can break high current at high voltage, a glass fuse will not do that, it will ARC instead and the meter may exploded if used at industrial level power and voltage. The Brymen fuse is also much more expensive.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 30, 2022, 05:10:06 pm
content: Safty wise the FF fuse is not necessary better than the F rated fuse. For the saftely the ceramic case fuses may break a higher fault current and are less likely to explode. Even the glass one are usually OK with 250 V, if the peak current is not very high.There is a slightly high chance to damage the meter with the F rated fuse. However the most critical case is a current slightly higher than the fuse rating ( so even the FF rated fuse does not blow) for an extended time. So here the fuse speed does not make a real difference, if it does no come with tighter tolerance.It somewhat depends on the protection and shunts used: chances are they have 1 A diodes and these should be OK with the cheaper fuse too. So better have one of the cheaper fuse than no fuse at all.If the shunts are rather low power and there is a seperate shunt for a ~50 mA range, it may still overheat with the original type of fuse.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 30, 2022, 05:23:35 pm
content: In my experience the fuse blows in a DMM when you forget to change sockets after a current measurement and go to measure a voltage (I haven't done it for alongtime now...) and is effectively a short-circuit, and blows pretty quickly regardless of the fuse rating.If it were a marginal out-of-range current measurement, which as stated wouldn't necessarily blow an FF particularly quickly, I'd be looking carefully at my measurement technique and why I chose to use the incorrect range after seeing the reading I was getting.Even in-range measurements have a duty cycle to them on current ranges, and extended measuring will cause inaccuracy and potentially damage the meter.The PG017 is a nice basic little meter, kudos to you for keeping it alive all these years!

author: Mario87
author_rank: Frequent Contributor
timestamp: April 30, 2022, 07:03:54 pm
content: Quote from: BeBuLamar on April 30, 2022, 04:55:59 pmYou are missing the point, if I buy these fuses they are only for this DMM, they come in packs of 10, so if I only use 1 or I use all 10 it doesn't make a difference, the cost is still £25 when the entire meter cost £30 over 10 years ago. Economically it's pointless unless there is a real true valid (ie safety) reason.Quote from: HKJ on April 30, 2022, 04:57:46 pmIf there is no risk to my safety, then I think I will stick with an F-rated fuse. The meter will mostly be used for 12v stuff (car, motorbike, etc), but could be used occasionally to check if 240V mains is isolated and to ensure I am not accidentally about to work on a live circuit.Quote from: Kleinstein on April 30, 2022, 05:10:06 pmEven the cheap fuses that are £2 a pack from RS which are F-rated are ceramic fuses, so should also be less likely to explode. From what is being said here it seems an F-rated fuse should be fine, especially for my own safety, but might be a slight higher risk of damage to the meter itself (which I am not THAT fussed about) and is only a risk if I use it incorrectly.To be honest, I am not even sure when or why the fuse in this meter went pop. All the main functions I usually use (resistance, voltage, capacitance) work fine except for diode test mode and I have not tested it for measuring current as it's not something I tend to do often. It was trying to figure out why diode mode wasn't working that led me to find the fuse had popped. In diode mode I can see there is 3V at the input of the fuse, but obviously nothing on the test leads.Hence I am now looking for a new fuse.Quote from: AVGresponding on April 30, 2022, 05:23:35 pmThis meter has 2 fuses (500mA one that has popped) and a 10A one that is fine. There are only 3 sockets, one of them is only used for 10A range measurements, but I am 99.9% sure I have never done measurements like that with this meter. Actually, I am pretty sure I have only ever used it for AC / DC voltage, resistance, capacitance & diode test.So really not sure when or why this fuse went as all functions I usually use still work except for diode test (which was why I went looking and found the bad fuse where diode test has 3V on the input of the fuse, but nothing at the probe socket).Yeah, this meter has served me well over the years, and done most of what I need, there have been times I have thought "it would nice if I had a meter that did XYZ also", but never felt the need to seriously upgrade until recently.Will be good if I can keep it alive a bit longer.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 30, 2022, 07:32:28 pm
content: It is a bit unusual to have the fuse also effecting the diode mode. Normally the fuse is only for the current ranges. Modern meters usually have a PTC to protect the ohms and diode mode. With 3 terminals and thus the lower current via the same terminals as voltage / ohms it at least makes some sense to also have the fuse also in the ohms/diode current source path, though there should still be some additional protection (e.g. PTC) and the fuse would be only a 2nd line of defense.

author: AVGresponding
author_rank: Super Contributor
timestamp: April 30, 2022, 08:24:45 pm
content: Quote from: Mario87 on April 30, 2022, 07:03:54 pmI still have the first meter I bought, a Radio Shack/Micronta 22-195 that I got back in 1988 or so, and it still works, despite a few misadventures with the aforementioned method of blowing fuses. Yes, it's primitive compared to other more modern, premium brand stuff, but it does its job and served me well for many years before I upgraded.

author: Fungus
author_rank: Super Contributor
timestamp: April 30, 2022, 09:00:14 pm
content: Go with the cheap ones.Just be aware of what's inside and don't lend it to anybody who's on their way to an industrial job and forgot their Fluke.Remember the fuse only does something on the amps range and you won't ever be poking probes into a mains socket on the amps range anyway. Right?Quote from: Mario87 on April 30, 2022, 04:44:23 pmIf you're only using it on the bench with low voltages then there's no risk at all.The only "risk" in a domestic situation would be something like messing around with mains isolation transformers which will defeat the RCD/GFD circuit breakers in your distribution panel. Mains isolation transformers are dangerous anyway and the type of fuse in your meter won't add muchextrarisk.Quote from: HKJ on April 30, 2022, 04:57:46 pmNot true.

author: Mario87
author_rank: Frequent Contributor
timestamp: May 01, 2022, 11:38:11 am
content: Thanks all, have just ordered a pack of the F-rated fuses, at the very least I made sure to buy ceramic and not glass fuses so the only difference is the replacement is an F instead of an FF