Why does my multimeter fuse keep blowing ? - Page 1
Forum section: electronics / beginners

author: HelicopterJon
author_rank: Newbie
timestamp: February 20, 2017, 11:51:46 am
content: Hi all - I hope someone can help me.I've recently started electronics and have a simple circuit showing how a transistor switches a LED.When powered up at 5V, the power supply shows the current at 0.01A.If I put my multimeter in series with the circuit it reads 0.013A on the 10A range. However, if I put the meter on to the mA range and repeat, the fuse blows. The fuse is rated at 500mA and has now been replaced twice in case the original was faulty. This meter is a Uni-T UT71D and does alert me if the test leads are not connected correctly for the range selected.I retried with another (cheap) meter and it read 133mA no problem.Any thoughts? It sort of defeats the object of having a multimeter if it's "not multi"Thanks- Jonathan

author: Deridex
author_rank: Regular Contributor
timestamp: February 20, 2017, 12:00:05 pm
content: The First thing that went through my mind when reading this: Some circuits have a way bigger inrush current than the later average current is. this can blow a fast reacting fuse. I don't remeber any other reason for this at the moment.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 20, 2017, 12:13:21 pm
content: So your power supply says 10mA, the Uni-T says 13mA on the 10A range and the other meter reads 133mA? Definitely something wrong there.Can you post the schematics, and maybe a picture of the circuit?

author: Wytnucls
author_rank: Super Contributor
timestamp: February 20, 2017, 12:27:01 pm
content: Silly thing but, are you sure the 500mA fuses were not ruptured before you installed them in the meter?

author: rsjsouza
author_rank: Super Contributor
timestamp: February 20, 2017, 12:40:45 pm
content: Quote from: PA0PBZ on February 20, 2017, 12:13:21 pmThat would be critical to evaluate what is going on. 133mA on a simple transistorized circuit that lights up a LED is way too much current. If you are actually getting this, I suspect the wiring is wrong and the current is being limited by the transistor maximum collector current. If that is what is going on, I can imagine that an initial current much higher than 500mA has a slight chance of blowing the fuse.

author: HelicopterJon
author_rank: Newbie
timestamp: February 20, 2017, 01:49:23 pm
content: Thank you very much for the replies so farThe circuit is attached. I was trying demonstrate to myself how a transistor switches.@Wytnucls - I got a 10 pack of brand new fuses this morning. They are the quick blow type (as stated on the meter) and I don't think they are suspect. I have some slow blow on order.Jonathan

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 20, 2017, 01:52:34 pm
content: No way you can get 133mA through that circuit, please post a picture of what you built.

author: Wytnucls
author_rank: Super Contributor
timestamp: February 20, 2017, 02:27:52 pm
content: The only way that fuse would blow is if the meter is faulty and somehow shorts the 9V battery across the mA shunt.

author: HelicopterJon
author_rank: Newbie
timestamp: February 20, 2017, 02:58:24 pm
content: Due to my advancing years I read 133mA. It should have read 13.3mA

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 20, 2017, 03:00:49 pm
content: Chinese resistors?

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 20, 2017, 03:07:09 pm
content: Quote from: HelicopterJon on February 20, 2017, 02:58:24 pmThat at least solves one of the mysteriesNow for the blowing fuse: Did you triple check the value of the resistors, did you measure them and did you measure the LED and transistor in ohms mode?It seems that 13mA is what you can expect of that circuit, so what current do you measure with the Uni-T if you simple hook up the 1K resistor to the 5V power supply?

author: HelicopterJon
author_rank: Newbie
timestamp: February 20, 2017, 03:12:04 pm
content: After pulling out what is left of my hair, I checked again the wiring. My little circuit was fine, it was how I had the meter wired inThanks again to those who have replied and apologies for wasting your time.

author: PA0PBZ
author_rank: Super Contributor
timestamp: February 20, 2017, 03:14:59 pm
content: Quote from: HelicopterJon on February 20, 2017, 03:12:04 pmIt happens to all of us, good that you solved it

author: SingedFingers
author_rank: Frequent Contributor
timestamp: February 20, 2017, 03:16:05 pm
content: We've all done it. If it's any consolation, this bag lives on my bench all the time:

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 20, 2017, 05:40:04 pm
content: Quote from: HelicopterJon on February 20, 2017, 11:51:46 amEr... no, the meter alerts you if the test leads are not connected _to the meter_ correctly for the range selected. As you have now discovered, fortunately in a low-power circuit, the meter doesn't know or care if you have it connected improperly _to the circuit_ under test -- and can't inform you except by blowing fuses.This is why, when asking questions about probing a circuit, you should always include in your schematic exactly _where_ you are probing the circuit with your meter or scope. Simply posting the schematic is nice and necessary, but we also need to know where/how the meter is connected; after all it's part of the circuit too. Anyhow, it's good that you've got the problem sorted now.Blowing the low-range current fuse, and not realizing it, is the root cause of many many "meter current range not working" complaints and questions I've seen on this forum. There are a couple threads active right now that probably have this as the root cause or at least a major contributor of difficulty in understanding what's going on. It gives us used-to-be-newbies something we can help with, at least!

author: rsjsouza
author_rank: Super Contributor
timestamp: February 20, 2017, 07:19:39 pm
content: Quote from: PA0PBZ on February 20, 2017, 03:14:59 pmIt certainly does happen to all of us. Not only that, but thanks for actually replying with something. Unfortunately it is not uncommon for some folks to ask for gobs of information only to vanish without giving back any satisfaction as to what exactly has happened...

author: challie2
author_rank: Contributor
timestamp: February 21, 2017, 12:42:20 am
content: It might be the type of fuse your using as far as i am aware although dont take my word for it multimeters use slow blow fuses, possibly try swapping them with your cheap meter(ONLY IF SAME MAX Amp Rating) slow blow fuse allows for inrush or surge current and only go poof when there is a steady state current that exceeds the fuse rating or it meets its end life from usage.

author: Rick Law
author_rank: Super Contributor
timestamp: February 21, 2017, 01:03:06 am
content: Quote from: HelicopterJon on February 20, 2017, 02:58:24 pmDon't feel bad. I know how it is. Sometimes, I stupidly refuse to stop and grab my reading glass...

author: imidis
author_rank: Frequent Contributor
timestamp: February 21, 2017, 01:05:44 am
content: Indeed, don't feel bad at all. I've had many times trying to figure something out and then the aha momentThen it's just like doh!