My ammeter stopped working. - Page 1
Forum section: electronics / testgear

author: chipwitch
author_rank: Frequent Contributor
timestamp: March 18, 2014, 12:47:16 am
content: I have a UEI DMM. Model 383, I believe. It's a handheld unit. I know it's old, cheap and I should get a good meter...I tried to use the ammeter today, and it won't give a reading. The display shows "0.00" and when it's connected to a circuit, it displays, "-0.00"I've only just used that feature recently and it worked a week or so ago. I was playing around with a generic clamp probe and may have blown the fuse then on mains power. Anyway, I tested it using a lit LED. I got zero on all scales. There are two jacks, one for low and one for high amps (2A max and a 10A max). I tried all scales with both jacks.I opened it up expecting the fuses to be blown. The 2A buss fuse was blown, but the 10A fuse was not. Do you think the 2A fuse could prevent the 10A jack from working? Or is 20mA too low to be sensed on the 10A jack? It shows 3 decimal places on the low scale of the 10A jack. Looks like it would register. I could try a higher current. Or, I could short out the 2A fuse to test it, but since it's my ONLY tester, and I can't really afford a new one right now, I'd rather not muck it up. Anyone got any ideas?

author: lpc32
author_rank: Frequent Contributor
timestamp: March 18, 2014, 01:11:10 am
content: Just test something higher.

author: chipwitch
author_rank: Frequent Contributor
timestamp: March 18, 2014, 04:52:50 am
content: no joy. I couldn't find a suitable load so I used 4 parallel 22 ohm power resistors. The PSU is 1-10v, 3A max. The load drew down the voltage to about 8.75. That's all my psu could deliver. Even that started warming up the regulator after about 2 seconds. That would give me a draw of about 1.6A. Obviously it was drawing power. Still zeros on the display.

author: retiredcaps
author_rank: Super Contributor
timestamp: March 18, 2014, 11:37:03 pm
content: Maybe a resistor on the multimeter pcb is open in 10A path?

author: idpromnut
author_rank: Unknown
timestamp: March 18, 2014, 11:45:34 pm
content: Rewind that a sec, you tested the current range using a lit LED... how did the LED light if the current range has a blow fuse (or other type of damage)?I would first start by testing the continuity of the amps ranges. If that works, then you probably have a more complicated issue than just the fuse(s).

author: chipwitch
author_rank: Frequent Contributor
timestamp: March 19, 2014, 01:24:16 am
content: Right! Good catch!I mis-spoke. Itwaslit. I broke the circuit and seriesed the meter with the led. It did NOT light. Which, points out my err in my previous post where I calculated the current that would have been drawn on the power resistors. Clearly there could NOT have been any current drawn with the ammeter in place.<edit> idpromnut, I would have given you an official "thumbs up" or something, but apparently the forum doesn't have such fluff. Thanks for the observation.

author: chipwitch
author_rank: Frequent Contributor
timestamp: March 19, 2014, 06:14:47 am
content: Idpromnut, you mean I can actually put my continuity tester on the probes?really?

author: PA4TIM
author_rank: Super Contributor
timestamp: March 19, 2014, 07:23:05 am
content: It could be the led did not burn with the amp meter in series because the burden voltage of the meter. If the psu throws out 1,6A with the meter in serie , the fuse must be good and the meter range/function must be dead. (or you do something wrong).The fuses are part of the amp circuit so you can test them with it's own continuity tester.

author: idpromnut
author_rank: Unknown
timestamp: March 19, 2014, 03:12:58 pm
content: Quote from: chipwitch on March 19, 2014, 06:14:47 amPA4TIM beat me to it.GL!

author: chipwitch
author_rank: Frequent Contributor
timestamp: March 19, 2014, 06:18:12 pm
content: Quote from: PA4TIM on March 19, 2014, 07:23:05 amThat was what I suspected initially (burden voltage). Now, I'm not so sure.I must retract my earlier comment about the 1.6A with the power resistors. I don't know that that is how much it was drawing during the ammeter test. What I did was connected it without the ammeter to test the voltage drop across the load, then calculated the current I should expect displayed on the ammeter. Then I hooked the ammeter up and it displayed nothing. I said it was drawing 1.6A. While that is possible, it does not include the possibility that there was 0A. I only have the one meter, but I could connect my scope to check the voltage drop during the ammeter test. That would tell me if I have a load or not.I have some 10W leds. I could also connect one direct for a short burst. That would tell if there's a load during the test or not too.

author: chipwitch
author_rank: Frequent Contributor
timestamp: March 19, 2014, 06:23:33 pm
content: Quote from: idpromnut on March 19, 2014, 03:12:58 pmI just wasn't sure if there might be some sensitive electronics that would be upset about the applied voltage/current from the continuity tester. Nor that it would tell me what I needed to know. If the point of testing is just to check the fuses, I did that by removing them. I gathered that you were saying something could be gleaned from testing the condition of the ammeter circuit itself? Is that not the case? You were just testing the fuses?

author: idpromnut
author_rank: Unknown
timestamp: March 19, 2014, 06:53:12 pm
content: Well, the advantage of checking the continuity through the current range is that you don't check just the fuses, but the shunt, etc as well. But yes, essentially, that test would only tell you that either the fuses are blown or some other part. To determine past that, you will need to pop the meter open to see what's going on inside, or (and be careful obviously with this) jumper the fuses with a beefy piece of wire and again perform a continuity test.The thing I like with the continuity tester is that you are fairly certain you won't blow anything up, whereas testing with a lamp/LED/etc you have the (small) chance of an oopsie moment.And if you do pop the meter open to have a look inside, post some pictures! *shameless teardown/repair request*GL

author: PA4TIM
author_rank: Super Contributor
timestamp: March 19, 2014, 06:57:28 pm
content: Just remove the fuse and measure the resistance of the fuse with the meter.To test the circuit take a 5W car-lightbulb. Connect it to your PSU and set the amp meter in serie. (just to be sure, choose range, put red in 10A jack, black in common. connect red to the plus of the PSU, black to the lamp, from the other lamp terminal to the minus of the PSU)The meter measures current by measuring the voltagedrop over the internal shunt and in a good meter a fuse is in serie too, (the burden voltage is that voltdrop named) So the fuse can be dead, and/or the shunt. In that case the damage can be limmited to the shunt but in cheap meters often the meter is killed too (and it can injure the user too)If you open the meter you should look for smoked parts. Things like fuse, resistors, protection parts, burned traces , burned switch contacts etc.

author: chipwitch
author_rank: Frequent Contributor
timestamp: March 19, 2014, 07:09:57 pm
content: Got it. PA4tim and Idpromnut, great pointers. I'll tear into it and report back.

author: chipwitch
author_rank: Frequent Contributor
timestamp: March 19, 2014, 09:39:46 pm
content: Idpromnut, I have to say, I felt a little 'dirty' taking pictures and uploading them so you and everyone here could gawk at the insides of my multi-meter. *sigh* What I won't do for a little attention!!? I posted the teardown for you over in the Test Equipment forumhttps://www.eevblog.com/forum/testgear/teardown-uei-dm383/It's just a cheapo DMM. Not much to look at. Kinda like me.Okay, so I tested the continuity. There's continuity, read 0 ohms or very near, which I guess is ideal, on both 2 and 10 amp jacks. 10A fuse is good. Just needed to jump the two amp. Right. I guess that's it then.Really appreciate all your help, guys.