Measuring a circuit's current draw - Fluke 87V - Page 1
Forum section: electronics / beginners

author: marcusone
author_rank: Contributor
timestamp: January 16, 2013, 02:02:06 am
content: So i have a Fluke 87V, and inserted it into/between the circuit; in series (I believe that's the correct term), to measure how much a buzzer i have draws (supposed to be 50mA) - as well as just to learn/play around.If I put the red lead into the mA slot on the Fluke 87V, the circuit doesn't work (it has a "timer" capacitor before the buzzer sounds), the capacitor doesn't charge up enough to trigger the timer (I watch it charging on another DMM). But if I put the lead into the 10A slot on the Fluke 87V, it works and measure the 50mA (well, 0.0495ish).Can someone explain why this happens? Doesn't seem to matter where in the circuit I insert the DMM... at the neg terminal, positive for entire circuit, or just at the buzzer.I don't believe its the fuse, as the Fluke beeps if you put the leads in wrong (set to Ohms and put red in either A or mA slot, it will beep, which I believe also tests the fuses?).Thanks,

author: mstevens
author_rank: Regular Contributor
timestamp: January 16, 2013, 02:14:40 am
content: The ammeters have fuses. A 10A fuse for the 10A jack and a 400 mA fuse for the mA jack. The 400mA fuse may be blown.

author: marcusone
author_rank: Contributor
timestamp: January 16, 2013, 02:23:51 am
content: Quote from: mstevens on January 16, 2013, 02:14:40 amI know they have fuses... Even thought it beeps to indicate leads in wrong slot (thought that was also a test of the fuse as well?).I opened the unit and using another DMM did a continuity test on the fuses. The 10A causes a beep and the 400mA doesn't... I assume it blew then??

author: Spawn
author_rank: Frequent Contributor
timestamp: January 16, 2013, 02:26:23 am
content: yes

author: robrenz
author_rank: Super Contributor
timestamp: January 16, 2013, 02:40:24 am
content: The burden voltage of the meter is 1.8mv/mA on the 60mA and 400mA ranges. The burden voltage of the meter is 30µV/mA on the 6A and 10A ranges. There is 60 times more loading of the circuit on the mA ranges. That alters the function of the circuit the meter is in series with. At 50mA for the buzzer that is 90mV voltage drop across the shunt in the meter. That is a lot of additional loading the circuit was not designed for.Edit: I missed the blown fuse also.

author: UPI
author_rank: Regular Contributor
timestamp: January 16, 2013, 03:16:26 am
content: You might consider shorting your leads together until the buzzer gets started. Maybe it will continue to work with the meter in-line once it has fired off.Edit: Sorry, I missed the comment about the blown fuse.

author: marcusone
author_rank: Contributor
timestamp: January 16, 2013, 04:06:13 am
content: Quote from: robrenz on January 16, 2013, 02:40:24 amIs it blown? I find it strange (although I'm admittedly a noob, why posting here, so perhaps I did blow it one of the first times trying to read mA's and didn't realize it). I've ordered a new fuse to try.On your comment... its a 9v circuit using a test bench power supply and i connect the meter in before the circuit gets it power... I get readings using a cheap meter's mA setting, so i'm leaning towards blown fuse

author: JuiceKing
author_rank: Regular Contributor
timestamp: January 16, 2013, 05:20:46 am
content: Quote from: marcusone on January 16, 2013, 04:06:13 amThe 87V manual has a test procedure to determine if a fuse is blown or not that can be done without opening the case or using a second meter. Good to have a spare on hand. Like an umbrella, just having one around seems to keep bad weather away.

author: ConKbot
author_rank: Super Contributor
timestamp: January 16, 2013, 05:48:07 pm
content: Quote from: JuiceKing on January 16, 2013, 05:20:46 amOn almost every meter, you can check the fuse/shunt by putting the + lead in the Volts/Ohms/Diode socket, and probe the amp range socket of choice with the meter on ohms. Its all the same "common" terminal.I find myself checking meters this way a lot at work when they arent used exclusively by me.

author: funk1980
author_rank: Contributor
timestamp: January 17, 2013, 10:17:11 am
content: I've had blown fuse in my Fluke 87. The test procedure in the manual can tell you wether it's blown or not. No need to open it up. Too bad these special fuses cost aroung 10 bucks each!

author: nanofrog
author_rank: Super Contributor
timestamp: January 18, 2013, 02:56:04 am
content: Quote from: funk1980 on January 17, 2013, 10:17:11 amCheap insurance IMHO, and well worth the $10 vs. a serious injury, or worse, dead.