3 replies to this topic

[Matt Brailsford]

Hi Guys, I know this probably sounds like a dumb question, but I just wanted to get clarification on whether I'm readin my multimeter right? I'm using one of these multimeters (http://www.quasarele...r-3.5-digit.htm) to try and figure out the amperage requirements of an FS-GR3C RC receiver. To do that, I have the positive lead of a 5v power supply (it's actually running off the netduino 5v supply, which in turn is running off a 9v square battery) connected to the positive lead of my multimeter, and the negative lead of the multimeter connected to the positive input of the receiver. The negative input on the receiver is then connected to the negative lead of the battery. I turn on the multimeter, and set it to the 200m A setting which gives me a reading of 0.2, which I believe is 0.2mA. (by the way, I have the positive lead on the multimeter connected to the 10A input, as for some reason I get no amperage reading on the other input of the multimeter. Blown fuse?) The reason why I am questioning this, is that I thought I'd run a test to make sure I was doing it right by checking the amperage requirements of the netduino board in the same way. Now from what I have read, the requirements of the netduino should be around 40mA, however when I measured it, it was only drawing 0.8mA according to the multimeter. So my question is, am I measuring the current correctly? if not, what am I doing wrong? and if so, how come the netduino was reportedly only pulling 0.8mA? Many thanks for your help. Matt

[Mario Vernari]

Hello Matt.
Everything you've done seems right.
Most of the times the DMMs (digital multimeters) don't use a fuse on the 10A shunt. It's simply a thick wire, pretty hard to fuse!
Instead, the standard input could/should be fused, but...I may guess they use a auto-restoring fuse (how do you call it?). Once the current rises above a certain threshold, the "fuse" opens the circuit. As soon you detach the leads, the "fuse" restores automatically.
I repeat, I'm not sure about this solution on your DMM, but if I were you, I wouldn't be worried.
To test the fuse health, simply check your appliance: if is powered, the fuse is good.

BTW, I guess you shouldn't take care about the knob indication when you plug the 10A input. Simply, turn it to "10A" and that's the only choice allowable for that DMM.
That will answer to your (right) readings: 0.2 stands for 200mA (in a 10A range).

Finally, consider that any DMM has a relevant error on the readings. On digital meters you have always two errors (to combine):

• +/- N (least) digit (e.g. N=1);
• a percentage related to the full scale (e.g. 1%)

Thus, the 0.8 is far from being the real current flowing through the Netduino.
Supposing N=1, the actual value could fall in the range 0.7..0.9. Afterward, you should consider the full-scale percentage: supposing 1% out of 10A, is 100mA...

It's much like weight a spoon of sugar with your bathroom scale!
Cheers

[Matt Brailsford]

Hi Mario, Thanks for your reply, I really appreciate your help. I wasn't aware of the errors on a DMM, so knowing that now, makes a lot of sence. Thanks again Matt

[Magpie]

Hi Although your probably already on the right track. A couple of things, I personally haven't come across Multimeters without fuses, It must be a real cheap one. They usually seem to have one fuse for each current input. The large one is usually expensive, like $10 for a fuse. To check whether your meter is any good. Try measuring the current through a 1 percent resistor. Use ohms law to calculate what current should be passing through it using your 9 volt battery. Dont use too small a value of resistance as the resistor will burn. Also check your batteries terminal voltage under load, why not. Sometimes I calibrate the meters at work, very boring, you have to do each position on the switch.