Pulse motor build off time.

[gyulasun]

It's interesting, isn't it, that he can use a DMM as a voltmeter to get a voltage reading, then simply change the switch setting and get a current reading, without changing any wiring connections. I sure wish I had a DMM that worked that way, it would save a lot of trouble.

Hi TinselKoala,

I was also surprised to see how he checked the current, just turning the usual rotary switch but not re-plug from the Volt input to the Amper input...  and I managed to capture a snapshot from his video where the labels are just readable enough, see the attachment.

So there are such DMMs that are able to measure current and voltage using the same input plugging to both, I did not know about this. Here is a link to this meter type he uses:  http://www.southwiretools.com/tools/tools/10040N   and there are only 3 inputs instead of the usual 4: from left to right there is the 10A, Common and the 3rd is all the rest (Volt, Ohm, Temp ect).

Gyula

[TinselKoala]

Hi TinselKoala,

I was also surprised to see how he checked the current, just turning the usual rotary switch but not re-plug from the Volt input to the Amper input...  and I managed to capture a snapshot from his video where the labels are just readable enough, see the attachment.

So there are such DMMs that are able to measure current and voltage using the same input plugging to both, I did not know about this. Here is a link to this meter type he uses:  http://www.southwiretools.com/tools/tools/10040N   and there are only 3 inputs instead of the usual 4: from left to right there is the 10A, Common and the 3rd is all the rest (Volt, Ohm, Temp ect).

Gyula

What do you mean, "The usual 4" inputs? I have six or more DMMs from different manufacturers that have only the usual THREE inputs. The only one I have that has 4 inputs is the Fluke 83, and its "4th" input is a fused high-current Amp input. The other three usual inputs are assigned, one to volts and ohms, one to mA and uA, with one common for them all. The cheapo 3-jack meters have one _unfused_ 10A input, and one fused  V/mA/ohms input, and one common.

The point is not how the plugs in the meter are wired but how the meter is connected to the circuit. If you monitor voltage, you connect the meter directly _across_ the output of the voltage source you are monitoring. If you then simply switch the meter to the "amps" range, you are then creating a short-circuit through the meter. This will of course give you the greatest current reading you can get from your source: the meter in the amps range is a _very_ low impedance "load" on the source and the true voltage (unless regulated) will drop.  This is not the way power output is measured.

But I have exchanged notes with Lasersaber and he confirms that he did it this way deliberately to see what the short-circuit current was capable of reaching with his motor. He's putting a note in the Description of the video to clear up any confusion. Of course he is smart enough not to use such readings as a claim of "power output" and later on he'll be providing proper power readings: Voltage meter (high impedance) in parallel across a load, and Current meter (very low impedance) in series with the load, simultaneous readings from two meters.

The very best current meters have _zero_ resistance or as close to it as possible. If you take apart a cheapo DMM you may see the current-viewing shunt: A piece of heavy copper wire. These meters measure current by looking at the voltage drop across this very low resistance shunt and displaying the result as a current. Even analog, moving-coil panel meters will usually measure current in a similar fashion: they will have a shunt -- a veritable short-circuit -- between the meter terminals that handles most of the current while the meter movement itself only gets a small part of the current. This is done so that the "insertion loss" of having the meter in the circuit is small.

[gyulasun]

Hi TK,

On "the usual 4" inputs I mean 1)the High current, 2)the mA, 3)the Common and 4)the Volt Ohm etc inputs.
I have a FLuke 87 which should be similar to your Fluke 83, with 4 inputs.
In the cheapo category there are types also with 4 inputs like this here:
http://www.ebay.co.uk/itm/LCD-AC-DC-Digital-Voltmeter-Multimeter-Ammeter-OHM-Auto-Voltage-Current-Tester-/161369748719?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item259262f4ef 

You are correct there are many DMMs which have only 3 inputs and probably there are many types with 3 inputs vs the 4 ones. In my enviroment, when I was active, the 4 input DMMs ruled, though we had several types with 3 inputs too.

I agree, the point is how the meter is connected to the circuit when V or I measurements are done and I forgot about this completely when answered, very sorry for this, I must have had a 'short circuit'.   

Thanks for your kind answer.

Gyula

[TinselKoala]

You're welcome and thank you for the response. I'm always kind... just not always to everybody. 

Meanwhile, back at the lab...


http://www.youtube.com/watch?v=jGmbN1CzmsQ

[Lakes]

Hmm... Attach piezos to soles of shoes, add led... night runners??