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I did the following to measure the resistance of some DC motors I have and I'm wondering if I did it properly and if my conclusions are sound :

I had 4 motors (same model) already connected to a breadboard in parallel. I have a semi-broken "Universal AD-DC converter" variable power supply that is set to output 4.5v. When I plugged that on my breadboard (motors started turning) and measure the voltage straight on the adapter, it was fluctuating around 10 to 11 volts (Unregulated I guess). I then measured the current and it was about 0.6A (even though the adapter says "200mA Max").

It means my circuit had a 16.6 Ohms resistance, so each motor is around 65 Ohms.

Makes sense?

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  • \$\begingroup\$ I say "semi-broken" because it outputs power, but the voltage doesn't seem to relate to the setting. \$\endgroup\$ – Tipx Apr 26 '12 at 3:13
  • \$\begingroup\$ If I check the resistance while the motor isn't turning, won't the resistance be really low, like 0.1ohm? \$\endgroup\$ – Tipx Apr 26 '12 at 23:23
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It may be 65-ish ohms per motor. It also may not be. The 10-11 v output was likely measured on a multimeter. The fluctuations may mean the output is bouncing around. The average voltage over some time scale reported by the dc multimeter may not be a good description of the output. Likewise for the current. The real output may be swinging from high to low in cycles. In which case the linear division of dc voltage by dc current may give a somewhat wrong result. This would not be a concern with a known good stable supply operated within specs. If you have access to an oscilloscope you can tell pretty quickly.

To get a dc resistance measurement you may need to disconnect the motors and measure their resistance.

Also, are you sure it's the supply that's broken and not the load?

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  • \$\begingroup\$ Sadly, I don't have access to an oscilloscope. I read websites saying that measuring the motors without any current won't give reliable values. When I measure any single motor, I get something like 0.1ohm. As for the power source, I'm not totally sure. \$\endgroup\$ – Tipx Apr 26 '12 at 23:56
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Even a power supply with the worst regulation won't output a higher voltage than rated when massively overloaded. You would expect an unregulated supply to output a higher voltage at no load, lowering to rated voltage somewhere in the middle of the current range and dropping to lower at full rated current. Beyond this, the regulation may become even worse, but normally the limit is a power limit - it will start getting hot.

If it is adjustable (you say it ignores the setting), then this is usually achieved by a linear regulator. For it to be unregulated and have a switch, it would need multiple taps on the transformer, which is expensive and not normally done.

So it sounds like your power supply may be broken. If you have any power resistors of between 20 and 30 ohms then you could perform a simple load test. They will need to dissipate at least a watt (if it is doing 4.5V at 0.2A), or you could parallel 4 or 5 1/4 watt resistors. Keep an eye on the heat, as if it is outputting 10V+ then much more power will be dissipated.

On the flip side, this could be an issue with your meter. DC brushed motors introduce vast amounts of broad spectrum noise onto power supplies. This can confuse meters - especially cheap ones. The worst case is for cheap RMS meters. You could be getting spurious readings.

However, it may be worth taking a step back. Are you actually intending on measuring resistance? The windings in the motor will have a resistence, but this needs to be measured with the motor stationary, ideally using a meter on the resistance range. But why would you want to know the resistance of a simple DC motor?

Are you actually trying to characterise the motor for some other purpose? In that case you really need to perform current measurements under no load, stalled and for several points in between.

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  • \$\begingroup\$ The reason why I wanted to measure the resistance is the following: I'm really new to the hobbie (just a hobbie for me, not a job :-P) and I want to design a circuit to feed the proper amount of current to the motors. I figured I needed to know the resistance of the motor to be able to get only the good amount of volts and amps to the motor. \$\endgroup\$ – Tipx Apr 27 '12 at 2:43
  • \$\begingroup\$ Are these small permanent magnet DC motors? They can't really be characterised by a single measurement. When running with no load, current will be low. As load increases, current increases. \$\endgroup\$ – Cybergibbons Apr 27 '12 at 11:21
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Here's the formula... $$ \frac{1}{R_{eq}} = \frac{1}{R_1}+\frac{1}{R_2}+.....+\frac{1}{R_n} $$ I just multiply it by the number of resistors so 16.6 X 4 is 66.4.

But it would be better if you measured one at a time, or just use a multimeter!

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    \$\begingroup\$ use $$ to generate a math equation \$\endgroup\$ – CyberMen Apr 26 '12 at 19:57
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What you measured is the impedance; which is the net of resistance and inductance of the motors. To measure the resistance, you only have to apply about 10% of rated voltage while locking the rotor to prevent the motor rotation, then measure the current: \$ V\div I=R\$

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