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My relay coil 12v measure 100 ohm of resistance between pin 86 and 85 or between 85 and 86. A diode IN4001 is added in parallel to the coil.

  1. However, after adding the diode, the coil has the same reading 100 ohm when measured between 86 and 85, and again vice versa, 85 and 86. Why? Is not the relay supposed to show resistance only in one direction????

  2. Why the relay coil resistance still stays at 100 ohm after adding a diode in parallel with the coil??

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3 Answers 3

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A diode 1N4001 is added in parallel to the coil.

Your meter (on a low ohm scale) might inject 1 mA into the "thing" it connects to in order to determine resistance. 100 ohms (the relay coil) and 1 mA produces a potential difference of 0.1 volts and this is way lower than what any normal silicon diode needs to start any meaningful conduction. So the diode appears open circuit.

Is not the relay supposed to show resistance only in one direction?

If the diode were wired in series then this would be the case but, you have the diode wired in parallel.


Edit - diode current at low voltages

enter image description here

The above is for a 1N4148 diode (rather than a 1N4001) because the data is available and therefore it can be extrapolated (as I have done in red). So, 0.1 volts divided by 40 nA = 2.5 MΩ and, compared to 100 ohms, it conducts 0.004 % of the current.

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The problem would be unique to digital multimeters, in which the voltage applied by the prods for resistance measurement would be quite low (less than 0.6V) for the silicon diode to conduct.

With an analogue multimeter the applied voltage would be high enough (equal to multimeter battery voltage) and the silicon diode would conduct.

Measurements across a 1N4001 in parallel with a 100Ω resistor, made with an anlogue multimeter (battery voltage 3V), gave a reading of 100Ω in one direction and 18Ω in the other.

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A DC relay coil requires DC current, but that doesn't mean that it passes current in only one direction. After all, electrically, it's just a long thin wire. In fact, until you add the diode, the relay works equally well with either terminal positive with respect to the other.

Relays with AC coils (still just wire) are constructed differently in the magnetic circuit. First of all, they depend on reactance as well as resistance to limit the current. And they often have shaded poles in order to avoid any buzz or chatter from the power line frequency.

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  • \$\begingroup\$ The OP's point related to adding a diode across the coil and not noticing any change in the measured resistance. \$\endgroup\$
    – Russell McMahon
    Jul 14, 2020 at 20:11
  • \$\begingroup\$ @RussellMcMahon Yes. But Dave did teach us about a shading ring for AC relays, which is great to know about if you wanted to know why they don't chatter. That may be half a credit if the question had asked about AC. ;) \$\endgroup\$
    – jonk
    Jul 15, 2020 at 9:05

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