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I have a 5v(coil) dpdt relay. Running it with 9v is safe or not? How can i reduce the voltage from 9v to 5v ? How much resistor i need ?(i do no want to use voltage regulator pls)

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    \$\begingroup\$ This is difficult to answer properly without having proper specs of your relay. Please enhance your question accordingly \$\endgroup\$ – Ariser Feb 13 '17 at 12:12
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Place a ~4 V zener diode in series to drop the voltage.

schematic

simulate this circuit – Schematic created using CircuitLab

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You will put a resistor in series with the relay coil. The resistor value is selected in conjunction with knowing the DC resistance of the relay coil. The idea to select the resistor value so that the coil drops 5V and the resistor drops about 4V.

For example if the coil resistance is 50Ω then the series resistor has to be about 40Ω.

Here is the general formula:

$$\frac{5V}{R_{COIL}} = \frac{4V}{R_{RESISTOR}}$$

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  • \$\begingroup\$ I dnt knw the coil resistance. It is not written in the relay \$\endgroup\$ – Subhajit Sarkar Feb 13 '17 at 10:20
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    \$\begingroup\$ You have two options. (a) Look in the relay data sheet. (b) Measure the coil resistance with your multimeter in ohms mode. \$\endgroup\$ – Michael Karas Feb 13 '17 at 10:22
  • \$\begingroup\$ Yes good idea i can measure the resistance using my multimeter \$\endgroup\$ – Subhajit Sarkar Feb 13 '17 at 10:23
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You should take a look to the relay datasheet, but usually 9V into a 5V relay will fry the coil.

I'll advice against using just a resistor. At least add a 5.1 volt zener diode (1N4733A, for example) as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

Choose a value of R1 that simultaneously protects the zener diode and allows enough current into the relay coil. For a low sensitivity coil with 55 Ohm +/-10% DC resistance operating at 5.1V +/-5%, a R1 of 33 Ohm +/5% would do the trick.

For R1 = 33 Ohm, worst case zener current through D1 would be about 53 mA and 0.25W dissipation so it should be OK. Worst case dissipation in R1 would be about 0.55W, so you should use a 1W resistor.

You may like to choose an even lower value of R1 if you foresee that the 9V supply may have some tolerance and deliver less than 9V. It's up to you, you should make the calculations and decide.

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    \$\begingroup\$ You will also need a flyback diode (1N4148) if you plan to switch the 9V into the coil using a transistor, in order to avoid damaging it. \$\endgroup\$ – Enric Blanco Feb 13 '17 at 11:42
  • \$\begingroup\$ There's no reason to use the zener, you are just wasting even more power. \$\endgroup\$ – Chris Stratton Jun 14 at 13:52

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