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I am building a circuit for user to interface with relays, and I am trying to build a protection to the circuit against fly-back from the inductor coil of the relay. Since there is no specific relay it is quite hard to choose the parts. All that is certain is the relay must be 5V and draws up to max of 250 mA (my recommended limit), but a protection for higher amp rating would be better.

The protection in my circuit is a diode - resistor combo. with the diode being 1n4007 which is greatly way more than what I need (which is good). the resistor is a bit tricky and what I am not sure about.

Assuming from what I know about inductors, is that it is a current source, and the voltage will depend on the resistor.

Here is my computation

V = I × R
5 = 0.25A (largest 5V coil current I found) × X ohms
X = 5/0.25
X = 20 ohms

20 ohms is for my absolute max, but for lower current loads it could go up to 50 ohms + (assuming 5V is our desired voltage). I don't know which is a better safe side value — is higher the better or the opposite? My guess is lower, but how low a resistor that generates 1 volt?

On a similar note, the power rating required for this resistor:

P = I × V
P = 0.25A × 5V
P = 1.25 watts

1.25 watt resistor are hard to find on resistor values that low. Will a 1/4 or 1/2 watt resistor work since the current is not continuous and it goes down?

Please help me if what I know is correct, thank you.

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You are getting muddled.

It's not the 5 volts that is used to power the relay that needs to be factored in when designing a resistive flyback suppressor, it's the maximum voltage that your driving circuit can withstand under flyback conditions that has to be considered.

For instance, consider the situation of using an NPN (or N channel) transistor to activate a relay. When the relay is deactivated, the inductive 250 mA would produce a peak voltage of 25 volts in a 100 ohm flyback resistor. This means the collector (or drain) of the transistor would see a peak voltage of 25 volts plus 5 volts (30 volts).

Therefore, if the transistor has a voltage rating greater than 40 volts (a little added margin) then you would be good to go.

Regarding the power rating you don't need to pick a resistor that has a rating equivalent to the instantaneous value of the power surge; resistors are usually very good at handling largish short power surges so, if the flyback surge is done in less than 10 ms (fairly normal) then pretty much any quarter or half watt resistor will do the job.

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  • \$\begingroup\$ Just, clarifying; im using a N channel MOSFET(BSS138) and the specs i should look for is the Gate-Source Voltage which is ±20 V in my case. So using a 20 ohm resistor is sufficient because it is way below that rating ( 20 > 5 + 5 ) \$\endgroup\$
    – Jake quin
    Dec 4 '19 at 23:50
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    \$\begingroup\$ It's the source-drain voltage that is the one taxed by the flyback voltage. Given that this is rated at a max of 50 volts, the resistor value can be chosen to limit the voltage to (say) 5 + 35 volts flyback i.e. a resistor value of 140 ohms with a current of 250 mA. \$\endgroup\$
    – Andy aka
    Dec 5 '19 at 8:07
  • \$\begingroup\$ is there an advantage for going the highest possible resistor for the current to quickly dissipate, or is a lower resister is preferred? \$\endgroup\$
    – Jake quin
    Dec 5 '19 at 21:07
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    \$\begingroup\$ Higher value resistor means the energy stored in the relay’s coil is dissipated more quickly hence it turns off more quickly. \$\endgroup\$
    – Andy aka
    Dec 5 '19 at 22:14
  • \$\begingroup\$ thank you sir, your answer really helped me understand \$\endgroup\$
    – Jake quin
    Dec 6 '19 at 0:54
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First decide what you want your diode/resistor combination to do.

If it's to protect the driving transistor against the high voltage flyback, then zero resistance is ideal, and the flyback will be clamped to <1 volt above the rail. If you know the maximum collector voltage rating of your drive transistor, then you can choose a resistance to allow up to, but not exceeding, that voltage to be generated.

If it's to turn the relay current off quickly, then use as large a resistor as you can, to get as high a voltage as you can for rapid current fall, subject to not destroying your transistor by overvoltage. A zener diode might make a better clamp here, as the voltage is then more or less independent of current.

If you do want to use a resistor, then the power rating is unimportant for most purposes, as it's the coil energy that gets dumped into the resistor once per relay operation. Only very frequent relay operation will tax the resistor power dissipation. Unfortunately, few low power resistors have an energy pulse spec. If it's a hobby project, then just use 1/4 watt, and feel it after several relay operations. If you're selling these things in the 1000s, then you might want to get a pulse energy spec from the resistor manufacturers, or you might not.

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