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I'm designing a circuit to drive a single coil latching relay. This uses two MOSFETs, arranged so current flows one way through the relay coil to switch it on, and then the other MOSFET activates to reverse the coil current to switch the relay off. Both switch on and switch off is only a short duration, as the relay holds its set/reset state without coil current.

So to my question: Are free wheeling diodes required to deal with the back EMF when the current is switched off? If so, how could they be arranged, as the polarity of the coil reverses depending on which mosfet has operated?

enter image description here

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  • \$\begingroup\$ Please post a schematic. \$\endgroup\$
    – winny
    Dec 3, 2022 at 23:12
  • \$\begingroup\$ It's harto answer your question without at least a rudimentary schematic drawing. Show us what you have in mind. \$\endgroup\$ Dec 3, 2022 at 23:14
  • \$\begingroup\$ I'll post a schematic, copied from an existing design \$\endgroup\$ Dec 3, 2022 at 23:15
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    \$\begingroup\$ As an excercise: replace your relay coil with a current source and see if the body diodes in the mosfets are sufficient to allow flow in either direction. \$\endgroup\$
    – Bryan
    Dec 3, 2022 at 23:50
  • \$\begingroup\$ I think you will need a RC snubber. \$\endgroup\$
    – Drew
    Dec 4, 2022 at 3:25

2 Answers 2

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Looks 'okay' to me as-is, provided that Icoil * R1 + 12V does not exceed the 60V Vds rating of Q3. And you have to drive the inputs in a fairly pathological manner to get even that (switch IO8 low simultaneously with switching IO7 high). Normally it's the lower value of Icoil (12/(220+Rcoil) rather than 12/Rcoil).

However, it would be easier on Q3 and create less EMI if there was a single diode such as an LL4148 from Q3 drain to +12. Otherwise the MOSFET body diodes take care of it.

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  • \$\begingroup\$ Thanks for your help. The 12v relay is rated at 50mA, so would be well within the capability of the Vds rating of Q3. I'll model adding a 4148 to Q3 \$\endgroup\$ Dec 4, 2022 at 17:24
  • \$\begingroup\$ Vds is voltage rating. \$\endgroup\$ Dec 4, 2022 at 17:28
  • \$\begingroup\$ Just confirming that 1N4148 or similar would be biased the same way as the body diode i.e. cathode connected to supply voltage and anode connected to drain. LTSpice shows a small surge current when the relay current is removed, in both directions \$\endgroup\$ Dec 4, 2022 at 19:19
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    \$\begingroup\$ Sounds correct. If it was backwards there would be a huge destructive current when Q3 turned on. ;-) The 'surge current' you are seeing is no more than the relay coil current. \$\endgroup\$ Dec 4, 2022 at 19:32
  • \$\begingroup\$ "Vds is voltage rating" I was providing the coil current to indicate that R1 x Icoil (220 *0.050) + 12V was less than the Vds of the Mosfet. In this case the total voltage is 23v against a 60v rating so it should be fine. \$\endgroup\$ Dec 12, 2022 at 0:24
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Here's the schematic of a single coil magnetic latch relay that employs a capacitor and an SPDT switch to generate the 'set' and 'reset' pulses.

The capacitor charge pulse sets the latch relay while the inverted discharge pulse resets it.

enter image description here

No freewheeling diodes are required.

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  • \$\begingroup\$ How do you propose that this answer is useful? I mean, of course there's no need for diodes, since there's no transistors to protect. \$\endgroup\$
    – enhzflep
    Dec 4, 2022 at 19:28
  • \$\begingroup\$ Free wheeling diodes protect switch / relay contacts too. In this scheme the switch may as well be replaced by transistors with no need for any freewheeling diodes. \$\endgroup\$
    – vu2nan
    Dec 5, 2022 at 3:51
  • \$\begingroup\$ "Free wheeling diodes protect switch / relay contacts too." How would you know ? In this case the relays are switching 16A at 230VAC, good luck finding a diode that will protect the contacts. \$\endgroup\$ Dec 12, 2022 at 0:18
  • \$\begingroup\$ Hi David, There's no question of freewheeling diodes in an AC circuit! \$\endgroup\$
    – vu2nan
    Dec 12, 2022 at 4:10

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