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I'm pretty new to electronics. I've however recently used mechanical and solid state relays successfully. Now, I am just started trying to use latching relays. I am currently trying to create a set up whereby my raspberry pi would trigger the latching relay to both set and reset positions, thereby switching the main circuit on and off. However, I am encountering some problems as the relay does not seem to latch when the appropriate current is applied to set and reset coils. The schematic of my relay is:

latching relay schematic

The datasheet can be found here: http://docs-asia.electrocomponents.com/webdocs/1398/0900766b8139805d.pdf

Pins 1 and 6 will be connected to GPIO pins of my raspberry pi, while the main circuit that will be switched on and off will be connected across pin 10 and 3. Since I read that the operate voltage of a latching relay is 3.75V (based on nominal voltage of 5V), I used an adjustable voltage regulator to convert my 9V power supply to around 3.75V. I applied this 3.75V voltage to pins 1 and 6, one at a time, and the GND of pins 12 and 7 to the emitter pin of my transistor which drains to my raspberry pi GND. I also applied 9V across pins 10 and 3, where pin 3 leads to GND of the circuit. When I ran my looping raspberry pi script to cease the voltage across pins 12 and 1, I verified with my multimeter that the voltage dropped to zero from around 3.75V before rising back up again after a few seconds. I expected that current will pass from pin 10 to pin 3. However, the latching relay did not work and I confirmed with my multimeter that no current passed from pin 10 to 3 when set voltage is applied, and I did not hear any expected clicking noise during operation. I then activated the reset coil instead and find that there is still no current passing from pin 10 to 3, or any latching sounds produced. Could any advise me on this issue?

I've also drew a schematic as well. The schematic for the relay is not the exact one I used, since there are limited options on the circuit lab sketch, but this diagram should otherwise be about the same as my current set up with one GPIO pin used.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Why should current pass from pin 10 to pin 3? They're separate contacts on the relay. 3,4 and 5 are one contact and 10, 9 and 8 are the other. \$\endgroup\$ – Finbarr Jan 12 '18 at 9:44
  • \$\begingroup\$ Thanks. Measured between 4,5 and 9,8. Relay's working now. \$\endgroup\$ – Craver2000 Jan 12 '18 at 11:52
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Just to be sure we're on the same page, relay coils are between pins 1-12 and 6-7, not between 3-10. Also note that Q2 has some voltage drop even when turned on, so if you set the PSU to the latching voltage, the relay will see a smaller voltage and may not activate. Set the PSU to "rated" voltage, i.e. 5V. You could even leave it at 9V, since the relay is guaranteed to withstand up to 10.9V, though that's getting pretty close to the limit which is OK for a prototype but not advised for long-term usage.

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  • \$\begingroup\$ Took your advice and used the 5V power supply rather than the aforementioned converted voltage. Relay appears to be working now. I also wanted to ask whether if you think I should get a 9V rated latching relay? The reason is because in my circuit set up, the 9V PSU will be available for feeding into the coil of a 9V relay if I get one, however I need to use and implement a 5V voltage regulator in my circuit if I stick to the 5V relay, and thus potentially creating inefficiencies in my circuit (from voltage conversion). What would you recommend? \$\endgroup\$ – Craver2000 Jan 12 '18 at 11:57
  • \$\begingroup\$ @Craver2000 If you have 9V, then using a 9V relay and saving yourself the trouble with a voltage regulator indeed sounds logical. \$\endgroup\$ – Dmitry Grigoryev Jan 12 '18 at 12:07
  • \$\begingroup\$ If you want to use the 9V supply I'd use the 9V version of the relay. It operates at 6.75V and will take up to 19.6V so you have plenty of margin either way. \$\endgroup\$ – Finbarr Jan 12 '18 at 12:07
  • \$\begingroup\$ If the datasheet for a 9V latching relay only states max switching current and voltage for AC, can these still be used to switch DC currents as well? \$\endgroup\$ – Craver2000 Jan 12 '18 at 12:35
  • \$\begingroup\$ @Craver2000 Your datasheet seems to provide both AC & DC ratings: 110VDC / 0.3A - 33W 30VDC / 2.0A - 60W 120VAC / 0.5A - 60VA 240VAC / 0.25A - 60VA \$\endgroup\$ – Dmitry Grigoryev Jan 12 '18 at 12:56

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