I want to make a circuit which uses a latching relay to disconnect a battery from its circuit, but it will still have a path through the relay's coil and the BJT used to switch it. How significant to the battery is the BJT's leakage? (I would use the collector-emitter junction to control the coil current, and a resistor to the base to control the CE junction)

enter image description here

I couldn't (quickly) find a symbol for a latching relay, so I modified the one it had for just "relay". The assumption implied in the schematic is that the BJT throws the relay to the upper contact and the momentary switch makes the lower contact close. It might need a capacitor to make the armature reach the trip point before electrical contact is broken (i.e. if this is a problem I'm guessing a cap would be a way to solve it)

SECOND EDIT Sorry about the bizarre appearance of grafting in the photos. I wanted to show that a standard (yet heavy guage wire suitable for increased current flow) USB phone charging cable (needed to supply power to the Raspberry Pi) would not have to be modified to connect with my circuit. The circuit is supposed to indicate 2 separate wires for + and - emerging from the inline connector's cable jacket.

When the answer to this question pointed out that the diodes which suppress the back-EMF spike (snubbers?) were missing I noticed that the polarity of the "coils" was reversed as well. I think (hope) this is now correct. I have used latching relays before which had two coils per relay. The datasheet said that this allowed for two options

  • Both coils could be used, so that one coil would throw the armature to one set of contacts, and the other coil would be for the other contacts (like SET and RESET?). Each coil would have the same relative polarity so that the potential at one of the terminals marked with, say, a + symbol would be at the same potential as the other + terminal. But it didn't matter what the absolute potential was because of their design which allowed the second option
  • Only one coil could could be used to both set and reset the armature which closed the contacts. This was done by simply reversing the polarity of the current being delivered to energize the coil when changing the state of the contacts. It didn't matter which coil was used when using the polarity reversal method.

(If I have been so awkward so as to give a "for dummies" presentation of this material, it's only because that's the only method at my disposal (which is dictated by my level of "expertise"!) ... ;+D

  • \$\begingroup\$ Please show a schematic diagram of your connections. \$\endgroup\$ Commented Feb 6, 2021 at 20:44

1 Answer 1


Your circuit diagram is weird, it looks like you have shorted the battery with the relay. And the USB (power?) to the RPi I'm assuming is not being cut by the relay. You have some other load connected to the relay? Anyhow the current through the BJT when off would be in the nA range. You should add a freewheeling diode to the transistor also to not blow it up when turning off the relay coil. enter image description here

  • \$\begingroup\$ Sorry for the confusion, I thought that slapping a photo of a USB cable onto the schematic would suggest that current would flow into and out of the cable. This whole thing was to have a way for the Raspberry Pi to shut down completely. A stock unit still has LED's lit etc after a programmatic shut down. As for the photos, I won't make that mistake again! I'm sure that there is a widely used method (either through standards or popular usage) to indicate a USB connector being used (note to self: try using Google next time and don't be in such a rush to post your embarrassment)) \$\endgroup\$
    – TRS-80
    Commented Feb 7, 2021 at 0:13
  • \$\begingroup\$ Don't worry about the pictures. But if I understand this correctly the whole contraption is a way of shutting down the RPi? \$\endgroup\$
    – Dejvid_no1
    Commented Feb 7, 2021 at 1:36
  • \$\begingroup\$ Yup, that's all it's meant to do. On. Off. For context, this is coming from a person who used 17 GPIO's, each with its own opto-,coupler, to make a parallel bus so that an RPi could exchange data with an Arduino because of some obstacle that stood in the way of getting my brain to wrap around the profound nature of an invention such as THE UART: Something I couldn't articulate meant I didn't have the whole story nailed down and so I was forced to use 11 lines to send, return on 4, 2 as ctrl. Seriously, I did this. I'm not proud but I'm being honest. I can see this getting migrated to psych. \$\endgroup\$
    – TRS-80
    Commented Feb 7, 2021 at 7:53
  • \$\begingroup\$ To keep your sanity you might want to have a look at something like this: lowpowerlab.com/guide/atxraspi \$\endgroup\$
    – Dejvid_no1
    Commented Feb 7, 2021 at 14:25
  • \$\begingroup\$ I appreciate the link. 'tis a cool device! \$\endgroup\$
    – TRS-80
    Commented Feb 10, 2021 at 13:20

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