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I'm building a small stripboard audio switchbox using some hamlin 5v reed relays (HE3621A0550, mouser link here).

A small SPST momentary switch activates the relay. Wondering if the best practice is to ground the relay from the switch (sink) or send current to the relay (source) from the switch. Since the switch is SPST, the other end (ground or current) would be permanently wired to the relay from the switchboard.

I am leaning towards sinking but I wanted to make sure this was a safe practice.

Any other general tips to ensure a long lifespan and good, safe operation for these relays?

edit: I am aware that the Hamlin relay I selected is not latching. The momentary switching is intentional.

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  • \$\begingroup\$ it is exactly the same, just pick the topology that simplifies the pcb or the wiring or whatever metric you come up with. \$\endgroup\$
    – Vladimir Cravero
    May 17, 2017 at 20:53
  • \$\begingroup\$ "ensure a long lifespan" is meaningless... What voltages and currents are being switched, what is the frequency of switching, how long do you expect it to work for. Also, be wary of using mechanical switches to drive mechanical relays, switch bouncing can cause some unfortunate results. esp with a relay that fast. \$\endgroup\$
    – Trevor_G
    May 17, 2017 at 20:57
  • \$\begingroup\$ That's what I figured, though I thought that sinking/grounding would be slightly superior as it would amount to more shielding for the high impedance signal which would be a little better practice for this project. \$\endgroup\$
    – user149482
    May 17, 2017 at 20:59
  • \$\begingroup\$ Yeah I was worried about bouncing. Should I put something across the switches to help with that? \$\endgroup\$
    – user149482
    May 17, 2017 at 21:00
  • \$\begingroup\$ A capacitor across the relay would help with the debounce but it might need to be large depending on the switch performance, then some small series resistance might also be prudent as long as you keep the voltage over 4V at the coil. \$\endgroup\$
    – Trevor_G
    May 17, 2017 at 21:07

2 Answers 2

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In general, it makes no difference to switch V+ or ground.

However common practice in automotive is to switch ground in some cases and switch power in others.

  • in case of accidental wire shorts, the low side switch is safer if the high side is more protected.

What does matter is how the release current is snubbed with a reverse diode across the switch to the opposite rail.

Case in point, Weller for decades used a thermostatically controlled reed switch to regulate the soldering iron to a controlled temperature. They failed to recognize this little coil and wires spews out enough EMI to cause false triggers on scope or long leads in breadboard edge sensitive logic.

So at a minimum a diode is needed. Then twisted pair (UTP) wires everywhere +/- current flows to switched load. Then sometimes shielded or STP wires and then sometime a CM ferrite sleeve choke.

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  • \$\begingroup\$ Yup make sure you buy the ones with the diodes built in. \$\endgroup\$
    – Trevor_G
    May 17, 2017 at 21:04
  • \$\begingroup\$ Yeah I chose the relay (HE3621A0550) with a built in diode. That should be sufficient right? \$\endgroup\$
    – user149482
    May 17, 2017 at 21:07
  • \$\begingroup\$ @user149482 yes as long as the switches and relays are close together. If you have long wires between them additional diodes at the switch end may be prudent. \$\endgroup\$
    – Trevor_G
    May 17, 2017 at 21:08
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    \$\begingroup\$ no that depends , WELLER did the same thing and relied on the internal diode and I can recall dozens of times others would say how did my circuit trigger... I would say.. turn off the Weller. I have another 2 cent answer with arrows that shows current flow to explain why \$\endgroup\$
    – Tony Stewart EE75
    May 17, 2017 at 21:10
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    \$\begingroup\$ Another cheap trick is twist fine insulated wire around inductive wires and ground at source to act as partial shield for EMI if near high impedance sensor wires, otherwise X (don't care) but if it ends up in your preamp audio, don't say I didnt warn you \$\endgroup\$
    – Tony Stewart EE75
    May 17, 2017 at 21:24
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it really makes no odds if you are using a switch. With a transistor, common practice is to have the emitter to ground (mostly because it's easier to drive the base from standard logic) but there is no reason at all that it must be so.

The relay doesn't care. Do what is easiest!

(Don't forget your diode to clamp back EMF's across the relay coil, or your switch might develop problems in the future.)

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