Relay Bypass Latching/Non-Latching?

[redbagy]

Hi! Most implementations of a momentary/latching switch relay bypass I've seen use a latching relay instead of a non-latching one.

Pros of a latching relay would be that only a short 'high' current is needed instead of a constant one. However with a non-latching relay, the relay bypass would default to bypass when no power is supplied (which would be useful in a couple of situations perhaps?).

What are your thoughts about this?

[DIY Bass]

I have 2 pedals which have relay bypass.  I am not sure, but I assume they are both non-latching.  I have always thought it was quite elegant.  They are not driven by a microcontroller.  They are 5V relays, and there is a 5V regulator on the board.  The footswitch just switches the 5V to the relay on and off.  The pedals do "remember" the on/off state, but there is only 2 (or 3 if you want the "off" state actually grounded rather than floating.  The LED is also linked to the 5V line, so it goes on and off at the same time.  Makes replacing a dead footswitch so much easier if ever required.

[PRR]

Popular-price latch relays are relatively recent and still cost more than a CMOS flip-flop. I think you have a narrow view of the field.

Yes, dumb relays are nice because you know which way they will come up.

[ElectricDruid]

One advantage of relay switching for me (from a microprocessor programming angle) is that you can do both momentary and latching switching on the one switch. It's pretty easy to program up a footswitch to give you a latching toggle when given a short tap, but to give a momentary only-on-when-held-down action for longer presses. This adds a little bit of versatility for only a bit of extra coding - the relay hardware is the same as the "normal" on/off action. You *could* do this with either latching or non-latching relays, but honestly, I'd have said non-latching is easiest/cheapest.  I also like the feel of soft-touch SPST footswitches with no "CLUNK!" - some people hate them, but for me, that's an improvement, and relay switching lets you use them.

Unless there was some desperate reason why I needed the lower overall power consumption of a latching relay, I think I'd always use non-latching. But then again, if I was trying to minimise power consumption that bad, I'm not sure I'd want a relay at all...

[mark2]

I've worked a lot with both latching and non-latching, and honestly can't see, hear, or notice any worthwhile difference (including with hold-to-momentarily-toggle functionality). So I just prioritize price and availability first, then current consumption second.

At the moment most everything I'm doing is using latching. But not for any principled reason.

[Ripthorn]

I use non latching relays, but the benefit of latching is that you can use the short switching pulse to shunt the signal output using a bjt or FET to eliminate any risk of popping. This is how some commercial pedals do it, but requires a separate mcu output pin for the on vs off functionally.

[mark2]

I use non latching relays, but the benefit of latching is that you can use the short switching pulse to shunt the signal output using a bjt or FET to eliminate any risk of popping. This is how some commercial pedals do it, but requires a separate mcu output pin for the on vs off functionally.

I'm sorry I'm feeling really dense today. Can you describe more about what you mean by "shunt the signal output using a bjt or FET to eliminate any risk of popping" ?

[ElectricDruid]

I'm sorry I'm feeling really dense today. Can you describe more about what you mean by "shunt the signal output using a bjt or FET to eliminate any risk of popping" ?

He means that they use the relay pulse to switch on a FET/BJT that shorts the signal to ground during the relay switch. That ensures that at the moment that metal contacts are banging together, there's no voltage on them at all. That helps eliminate pops when switching.
As mentioned, you can do the same thing with non-latching relays too, but you need a separate short pulse from somewhere, since that's not what you use for the relay. That implies either an extra MCU output pin, or a monostable.

[mark2]

Got it, thanks!
So, it's mainly helpful when you're out of gpio pins (edit; or of course if you don't have any when you're not using a uC). Otherwise I'd probably use a separate pin for the transistor anyway, in case I wanted to tune the timing differently than the relay switching.