Fun with relays

[sjaltenb]

I picked up a relay from radioshack ( i know i know) the other day, a Tyco 12V and decided I'd play around with it to see how it works.

Im guessing its a non-latching, type. Basically if i apply 9Vs to it, it flips to the other side, and if I remove the voltage, flip back to the other.

Seems simple enough! And a a perfect application for my problem ( i want to switch two effects loops that are not side by side ). I figure I'll just got a latching stomp switch and either hold current to the coil or release it. Anyone see any problems? The only thing I can think of is that during power down, the coil will release, but when I power the board back up, it should return to its previous state, right?

I guess this is why I shouldn't go the simple route, I should do it like R.G.s design. It appears these relays are reluctant to pass guitar signal anyway 

[JKowalski]

Make sure you have a diode to save the coil from back EMF:

http://www.youtube.com/watch?v=aSmMFog10D0

Exaggerated example but it helps to see the problem: Basically, when you put voltage across the coil, the current increases until it reaches a maximum determined by the resistance of the coil. When you remove the connection abruptly, the current cannot instantaneously change to zero so what happens is that the voltage across the coil increases tremendously until the switch actually arcs over (as you can see in the video, though that example has alot of potential energy compared to this application). This can damage your switch, obviously.

Putting a diode parallel with the relay (facing backwards to the power supply voltage) will cause any voltage that appears across the coil higher then the supply voltage to be cut off because the diode becomes forward biased by the increasing voltage and drains off the excess energy.



They should pass guitar signals fine because relays are just switches with ideally 0 ohm resistance... I don't know what you mean there.

[sjaltenb]

Thanks for that! I learned something new, I was sure I would if I posted, so thats why I did 

Well, thats what I was wondering about, i couldn't measure any resistance across the switch, but when i tested it with my guitar, I couldnt get signal to pass through 

I thought that it maybe was not a 'small signal relay' which I have seen, but, as long as there is no resistance, why would it matter? I'll try again today and do it with jacks instead of an audio probe, etc.

[cpm]

the current through the coil(s) must be taken into account. Make your numbers from the datasheet: eg, a 90 ohm coil with a 6v supply will eat ~60mA. Add that for the number of relays used... to get your actual power needs (beware battery life)

you may try a small R in series with the coil for less current, but if you lower the current too much the coil wont activate. Just try some values

[sjaltenb]

Yeah, not sure what I was doing wrong earlier but it passes the guitar great.

I'm bout to head up to the shack to grab the other one and wire the little deal up. I think I'll keep a 3PDT stomp switch in there, and simply flip the current on or off with one pole, and let the other poles handle the LEDs.

It says 60ma on the specs for the current, I'm hoping that since it is working with 9v instead of 12 that number will be a little lower. Shouldn't matter, as I am using a PSU.

Thanks guys, I'll post a pic in a little while...one of my first complete DIY projects (no pcb, etc)

[sjaltenb]

Here they are! Simple, yet effective! They will be used two places on the board, to flop between two Tube Drivers, and to flop between the CE2 and the Mistress (both sets are not together in the chain, thus, the double relays)

3PDTs will control the current to the coil and the LEDs.

[sjaltenb]

Of course, nothing is ever easy! POP!!

I ended up having to use two sets of relays to do the switching. One set controls the two send/returns to the tube drives and the other set controls the On/Off function. I had to do this because I have a set of effects that are between the two being switched. Thus, I had to switch the inputs and outputs as well as the S/Rs.

Anywho, Im doing a bunch of reading. Looks like I'll need to try a few pulldown resistors and caps to slow the switching. I believe it only happens when release the voltage to the relays. Otherwise, the design works as I had hoped!

Thanks for any suggestions. I'm going back to read R.Gs stuff again

[Ronsonic]

Kowalski's right about the need for a diode, but check. Most relays now produced have the diode built in. Real pain when working on an older circuit that took advantage of the relay's indifference to polarity.