relay switching

[lowell]

hey all,
I'm just curious about relays and why one might use them for switching... I guess channel switching specifically. Why the need for the relay, could this not be done w/ an ordinary toggle or stomp switch? I'm asking because I don't know... could someone shed some light on this subject? I'm thinking I might want to use a relay for channel switching in a 5e3X2 that I recently built.

[Enzo]

It is not channel switching per se, it is switching chores in general. All a relay is, really, is the same thing as your toggle switch except instead of your fingers moving the switch, there is a little electromagnet that moves it back and forth. A relay can have as few as one switch, or two, or many many. The common little pc board mounted rectangluar ones are usually DPDT.

A relay offers several advantages. one is that you can operate many sets of contacts with one switch. For example I can have one set that selects which volume control is active, and another that selects some gain element like a cathode bypass cap. One button controls both.

A relay can be right on the board next to the circuit it controls. This eliminates long wires from some point in the circuit up to the front panel and back. It doesn;t matter how long the wire is that carries DC to the relay coil, but it does matter how long the signal wires are running around.

One panel switch can operate several relays scattered here and there. Maybe not on a small Fender clone, but on some amps, channel switching might require 6 or 8 switching contacts somewhere. Good luck finding that 8 pole double throw toggle switch for your front panel.

ANd a relay can make your control, remotable. You cannot flip your toggle switch while standing at the mic playing and singing, but stomping on a foot switch is simple, and that footswitch can control just about anything.

I could have a sensitive high gain circuit that needed switching - like an input - and a high level signal that also needs switching - like an output. I don't really want to bring those right next to each other in a switch, and have their wires wrapped around each other. But I can mount a small relay near each circuit and control them both from one switch. Now the circuits stay as far apart as before.

In more complex designs, multiple relays can be controlled in more than one way. For example one panel switch might turn on a certain group of relays, and a different switch turns on a different set of relays, but some of the relays can be in both sets. For instance in a three channel amp, relays for a gain cap might want to turn on for channels 2 and 3 but not 1. It is very simple with a few diodes to "program" any combonation of relays to energize for any switch on the panel.

There are other things used for the same purposes, like JFETs and LDRs. You can stick them all over the place and control them all with a common switch in similar fashion. There are plusses and minuses for each method, relays included.

Those are a few things off the top of my head.

[lowell]

Enzo,
thanks, more than I hoped for. However you've spurred some other q's. I'd really like to see an example or if you can put one into words that'd be great. For example say I wanted a footswitch to add a gain stage to the preamp before the PI. Now, I know how I'd wire this to be toggled, but how is wired w/ the relay... does the stomp switch "connect" dc to the relay and then the relay itself performs the switching task? Also, how do I pick a suitable relay for a specific task.

[Enzo]

does the stomp switch "connect" dc to the relay and then the relay itself performs the switching task?

Exactly, that is the very description of a relay.

Your new mantra: "a relay is just a switch."

GO chant that for a while until enlightenment is achieved.

Relays have poles and throws jist like switches, the only difference is how they are activated. If you can wire something to a switch, then you can wire it to a relay. Relays either have printed right on them or on the data sheet, their internal wiring. Wire the contacts inside it just like a toggle switch. The there will be the pair of terminals for the coil. You need a power source appropriate for the coil, and a way to switch it on and off. We generally use DC for that, and so you need some sort of suppressor across the coil, typically a diode wired for reverse bias.

You probably want what we call a small signal relay. The contacts are handling signal and small currents, so you don't need a huge clanking relay. RElay coils come in a variety of voltage ratings: 5, 6, 12, 24, etc. They come in AC and DC versions. They come in momentary cuty and continuous duty. We want continuous, and I rarely see relays not rated for continuous in the catalog, at least not in types we would use.

The voltage doesn't matter, pick one you can convenently provide in your amp. No point in using 24v relays if all you have in the amp is 6v and 400v.

A common arrangement would be to connect one end of the relay coil to power - let us use 12VDC for example. Then the other end of the coil would get grounded to turn the relay on. Unground it and the relay relaxes. Relay contacts will be marked NC and NO - Normally CLosed and Normally Open. Normally refers to the coil NOT energized. When the coil is energized, then the NC contact opens and the NO contact closes.

We usually put the switch in between the coil and ground - what they call low side switching. Turning the switch on and off turns the relay coil on and off. That switch can be a pushbutton or toggle on the front panel, or it can be a foot switch. Want both? Add a jack between the panel switch and ground with the switch wires to the tip contact and teh grounding cutout contact wired to ground. Now when nothing is on that jack, the cutout contact completes the circuit to ground. Plug a footswitch into the jack, and now it is in series with the other switch and the relay coil. The panel switch MUST be left in the ON position for the footswitch to work. I am sure you have seen commercial amps like that - the panel button must be pushed in for the footswitch to work.

Doesn't Ted Weber or someone make little relay boards?

[lowell]

Enzo,
thanks that does clear up alot. Is there ever a need to incorporate a transistor of any sort w/ the relay to make this work... or just directly to a power source? Also, for channel switching or simple tasks like adding a gain stage via a switch is a relay optimal or a transistor switching arrangement?

[Enzo]

Is it optimal to play an A chord at the fifth fret or back at the nut? Relays, like transistors and LDRs, are just another tool in the toolbox. Often you can select one way or another for the same task. There is no hard rule. I like relays, but then I also have a backgroud that includes a LOT of relay work. I can read relay logic trees readily. You won't see that in guitar amps, but it is a part of me. SOmeone else might prefer to make the same circuit functions from JFETs or LDRs.

A transistor can act like a switch. The very JFETs we mention are examples. But it is quite common to use a transistor to switch the relay on and off. One advantage of this would be that the relay current stays local. Instead of having the coil current run out a footswitch cable and back, the small base current for the transistor is all there is.

[chipaudette]

Before you go too far down the road with relays, I'd like to reinforce Enzo's discussion of getting the DC source. Many tube guitar amps only have a DC source at 300-500 V. That is not helpful for driving relays.

Some guitar tube amps (non-cathode-biased amps, at least) have another DC supply at -50V or so. That supply generally doesn't have enough current capacity to drive a relay, so it's unusuable.

Generally, you've got to create your own DC supply off the 6V AC heater supply. You'll need a bridge rectifier and some filtering (or a 5V regulator chip). It's not hard to do, but it's parts that you've got to add in addition to your relay. Some people don't like taking that extra step.

Chip

[lowell]

ok thanks guys... that clears up a lot... yeah using the filament winding for the DC source (ofcourse rectifying and filtering it) makes sense.

[Enzo]

You could use the vias tap for a relay supply, sometimes, but it depends upon the current you need. Not all relays of a given voltage rating are the same. And not all bias windings are the same either. And how many of them. How many relays, that is.

On the other hand, if you go with JFETs, the gate requires negligible current, so it is not an issue.

And it also depends on the amp you are working in. More and more amps these days have a low voltage winding intended for things like relays and op amps. Certainly not hard to find PTs with one.

[lowell]

Enzo do you think that the bias tap on a power transformer for a Fender Tweed Twin could handle 208ma of coil current? I wrote Mercury about this too.

[Enzo]

probably. Put that much of a load on it with resistors and find out. That is less than a quarter of an amp.

[lowell]

Find out how? What's gonna happen if it's ok or not ok? I obviously don't wanna damage the transformer.

[Wilder Amplification]

Why on earth would you need 280mA of coil current for small signal switching when they make high sensitivity small signal relays that only draw 30mA of coil current? Check out the Omron G5V-2-H1-DC5 DPDT 5V 30mA relays. I use them in my amps and they work great. For (2) channel switching relays + a solo boost relay I'm only pullin' 90mA of coil current.

Here's a link to those relays at Mouser -

G5V-2-H1-DC5 Omron Electronics Low Signal Relays - PCB

Mouser mistyped the specs and shows the spec for the "H-DC5" as the spec for the H1-DC5 and vice versa. But I've personally tested the H1-DC5 and they do indeed only draw 30mA of coil current a piece.

Another great DPDT relay is the Takamisawa RY12W-K DPDT relay, which only draws 12.5mA of coil current and is perfect for small signal switching.

I also use a transistor to handle the coil current, which also allowed me to add panel channel indicator LEDs and the transistors to control both the LED + the relay. Overcomplicated? Of course...but there is a means to an end with transistor controlled relays I assure you.

Another great high sensitivity relay for small signal switching is the Takamisawa RY12W-K. They're DPDT and they only draw 12.5mA of coil current.

[lowell]

I have 5 relays in my amp totalling 208ma when all are energized.

Jon I don't NEED to use jfet controlled relays as I have relay transformers, just am wondering how I can cut costs and if the bias supply thing is doable or not. That is the most simple and cheapest solution for now. Thanks for the info though. I'll take a look at those relays.

[Wilder Amplification]

I have 5 relays in my amp totalling 208ma when all are energized.

Jon I don't NEED to use jfet controlled relays as I have relay transformers, just am wondering how I can cut costs and if the bias supply thing is doable or not. That is the most simple and cheapest solution for now. Thanks for the info though. I'll take a look at those relays.

Not sure what you mean by "relay transformers" but the reason for the BJT transistors in mine was to isolate the relay current from the foot switch. I also plan to implement MIDI capability in the somewhat near future and most logic chips cannot source/sink relay coil current.

With 5 of the Takamisawa RY12W-K DPDT relays you can reduce your coil current by 70%, which IMHO is a HUGE reduction in coil current. Why draw more than you need to? Of course you'll need a 12V supply to drive 'em but that shouldn't be too hard to fab up.