Still struggling with powering my relays

[hamfist]

OK, I'll admit it, I'm still struggling to power a couple of relays I want to use for channel-switching.
I've tried a couple of circuits from the forum, tapping from the heater filament supply and , for various reasons, it's still not working for me. This would seem to be at the edge of my abilities so ....

What would be wrong with buying a small extra transformer, tapping into the mains supply within the amp, and powering the relays that way ?

I could either get a 12V DC output transformer, and use the output direct to power the relays - am I right that the bad point of that is that the entire relay coil circuit current would need to travel out to the footpedal switch, and back again to the amp, as the circuit would need to be kept isolated from the ground.

Alternatively, I could get a 12V AC centre-tapped output transformer. If I grounded the centre tap to the amp's chassis, I could then use a rectifying circuit and a 5V voltage regulator to power the 5V relays that I already have.
the good point of this is that the footswitch would only ground the circuit. people seem to think that's the way to go.

Are both these options feasible ? What are the issues I might find with both ?

I am desperate to get the channel-switching going in this amp !!

Thanks, in advance, for any advice.

[Don Moose]

What would be wrong with buying a small extra transformer, tapping into the mains supply within the amp, and powering the relays that way ?

I could either get a 12V DC output transformer,

Alternatively, I could get a 12V AC ... rectifying circuit and a 5V voltage regulator to power the 5V relays that I already have.

Are both these options feasible ? What are the issues I might find with both ?

Both options are feasible, but there is also the option of finding a 5VDC wall wart - you only need a couple hundred mA (check your relays' coil currents, add them up, multiply by 1.2 to 1.5) and a couple hundred uF filter for that rail.

If you have a bunch of old electronic doo-dads, you probably have something suitable in your techdebris box.

Hope this helps!

[Chevy]

OK, I'll admit it, I'm still struggling to power a couple of relays I want to use for channel-switching.
I've tried a couple of circuits from the forum, tapping from the heater filament supply and , for various reasons, it's still not working for me. This would seem to be at the edge of my abilities so ....

What would be wrong with buying a small extra transformer, tapping into the mains supply within the amp, and powering the relays that way ?

I could either get a 12V DC output transformer, and use the output direct to power the relays - am I right that the bad point of that is that the entire relay coil circuit current would need to travel out to the footpedal switch, and back again to the amp, as the circuit would need to be kept isolated from the ground.

Alternatively, I could get a 12V AC centre-tapped output transformer. If I grounded the centre tap to the amp's chassis, I could then use a rectifying circuit and a 5V voltage regulator to power the 5V relays that I already have.
the good point of this is that the footswitch would only ground the circuit. people seem to think that's the way to go.

Are both these options feasible ? What are the issues I might find with both ?

I am desperate to get the channel-switching going in this amp !!

Thanks, in advance, for any advice.

Not an expert at this, but a few thoughts....
The heater filament supply plan messes with something that is crucial to the operation of the amp... the heater supply. Taking that current outside the amp via footswitch cable could wind up in problems somewhere down the road due to grounds/shorts, etc. So IMHO, it would make mucho sense to go with an additional mini xformer or a wall wart. If you get a short or ground on that, no biggy. Amp still works.

Unless you use some sort of current amplifier (ie transistor switch) to reduce the current required to the footswitch, yes, all the current in the relay winding will typically need to travel down the cable to footswitch and back. Doesn't matter if you just provide a ground when you step on the switch, you will still have the full current travelling out to the pedal and back.... just means you've grounded one side of the supply to power the relays. The advantage of using a ground-referenced supply is that you don't need a special insulated jack installed on the chassis.
Something else to consider is the typical LED indicator..... it will need several mA of current anyways.

One way of doing it, I've seen done at a "pro" shop, is to use an additional suitable small xformer, and not ground reference any of the AC leads at all; the "minus" side of the DC supply was grounded, including the common leg of the regulater. So you get a +5V DC supply referenced to ground. Seemed to work just fine.... no hum. Had one indicator LED on the footpedal as well.

Hope this is of some help.

[hamfist]

Not an expert at this, but a few thoughts....
The heater filament supply plan messes with something that is crucial to the operation of the amp... the heater supply. Taking that current outside the amp via footswitch cable could wind up in problems somewhere down the road due to grounds/shorts, etc. So IMHO, it would make mucho sense to go with an additional mini xformer or a wall wart. If you get a short or ground on that, no biggy. Amp still works.

Unless you use some sort of current amplifier (ie transistor switch) to reduce the current required to the footswitch, yes, all the current in the relay winding will typically need to travel down the cable to footswitch and back. Doesn't matter if you just provide a ground when you step on the switch, you will still have the full current travelling out to the pedal and back.... just means you've grounded one side of the supply to power the relays. The advantage of using a ground-referenced supply is that you don't need a special insulated jack installed on the chassis.
Something else to consider is the typical LED indicator..... it will need several mA of current anyways.

One way of doing it, I've seen done at a "pro" shop, is to use an additional suitable small xformer, and not ground reference any of the AC leads at all; the "minus" side of the DC supply was grounded, including the common leg of the regulater. So you get a +5V DC supply referenced to ground. Seemed to work just fine.... no hum. Had one indicator LED on the footpedal as well.

Hope this is of some help.

THanks very much.

I've actually now just bought a small 6V transformer, which I will be using with a voltage doubling rectifying circuit, to drive a 5V regulator and relay.
When the footswitch arrives, and I've sorted out one or two other issues with the amp. I'll be attempting to finalise the channel-switching stuff.

I'll be sure to let everyone know exactly what works and what doesn't !

[52 Bill]

I've actually now just bought a small 6V transformer, which I will be using with a voltage doubling rectifying circuit, to drive a 5V regulator and relay.

Why are you thinking of using a voltage doubler and regulator for this? The six volt transformer should give you around 8-9 volts dc.

[hamfist]

Why are you thinking of using a voltage doubler and regulator for this? The six volt transformer should give you around 8-9 volts dc.

Six volt (AC) transmformer gives about 5 volts DC with a full wave rectifier, I have found (diode loss etc). Not quite enough for a 5V regulator.
Someone on this forum (I forget who) suggested that a voltage doubling circuit might have some sort of "humbucking" (or even a better filtering) effect than a non-doubling rectifier circuit. It seems a pretty simple circuit to make, and I have all the parts, so I thought I'd go for it.
Using a voltage regulator seems to be a popular and well used way of supplying the exact voltage for a relay.
The 5V regulators I have, can take any input voltage from about 6 up to 30V.
So a doubled 6V AC sgould get me about 9-10V DC ....driving my voltage regulator nicely.

[Chevy]

Six volt (AC) transmformer gives about 5 volts DC with a full wave rectifier, I have found (diode loss etc). Not quite enough for a 5V regulator.
Someone on this forum (I forget who) suggested that a voltage doubling circuit might have some sort of "humbucking" (or even a better filtering) effect than a non-doubling rectifier circuit. It seems a pretty simple circuit to make, and I have all the parts, so I thought I'd go for it.
Using a voltage regulator seems to be a popular and well used way of supplying the exact voltage for a relay.
The 5V regulators I have, can take any input voltage from about 6 up to 30V.
So a doubled 6V AC sgould get me about 9-10V DC ....driving my voltage regulator nicely.

Then again...... if you want, you can go bare bones and make this work without any regulation at all.... just some full wave rec'ing, good filtering, and perhaps a few voltage dropping diodes if necessary. If you're getting bang on 5VDC with the xformer and power supply, the relay won't likely mind a + or - variation due to typical line voltage fluctuation. You'd just need to make sure you still have the required relay pull-in voltage when the line is down to say 110V or whatever it is at it's lowest in your area. On the hi side, the relay may also be happy with up to 7V across it as well; don't know the specs, though.

But.... if you're planning to have a series LED for your footswitch pedal indicator, don't forget to factor in the voltage drop across that, too.

Will you be running a standard mono guitar cable to the pedal?

[hamfist]

Then again...... if you want, you can go bare bones and make this work without any regulation at all.... just some full wave rec'ing, good filtering, and perhaps a few voltage dropping diodes if necessary. If you're getting bang on 5VDC with the xformer and power supply, the relay won't likely mind a + or - variation due to typical line voltage fluctuation. You'd just need to make sure you still have the required relay pull-in voltage when the line is down to say 110V or whatever it is at it's lowest in your area. On the hi side, the relay may also be happy with up to 7V across it as well; don't know the specs, though.

But.... if you're planning to have a series LED for your footswitch pedal indicator, don't forget to factor in the voltage drop across that, too.

Will you be running a standard mono guitar cable to the pedal?

As I have the 5V regulators already, it should be very easy to sling one in the circuit. it just guarantees the right voltage, with no stress, for a doofus like me !

I'm not planning an LED in the pedal, so no need to count that into the equation.

Yup - just a plain old mono cable to the pedal

[52 Bill]

When you got the 5vdc from the 6vac source, were you trying to use the filament winding of your amp? Is the filament winding center tapped? If it is, then I would expect you to get around 4.5vdc.

Have you tried the separate 6vac transformer yet? The theory here sez that the dc output should be the ac voltage x 1.4, so 6vac x 1.4= 8.4vdc.

[hamfist]

When you got the 5vdc from the 6vac source, were you trying to use the filament winding of your amp? Is the filament winding center tapped? If it is, then I would expect you to get around 4.5vdc.

Have you tried the separate 6vac transformer yet? The theory here sez that the dc output should be the ac voltage x 1.4, so 6vac x 1.4= 8.4vdc.

Yes, it was my amps heater filament winding, which IS centre-tapped (and grounded).
I've bought a new 6V transformer, but not had the chance to try it out yet. the theory is generally beyond me at the moment. When I give the transformer a go, I 'll try just a basic rectifier first, and see how much voltage I get.

[oc disorder]

A filter cap 1000uF (or greater) 16v after the bridge will raise the average......
Some info here (scroll down)
http://www.freewebs.com/valvewizard/heater.html

[Chevy]

Shorts/grounds & a regulator

Hamfist,

By the way..... I mentioned that you could could opt to go minimalist and just use the rectified DC without a voltage regulator... one problem with this is in case of shorts, how will you protect the new mini power supply? Fuse it?

That's a major advantage of putting in the regulator; if you happen to short/ground the pedal or cable, the regulator will go into protection and limit the current. So that's a great reason to use a voltage regulator. Makes it sorta foolproof.

[hamfist]

Hamfist,

By the way..... I mentioned that you could could opt to go minimalist and just use the rectified DC without a voltage regulator... one problem with this is in case of shorts, how will you protect the new mini power supply? Fuse it?

That's a major advantage of putting in the regulator; if you happen to short/ground the pedal or cable, the regulator will go into protection and limit the current. So that's a great reason to use a voltage regulator. Makes it sorta foolproof.

THat's a good safety point for using a regulator !

As for fusing, I intended the new transformer to be fused by the mains fuse (which will hoefully be able to cope with a few more mA).

[Enzo]

What is your mains fuse? 4 amps maybe? 3 amps? Now multiply that by the mains voltage. This will give the power that has to flow before the fuse thinks about blowing. In my examples, 360 watts or 480 watts at 120v mains. For your little transformer to get any fuse protection at all, it would have to draw that much power.

That is why 5 watt power resistors can burn up without blowing the fuse in an amp.

COnsider a 6v 1 amp transformer is rated at 6 watts. If it tried to carry 360 watts it would melt into slag long before it would stress that fuse at all. If your circuit failed and tried to draw 10 amps (and we will pretend the voltage would still be 6v) that is merely 60 watts. Half an amp from the 120v mains. The fuse wouldn't care. Your transformer would melt and the rest of the amp wouldn't know the difference. In reality, the 6v would drop way down, meaning even less of a strain on the mains fuse.

The mains fuse offers no protection to your small extra transformer.

The point of the mains fuse is really to protect you and your home, not the amp. It blows to prevent the amp from catching fire in a failure.

[hamfist]

What is your mains fuse? 4 amps maybe? 3 amps? Now multiply that by the mains voltage. This will give the power that has to flow before the fuse thinks about blowing. In my examples, 360 watts or 480 watts at 120v mains. For your little transformer to get any fuse protection at all, it would have to draw that much power.

That is why 5 watt power resistors can burn up without blowing the fuse in an amp.

COnsider a 6v 1 amp transformer is rated at 6 watts. If it tried to carry 360 watts it would melt into slag long before it would stress that fuse at all. If your circuit failed and tried to draw 10 amps (and we will pretend the voltage would still be 6v) that is merely 60 watts. Half an amp from the 120v mains. The fuse wouldn't care. Your transformer would melt and the rest of the amp wouldn't know the difference. In reality, the 6v would drop way down, meaning even less of a strain on the mains fuse.

The mains fuse offers no protection to your small extra transformer.

The point of the mains fuse is really to protect you and your home, not the amp. It blows to prevent the amp from catching fire in a failure.

OK, makes sense. Do you have any other suggestions for me ?

BTW, the transformer is a little 250mA 6-0-6V Centre tapped one. I was intending to either use one half of the winding with a voltage doubling rectifier, or the full winding with a normal rectifier.