powering a relay

[Enzo]

SO make the 7905 work if you already have it, the relay doesn;t care what polarity you use. REverse the diode rectifiers and filter cap to make it negative. Remember the 79M05 may have a different pin order than the 7805. If the supply is now negative, then reverse the diode across the relay coil as well. SO now the relay runs off -5 instead of +5.

[Vaughan]

As drawn it should be a 7805, yes.

You are rectifying 3.15VAC, which yields about 4.5vDC once filtered. If the 5v relays work at all I bet it has little to do with the regulator. it cannot be working within its regulating range. It certainly needs an input voltage higher than the output to function. Try replacing it with a wire and see if the relays work just as well.

Mmmmmm, quite correct Enzo. The reason it has a regulator in this circuit is because the little PCB I've used was taken from an old "Wall Wart" which was supplied from a 12v Tranny. I've now altered the circuit so that the 7805 is now fed from a voltage tripler. The input to the 7805 is now way above the minimum 7V supply needed.
I'll post the circuit for those interested as soon as I've drawn it out.
I've really learnt a lot from this thread, and sorry to those tried to follow the incorrect schematic.

[Vaughan]

Mmmmmm, quite correct Enzo. The reason it has a regulator in this circuit is because the little PCB I've used was taken from an old "Wall Wart" which was supplied from a 12v Tranny. I've now altered the circuit so that the 7805 is now fed from a voltage tripler. The input to the 7805 is now way above the minimum 7V supply needed.
I'll post the circuit for those interested as soon as I've drawn it out.
I've really learnt a lot from this thread, and sorry to those tried to follow the incorrect schematic.

This is my new schematic

[hamfist]

SO make the 7905 work if you already have it, the relay doesn;t care what polarity you use. REverse the diode rectifiers and filter cap to make it negative. Remember the 79M05 may have a different pin order than the 7805. If the supply is now negative, then reverse the diode across the relay coil as well. SO now the relay runs off -5 instead of +5.

This is way beyond my understanding. How would the grounding scheme look, if you run the relay on negative voltage ?

[Enzo]

The 7805 and 7905 work the same way, just that one of them takes a positive voltage in and regulates it down to a lower positive voltage, while the other takes a negative voltage in and regulates it to a lower negative voltage.

I am ignoring the tripler for this discussion, I am looking at the original circuit with a simple rectification of the 3v.

When you rectify the AC voltage, the only thing that determines the polarity is which way the diodes face. You have them pointing to the right for your positive voltage. If you were to reverse them so they pointed left, then you would have negative voltage coming out.

You have a filter cap after the rectifiers. If we are changing from positive to negative, we would have to reverse that cap.

You have a diode across the relay coil to kill voltage spikes. It is wired in reverse so it doesn't short the power supply. If we change polarity, that diode would also have to be reversed.

The relay is just a coil of wire that activates the moving parts when current flows through it. It has no inherent polarity. if you connected the relay coil across a 5 volt battery - without any other stuff - it would energize. And it wouldn;t matter which way we connected the battery. In the same sense as a magnet attracts a piece of iron. It doesn;t matter which end of the magnet you use, it will still be attracted to the iron.

And the ground? It simply completes the circuit as it always did. A switch and relay is a very simple circuit. You can wire it up to run off any appropriate voltage supply, polarity is just a detail. But we do have to pay attention to all the parts in the circuit so they make sense with the pollarity we chose.



Imagine you have a battery and a light bulb. And "ground." If I connect one side of the battery to ground, and one side of the bulb to ground, then when I touch the free ends of each together, the bulb lights. Current flowed from the battery through the bulb, through ground, and back to the battery. The polarity of the battery doesn;t matter there. I could reverse it and the bulb would still light up when I complete the circuit. In the relay circuit we have the power supply, the relay, and the switch. Just like the battery adn bulb, it doesn;t matter what polarity we use from the power supply. All that matters is since we added things like voltage regulators and a diode across the relay coil we need to make sure THOSE things agree with the polarity we chose.

[hamfist]

Nice one Vaughan !

In actual use, how are you finding the circuit ? Quiet ?

Where did you ground the power, after the footswitch ?

[Vaughan]

Relay Power

Hi Hamfist

I've just finished testing the circuit in situe. The tripler gives a good 11V, more than enough for the 5V regulator. O/P from the regulator is a smidge above 5V (5.03V on my digital meter). Just run the amp up to full power with the input fed with a low signal from a audio generator. When the relay operates from the foot switch there are no clicks and no hum that I can hear.

Vaughan

[hamfist]

Hi Hamfist

I've just finished testing the circuit in situe. The tripler gives a good 11V, more than enough for the 5V regulator. O/P from the regulator is a smidge above 5V (5.03V on my digital meter). Just run the amp up to full power with the input fed with a low signal from a audio generator. When the relay operates from the foot switch there are no clicks and no hum that I can hear.

Vaughan

That's great Vaughan. I've got it all up and running myself as well, voltages much the same as yours. Switches channels quietly and cleanly.
The only real difference I did from your circuit was that I had all caps grounded at all times (not just when the footswitch is activated, for some of your caps). My way, all the caps are charged and ready to go at all times. I'm not sure it really makes any noticeable difference. It's just the way I did it.

[BK-Amps]

You're making it too complicated.

You can just run a bridge rectifier off the heater taps (6.3V) and filter with a 4700u or so cap. DON'T ground it. Make the return leg of the relay connect to the negative end of the rectifier and cap. You can also use an NPN transistor to switch this by forming a voltage divider between + and - and feeding the middle to the base (on), or grounding the base (to the minus, side not earth) when you want it to switch the relay off

This is a floating relay supply, is quiet, and works fine.

Edit: If you want a footswitch, you can connect the leads to the NPN base and minus side of floating supply. You also have to use a connector that is isolated from the chassis.


BK

[hamfist]

You're making it too complicated.

You can just run a bridge rectifier off the heater taps (6.3V) and filter with a 4700u or so cap. DON'T ground it. Make the return leg of the relay connect to the negative end of the rectifier and cap. You can also use an NPN transistor to switch this by forming a voltage divider between + and - and feeding the middle to the base (on), or grounding the base (to the minus, side not earth) when you want it to switch the relay off

This is a floating relay supply, is quiet, and works fine.

Edit: If you want a footswitch, you can connect the leads to the NPN base and minus side of floating supply. You also have to use a connector that is isolated from the chassis.


BK

BK, your way doesn't actually sound less complicated. Also a bridge rectifier wouldn't actually be suitable to replace Vaughan's circuit shown above (or my own circuit) as we are dealing with a centre-tapped transformer circuit.

As for many things, it would seem like are many different ways to arrange appropriate power supply for a relay.

[BK-Amps]

.. as we are dealing with a centre-tapped transformer circuit.

Doesn't matter. You leave the CT grounded to earth. This method allows you to continue to use the tap for you heaters. You can also elevate your center tap if you choose.

BK

[hamfist]

Doesn't matter. You leave the CT grounded to earth. This method allows you to continue to use the tap for you heaters. You can also elevate your center tap if you choose.

BK

Do you have a schematic. As a bit of a noob, I can't really get my head round it without one.

[hamfist]

OK, here's BK-amp's schem.

[hamfist]

Just as an update to the thread, I have now implemented a circuit almost identical to Vaughan's (see below). I was getting a small "pop" on switching channels. Then I put a 0.1uF cap across the footswitch terminals ....now I have absolutely silent channel-switching.

[Steve A.]

You're making it too complicated.

You can just run a bridge rectifier off the heater taps (6.3V) and filter with a 4700u or so cap. DON'T ground it. Make the return leg of the relay connect to the negative end of the rectifier and cap. You can also use an NPN transistor to switch this by forming a voltage divider between + and - and feeding the middle to the base (on), or grounding the base (to the minus, side not earth) when you want it to switch the relay off

This is a floating relay supply, is quiet, and works fine.

Edit: If you want a footswitch, you can connect the leads to the NPN base and minus side of floating supply. You also have to use a connector that is isolated from the chassis.


BK

BK:

Here's a drawing of how I get 6vdc from a 6.3vac filament supply. It can also be used to power relays. (The 6vdc is from the + terminal to ground- I forgot to draw that in!)

http://www.blueguitar.org/new/schem/...r/ac_to_dc.gif

Nothing fancy- just a bridge rectifier and a few resistors and caps. You may need to adjust the value of the resistors to match the load...

Steve Ahola

P.S. This is used with a PT that has a 6.3vac filament tap- if there was a center tap it would be taped off. Or you could just isolate the ground lug from ground and use it with any 6.3vac filament supply.