Need help with footswitching and LEDs

[Gaz]

I'm troubleshooting a simple foot switch setup, and I'm pretty stumped.

I'm powering 2 relays from a separate transformer rated 12v @ 300mA. I'm bridge rectifying it up to 18 volts or so, and regulating it back down to 12v.

I understand that doubling the voltage of the PT reduces the current by half, effectively giving me a 150mA supply. Each relay only draws 12mA, so 24mA total.

The relays function fine, but when I hookup up the standard 2-button Marshall style foot switch with LEDs, the relays do not fire. When I disconnect the LEDs inside the pedal the switching works again.

I presumed the LEDs were drawing too much current even though the pedal comes stock with 1k current limiting resistors. I increased them to 4.7k and the switching worked!

Sounds okay, but it was really just an educated guess on my part, and I want to understand how much current the LEDs were/are actually drawing, and if the switching will be reliable.

[Merlinb]

I understand that doubling the voltage of the PT reduces the current by half, effectively giving me a 150mA supply.

I thought you were bridge rectifying 12V to get close to 18V? No doubling going on there...

The LEDs were sucking roughly 12V/1k = 12mA each, so your total was close to 48mA; not a problem unless your reservoir cap is on the small side...

[Enzo]

Are you sure they prevented the relays from working? ANy chance the relays are energized and can't release?

My initial reaction is that the LEDs are not loading down the power supply. My thought is that the current through them is enough to keep the relays from relaxing.

[Gaz]

Thanks, Merlin, you are correct, which leads me to believe that Enzo is right, that the relays "can't relax." Here's a link to the schematic for the footswitch: Google Images

Is there a fix for this?

[pdf64]

Why not use the 4k7 resistors - aren't the LEDs bright enough?
Another option is to connect the LEDs in series with the relay coil. No dropper resistor needed.
If you must use the Marshall footswitch without any mods, then could reduce the regulated voltage levels (with relays appropriate to that voltage). At 5 or 6V, the relay coils will need much more current than the LEDs.

[R.G.]

I'm powering 2 relays from a separate transformer rated 12v @ 300mA. I'm bridge rectifying it up to 18 volts or so, and regulating it back down to 12v.

I understand that doubling the voltage of the PT reduces the current by half, effectively giving me a 150mA supply. Each relay only draws 12mA, so 24mA total.

Actually, it's more complicated than that.

Transformer currents are rated in amps RMS, and they presume a sine wave in that rating. The current into rectifiers and full wave bridges in particular violate that presumption. The current in a full wave bridge flows in short, very sharp pulses at the peak of the AC wave, and not at all in other parts of the wave. When you look at the RMS value of the transformer currents for a DC current of X amount, you'll find that the transformer sees 1.6 to 1.8X in terms of RMS current as far as heating is concerned. For a continuous supply of 1A out of a bridge rectified and capacitor filtered power supply, you need a transformer rated for 1.6A to 1.8A, with the bigger values being where you get with huge filter caps like everyone likes to use.

It gets worse if you look at voltage sag. That 3.6W transformer (12V * 0.3A) has a significant winding resistance and may sag 12-15% for AC loads. It's 12Vac out only when it supplies 300ma. It's higher than that otherwise to provide the voltage to let it sag down to 12V at full load. But a FWB supply is not pulling currents of 300ma*1.414 = 0.424A at 300ma out. It's pulling more like 4A peaks. So the sag is far worse for a FWB supply than it is for a resistive load on the transformer because of the sharp peaks.

The point of saying all that is that you can run into situations where the loading looks mild, but overloads your transformer.

The relays function fine, but when I hookup up the standard 2-button Marshall style foot switch with LEDs, the relays do not fire. When I disconnect the LEDs inside the pedal the switching works again.

I presumed the LEDs were drawing too much current even though the pedal comes stock with 1k current limiting resistors. I increased them to 4.7k and the switching worked!

I don't know the insides of the standard 2-button Marshall style footswitch. Got a pointer to a schemo?

[Enzo]

Unless shown otherwise, I tend to assume the typical two button switch has a LED/resistor series across each switch. When switch is open, the LED is lit by current through whatever the switches are controlling - typically a relay coil. Switch closed shorts across the LED. Trick is to arrange LED current below hold threshold for relay.

[R.G.]

That definitely fits with things getting better with bigger resistors. My experiences with old Vox footswitches leads me to be very suspicious of what's inside a footswitch.

[Jot]

That definitely fits with things getting better with bigger resistors. My experiences with old Vox footswitches leads me to be very suspicious of what's inside a footswitch.

Apologies for hijacking this thread, but R.G.'s comment is a red flag for me. I recently bought a Carvin 4-button footswitch that I planned to adapt for use with a Thomas Vox Buckingham head so that I can use the otherwise-unavailable built-in fuzz. Turns out that each of the switches on the Carvin pedal has an LED that's wired in parallel with the switch rather than in series. The DIN plug on the Carvin pedal isn't compatible with the Vox head, which is just as well, since I probably would have been tempted to just try it and see what happened.

Can anyone tell me whether it's safe to just go ahead and solder the proper DIN plug onto the cable (I have the Vox pedal schematic, so I'll avoid the lead that's meant to power the incandescent fuzz-indicator bulb on the original Vox footswitch), or whether I should rewire the Carvin pedal LEDs from parallel to series, or (probably the safest option) whether I should remove the LEDs and just use the Carvin pedal without them? FYI, the Carvin pedal uses conventional on-off switches and a 5-wire cable.

[R.G.]

Thomas used switches to ground for all functions, but they used a completely separate indicator system to run the 24V incandescent bulbs they used for indicators. LEDs were not commonly available (if even invented!) at the time Thomas designed that. So they provided a resistor to 24-30Vdc for each light, and then made each switch with an indicator on the pedal be double pole, one pole for function, one for lights. I'll have to go look at the schemos, but I don't think the Buck had any indicators at all on its footswitch. Which leads to the next observation: all Thomas Vox footswitches were NOT alike, and were not even necessarily proper sub/super sets of one another. PM me for help if you like.