Power relay from rectifier winding?

[daz]

Relays come in two flavors, AC and DC. You can usually run anay type on DC, but if you have a DC relay and power it with AC, it will buzz. Even if you rectify the 5vAC, you still need to filter it, at least a little, did you?

Re: pops and stuff. The main reason we get pops in guitar amps when relays switch is the unterminated switch contacts in the audio path, not the coil circuit.

Yes, as per Juan's advice. Plus using a 1000uf jacked it up to 8v as he suggested and i was ale to use a 12v relay because i didn't have anymore 5v ones. so it's working. thanks all.

I do have one more Q tho...if i add a few LEDs in line with the power as an indicator it works intermittantly. One will work every time but burn out. Any simple ideas to make that happen?

[J M Fahey]

LEDs stand 50mA TOPS and itīs advisable to run them at no more than 20 or 30mA for long life; your coil must be passing way more than that.
To boot, you waste 1.9V you just earned with the cap upgrade.
Wire the LED in parallel with the coil, with a series resistor to limit LED current to 20/30 mA

So if, say, you have 8VDC, R=(8V-1.9V)/0.02A= 305 ohms, use 270/330/470 ohms, all will work, brightness will vary slightly.

Power dissipated= 6V*0.02A = 0.12W or less so 1/4W is fine.

[loudthud]

If you are worried about the coil Voltage being too high, a resistor in series with the coil will limit the Voltage. Just measure the coil resistance and calculate the required series resistance.

[R.G.]

An even more present question is what are you switching with the relay, and is there another way to get that done?

Relays are workable, but crude devices.

[daz]

LEDs stand 50mA TOPS and itīs advisable to run them at no more than 20 or 30mA for long life; your coil must be passing way more than that.
To boot, you waste 1.9V you just earned with the cap upgrade.
Wire the LED in parallel with the coil, with a series resistor to limit LED current to 20/30 mA

So if, say, you have 8VDC, R=(8V-1.9V)/0.02A= 305 ohms, use 270/330/470 ohms, all will work, brightness will vary slightly.

Power dissipated= 6V*0.02A = 0.12W or less so 1/4W is fine.

Thanks, but I can't wire it that way with a 2 conductor cable and have the LED at a footswitch and I'd rather just pass than bother with that.

An even more present question is what are you switching with the relay, and is there another way to get that done?

Relays are workable, but crude devices.

it's adding a switchable load at the effects loop via a relay activated pot to get a slight volume boost when needed.

[Dave H]

I do have one more Q tho...if i add a few LEDs in line with the power as an indicator it works intermittantly. One will work every time but burn out. Any simple ideas to make that happen?

Put a resistor in parallel with the LED to reduce the LED current to say 10mA. eg. If the relay takes 50mA you need 40mA to go through the resistor so the resistor value is R = 1.9/0.04 = 47ohms. As the LED drops 2V from the 8V supply it would be better to use a 5V or 6V relay. To accurately work out the resistor value we'd have to know the coil resistance.

[Dave H]

An even more present question is what are you switching with the relay, and is there another way to get that done?

Daz, where is the pot (rheostat)? If you had it in the footswitch you could just wire the switch across it without using a relay. To get a switchable boost I switch the first stage cathode bypass cap directly with a footswitch (no relay). For indication there's a low current LED powerd by a 9V battery on the second pole of the switch. It lasts for years.

[SoulFetish]

LEDs stand 50mA TOPS and itīs advisable to run them at no more than 20 or 30mA for long life; your coil must be passing way more than that.
To boot, you waste 1.9V you just earned with the cap upgrade.
Wire the LED in parallel with the coil, with a series resistor to limit LED current to 20/30 mA

So if, say, you have 8VDC, R=(8V-1.9V)/0.02A= 305 ohms, use 270/330/470 ohms, all will work, brightness will vary slightly.

Power dissipated= 6V*0.02A = 0.12W or less so 1/4W is fine.

The fairchild MV50154 (good 'ol fashioned red LED) can take a continuous forward current of 100mA (max continuous) and has as linear a forward voltage drop as I've seen. I used these in a build to switch between cathode biased EL84s and simulated "fixed" bias using these LEDs and even when driven into class B at around 130mA, they didn't break a sweat. But, this is the only one I've ever found which didn't have the limitations you are describing above.

Edit: looks like fairchild no longer makes them but Everlight does -
http://www.mouser.com/ds/2/143/ds300031-40261.pdf

[daz]

Daz, where is the pot (rheostat)? If you had it in the footswitch you could just wire the switch across it without using a relay. To get a switchable boost I switch the first stage cathode bypass cap directly with a footswitch (no relay). For indication there's a low current LED powerd by a 9V battery on the second pole of the switch. It lasts for years.

I realize that but the whole reason i'm doing it this way is i don't want to run the signal the necessary 10-15 feet to the pedalboard and another 10-15 back. I only use 2 1' cables in it for a DSP that sits atop the amp. Anyways i've decided this is all becoming a pain and i fear added noise so i think i'm going to pull it all and go back to the way it was then build a small box with the relay and put and a power jack and it;s sit atop the DSP and be powered by a wall wart. I could get it to work with the right value components,as i have done this many times but it;s always been external to the amp, usually in a small rack i used to use. But i don't use that now so a small box will do. Send>DSP>switch box>return.

[daz]

The fairchild MV50154 (good 'ol fashioned red LED) can take a continuous forward current of 100mA (max continuous) and has as linear a forward voltage drop as I've seen. I used these in a build to switch between cathode biased EL84s and simulated "fixed" bias using these LEDs and even when driven into class B at around 130mA, they didn't break a sweat. But, this is the only one I've ever found which didn't have the limitations you are describing above.

Edit: looks like fairchild no longer makes them but Everlight does -
http://www.mouser.com/ds/2/143/ds300031-40261.pdf

I remember i used to be able to get a similar one from radio shack years ago. It only drew something like 2 mA and could handle a 9 or even 12v power supply as i recall. I used to use em all the time.

[Chuck H]

Daz, where is the pot (rheostat)? If you had it in the footswitch you could just wire the switch across it without using a relay. To get a switchable boost I switch the first stage cathode bypass cap directly with a footswitch (no relay). For indication there's a low current LED powerd by a 9V battery on the second pole of the switch. It lasts for years.

I've done this with cathode circuits too. Works great But I don't think I'd do it with a signal lead. Ever notice the hum/buzz addition when you plug in the reverb footswitch into an old Fender? Then there's the typically high impedance of most signal lead circuits where adding a length of shielded cable would surely result in a significant capacitance bleeding highs out of the tone. But...

If daz's circuit is just a switched load, and the cathode for that stage is fully bypassed, it might be possible to use the higher, boosted load value hard wired in the amp and add a resistance in series with the cathode bypass cap for the gain reduction. Similar to what you outlined above. You might even be able to use the LED indicator as part of the cathode bias/bypass circuit and eliminate the need for a battery. I'm envisioning a working circuit in my head now for something like that.

[daz]

By the way, can you determine what amount of current a relay will require by the voltage and coil ohm specs?

[daz]

Put a resistor in parallel with the LED to reduce the LED current to say 10mA. eg. If the relay takes 50mA you need 40mA to go through the resistor so the resistor value is R = 1.9/0.04 = 47ohms. As the LED drops 2V from the 8V supply it would be better to use a 5V or 6V relay. To accurately work out the resistor value we'd have to know the coil resistance.

I tried that but the smallest value R that still allows the relay to trigger won't allow the LED to light. the problem is the voltage of the supply is barely enough to trigger the relay with nothing else added, so it's shaky at best. I need to get the proper value relay. But in any case as i said above i think i'm going to $shit-can this anyways and build it into a external box with a wall wart.

[Chuck H]

By the way, can you determine what amount of current a relay will require by the voltage and coil ohm specs?

Pretty much. Just use Ohm's Law. The voltage across the coil is known. Divide that by the coils resistance to approximate current. There is the additional property of impedance @ 120Hz, probably insignificant? Anyway, the relays working current should be in the product data.

[daz]

Pretty much. Just use Ohm's Law. The voltage across the coil is known. Divide that by the coils resistance to approximate current. There is the additional property of impedance @ 120Hz, probably insignificant? Anyway, the relays working current should be in the product data.

Ok, as i have said in the past when it comes to math i'm even dumber than i am when it comes to electronic theory. If i have a 12v relay and the coil is 150 ohms am i correct that it would pull 12.5 mA? Bt the way, most relays i see show the contact rating but not the coils current draw. I suppose if you can find the manufacturer's PDF it would but i buy most components at electronics surplus places and they often have little info on the relay.