Switching roadblock during ODS build

[Fischerman]

I'm finishing my ODS this weekend (hopefully) and decided to test out the power supplies before I finished wiring up the preamp. A lot of this switching wiring will be buried under bundles of shielded preamp cables so I wanted to test it first (and boy am I glad I did!). But I have some concerns.

I'm bridge rectifying the 5vac taps (~6.5vdc at first 1000uF cap) and using an adjustable Vreg set to the max setting (it was all Radio Shack had) but in any case, I think the power supply is working OK...I'm getting about +5.5vdc as my final filtered relay supply with both relays OFF and only a red LED (with a 10k series resistor) going.

I'm only using two relays, PAB and Channel Switch (CS) and they are NTE R40-11D2-5v/6v relays and the coil resistance is ~167ohm (per the data sheet). The data sheet lists the pick-up voltage as 70% the rating so I assume that's 70% of 5vdc or 3.5vdc.

Checking the PAB relay it seems fine and works fine. When OFF, the voltage to ground from both sides of the coil is 5.5vdc. When switched ON, the supply side drops to 4.55vdc and the ground side drops to 0vdc.

But the CS relay has a Green LED in series with the ground connection (CS Relay ON = Clean mode). So when switched ON, the supply side of the CS coil drops to 4.64vdc but the ground side of the relay only drops to 2.1vdc (not 0vdc...because of the LED). So there is only ~2.5vdc across the coil. But it works...the relay switches. Is is only 2.5vdc across this coil a potential problem?

Some other info: I started with a 150ohm resistor in series with the Green LED...but then the Red LED wouldn't completely turn OFF when I switched channels (it still doesn't, but with that 150ohm series resistor the Red LED was 'noticeably ON', now it's just very, very dim). The Red LED originally had a 1k series resistor but now has a 10k (in an effort to get it even less illuminated when on Clean channel).

This adjustable Vreg (LM317T) uses a 240ohm resistor and a 5k pot but I chose them to get close to max Vout (220ohm and 4.7k). I have since obtained some NTE962 6v Vregs...but my rectified DC is only ~6.5vdc so I'm not sure if changing to this 6vdc Vreg will actually get me that extra 0.5vdc.

Any help appreciated. My intention was to have a Green LED and Red LED on the front panel for the two channels...and it all appears to be working...I'm just concerned that there isn't enough voltage across the CS relay coil. If needed I can VISIO up a schematic...I still live in the dark ages and do my stuff with a pencil and Engineering paper...and I don't have a scanner.

[Bob-I]

I really can't follow the schematic. A drawing would really help out, even a scan of your hand drawn schem would be fine.

[tele_player]

Without commenting on the rest of your circuit, I'd say you're cutting it close running that off the 5vac tap. A voltage doubler would be a good idea.

Voltage regulators need some extra voltage to ensure regulation, and if you get a 10% brown-out on your AC line, everything is going to drop, you'll definitely be under the dropout voltage for the regulator, and your relays might stop working.

This is not a very good design.

[Fischerman]

Thanks for the responses. I agree that it's not a good design...I don't really know what I'm doing (obv) so if it was it would have been a fluke. I've never used a voltage doubler but I will look into it.

I think I got it working 'nominally' though. I moved the connection for the Green LED positive leg to the supply side of the CS relay...essentially putting the Green LED in parallel with the CS coil instead of in series with it...I also added a 180ohm resistor in series with the Green LED (since it's no longer in series with the 167ohm coil). I also swapped out the Red LED for a 2.6v Blue LED (the Green is a 2.1v LED and the Red was 1.8v).

Now, with both relays ON both supply sides are at 4.4vdc and both ground sides are at 0vdc (with only one relay ON the supply is @ 4.5vdc). The LEDs work and the Blue LED shuts completely OFF when I'm on the Clean channel (the LEDs flip-flop perfectly now). The power supply never drops below 4.4vdc...it's at 5.5vdc with no relay on and only the Blue LED illuminated. The Blue LED has a 15k resistor in series with it (it's clear plastic but illuminates Blue). Once I get a chance I'll make a schemo and post it.

Thanks again.

EDIT: Also FWIW, I found out that I was right AT the pick-up voltage for this relay. If I turned the PAB relay on first...I couldn't get the CS relay to pick-up...even if I switched the PAB off it still wouldn't pick-up but maybe 50% of the time. But if I turned the PAB relay off and then switched the CS relay on it would pick-up...and then I could turn the PAB relay on and the CS relay would stay pulled-in. So apparently 2.5vdc is right at where this relay will pick-up and I'm at 4.4vdc now so I feel comfortable.

[tele_player]

If the supply voltage is changing when the load changes, the regulator isn't working, probably because you're below the dropout voltage. You could just leave it out.

Search the forums and you should find the schematic for powering 5v relays off the 5vac filament supply. This uses a doubler, and could be regulated with a 5v regulator to supply a solid 5v DC that won't vary with supply voltage or load current (within reason.)

[dimitris]

Check at Brown note's D'Lite forum the foot switch installation thread.
There is a detailed schematic with 3 relays getting power from 5 volt filaments.

[Fischerman]

Thanks Dimitris.

FWIW, I yanked out the adjustable Vreg and the two resistors and put in a NTE962 6v Vreg. The supply voltage with both relays off dropped 0.2vdc to about 5.3vdc but now the voltages with one/both relay on are 4.6/4.5vdc respectively.

I have a 5v Vreg around here somewhere but I can't find it at the moment. It all works fine but there is still that small drop.

[Fischerman]

I found the 5v Vreg...the voltage is still drops to 4.65vdc with both relays on. I made a quickie schemo but when I convert VISIO to jpeg it looks terrible but I think it's still readable.

The Blue LED is a little brighter than the Green when no footswitch is used (there is a pull-pot on the front panel that also switches channels...the panel switch is in parallel with the FS)...but when I plug in my footswitch the LED in the FS makes the Blue LED about perfect.

I think I'm gonna go with this and move on with the build...I'm so close...but my neck is killing me.

[img:678:300]http://i108.photobucket.com/albums/n4/m ... Relay2.jpg[/img]

[Fischerman]

Couple of pix of the build. The Send/Return jacks are out to facilitate soldering. Most of the audio wiring hasn't been done...that's pretty all that's left. There are only 3 boards total...the power supply, the audio, and the filter cap board in the doghouse. The relays are in the middle of the audio board in sockets. Bandmaster Reverb chassis, HRM pots are on front panel. FX loop Return Level on front panel.

[img:600:800]http://i108.photobucket.com/albums/n4/m ... layend.jpg[/img]
[img:600:800]http://i108.photobucket.com/albums/n4/m ... pboard.jpg[/img]

[heisthl]

Very nice - looks spaceous for a Bandmaster conversion - Good Job!

[Fischerman]

Thanks heisthl. Your Fender conversions have been a big help. You can't really see it but I did one of those long ground busses like you did in one of your builds (I stripped a piece of 18gage copper wire)...this is the longest ground buss I've ever used...it's huge.