Phantom Power Toggle Switch Issues

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CurtZHP

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Been having a particular problem pop up on two (maybe three) separate preamp builds, and you know what the man said....

"Once is happenstance, twice is coincidence, three times is enemy action!"

Basically, the issue is the failure of the phantom power switch.  It will work fine for a little while, then eventually just quit, leaving me with phantom all the time or not at all.

The first time was when I was working on my last tube preamp project.  I had TWO switches fail.  I didn't think too much of it because those two switches were dug from a box of junk parts, so who knows what shape they were in?  So far so good on their replacement.  The second instance involved a solid state build.  Soon after using it a few times, the brand new switch failed.  I have a possible third instance in that a two channel version of the solid state unit hasn't lost either phantom switch yet, but when I switch them on, I swear they make a slight popping sound instead of the usual click.  I imagine what I'm hearing is arcing going on inside the switch.

So, naturally, I'm thinking that these toggle switches just aren't capable of switching 48VDC.  I've read in a couple places that the typical off-the-shelf miniature toggle isn't designed to handle that, but can switch AC all day long.  I've also read in a couple places that that's essentially bollocks.  I'm inclined to believe that, since I have another preamp that I built from a kit every bit of twenty-five years ago, that uses the same type of switches for phantom power, but has never failed.

I've attached a schematic of the power supply I'm using for the solid state preamp.  Could it be that the phantom stage is pushing too much current?  Would simply adding a current limiting resistor to the 48V output solve the issue?  I thought about putting the phantom switch on the AC side of the phantom supply circuit.  That would likely work for a single channel unit, but not for a two channel unit.

 

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Bo Deadly

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Post the schem w/ switch and any parts connected to it including the two 6k8 feeding the signal lines.

Usually there's a 100R / 220u -ish RC in there. If you just have C, that's definitely the problem. You need the R to limit current. Even with 100R, that's 0.48A for a moment. Or if you have R but C is too big, that's also bad because it won't be for a moment, it will be longer. Even 220u is a little large. 100u would be much more polite. If you don't have R at all, that's 10's of amps. For an instant. But that's long enough to completely torch that switch.
 

CurtZHP

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Just to make it easier to see, I lopped off everything but the input stage.

I've got a 100uF in place already, but am lacking the 100R you suggested.  (The 22K is just there to keep the LED from going "poof!")

I do have a rather beefy (2W, I think) 220Ohm resistor on hand.  Would that be large enough to offer protection?
 

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CurtZHP

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Incidentally, I dug up the manual for the kit build I mentioned earlier.  Looks like they have a 100R resistor in series with the 48V feed, with a 47uF cap to ground right after it.  I guess that explains why those switches still work after 25 years.
 

Bo Deadly

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Yup. Without it you're instantaneously charging that cap to 48V. It could literally be putting 10-20 AMPS through it. So that popping noise was probably indeed arching. The plating on the contacts is getting burned off. You should definitely replace any switch that was ever stressed from that condition.

And 220 is a little high. Realize that the resistor is going to cause a voltage drop. It's only a volt or two but still. No reason not to just use 100R / 100u.

Incidentally, if that input transformer is center tapped, you can also deliver phantom through the center tap through a 3.4K resistor and get better CMRR.
 

CurtZHP

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squarewave said:
Yup. Without it you're instantaneously charging that cap to 48V. It could literally be putting 10-20 AMPS through it. So that popping noise was probably indeed arching. The plating on the contacts is getting burned off. You should definitely replace any switch that was ever stressed from that condition.

That's what I was afraid of.  Good thing I purchased replacements on my last parts order.  Wish I'd thought of the extra resistors too.

And 220 is a little high. Realize that the resistor is going to cause a voltage drop. It's only a volt or two but still. No reason not to just use 100R / 100u.

That was my other concern, knowing there'd be a drop, wondering how high I could go without eating into the 48V supply.  Granted, the true output of this supply is likely somewhere around 50-52V unloaded, so I might get away with it.  I'll try it an measure.

Thanks again for the help!
 

JohnRoberts

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Phantom supply is not very precision but indeed trying to instantaneously charge 100uF to 50V could easily draw several amps. It will depend somewhat on the output impedance of your 48V supply.

The drop from 100 ohms for one mic should not be problematic.

JR
 

Bo Deadly

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JohnRoberts said:
Phantom supply is not very precision but indeed trying to instantaneously charge 100uF to 50V could easily draw several amps. It will depend somewhat on the output impedance of your 48V supply.
But isn't it just draining one filter cap into another? Although I can imagine the effects of power lost due to arching before the switching contacts actually close might be more of a factor. After that, ESR.
 

JohnRoberts

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squarewave said:
But isn't it just draining one filter cap into another?
OP posted regulated supply with 220uF capacitor on output .


Although I can imagine the effects of power lost due to arching before the switching contacts actually close might be more of a factor.
I am not aware of  mechanism where arcing dissipates significant energy, perhaps a tiny amount to ionize gases nearby.
After that, ESR.
Ding ding ding we have a winner.... The ESR of both capacitors in series, plus any wiring in series, PCB traces, even switch contact resistance. In theory the current spike could be infinite, in reality not.

Looking at the OP's schematic he shows a regulator with a 220uF output capacitor. Instantaneously this will form a capacitor divider driving the smaller cap to roughly 15-20V before the 48V regulator starts charging it higher (ASSuming it has current limiting) .

A 100 ohm R in series could eliminate much drama.

JR
 

mjrippe

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Excellent diagnosis and discussion folks!  I have not had problems in the past but will incorporate a current limiting resistor in the future.  Thank you.

Mike
 

Bo Deadly

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Also note that you also have the same problem when switching off. Meaning the cap is discharging instantly through the switch to ground. So when you add the 100R, will need to put it on the common of the switch so that the cap is discharged through it to ground.
 

CurtZHP

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squarewave said:
Also note that you also have the same problem when switching off. Meaning the cap is discharging instantly through the switch to ground. So when you add the 100R, will need to put it on the common of the switch so that the cap is discharged through it to ground.


Should I also have a resistor between the switch and ground?
 

CurtZHP

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PRR said:

That's where I was getting ready to put it, but one more question (sort of...).
I was scratching around just connecting the 100R to the common lug on the switch, which would be easy, but I'm concerned about whether its orientation relative to the LED resistor matters.  Placing it exactly where you have it would require hacking an existing PCB.  Certainly doable, but absolutely necessary?
 

emrr

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CurtZHP said:
That's where I was getting ready to put it, but one more question (sort of...).
I was scratching around just connecting the 100R to the common lug on the switch, which would be easy, but I'm concerned about whether its orientation relative to the LED resistor matters.  Placing it exactly where you have it would require hacking an existing PCB.  Certainly doable, but absolutely necessary?

No that was the easy place to put it on the drawing.  Think about the ratio (100 and 3K4), and the resulting voltage drop into a short (max current).  Run the math on it. 
 

CurtZHP

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EmRR said:
No that was the easy place to put it on the drawing.  Think about the ratio (100 and 3K4), and the resulting voltage drop into a short (max current).  Run the math on it.

So it sounds like I could just put the 100R directly on the common lug of the toggle switch and be done with it.
 

CurtZHP

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I'm also debating replacing the 100uF with a 47uF.  I imagine that wouldn't store as much charge, so would reduce the current rush either way.
 
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