9v drops to 6v when I plug in my pedal - is it faulty?

[jamierichards1234]

Hi,

Total beginner here so sorry if this is a stupid question..

I've built a pedal with 2 x deprofundis delay circuits (pt2399) in it and it seems to work, but when I plug it in my 9v supply drops to approximately 6v. Is this normal behaviour for a voltage to drop like this or is there likely to be an error somewhere in my build?

It doesn't seem to make any difference if I run it from the 'high current' output of my Voodoo lab pedal power or from a normal one.

I was getting ambitious and hoping to put another circuit in the same box but I guess this might not be possible!   


Thanks for the help,
Jamie

[vigilante397]

A wise man once told me "The only stupid question is one you already know the answer to." You don't know the answer, therefore not a stupid question

Usually a drop in voltage from your supply means that the circuit is trying to pull more current than your supply can provide. PT2399-powered delays will pull more current than say a fuzz face, but it still shouldn't be anywhere near enough to make the voltage sag on a wall-wart, let alone a Voodoo Lab pedal power. It would therefore be safe to assume something is wrong. A short to ground will pull a hell of a lot more current than a delay will, so that would be something to look for. Look for anything connecting to something it shouldn't be.

The strangest part of this situation is, of course, the fact that the circuit still works. Do you know how to measure current? I would be interested to know how much the pedal is trying to pull. It wouldn't point to a specific culprit, but if your 40-50mA pedal is trying to pull more than the 250mA available from your high-current output then there is cause for concern.

[GibsonGM]

^ ^  This.   In order to have that kind of sag, your 'device under test' must be pulling more than what the supply is rated for.   

Measure current draw, and perhaps, with all power removed and a jack plugged in to 'turn the pedal on', measure the resistance between the "+" and "-" at the PCB.

[ElectricDruid]

Also, if you've got two delays in one box, you could try separating them again and seeing if they drag down the voltage individually. It might well be the case that one of them is fine and it's the other one that has the short.

Edit: To answer the question in the subject line, as others have said...YES!

Tom

[jamierichards1234]

Thanks everyone for getting back to me.

Looks like I've been a real idiot here...

I decided to test the current draw of the pedal as you suggested, so I cleared all the mess of excess wires and circuits off my breadboard (which I've been using to power the pedal) and miraculously the voltage drop now seems to have disappeared. I thought everything was disconnected but there must have been something there still attached and causing the problem.

Sorry to waste your time!

I guess there's a lesson there to keep things tidy!

P.S. I decided to measure the current draw anyway and it was 130ma. Does that seem reasonable for 2x deprofundis delays?

[antonis]

sorry if this is a stupid question..

I've been a real idiot here...

It seems to me you need a therapist with good electronics skills..

BTW 130mA current draw is more than reasonable for your 2 X Oscar_Wilde delays..

[vigilante397]

P.S. I decided to measure the current draw anyway and it was 130ma. Does that seem reasonable for 2x deprofundis delays?

I agree with antonis, that is a bit more than I would expect (unless you have several giant LEDs on there). A single PT2399 should pull between 15 and 30mA, and a single TL072 should be between 2.8 and 5 mA. The 78L05 isn't 100% efficient so you can add 3 to 5mA quiescent current for that. So based on the datasheets (and without LEDs) I would expect between 42.6 and 80mA.

[ElectricDruid]

Ok, wild guess here - the two circuits have *both* got voltage dividers that make bias supplies for different bits of the circuit. Maybe they've even both got 5V regulators for the PT2399. All that stuff is in parallel, so its effective resistance drops...and drops..as you add more bits.

If you've got one 5V supply, you don't need another. Pull it out and use the other one. Similarly with bias networks.

</wild guess>

Tom

[vigilante397]

The bias network is a 10k/10k voltage divider, which means a single pedal would waste 0.9mA on the bias network. Put another one in parallel and you have an effective resistance of 5k from VCC to ground, which means 1.8mA.

I agree it would be better practice to combine the power supplies of the two, but I don't think running two power networks in parallel in one box would be any different than having them in separate boxes with a OneSpot daisy chain power supply. That being said, there is a fair amount of power filtering (100u on the input, 100u on the 5V rail, 47u on Vref), so a pair of them in parallel would have an inrush that would definitely make the line sag at startup, as I see no current-limiting on the input (at least on the tagboardeffects layout).

Jamie, that current measurement is after the pedal has been running for a bit, right? Can you walk us through how you measured the current?

[ElectricDruid]

Jamie, that current measurement is after the pedal has been running for a bit, right? Can you walk us through how you measured the current?

Yes, good question.

T.

[antonis]

The bias network is a 10k/10k voltage divider, which means a single pedal would waste 0.9mA on the bias network.

I like your maths, Nathan...
(generous for current but costly for power..)

My maths call for 450μA on the bias network , so let's shake hands on a mid-point of 675μA..

[Rob Strand]

Hmmm, If 6 was 9.

[vigilante397]

My maths call for 450μA on the bias network

Right, good call Serves me right, trying to do math at work

[jamierichards1234]

Thanks for all the replies. I'm only using the one power supply. I measured the current again today and it's now showing 88mA which seems more reasonable, but I'm a little confused that the reading has changed. As far as I can tell the only thing that's changed is my 9v power brick (someone borrowed my other one).

Ah well. It seems to be working now so that's all good.

P.S. I only used one 5v supply and one bias network. I wasn't sure at the time if it was the right move but I guess it was.

Cheers

Jamie

[vigilante397]

88mA is much more reasonable. I feel much better