Power Supply Question

[Falken]

Hi Guys, Gals!

New to the forum.  Just wanted to ask a question and get some feedback.

Once upon a time I found an old Netgear wall wart, 12v 800ma, and thought to myself, "Hey, this maybe could power a pedal chain."

So I set about turning it into a 9v power supply using a L7809 regulator.  I had a tiny project box on hand, put 2 barrel jacks, an led, and 3pdt stomp switch in it, along with a small circuit containing a diode for polarity reversal protection, a resistor for the led, 2 caps, and the L7809 regulator.  The 12v wall wart output 15v into the box, and 8.95v comes out.

Now my question and my problem.  I have a DIY Orange Squeezer, couple Fuzz Faces, some Danelectro pedals that do LED clipping, all are analog and seem to run fine, source voltage maybe gets down to 8.6v with everything running.  Now...  I have a Ditto pedal by I think TC Electronics, and an Octave pedal from Mooer.  Both I believe are digital pedals.  I plug either one of these pedals into the chain and the voltage nose-dives.  Especially the Mooer, it drops to 5v.  I can run ONE of the digital pedals off the supply, but nothing else along with it.  According to Mooer the pedal draws 128ma, don't know about the Ditto. 

What could be going on?  I thought it would be a bit more robust, but is a 12v regulator PS going through another 9v regulator just killing the available amperage/headroom?

Am I going about this the wrong way?  I mean, best thing would be to build an isolated PS with multiple legs or transformers, rectify and regulate each one, but I haven't gotten that far yet.  I'm just an amateur pedal builder gaining knowledge.

Thanks in advance for any and all replies.

[R.G.]

Once upon a time I found an old Netgear wall wart, 12v 800ma...
The 12v wall wart output 15v into the box, and 8.95v comes out...
all are analog and seem to run fine, source voltage maybe gets down to 8.6v with everything running. 

When you say "source voltage" do you mean the output of the "12V" supply or the output of the 9V regulator?

Here's what happens with regulators. They need some voltage higher than the output to work right. For standard 7800 series regulators, this is usually taken as 2V. It's less for some other types of regulators, but for 7800s, 2V is a good rule of thumb. That means that your regulator needs at least 11V out of the "12V" supply to work properly. When the input voltage to a linear regulator gets too low, it tends to follow the input voltage down at the dropout voltage between the two. So if your input "12v" dropped to 10V, I would expect the 7809 output to drop to something like 8V more or less.

When you note that with your analog pedals the source voltage drops to 8.6V, if that's the "9V" output, it means your "12V" has dropped to less than about 11V, and the regulator is no longer regulating, it's just passing through 2V less than the input voltage. If it's the nominal "12V" supply that you're measuring, then its the same problem, only worse. The 12V supply is not keeping up with the current demands no matter what it says on the regulator.

Now...  I have a Ditto pedal by I think TC Electronics, and an Octave pedal from Mooer.  Both I believe are digital pedals.  I plug either one of these pedals into the chain and the voltage nose-dives.  Especially the Mooer, it drops to 5v.  I can run ONE of the digital pedals off the supply, but nothing else along with it.  According to Mooer the pedal draws 128ma, don't know about the Ditto. 

What could be going on?  I thought it would be a bit more robust, but is a 12v regulator PS going through another 9v regulator just killing the available amperage/headroom?

The issue is not two regulators. It is that the 12V supply is not keeping its voltage high enough under load. Go measure what your "12V" supply does as you add pedals to the "9V" output. I think it will show a significant sag.

Am I going about this the wrong way?  I mean, best thing would be to build an isolated PS with multiple legs or transformers, rectify and regulate each one, but I haven't gotten that far yet.  I'm just an amateur pedal builder gaining knowledge.

Well, maybe. I was once a firm believer in multiple isolated power supplies, but a lot of real world experience has shown me that the multiple isolated supplies is an over the top solution in many cases. There are many large complicated pedalboards that are powered by one single 9V supply and work just fine. In my opinion, that statement that "the best thing" is a bit of internet hype. There is a grain of truth, but it's not always, or even often, necessary.

First the disclaimer: I work for the company that makes the supply I'm about to describe. Factor that into your considerations. However, this is also where much of my real world experience comes from. The guy I work for convinced me to look at the real world results versus theoretical. He was right.

There is at least one 9V power adapter on the market that is quiet and powerful enough to run dozens of pedals at once. It can be bought for on the order of $20-30. If you just want to build your own, go for it. It's good experience. But if you would rather focus on building effects than power supplies, there's a good solution that's cheap and effective.

[Falken]

Wow, RG.  From Geofex?  Love it.  I built 3 fuzz faces after reading your Fuzz Face page.  My wife says thank you.

When I say "source voltage", I mean the voltage coming from the secondary, ie MY regulator.  I'll check the 12v side and see what the voltage sag is there.  I know that the voltage-in reads 15v without any load, and it is "rated" for 12v at 800ma.  I'm just surprised that this wall wart can't stand up to a 128ma pedal.  Or, is there anything innately different with digital effects?  Do they, on average, draw more amperage than an analog effect?  I assume this can vary wildly of course, and through experience digital effects are much more sensitive to voltages, (ie drop below 8v and the effect might not turn on)

Thanks for the reply RG.

TL;DR  "Get or build a better power supply." - RG

[karbomusic]

There are many large complicated pedalboards that are powered by one single 9V supply and work just fine.

I'd have to agree. Not trying to slight the 300.00 power supply builders but I have a lowly newer PSA120. It's the most stable and quietest PSU I have, silly huh? It runs almost my entire pedal board (10 neg ground pedals). All those only come to 140 mA so I know I'm within it's spec. It obviously can't run NPN/PNP together so OK, I have two (one wired pos ground)  so for 40.00 I run both types and huge pedal board without issue, hum or noise.

[Falken]

There are many large complicated pedalboards that are powered by one single 9V supply and work just fine.

I'd have to agree. Not trying to slight the 300.00 power supply builders but I have a lowly newer PSA120. It's the most stable and quietest PSU I have, silly huh? It runs almost my entire pedal board (10 neg ground pedals). All those only come to 140 mA so I know I'm within it's spec. It obviously can't run NPN/PNP together so OK, I have two (one wired pos ground)  so for 40.00 I run both types and huge pedal board without issue, hum or noise.

Wow 140ma.  That makes my Mooer Pure Octave @ 128ma a real power hog.

/edit TC Electronics Ditto Looper says min 9v 100ma or great PS.

[duck_arse]

the "L" series regs are only rated for 100mA output. you may be overloading it with the digitals.

[Falken]

the "L" series regs are only rated for 100mA output. you may be overloading it with the digitals.

I'm not sure that is correct.  I believe they are rated for 1.5A output, and 100mV load regulation.  Someone correct me if I'm wrong.

[samhay]

Is the regulator in a TO-92 or TO-220 package?

[Falken]

Is the regulator in a TO-92 or TO-220 package?

TO-220

http://www.mouser.com/ProductDetail/STMicroelectronics/L7809CV/?qs=sps7W/wBcGH358Eg/iFhSA==

[R.G.]

The TO220's are all 1+ amps.

Good point, DA! I saw "L7809" and thought it was one of the big ones, but if it was "78L09" it would indeed only be 100ma, and that could well have been the issue. And you did exactly the correct question, Samhay - big package or small one.

@Falken: we're back to measuring the DC on the 12V supply as you add loading in the form of pedals.

[Falken]

The TO220's are all 1+ amps.

Good point, DA! I saw "L7809" and thought it was one of the big ones, but if it was "78L09" it would indeed only be 100ma, and that could well have been the issue. And you did exactly the correct question, Samhay - big package or small one.

@Falken: we're back to measuring the DC on the 12V supply as you add loading in the form of pedals.

Alright, test results.  Things are weird.

PS:No load, 15.2v to 15.5v IN, 8.93v OUT
PS:3 Analog FX  14.2v IN, 8.93v OUT
PS:Digital Octave FX  8v to 12v IN, 5.8v to 6.3 OUT
PS:Digital Pedal + Analog Pedal FXs  14v IN,  7v OUT

So...  It works as planned with Analog Effects, and the power supply seems to operate as planned.  If I put it on a digital, all hell breaks loose.  BUT, and this seems weird, the "vitals" improve if I add an analog along WITH the digital pedal.  Any ideas?  Seems to be, 2 things.  Either something weird is going on with my wall wart OR my digital pedal is doing something very odd.

Thanks for all the replies.  You are all being very helpful and have my sincere appreciation.

[R.G.]

I'm wondering if your "12Vdc" power supply is supplying rectified but not filtered DC. What happens if you put a BFC (Big Freakin' Capacitor) across the input to the 9V regulator - like maybe 1000uF?

[Falken]

I'm wondering if your "12Vdc" power supply is supplying rectified but not filtered DC. What happens if you put a BFC (Big Freakin' Capacitor) across the input to the 9V regulator - like maybe 1000uF?

Thinking ripple?  I'll give it a shot.  I biggest I have is 470uF, I'll give that a shot, and if nothing I'll put 2 in parallel.  Thanks for the reply.

[R.G.]

Yeah, ripple. Some effects have a biggish cap right inside the power jack, and I wonder if the addition of pedals is adding on filter caps and causing the odd results. If it gets much better with even more capacitance, that would be a good indicator.

[Falken]

I think we have a winner RG.

I placed a BFC across the input.  With just the digital octave pedal, I'm getting a steady 8.9v, with both digital pedals and 3 analog pedals all running together I'm getting 14v IN and 8.6v OUT.  I think that about does it.  I'm getting a little feedback if cranked, but that might be due to having the box open and using alligator clips to the capacitor.

Thanks for all your help.

Consolation questions:  I have a diode inline with the positive from the 12v supply, as a polarity reversal protector.  1) Is that needed?  I was thinking that the L7809 has a lot of safeguards of its own? but if I'm adding a cap to the input, I might want to protect that now, as the original 12v supply was center positive, and the output is center negative.  2), if I remove the diode, and get my .7v drop back, do you think that would help bring the ending voltage up a bit from 8.6v?

Thanks again for your replies and your time.  Feel free to ignore these final questions if you're busy helping others.  I think I'm heading in the right direction now.

[R.G.]

I placed a BFC across the input.  With just the digital octave pedal, I'm getting a steady 8.9v, with both digital pedals and 3 analog pedals all running together I'm getting 14v IN and 8.6v OUT.  I think that about does it.  I'm getting a little feedback if cranked, but that might be due to having the box open and using alligator clips to the resistor.

Good! Moving in the right direction. But if you're seeing 14V in and 8.6 out, that means your BFC isn't B enough. Double, triple, or quadruple it. Shoot, use 10X.  Your meter is kind of averaging the ripple-y voltage to get to 14V, but the 7809 output stays at 8.9x until its input drops under 11V - which the 8.6V reading says it's doing.

Consolation questions:  I have a diode inline with the positive from the 12v supply, as a polarity reversal protector.  1) Is that needed?

Not critical, but now hurting you from what I've seen so far.

I was thinking that the L7809 has a lot of safeguards of its own? but if I'm adding a cap to the input, I might want to protect that now, as the original 12v supply was center positive, and the output is center negative

That's not a bad idea, but the 7809 has a failure mode of its own. You need a diode with cathode to the 12V input and anode to the 9V output. This is normally reverse biased, but if you were to short the input, the capacitors in the pedals would feed backwards through the 7809, killing it. The 7809 is well protected for overloads and thermal problems, but not this one.   

2), if I remove the diode, and get my .7v drop back, do you think that would help bring the ending voltage up a bit from 8.6v?

A little, maybe, but that's not the problem. User a B-er BFC.

[Falken]

B-er, and F-er.  Got it.

[Transmogrifox]

The next obvious question coming to mind is what rectifier diodes come in that Netgear WW.  Is there a chance the peak inrush currents charging your BFC are causing a big thermal transient in the rectifier, causing diodes to stress and eventually fail after a small number of power cycles?

My "back of the envelope" calculations are this:
15V open circuit
12V at 850 mA
Rout = (15-12)/0.85 = 3.5 ohms
Ishort ~= Vopen/Rout = 15/3.5 = 4.25 Amps

The B-er and F-er the Cap, the longer the rectifier diodes have to supply inrush currents  up to ~4 amps until the cap is charged.  Each time you plug it in, these get hot, and if the hot they get is too hot it will eventually kill the diodes.

Here is a little hint to whether it would get too hot:
A typical rectifier diode in one of these will be something like the 1NX007 series.   The graph on the datasheet for Maximum Non-Repetitive Forward Surge Current shows something that looks like about 8 Amps for 100 cycles.  If this diode is a good representation of what your Netgear Wart is using, then you're probably safe to leave this thing short circuited for a long period of time, let alone charging a BFC.

I was about to suggest an inrush limiter circuit, but it looks like the wall wart transformer already does that for you

I almost didn't post because this doesn't add anything practical, but on the other hand it serves to alert any readers of this post later on to consider things like repetitive surge currents on diodes since it might be an issue in a different application.  For example a different wall wart of, say, 12V @ 2A supply might give reason to pause a moment and consider surge current and cap bank sizing. 

[R.G.]

The next obvious question coming to mind is what rectifier diodes come in that Netgear WW.  Is there a chance the peak inrush currents charging your BFC are causing a big thermal transient in the rectifier, causing diodes to stress and eventually fail after a small number of power cycles?

My "back of the envelope" calculations are this:
15V open circuit
12V at 850 mA
Rout = (15-12)/0.85 = 3.5 ohms
Ishort ~= Vopen/Rout = 15/3.5 = 4.25 Amps

The B-er and F-er the Cap, the longer the rectifier diodes have to supply inrush currents  up to ~4 amps until the cap is charged.  Each time you plug it in, these get hot, and if the hot they get is too hot it will eventually kill the diodes.

That's true.

However as a practical matter, the 4000 series diodes are good for up to 1A DC average going out of any filter cap you're likely to put on them.

In rectifiying line frequencies, you get pulses at the peak of the AC line sine wave and the peaks of those waveforms are what cause the rms current that heats the diodes. The problem is that this peak current is the very devil to calculate from anything that can be measured. The limitations are full of imponderables like the ESR of the caps, the forward resistance of the diode leads, the resistance of the switch and so on in addition to the resistances (and leakage inductances!) of the mains transformer.

It's a good thing to know about and at least consider, but in general the 4000 series is good up to 1A DC out with nearly any cap someone would really buy to put in a supply.

[Falken]

I was getting 8.6v with everything plugged in.  I added another 470uF cap in parallel and the input and output voltage stayed the same.  With 2 large caps, both 50v, my box is out of room.  I removed the diode, got the voltage drop back, and things improved a bit.  Now at 8.86v with everything on.

Was thinking for my next power supply, to get a transformer with dual 12v secondaries, rectifying, capping, regulating, filtering -  One leg for negative ground pedals, and the other for my positive ground fuzzes.  OR 2 trannies, either way.

Thanks everyone.  Help appreciated.