100 ohm resistor in Power supply

[arma61]

Hi

I know most of us use this 100 ohm resistor in series to the +9V to eliminate noise coming from the PSU.
Now I have 6 pedals mounted on a pedalboard, none of them with this resistor and using a (cheap) wall wart I hear that noise (sound like a F#, or is my guitar out of tune ?),
I've tried, as a good sucker!, to put one resistor in series with the +9V just from the socket to the filter board (7809 + some caps), so you can imagine what happened as soon as I turn on the pedalboard, I could toast some bread on this resistor  , so my question is :

before I start to dismantle the whole board, and being all pedals's power supply wired in parallel, is it enough if I put just one resistor on the last pedal? this will be allways on, (it's a cab simulator)



Thx 4 help
Armando

[alanlan]

If they are all home brew pedals, then I would add the resistor (and cap btw) to those pedals which appear to need it i.e. in the pedal.  If they are commercially built pedals I would filter them on your psu board separately but again, only if they appear to benefit from it.  It's hard to generalise and say you need to use a 100ohm+100uF cap filter on all pedals.  The fact is some might need it and others may not benefit at all.

Not a straight forward answer I know but it's not always so clear cut.

I don't think there is any benefit to be had in just filtering the last pedal unless that pedal needs it, but it won't help the others on your pedal board.

[Zben3129]

Why don't you use a higher wattage resistor?

Figure out what the max current you can draw from the supply is, and determine the required wattage from that. Remember that V = I * R and W = V * A

Zach

[arma61]

thank you m8s for answers

alanlan they are all home made and all fixed to the pedalboard by straps    , so I'll leave as last chance to modify all the pedals , also because, in this case, I'll have these Rs and Cs floating around or at least on a small perfboard. I think I'll go with Zac suggestion but I need some help 

I thought that sooner or later I would have met Mr. Ohm, I know what he said but I never understood how to put in "practice"  with my DMM

So, taking into account that the PSU can supply about 500 mA at 12.89V, what is the R, is the total resistance of all the pedalboard, including the filter board ? how do I measure that  ?

thx
Armando

[Ronsonic]

I sez don't bother. I mean you could just put a 100R in there and then measure the voltage drop divide by 100 and know the amperage drawn but why. Lift off, nuke from orbit and put a resistor and cap and the DC in of each pedal.

[Zben3129]

.5A * 13v = 7.5 VA

7.5VA roughly = 7.5W.

I would go with a 10-25watt resistor or so to give yourself a lot of room for error. However...

.5A * 100ohm = 6v

So if all your pedals were drawing a combined 500ma current across that 1 resistor, you would see a voltage drop of 6v. Not good.

Is there a reason you need 12v? You could use a 9v 1A rectifier and get a good 9vDC supply, which should also be quiet.

Zach

[alanlan]

I doubt the power supply will actually be required to supply 0.5A anyway for just 6 pedals.  However, this is just one reason why you should filter them separately if they need it and preferably within the pedal itself.  Also, you should perhaps think about getting a better adaptor.  It seems to be a false economy to use a cheapo noisy wallwart when you can get much better products for not much money.

[arma61]

thx to all, good ideas and suggestions from everybody.

I sez don't bother. I mean you could just put a 100R in there and then measure the voltage drop divide by 100 and know the amperage drawn but why. Lift off, nuke from orbit and put a resistor and cap and the DC in of each pedal.

I think I'll go that at the end!

Is there a reason you need 12v? You could use a 9v 1A rectifier and get a good 9vDC supply, which should also be quiet.

Zach

  Also, you should perhaps think about getting a better adaptor.  It seems to be a false economy to use a cheapo noisy wallwart when you can get much better products for not much money.

it's because I rather like to use, where possible, what I have hanging around than buying new stuff !! and God only know how many wall warts we have spreaded in all drawers!!! 

I've also found now in the basement, on my son's old bike, a sealed battery 12 V 1.5 A, it looks weak but I'll try to recharge and see, with it I should get rid of the noise.

Again thanks for help m8s (specially for refreshing muy math )
Armando

[Gus]

As posted about the 100 ohm and a cap should be in each effect.

It helps in a few ways.  the 100 ohm and cap forms a RC filter.

It limits the max current to worse case, 100 ohm shorted to ground of the supply voltage divided by the 100ohm,  For example 9VDC / 100 ohm = 90ma

Lets take say a effect at 1ma draw the drop will be 100 Ohm X 1ma= .1VDC

20ma draw, 100 ohm x 20ma = 2VDC drop

It helps to decouple each effect from each other if in each effect box.

The 100 ohm should be "looked" at and maybe DIYers should rethink the value.  Maybe lower current draw effects should have a higher value resistor and a cap and the higher draw maybe a lower value resistor and bigger value cap and maybe something like a poly switch like you can find on other electronics circuits.

  Polyswitches are cool things.

http://en.wikipedia.org/wiki/Polyswitch

A poly switch and current limit R and RC filter and a reverse diode.

Did I just post about polyswitches first on a effect forum?.  How soon will we see them?

Gus  12-20-2008

[snap]

Did I just post about polyswitches first on a effect forum?.  How soon will we see them?

Gus  12-20-2008

don`t think so: http://www.diystompboxes.com/smfforum/index.php?topic=5361.msg31175#msg31175 (Z.Vex 2002)
http://www.diystompboxes.com/smfforum/index.php?topic=36871.msg266137#msg266137 (P.Perry 2005)

[Gus]

Thanks for the links I guess I missed the two threads.

Now I believe you can find polyswitches on PC mainboard at places like the keyboard power line.  Early home PCs had fuse and sometimes if the keyboard was plugged in under power the fuse would open

http://catalog.tycoelectronics.com/catalog/cinf/en/c/22878/0?PID=409478&RQS=C~1^M~BYPN^TCPN~444259-000^RQPN~AGR600

I did not find a low enought current trip for most effect shorted current.   
http://catalog.tycoelectronics.com/TE/bin/TE.Connect?S=23159&M=FEAT&P=100007&U=&BML=10576,17593&LG=1