Power Supply Filtering Capacitor

[phillip]

When using a 100-ohm resistor in series with the power supply, does it matter which side of the resistor that the PS filtering cap goes on?  I'm making a new PCB and I can only manage to squeeze it in on the +9V side of the resistor, not on the other side.

TIA!
Phillip

[niftydog]

I can't imagine it would make much difference.. but I'd be trying to put it on the supply side of the resistor, rather than the load side.

[sir_modulus]

Well.....I'm no genius (far from it). Is the resistor current limiting? If it is then it doesn't matter since your using such a small voltage and if you have less than like 5V output ripple, then it dosne't matter which side (if you have 5V output ripple, you need to get a new power supply) as the defference is that the Vin is filtered for the resistor and onwards, or the resistor has to deal with the ripple(hence the ripple limit) , then the cap filters for rest of circuit. I could be completely wrong if this is like a envelope filter or something that is dependant on component order. So all in all: If it's for power, yes. For audio (or sig processing) than no.

P.S. Filter Caps are usually put across PS leads so you have it right.

[phillip]

if this is like a envelope filter or something that is dependant on component order. So all in all: If it's for power, yes. For audio (or sig processing) than no.

The resistor and capacitor are there to help prevent any hum or noise should an "iffy" power supply he hooked up to the circuit.

I was thinking (and hoping) that it didn't matter which side of the series resistor went on, since I can only squeeze it onto the PCB on the V+ side...not the load side.

Phillip

[Paul Perry (Frostwave)]

If it is the ONLY cap, then put it on the power supply side of the 100 ohm resistor, then throw away the resistor.
If you are driving the unit with a wallwart (which has a large cap inside it already) then put it AFTER the resistor, because the idea is that the resistor and cap form a voltage divider, from amn AC perspectrive.

BTW the argument about whether to put a cap across a wallwart, where some say this is pointless as the wart typically has 1000mfd inside, neglests the possibility that the internal cap may be dried up. Particularly as some warts run hot.

[phillip]

Thanks, Paul.  I'm probably going to end up dumping the 100-ohm resistor like you said and just jumpering it...I've never used a resistor in series with the PS before, and I've never had any problems since I always use either the Danelectro DA-1 "zero hum" or a Boss PSA-120, which run absolutely silently with all the pedals that I've used them with...even high gain pedals like the ToneBender MKII.

I always try to put a 47uF capacitor and 1N4001 diode in parallel with the PS...just in case.

Thanks again for help, guys

Phillip

[brett]

Hi.
Paul Perry is absolutely right.
There's also a hitch here - 100ohms is going to drop the voltage available to pedals if there's a significant load (e.g. a digital delay pedal).  It would usually be better to use a smaller resistor and a bigger capacitor.  I recently made a unit for a friend that uses a 3.3 ohm resistor, followed by a 2200uF cap, then ANOTHER 3.3 ohms and 2200uF cap.  That'll make for an ULTRA silent power supply that has minimal effect on the available voltage.  Those caps sound huge, but 16V units are quite small.  It fitted on a 3/4" by 1 1/2" PCB and into a 2 x 2 " box.
cheers