Series diode + resistor at the power source question

[midwayfair]

I've been doing perf layouts of some circuits for practice, and I've run across a few pedals with a power section like this:



Where there's a series diode followed by a small resistor. I think I understand what's going on here, but I'm wondering how much electronic benefit there is. The diode is clearly polarity protection, and I think the small resistor is there as isolation. Is there a benefit to using the diode this way over the more common 1n4001 to ground? It drops the voltage this way, as does the series resistor, but I don't see any specific considerations given to changing the bias (this is an op amp design after all).

[garcho]

The 100R forms a low LP filter with the caps, right? For filtering noise from the PS? Forward voltage drop from the diode is about 1V, I believe.

[ashcat_lt]

The benefit is that it actually protects against reverse polarity for good and always because the diodes just wont pass anything going the wrong way.  The other (more common) way the diodes will short out the reverse polarity supply for a second or two until it heats up and explodes.  Then, unless it just happens to take out one of the power supply traces on the Pcb, there wont be anything left to protect the rest of the circuit.  Use the one you've posted.

The resistor does help to set the cutoff of the filter with the caps, it also tends to limit the current demand, protecting the power supply somewhat from shorts downstream as well as "in-rush" demands when the thing is first powered up.

[slacker]

A reverse biased diode to ground is only good for polarity protection for short periods of time. It's good enough to protect the circuit if you accidentally try and stick the battery in backwards, or briefly connect a supply of the wrong polarity. If you connect a power supply of the wrong polarity for any length of time you risk burning out the diode, the diode will eat as much current as the power supply can feed it, and if this is more current than it can handle it will die. When it does it is likely to become an open circuit and then your polarity protection is gone and whatever bits of the circuit you were trying to protect will be damaged. Or you risk damaging the power supply as there's nothing to limit the current the diode can try and pull from it. I believe R.G refers to this as a battle to the death.

The series diode arrangement will protect the circuit all day long, when the power is the correct polarity the current through the diode is limiting to what ever the circuit wants. When polarity is reversed the diode is reversed biased to virtually no current can flow. The downside is the voltage drop, for some circuits this makes no odds, but for others with only 9 volts to play with it's not something you want.

EDIT: posted the same time as ashcat, but what the hell.

[Earthscum]

I remember a mantion that the resistor in series limits the current the diode sees in an AC situation so it doesn't burn out, as well as the aforementioned filtering. This is exactly how Boss did theirs when they used the AC supplies, if I'm not mistaken.

[midwayfair]

Thanks for the explanations, folks.

[amptramp]

Many DC wall wart supplies have a fullwave rectifier followed by an internal capacitor.  In some cases, the internal capacitor has a small resistor (about 1 ohm) in series with the capacitor but not the supply output leads in order to restrict inrush current in the diodes, so the DC output power will have quite a bit of ripple on it.  If you have a capacitor at the input to the stompbox with no series resistance, it would do most of the ripple reduction and carry most of the rectifier pulse currents down the power line.  The series resistor limits the pulse current which could radiate everywhere and cause 120 Hz interference (for a 60 Hz supply).  Pulse current would divide in the inverse ratio of the capacitors if there was no resistance and an inverse ratio of ESR (equivalent series resistance), which usually is the case.  You really don't want pulsed power around high-impedance signals.

The series resistor also limits the inrush current of the stompbox to protect the power supply.  It does some filtering in the sudio range, but unless you have a really large capacitor, 1000 µF or larger, it only tends to block high-frequency chatter in both directions on the power lead.