TUTORIAL Circuit Design - Power Supplies

[MattG]

Personally, I'm a fan of power sections that use MOSFETs for reverse polarity protection as opposed to a diode so you don't get a drop in voltage. I think that Spaceman Effects does it on almost all their pedals and it's what I generally use when I build my own circuits.

To be pedantic, there will be some current-dependent voltage drop due to RDSon. But as in temol's examples above, having a series resistor is useful for creating the low-pass RC filter with the bypass cap.

Another alternate solution for reverse-polarity protection is a diode between Vin and GND, with the annode on GND and cathode on Vin. There's no voltage drop, but in the reverse-polarity case, it protects the effect circuit but becomes a short to the power supply. That's another benefit of this MOSFET scheme - in the reverse polarity case, it's an open circuit, so protects the effect and won't kill the PSU.

At least at Digi-Key, that VP3203 is kinda pricey at $1.85 each... The VP2106 looks like the cheapest p-channel TO92 MOSFET (at DigiKey anyway). Any reason we couldn't get away with that instead? Slightly higher RDSon, and lower max continuous current, but I would think it would be acceptable for typical DIY pedals?

 

[MattG]

Just to hopefully add to the conversation, there are some pedals that use a simple capacitance multiplier for further filtering. I don't know if all Boss pedals do this, but many of the schematics I've looked at include one, e.g. OD-3. The Chauffeur Drive has an interesting power supply, which includes simple capacitance multipliers.

But I'm personally not in a big hurry to implement capacitance multipliers on my circuits, as it's not a "free lunch" kind of filter, see Rob Strand's comment on "Capacitance multiplier for filtering DC".

 

[leadfoot]

To be pedantic, there will be some current-dependent voltage drop due to RDSon. But as in temol's examples above, having a series resistor is useful for creating the low-pass RC filter with the bypass cap.

Another alternate solution for reverse-polarity protection is a diode between Vin and GND, with the annode on GND and cathode on Vin. There's no voltage drop, but in the reverse-polarity case, it protects the effect circuit but becomes a short to the power supply. That's another benefit of this MOSFET scheme - in the reverse polarity case, it's an open circuit, so protects the effect and won't kill the PSU.

At least at Digi-Key, that VP3203 is kinda pricey at $1.85 each... The VP2106 looks like the cheapest p-channel TO92 MOSFET (at DigiKey anyway). Any reason we couldn't get away with that instead? Slightly higher RDSon, and lower max continuous current, but I would think it would be acceptable for typical DIY pedals?

Yeah, there is some drop.

This article from RG Keen http://geofex.com/Article_Folders/mosswitch/mosswitch.htm is where I'd first seen it mentioned (using.a MOSFET instead of a diode). I don't see why you couldn't use a vp2106 or even something like a irf9540npbf I'd you're going to push a ton of current.

 

[YourGuitarist]

I've noticed that lower supply divider resistors shift my filter points down. I guess since there's a frequency at which the capacitors has enough impedance to not short to ground anymore. This changes a bunch of filters that operate on low frequencies.