Electronic – Diode between VCC and GND

diodespower supply

I was having a look at some guitar effect circuits and came across with a company that offers PCBs for some well known circuits. I have noticed that most of ther circuits have a 1N4001 (D1) placed between VCC and GND, without any kind of resistor in series.

This one, for example: circuit

At first I thought it was meant to avoid reverse biasing the transistors, but risking the power supply by placing it in parallel doesn't make much sense to me.

I think it would be better to place it in series to the source or, if needing to keep that parallel setup, at least put a resistor in series to the diode to avoid a direct short circuit.

I just would like to know if the circuit is wrong or I am missing something.

Thank you very much.

Best Answer

I nearly always put such a 'fools diode' in my circuit, even when I am breadboarding. Reverse power will kill most of the chips I use. Over-current protection is a function for the power supply, not for the powered circuit. This makes more sense:

  • most power supplies are current limited in the first place, think of basic batteries, and 7805-like regulated power supplies (but NOT NiMh accu packs!)

  • Having the protection in the attached circuit will not protect against an error in the circuit, or the wring to the circuit, so it must still be complement by a protection in the supply

  • I build much more powered circuits than I build power supplies, so it makes more economic sense to put the current limiting in the supply.

When I use a battery (NiMh etc) that does not inherently limit its current I add a fuse, polyfuse or the like (often the battery supplier has already done this). A series resistor is often not practical because it makes the voltage the circuit sees dependent on the current it consumes.

You circuit is fed by 9V, most likely it is intended to be fed by a 9V battery, which is current limited by itself (unless maybe when it is an NiMh 9V pack? or are those protected in any way?) If you are really worried I suggest you either

  • put a fuse or polyfuse between the battery and this circuit, or

  • as your circuit probably draws very little current and probably can live with a small voltage drop, put an schottky diode in series (an 1N5819 will drop ~ 0.4V at 100mA). This circuit was probably designed long before schottky diodes became common place.