Colorless writings, part 6 – The Diode Breakdown

What on earth are you talking about this time?

Doesn’t the title say it all? Ok, ok. I admit it’s somewhat ambiguous. I’ll start by talking about the diodes in general and you just try to hang on. Is that ok? Good.
Most of you know the basic use of diodes in stompboxes. First one everyone has encountered is definitely the bright red LED that lit up when your first guitar effect pedal was first engaged. The LED is a diode as much as all the other diodes are diodes. As the name Di-ode suggests, there are two terminals to every diode. If there were more, it would not be a diode anymore. It would be a triode or pentode and so on. At simplest, the diode will pass the current and/or voltage from one terminal to the other, but not the other way around. Most of you have played with an overdrive or distortion type of pedal? I know you have. The common way to create distortion is to use back to back diodes to make the signal clip, as in distort. Other very common use for diodes in guitar effect pedals is to use them for polarity protection. This use has two different applications. In series and in parallel. The diodes are also commonly used in electronic switching as rectifiers for switching JFETs. And then, of course, there’s the humble indicator LED. These must be the most common uses for diodes. While there are some applications for zener diodes to lower the voltage and a few others, the ones listed above are the most common.

The diode is simple semiconductor that is based on the idea of semiconducting material passing the voltage and current only in one direction. When someone had the eureka moment of placing two in parallel and in opposite direction of each other, the world changed. Earlier we had to rely on transistors driving each other to get the dirt out of our electronic guitar addons. After that moment we started having lots of harmonic distortion with ease.

All of the diodes used for clipping have their own character to the clipping. The most usual 1N914/1N4148 (or any other similar Si diode) has the tight feel on the breakage. Other commonly used clipper diodes are 3mm LEDs, germanium diodes, other small signal diodes like 1N60P and schottkys. LEDs compress the signal more and offer more output level. Schottkys are quite similar in character, but slightly tighter and more bassy. Germaniums tend to offer softer sounds to the clipping, but they drop the level a lot more than any other type. Different germanium diodes also have slightly different responses, but they all share the level issue. 1N60Ps are close to 1N4148, but they sound more brittle and glassy.

Some of you weren’t probably aware that any transistor can also be used as a diode? Just omit, snip or maim the collector and use the base as an anode and emitter as a cathode. Same thing applies to FETs as well. The current will pass from base/gate to emitter/source, but not the other way around.

So that’s for the clipping configuration, how about other purposes?

From the common ones, the one that’s simply more important than indicator LED to me is the reverse polarity protection. To protect the circuit from wrong power supplies, engineers have used diodes to rectify the supply voltage. There are two common, yet completely different ways to achieve this. This protection is very good thing to have, as some amplifying semiconductors are very delicate devices. Meaning that they will burn up in seconds if they get + voltage to where – should be and vice versa.

First, these is the method of placing a diode anode to ground and cathode to supply. This method has a valid point, as it doesn’t touch the voltage at all. But as a huge downside, the wrong polarity causes the diode to be only thing between positive and negative lead of the supply. Meaning that all the possible current that the supply is capable of giving out is shorted through that diode, leading it to literally break down. If there happens to be a fresh 9V battery as a supply, all the juice is pushed through that diode in just a few seconds. In worst case, the diode practically explodes and takes a chunk of the board with it. In best case the silicon semiconductor just burns up leaving the diode to act as a sort-of resistor. Either way, the effect will be completely fried if it gets the wrong polarity as its power supply. Sure it can be fixed, but it’ll need at least wire cutters (preferably an solder iron and a new diode) to get it running again. Unless the board is done for. Sadly, i’ve seen those cases too.

The second method isn’t as popular in main stream big brand designs as it is for boutiquers and small brands. This is to take the diode in series with the supply. Yes. It’ll affect the supply voltage. In worst cases, when using a diode that isn’t meant for this application, the voltage drop could be a Volt. That is usually way too much of a drop, so using a proper diode is more than crucial. 1N5817 Schottky is one of the good ones. The drop stays low and as with any diode using this method – the board or the diode itself doesn’t get hurt if it gets the wrong polarity. Circuit simply won’t work at all. In other words, a diode in series with the supply will eliminate all current flow from the circuit without breaking anything up.

 What about LEDs and the other types then?

The indicator LEDs are diodes just like their other counterparts. The just happen to emit light when current passes through them. Thus the name, Light Emitting Diode.

By other types in this context, i mean zener diodes in particular as those are the ones we commonly find in pedals. In addition to small signal diodes, schottkys and LEDs. Zeners have one very nice feature. They can be used to lower the voltage. In simplest terms, placed in parallel with the supply (anode to ground, cathode to supply), the supply voltage will be reduced to the value of the zener diode. If you use 8.2V zener in that position, the excess voltage will get dissipated through the diode, leaving the supply at 8.2 volts.

I feel like i haven’t even started on this diode subject, but the usual 1000 words are already up. So, against my standard behaviour, i’m going to leave you with a wikipedia link that explains more than you ever need to know about diodes. In detail.

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