Here’s another post in the “basics” series. This one is about reverse polarity protection.
Guitar pedals have a peculiarity: they are powered by a center negative supply . I only encountered this with guitar pedals, vast majority of other power adapters have center positive supply (it’s just how the cable is wired, I’m sure there are adapters that reverse polarity). This is, I’m sure, so that battery can be wired in a way to preserve it when the power cable is plugged in.
Anyway, you can see a potential for disaster. Most of the pedals are rugged enough (especially the simpler ones) but applying reverse polarity (attaching the wrong cable) might lead to damage. The first to go will probably be electrolytic capacitors, but depending on what’s in the pedal, could be anything.
Too much to read … watch video ;).
Let’s go through some simple protection options and see some common options for protecting our pedals.
Let’s have a look at the simplest one:
Super simple, and diodes are cheap. When correct polarity is used diode is conducting, reverse polarity – it acts as an open connection.
The downside of this is the voltage drop introduced by the diode. In the diagram above I put 0.5V, but it depends on the diode, depends on the current etc. For 1N4001, forward voltage is rated 1.1V, so, it could be anything from 0.5V to 1.1V (unlikely because our pedals are not drawing that much current). In this case anyway, this means that our headroom is lowered by 0.5V, and essentially, our supply is not 9V, but effectively 8.5V.
Even better option is using a Schottky diode, the diagram is pretty much the same:
There is not much more to be said about this, super simple, and our schottky diode has a lower voltage drop, so we only loose 0.2-0.3V of headroom. Note again, depending on the current drawn, this could go as high as 0.75V according to the datasheet. The above schottky diode can work safely as long as our pedal draws less than 350mA (for BAT48 diode anyway).
Anyway, compared to the silicon diode option, this is better way to go.
Here we come to the most complex (still very simple) approach, using a MOSFET. P-Channel MOSFET in this case is the right for the job:
Now, this is the proper job – very small voltage drop, depending on the needs you can use transistor that can support some crazy current draw. In my video I used TP2104 – that’s what I had handy (I had a power mosfet too but could not plug it into the breadboard, leads too thick).
When correct polarity is presented, the transistor is ON and it can have very, very small resistance. In the above case, that’s 6-10 Ohms for example. I used large resistance for my load so the current was small, the voltage drop did not even register. With reverse polarity, the transistor is OFF and only leakage current (nano amps) will flow.
The approach is by far superior, but also more complex. It requires 3 components compared the others. Zener diode is usually needed there for transistor protection, I didn’t use it in my video, but it’s a sensible thing to use it.
Silicon Diode in Parallel
OK, there’s another approach, and I must admit I used this few times. This is not a very good approach though, but let’s see the diagram first:
Obviously, the approach has the best headroom – no voltage drop whatsoever. When the reverse polarity is introduced, the diode turns on and lowers the reverse polarity.
Now, there are several issues with this. One is – it does not really prevent reverse polarity, it just limits it – if you watch the video, the reverse voltage is -1.1V.
Bigger issue is that there is no limit on the current that the diode draws. It will be only limited by the power source’s capability to deliver. Using battery, that can be a lot. On the video the diode got hot real quick, and that diode can take some serious current (relatively speaking looking at what a common pedal draws). One saving grace is that most common power supplies that we use only provides up to 50mA, so that might not be an issue, but in general, this approach is probably to be avoided.
Also, I’m not sure, but this might also damage the power supply, especially cheaper ones. So, while this will most likely work, I’d say it’s better to be avoided.
Btw, have a watch of that video, for this approach you’d have to use a serious diode, a small signal diode will not do … I deliberately tried one out and it went up in smoke in less than a second and then you don’t have any protection at all.
Which Approach to Use?
Reflecting on this, I’d probably use shottky diode first, and then, if really necessary, I’d use MOSFET. Using Schottky diode is very simple but effective approach and I don’t know how justifiable is using the more complex approach.
Have a look how did my little demonstration go:
There’s some explosions in there, you’ve been warned! (A gross exaggeration – I know 😎)