Recently it came to my attention that not everyone knows why we put diodes in reverse on the connections to some things. In fact, I recently gave some questionable advice in regards to what needs a diode for protection and what doesn’t. So, let’s get into it!
First of all, what is it we’re trying to do? Well, when you power things, it tends to sometimes be nice if you can turn it off and on and not just run it all the time. So, a circuit like below might be used:
It’s not important to exactly understand the whole thing. The key idea is that Q1 above is used as a switch. You connect a device, like a contactor or relay, up to this switch to control it. The above picture is of “low side switching” as the Q1 transistor switches the ground off and on. When the relay you have connected has no ground it does not turn on. When Q1 tells it to turn on, the ground connection becomes valid and it does turn on. “TRIG” in this picture is a signal from something like our GEVCU7 that can control the Q1 transistor. So, conceptually it’s pretty simple. If TRIG is powered then Q1 will turn on and allow power to flow from the OUTPUT line to GND (Ground). D1 is the key addition here. D1 is a diode. Diodes only allow voltage to flow one way. They have a “drop” which is the number of volts that are subtracted from any voltage that gets through. It isn’t uncommon for diodes to have a drop of between 0.2 and 0.7v. In this case, it’s pointing toward OUTPUT which means it wants to allow voltage to flow from ground to output. This is backward of the way voltage would flow if Q1 was on. For instance, if you had a 12 volt relay connected then 12V would flow from your power source, through the relay, through Q1, and to ground. So, “OUTPUT” is really at about 12V and the ground is at 0V. Normally, D1 does nothing because of this. It is backwards so it won’t allow current to flow. So, what does it do? Well, contactors and relays are interesting things. They are a form of an electrical device known as an inductor. Inductors store energy in them. When you open up the connection they are still full of energy. Where does that energy go? Well, it wants to flow back toward the open connection. In this case, Q1 would have turned off and thus there is no connection to ground any longer. The relay is trying to turn off but it has stored some energy. So, it tries to push that energy back out the opened connection. This causes a reverse voltage to form. Now you might see what D1 is for! If the contactor or relay tries to push energy in reverse on the “OUTPUT” wire then the diode will start to conduct and allow that energy to pass through to ground.
Take, for instance, the above pictured EV contactor. It is meant for allowing significant voltage and current to flow. We trigger it with Q1 above (or equivalent) because it takes a lot of power and little electronics don’t have the ability to control that much power directly. Because it takes a lot of power, it also stores a lot of power. If we didn’t fit a diode to redirect the flow of reverse power when it turns off, how bad could it be?
The above is from an oscilloscope capture of me disconnecting the pictured contactor from a 14V lithium battery pack. Note a few things:
- The upper voltage is 14V because I was measuring and disconnecting the +14V side
- The lower measurement was -96V!
- You can see ghost like lines that go MUCH lower. These are extremely rapid and short pulses that go down to hundreds of volts negative! The total timespan of these events is measured in millionths of a second. Alas, electronics can get damaged in millionths of a second.
So, as you may be able to imagine, electronics do not particularly like seeing -100V when they’re meant to be running at 14V. The contactor is OK with that. But, our poor Q1 above will not be so OK with that. So, we need the diode to shuttle the errant power to a safe place. Otherwise, it is nearly certain that Q1 will be blasted and destroyed in short order. In fact, during my tests there were no sparks when connecting the contactor but large sparks when disconnecting it. This is because connection was at 14V and disconnection caused hundreds of volts to arc against the terminal as I was disconnecting it.
So, I mentioned that I gave questionable advice regarding this topic. What was it? I said that maybe a fan might need a reverse protection diode as well. But, it probably doesn’t. Why? After all, fans also store some energy and have inductors in them. But, most fans these days are brushless DC. Brushless DC is one of the larger lies since “read my lips, no new taxes” but still it has a benefit here. A regular DC fan with no controller could cause a little bit of reverse energy flow but a brushless DC motor will not. This is because such a motor is electrically controlled and so when it loses power it just loses power and coasts down. Also, brushless DC motors are actually AC but who’s counting. There is no reverse energy that is going to flow when they turn off.
So, what’s the best advice? Search the internet for advice for any given item but mostly only relays and contactors need protection diodes in reverse. Things like DC/DC converters, fans, lights, and computers are not likely to cause any reverse energy zaps. But, even there, it doesn’t really hurt to have protection diodes, it just isn’t going to do anything.
