Join Barron Stone for an in-depth discussion in this video Protect against reverse current, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] One of the quickest way to accidentally destroy an electronic device is to connect it to the power supply backwards. It's an easy mistake to make if the power connector is not polarized, if you're not paying attention you might accidentally mix up the red and black power wires, or maybe you somehow stick batteries into a device backwards. Those are all mistakes that I've personally made at some point in time in the past. So to protect your electronic circuits against users like me, it's a good idea to include protection against reverse current if it could potentially damage your device.
Fortunately, that's easy to do by connecting a diode in series with the positive side of a DC power source. This reverse protection diode insures that the current will only flow in the forward direction so the DC power source will only ever apply a positive voltage to the circuit. If the power source is connected in the correct orientation current will flow through the diode to the device and everything works likes it's supposed to. But if for some reason that power source gets connected backwards or has some sort of malfunction the diode will become reversed by it and act like an open circuit.
That keeps current from flowing though the circuit in the wrong direction. So the circuit is still protected. One draw back of using a reverse protection diode is that you do loose some of the voltage from the DC power source to the forward voltage drop across the diode. For example. If I was connecting my circuit to a 5 volt power source and used a reverse protection diode whose forward voltage was 0.6 volts, my circuit would only actually get 4.4 volts at it's input voltage.
To minimize that forward voltage loss many circuit designs use a special type of diode called a Schottky diode whose unique semi-conductor composition gives it a much smaller froward voltage drop than standard silicon diodes. The forward voltage for a Schottyy diode is usually somewhere between 0.15 volts and 0.45 volts. But there reverse breakdown voltage is still large enough to make them useful for protecting against reverse current. Schottky diodes are special enough that they get their own schematic symbol.
Which looks like the standard diode symbol but with to 90 degree bends added to each side of the cathode line. Physically the packages for a Schottky diode looks very similar to a standard Rectifier diode. It's possible to tell them apart if you look closely at the tiny little print on them. I find that hard to read. So I usually check the forward voltage of my diodes using my DMM. To do that I'll set the selector nob to the diode mode. When I probe a standard Rectified diode, I can see that it has a forward voltage of around .6 volts which is what I would expect from a regular diode.
However when I measure the forward voltage of a Schottky diode I see that's much smaller than the regular diodes forward voltage. Now, you might be wondering why we don't just always use Schottky diodes in place of regular diodes. Since they have better characteristics. And the main reason is cost. Schottky diodes will usually cost several times more than their general purpose counter parts. Now that's not a big deal if you're only buying a handful of parts for a hobby project, But when engineers are designing electronic devices to be mass produced those small differences in cost really start to add up.
One other thing to consider when selecting a diode to protect against reverse current, is the amount of current that the circuit will draw. All of the current flowing into the protected circuit if first passing through this diode. So it's critical that the diode can handle all that current without burning up. I also need to be prepared for the unexpected. If something where to suddenly change in the load circuit that caused to draw more current than intended, that could potential melt the diode causing it to fail as a short circuit.
So my circuits no longer protected. To be extra careful and defend against that possibility I can insert a fuse in series with the protection diode. A fuse is a safety component protects electronic devices from drawing too much current and basically consists of a strip of wire. When the amount of current though the fuse is less than certain threshold, the fuse simply acts as a wire. But when the current exceeds the fuses threshold the fuse wire will melt causing it to act like an open circuit.
So if something changes in the load and it tries to draw too much current the fuse will break and cut off the power. I will need to replace or reset the fuse to get the circuit running again. But at least my circuit will be protected from whatever was going wrong.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals