What is the best Reverse Voltage Protection Circuit? || Repairing a Lab Bench Power Supply

A week ago, I wanted to charge up my lead acid battery with my lab bench power supply. So I set it’s voltage to 14.4V and its current limits to 1.35A just like the label of the battery recommends it. But apparently I had too much a drink. And as you can see connected - to + and + to -. which resulted in a small spark at the minus terminal and the fact that my power supply is now constantly shorted even when there’s no loads connected to it. In a nutshell, that means I cannot use it to power anything which on the other hand means it is busted. So in this video, let’s find out how we can repair such a damage and also how we can create a circuits that can protect our devices from such a reverse voltage. LET’S GET STARTED This video is sponsored by JLCPCB, where you can order custom PCBs with ease. Their digital manufacturing technology ensures high quality and accuracy for price of only $2 for 10 PCBs with 48 hours quick turnaround! For the first repair step, I obviously had to open up the lab bench power supply. After removing a few dozens of screws I removed the nuts that hold the main PCB in place and lifted the doubts in order to inspect the circuitry around the two power output channels. What stood out to me was a SMD diode which was connected between each one of the output terminals like it’s shown in this schematic. That means that when the battery load is hooked up properly to the output terminals The diode does practically not influenced the current and voltage values. But if we connect a reverse voltage to the terminals then the diode shorts the power source and protects the inner electronics from the harmful reverse voltage and current. The only problem is that the diodes will most likely not survive such a current surge. Which was true for my case as well. Since I could measure the forward voltage of the diodes at the still functional channel while the defected channel delivered way to low voltage values. So to fix my supply, I simply had to desolder the blown up diodes order some new ones of the same type and solder one of them in place. And after reassembling the lab bench power supply and powering its it seems to function correctly once again. Awesome! But the question remains whether this kind of reverse voltage protection is the best option? (Which let me spoil the surprise here, it is not.) First off, you would usually add a fuse in series to the plus terminal So that if there’s a reverse voltage it would get blown and thus interrupt the current flow instead of constantly shorting the battery loads which could end with terrible results. And the next problem is that a reverse current flow through the inner electronics is still possible. Which in my case was apparently not a problem. But, take for example, ElectroBOOM’s inverter which got completely busted by reverse voltage and also featured the same protection circuits. So, what we need to do is to completely stop flow through the inner electronics which can be achieved by simply putting a diode in series with the output terminals. If the battery is connected properly, current flows through it. And if reverse voltage is applied, the diodes blocks the reverse current. Sounds like a perfect solution! Well, unfortunately it is not that perfect. We can find that out by measuring the temperature of the diode while in use which turns out to be at around 47°C with a current flow of 2A. The problem is that this Schottky diodes, which already features a small forward voltage still creates a power loss of approximately 0.84W at 2A in the form of heat. And let’s not forget that the voltage at the load is not stable due to the variable voltage drop of the diodes according to the current flow So, “What is the best reverse voltage protection” you might ask? Well, through our earlier circuits, we know by now that we need some sort of switch with a very low resistance that turns on if the voltage is applied correctly and turns off if it is connected the wrong way. So the solution is a P-Channel MOSFETs like this IRF5305. And the well-known schematic like this, which you can find all over the internet. As the first example, let’s use a light bulb as a load instead of the battery to keep things simple. If the power source is connected properly here and I’m using a voltage of, for example, 12V then we got a voltage potential of 10.7V at the source. Because there is an initial voltage drop across the MOSFET’s body diode of around 1.3V. And since the gate is connected to ground so it’s 0V, we got a gate-to-source voltage of -10.7V which, according to the MOSFET’s output characteristics graph, turns it on. In this state, the MOSFET can reach a resistance of 0.06Ω which at a current draw of theoretical 2A would equal a power loss of 0.24W. Much better than the diodes. Of course, we still have the problem that the voltage drop varies with different current draws. But the effect is much less noticeable with the MOSFET. Now when the voltage is applied the wrong way we would get a theoretical gate-to-source voltage of +12V But the MOSFET only turns on with voltage lower than -2V, which means it will stay off. Sounds like an awesome protection circuits. But you have to keep in mind that with a lower power supply voltage the MOSFET’s resistance increases and thus its voltage drop which makes it horrible, inefficient and useless at low voltages. Also, my MOSFET got a maximum gate to source voltage of ±20V Anything above that will lead to the destruction of its. But of course you could add a resistor and a Zener diodes in order to limit the gate voltage to a suitable limits. Now since we know all the important facts about this reverse voltage protection circuits, it is time to replace these simple resistive loads with our battery. As you can see by applying the voltage the correct way, the battery charges like usual. But if we connect the battery the wrong way the fuse I added for safety reasons keeps popping which means there’s still a problem. The reason is that the battery adds its own voltage levels to the circuits so that the MOSFET is not closed when the reverse voltage occurs. Hence this circuits cannot be used for voltage sources, only loads. Thankfully though, while tinkering up a solution circuits I found Vince’s thoughts blog which offered a very minimalistic solution to the problem His circuits basically ties the gate voltage to source with a resistor in order to keep the MOSFET normally off. And only if the battery loads is connected the right way it powers an NPN transistor which pulls the MOSFET’s gates to grounds and thus turns it on. After building up the circuits and connecting it to the battery I can confirm that this circuit is not only very simple but also functional. So, feel free to try it out yourself! Of course, there also exists versions of the MOSFET circuits with N-Channel MOSFET instead of P-Channel types. (If you’re more into that.) And with that being said you’re now familiar with the different reverse voltage protection techniques. That will hopefully help you to not destroy your lab bench power supply like I did. If you enjoyed this video then don’t forget to like, share and subscribe. STAY CREATIVE AND I WILL SEE YOU NEXT TIME

Channel: GreatScott!

Views: 769,244

Rating: 4.9169445 out of 5

Keywords: reverse, voltage, protection, tutorial, circuit, best, diy, how to, make, project, guide, beginner, beginners, battery, charge, discharge, diode, schottky, compare, comparison, fuse, lab, bench, power, supply, repair, efficiency, MOSFET, mosfet, p channel, n channel, zener, resistor, gate, limit, liion, li ion, lipo, li po, electronic, electronics, greatscott, greatscott!, transistor, npn, pnp

Id: 7Tk5ghH_U2s

Length: 8min 51sec (531 seconds)

Published: Sun Sep 16 2018