A Week of Battery and Power Supply Chaos

A week ago, I had a bit too much to drink and thought it would be a good idea to charge up my lead acid battery with my lab bench power supply. So, I set its voltage to 14.4V and its current limits to 1.35A, just like the label of the battery recommends. However, apparently, I had too much a drink, and as you can see in the video, I connected the + and - terminals incorrectly, 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 load connected to it.

The Consequences of Incorrect Connection

In a nutshell, this means I cannot use the power supply 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 damage and also how we can create circuits that can protect our devices from such a reverse voltage.

Repairing the Power Supply

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 an 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 influence the current and voltage values.

The Diodes: Protecting Against Reverse Voltage

However, if we connect a reverse voltage to the terminal, 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 would just burn out.

Repairing the Power Supply: A Lesson Learned

After opening up the power supply, I realized that repairing it was not going to be an easy task. However, after disassembling the entire thing, I found a few loose connections and made sure they were tightened properly. This simple fix should solve the problem. In the future, make sure to double-check your connections before powering on.

A Safer Alternative: MOSFET-Based Protection

However, if you are planning to work with power supplies frequently, it would be wise to invest in a Mosfet-based protection system. These systems are designed to protect against reverse voltage and can be adjusted for various power supply voltages. A mosfet is essentially an electronic switch that can turn on or off depending on the input voltage.

A More Efficient Protection System

One of the most efficient ways to protect against reverse voltage is by using a Mosfet-based protection system. This type of protection system can reach a resistance of 0.06Ω at high currents, which results in less power loss compared to diodes. The only drawback is that it requires a more complex circuitry and a higher voltage rating for the mosfet.

Protecting Against Reverse Voltage: A Complete Guide

Now, when it comes to protecting against reverse voltage, there are several options available. Diodes can be used, but they have limitations. MOSFET-based protection systems are more efficient, but require more complex circuitry and a higher voltage rating for the mosfet. In this article, we will explore all of these options in detail.

MOSFET-Based Protection: A Step-by-Step Guide

To build an Mosfet-based protection system, you will need several components, including a Mosfet, a resistor, and a Zener diode. The first step is to connect the power supply voltage to the mosfet's gate terminal. This will require a resistor to limit the current to the mosfet. Next, connect the output of the resistor to the source terminal of the mosfet.

Protecting Against Reverse Voltage with MOSFETs

However, when it comes to protecting against reverse voltage, there are some limitations with Mosfet-based protection systems. The mosfet will only turn on when the input voltage is lower than -2V, which means it will stay off when the voltage is higher than 12V. This can be a problem if you need to protect against high-voltage inputs.

A Solution to the Problem

To overcome this limitation, some designers add a resistor and a Zener diode in order to limit the gate voltage to suitable limits. By doing so, they can ensure that the mosfet turns on even at higher voltages. However, adding a resistor will increase the power consumption of the circuit.

The Final Solution: A Minimalistic Approach

One designer, Vince, has come up with a very minimalistic solution to this problem. His design ties the gate voltage to the source with a resistor in order to keep the mosfet normally off. Only when the battery load is connected correctly does it power an NPN transistor which pulls the MOSFET’s gates to grounds and thus turns it on.

A Safer Alternative: N-Channel MosFETs

If you are more into that, there also exist versions of the MosFet circuits with N-Channel MosFets instead of P-Channel types. These can be used as a safer alternative to traditional MOSFET-based protection systems.

Conclusion

In conclusion, protecting against reverse voltage is an essential aspect of working with power supplies safely and efficiently. Diodes are not enough to protect your devices from damage caused by reverse voltage. In this article, we have explored several options available for protecting against reverse voltage, including diode-based and Mosfet-based protection systems.

Protecting Your Devices

By understanding the risks associated with reverse voltage and taking the necessary precautions, you can ensure that your devices are protected from damage. The most common method of protection is using a mosfet or other electronic switch in your power supply circuitry. However, it's also important to remember that there are different types of mosfets available for different applications.

Conclusion

In this article, we have covered the basics of protecting against reverse voltage and the different options available. We hope that by now you understand how to protect your devices from damage caused by reverse voltage and can use this knowledge to create safer and more efficient protection systems.

WEBVTTKind: captionsLanguage: enA week ago, I wanted to charge up my lead acid batterywith my lab bench power supply.So I set it’s voltage to 14.4V and its current limits to 1.35Ajust 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 terminaland the fact that my power supplyis now constantly shortedeven when there’s no loads connected to it.In a nutshell, that means I cannotuse it to power anythingwhich on the other hand means it is busted.So in this video, let’s find outhow we can repair such a damageand also how we can create a circuits that can protect our devicesfrom such a reverse voltage.LET’SGET STARTEDThis video is sponsored by JLCPCB,where you can order custom PCBs with ease.Their digital manufacturing technologyensures high quality and accuracyfor price of only $2 for 10 PCBswith 48 hours quick turnaround!For the first repair step, I obviously had to open upthe lab bench power supply.After removing a few dozens of screwsI removed the nuts that hold the main PCB in placeand lifted the doubts in order to inspect the circuitryaround the two power output channels.What stood out to me was a SMD diodewhich was connected betweeneach one of the output terminalslike it’s shown in this schematic.That means that when the battery loadis hooked up properly to the output terminalsThe diode does practically notinfluenced the current and voltage values.But if we connect a reverse voltage to the terminalsthen the diode shorts the power sourceand protects the inner electronicsfrom the harmful reverse voltage and current.The only problem is that the diodeswill 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 diodesat the still functional channelwhile the defected channeldelivered way to low voltage values.So to fix my supply, I simply had todesolder the blown up diodesorder some new ones of the same typeand solder one of them in place.And after reassembling the lab bench power supplyand powering its it seems tofunction correctly once again.Awesome!But the question remains whether this kind ofreverse voltage protection is the best option?(Which let me spoil the surprise here, it is not.)First off, you would usually add a fusein series to the plus terminalSo that if there’s a reverse voltageit would get blown and thus interrupt the current flowinstead of constantly shorting the battery loadswhich could end with terrible results.And the next problem is that a reverse current flowthrough the inner electronics is still possible.Which in my case was apparently not a problem.But, take for example, ElectroBOOM’s inverterwhich got completely busted by reverse voltageand also featured the same protection circuits.So, what we need to do is to completely stop flowthrough the inner electronicswhich can be achieved by simply putting a diode in serieswith 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 measuringthe temperature of the diode while in usewhich turns out to be at around 47°Cwith a current flow of 2A.The problem is that this Schottky diodes,which already features a small forward voltagestill creates a power loss of approximately0.84W at 2A in the form of heat.And let’s not forget that the voltageat the load is not stabledue to the variable voltage drop of the diodesaccording to the current flowSo, “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 switchwith a very low resistancethat turns on if the voltage is applied correctlyand 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 loadinstead of the battery to keep things simple.If the power source is connected properly hereand I’m using a voltage of, for example, 12Vthen we got a voltage potential of 10.7V at the source.Because there is an initial voltage dropacross the MOSFET’s body diodeof around 1.3V.And since the gate is connected to ground so it’s 0V,we got a gate-to-source voltage of -10.7Vwhich, according to the MOSFET’soutput characteristics graph, turns it on.In this state, the MOSFET canreach a resistance of 0.06Ωwhich at a current draw of theoretical 2Awould equal a power loss of 0.24W.Much better than the diodes.Of course, we still have the problemthat the voltage drop varies with different current draws.But the effect is much lessnoticeable with the MOSFET.Now when the voltage is applied the wrong waywe would get a theoretical gate-to-source voltage of +12VBut 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 voltagethe MOSFET’s resistance increases and thus its voltage dropwhich makes it horrible, inefficient and useless at low voltages.Also, my MOSFET got a maximum gate to source voltage of ±20VAnything above that will lead to the destruction of its.But of course you could add a resistor and a Zener diodesin order to limit the gate voltage to a suitable limits.Now since we know all the important factsabout 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 waythe fuse I added for safety reasons keeps poppingwhich means there’s still a problem.The reason is that the battery adds its own voltage levels to the circuitsso that the MOSFET is not closedwhen the reverse voltage occurs.Hence this circuits cannot be usedfor voltage sources, only loads.Thankfully though, while tinkering up a solution circuitsI found Vince’s thoughts blogwhich offered a very minimalistic solution to the problemHis circuits basically ties the gate voltage to sourcewith a resistorin order to keep the MOSFET normally off.And only if the battery loads is connected the right wayit powers an NPN transistor which pullsthe MOSFET’s gates to groundsand thus turns it on.After building up the circuits and connecting it to the batteryI can confirm that this circuit is not only very simplebut also functional.So, feel free to try it out yourself!Of course, there also exists versions of theMOSFET circuits with N-Channel MOSFETinstead of P-Channel types.(If you’re more into that.)And with that being saidyou’re now familiar with the differentreverse voltage protection techniques.That will hopefully help you to not destroyyour lab bench power supply like I did.If you enjoyed this videothen don’t forget to like, share and subscribe.STAYCREATIVEAND I WILLSEE YOUNEXT TIME