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I am designing a power bank which uses sustainable power generation methods for a school electronics project. It is based around the Arduino Nano.

At the moment I am planing to charge an 18650 cell using a hand cranked generator which I have designed as well as a small solar panel array.

One problem I had with the hand crank is that due to the nature of hand cranks its output voltage is not very stable even when using a linear regulator or a buck converter. 18650 charging circuits require a stable 5V input so they are not a practical option for me.

Instead I plan to connect the output of my generator directly to the 18650 cell to charge it. This will however become a problem if the battery gets too full and therefore gets overcharged.

I am already using the Arduino Nano to read the voltage of the 18650 cell and output this as a percentage to an LCD. I now want to add a function to the power bank where it continually checks the charge level of the battery using the data from my pre-existing program to make sure the 18650 is never overcharged.

To do this a couple of things need to happen and that’s where I plan to utilise some type of transistor.

First I need cut the connection from the 18650 to the rest of the circuit each time the voltage is checked (maybe every few seconds.) This is to make sure that the voltage read by the Arduino is the actual voltage of the 18650, if it were connected to the circuit and thus was under load the voltage would be less than actual and this effect is not linear so almost impossible to account for in code.

Next I need to have a second transistor which would cut off only the charge going into the battery in the event that it is full to prevent the overcharging problem.

The way I want to do this is by using an output pin on the Arduino to control a transistor to break the connection to the 18650 when the pin is high. When the pin is low I want the circuit to be closed.

However I have only ever seen BJTs and FETs used to make a connection when the control voltage (Arduino pin) is high.

Yes I could hold the Arduino pin high instead but this would waste more power.

Yes I could use a relay in a normally closed configuration, but I think the constant clicking would get annoying.

How can I achieve this normally closed relay effect With a transistor?

Sorry for the long winded explanation, but I thought it would be useful to offer some insight into the project to give you an idea of the setup.

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    \$\begingroup\$ Sounds like an XY problem to me so use a chip designed for charging and protecting your battery. They are available. \$\endgroup\$ – Andy aka Sep 12 at 11:41
  • \$\begingroup\$ Could you use a flywheel with a freewheel (sprag clutch - they can be 3-d printed) to even out the hand cranking? \$\endgroup\$ – Andrew Morton Sep 12 at 12:53
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There are a number of other possible solutions such as short-circuitung the generator that might be easier.

schematic

simulate this circuit – Schematic created using CircuitLab

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Yes I could hold the Arduino pin high instead but this would waste more power.

That's a wrong assumption. It is NOT TRUE. Microprocessors like the ATMega328 that is used in many Arduino modules are build using CMOS technology and that means that they can maintain a voltage (low or high) at any output and as long as no current flows then this does not consume any power.

If you would use a MOSFET (connected to the output of the Arduino) to switch on/off your relay or LED then no current would flow as a MOSFET uses similar technology so again, no current flows to keep the MOSFET on or off.

So no power is used to keep a MOSFET continuously in an on or off state. Only when it is switched on => off or off => on a little bit of power is needed.

So your question is based on a false premise.

Yes, normally of devices do exist: depletion type MOSFETs and JFETs as well.

But these aren't very suitable for switching things they're used in some niche applications for amplifying the voltage of an electret microphone or amplifying RF signals.

TLDR: you're asking for a device that's unneeded as we can solve the "problems" you see using simple circuitry. These are common solutions that all designers know about.

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  • \$\begingroup\$ Hi Bimpelrekkie, thanks so much for the explanation. I thought that MOSFET would use power when held high by an Arduino but now I know better. I’m only 17 and mostly self taught with electronics so still have a lot to learn. Really appreciate your help. \$\endgroup\$ – Billy Sep 12 at 12:42
  • \$\begingroup\$ note: if you use a MOSFET to hold a relay on, the MOSFET doesn't use power, but the relay does. \$\endgroup\$ – user253751 Sep 12 at 17:25
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So first off, charging a LiIon 18650 direct off a hand crank is a bad idea. They need precise control over the rate of charge or else they can explode. So using a hand crank has all sorts of potential problems if you dont have more intricate ways of controlling the voltage and current.

Second, yes you can use a FET to act a bit like a relay, but it requires an active circuit to do so so while it may make sense on the side that disconnects the main circuit it may not make so much sense on the crank side. If you wanted to do this on the crank side then I would probably consider a latching relay since it will likely stay in an on or off position for an extended period of time.

However in all honesty I'd scrap your entire approach and rethink it. What I would do is hook up a very large capacitor (maybe even a super cap) to the crank. This will ensure the voltage stays constant for an extended period of time and not change wildly, then hook this up through some charging circuitry.

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  • \$\begingroup\$ Hi, thanks Jeffery. GreatScott has a video on the subject of hand crank charging where I got the idea from. youtu.be/3fpsInXqo1E I have tried using a relator and capacitors for smoothing but didn’t have much success. Will try a few more things. I’ve got an 18650 charging circuit but have to regulate the crank to a stable 5V first, while loosing the least power possible. Tried an LM7805 but might have to get a buck converter instead. \$\endgroup\$ – Billy Sep 12 at 12:02
  • \$\begingroup\$ @Billy if your concern is that you want to loose minimal energy then an LDO is not the way to go, they are wasteful as all hell. You would want a buck-boost converter that will buck when you are over voltage and boost when you are under voltage. This should let you pull the most out of the capacitor. By using a nice big capacitor it ensures the voltage changes gradually which will likely be fairly important for a stable circuit. \$\endgroup\$ – Jeffrey Phillips Freeman Sep 12 at 13:13
  • \$\begingroup\$ was only using the LDO to test as thats what I had laying around. Will buy a couple buck converters, one for the panels and one for the crank. Won’t need a boost converter as I get about 8V out of the crank and 11V from the panels. The crank I designed uses a nema 17 stepper motor Coupled to a planetary gearbox to increase speed and therefore voltage, with a bridge rectifier on each coil to get DC. Each rectifier has a 4700uF capacitor on it to help with the smoothing. I think these caps should do the trick but I have a 6000uF that I could add before the buck converter as well if need be. \$\endgroup\$ – Billy Sep 12 at 13:44
  • \$\begingroup\$ You still need a boost converter, and a capacitor if you plan to not waste energy. As I said you can not hook the crank directly up to the battery as the current flow needs to be controlled. As you crank even if you create much higher voltages than needed all that energy will not be used by the charging circuit in real time, especially as the battery gets closer to being fully, it will basically trickle the energy into it. so if the battery is 90% full and you crank for 10 seconds without a capabitor and boost most of the energy is lost. \$\endgroup\$ – Jeffrey Phillips Freeman Sep 12 at 13:46
  • \$\begingroup\$ @Billy but with a proper capacitor and boost/buck if you crank for 10 seconds and charge the capacitor as an intermediary you will find even if the battery is at 90% when your done cranking it will continue to charge the battery for maybe 30 seconds to a minute as it slowly trickles that energy in, which otherwise would have been lost. \$\endgroup\$ – Jeffrey Phillips Freeman Sep 12 at 13:48
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You might be looking for a "depletion mode" MOSFET, these are on all the time, unless you drag the gate negative, which turns them off.

In the example schematic below I have shown an example schematic the arduino outputs 0v or 4v to turn the depletion device ON or off.

You could replace the Arduino with a zener diode, but you would need a low current zener, and a quite high value resistor, you could probably put another transistor in there to turn on the resistor only while the generator is cranking.

schematic

simulate this circuit – Schematic created using CircuitLab

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