#139 Is there a simple and cheap way to protect your super caps? How?

#139 Is there a simple and cheap way to protect your super caps? How?

Grüezi YouTubers. Here is the guy with the Swiss accent. With a new episode around sensors and microcontrollers. In video #133 we used super capacitors to
safely shutdown the Rapberry Pi. In the comments, viewers asked about over
voltage protection of the super caps. I will show you a big and small and cheap
possibility. And I will show you one concept which does
not really work. But why do we have to protect Super Capacitors? And how can we protect them? Most Super capacitors are only specified up
to 2.7 volt. If we need higher voltages, we have to connect
a few of them in series. To give the Raspberry a save shutdown I used
two caps because it runs on 5 volt USB. So, the basic diagram is like that: Two Super
capacitors in series across the 5 volt connector. If we charge now capacitors in series, the
current is equal in both capacitors. But, if the capacitance are not completely
equal, the one with the lower capacitance charges faster and its voltage can go over
the rated 2.7 volts. To avoid that, we have to connect a resistor
in parallel to the smaller capacitor. Like that, part of the current can flow through
this resistor and therefore is not used for overcharging the capacitor. Because we do not know, which of the capacitors
is smaller and exactly how much, and because some other things can happen, we add a small
switch to this resistor. It switches only, if the voltage of the capacitor
is close to the 2.7 volt. Like that, we can use quite small resistors
and bypass a big amount of the current. As soon as the voltage is in the save zone,
the resistor is switched off again. If we use such a concept for all our capacitors,
and our protection voltage is 2.7 volt, they are protected against over voltage as long
as the total voltage is lower than the number of capacitors times 2.7. In our case, the voltage would have to stay
below 5.4 volts. Which is ok with USB. The specification allows a maximum voltage
of 5.2 volts. Let’s start with the simplest concept: Zener
diodes. The easiest way would be to solder a Zener
diode across each super cap. But what is a Zener diode? Normal diodes conduct current only in one
direction and block it in the other direction (at least up to their very high blocking voltage). Zener diodes are somehow different. In principle, they also block current in the
“wrong” direction up to a certain voltage. But this voltage is low and exactly specified. You can get Zener diodes for many voltages. If we check one with our transistor tester,
it shows two diodes in opposite direction. The one with the voltage of 0.8 volt is the
normal diode. The other one with 2.4 volt is the Zener part. Let’s have a closer look: On the positive
voltage side, we see a curve of a normal diode. On the negative voltage side, we see the Zener
behavior. At the beginning, nearly no current flows. At a particular voltage, the current starts
to flow. But this is not at a sharp voltage as we can
see. The curve is only bent. Because all these things happen on the negative
voltage side, we have to use the Zener diodes always in reverse direction. This is, why I mirror the curve now. Just a small remark: Not all transistor testers
are able to measure Zener diodes properly. And all can only detect these diodes if they
have a low Zener voltage. So, let’s check how this simple concept
works. I have 2.4 volt and 2.7 volt Zeners. I start my test with no protection at all. Two 10 Farad Super Caps are connected in Series
and 5 volt is applied. If both are discharged at the beginning, they
load quite evenly. The difference is small, because their capacities
are similar. I discharge now the left capacitor a little
to simulate uneven capacities. If I load them again, the right one quickly
approaches 2.7 volts and I have to stop the experiment in order to not harm it. Let’s now protect this Cap with a 2.4 volt
Zener. To understand what happens, I measure the
current through the diode with the yellow Fluke meter. As we already know from the chart, the diode
already starts to conduct at below 2.4 volt. So, a part of the current is now “diverged”
through the diode instead of the capacitor. This part does not charge the capacitor anymore. So, the other cap can catch up. But because the current through the diode
is quite small, the protection is not big, and the voltage exceed 2.7 volts a little. If the imbalance between the two capacitors
would be bigger, this protection would not work anymore. And, we easily can imagine, that a 2.7 volt
Zener would not work at all. So, this is a simple protection, which works
somehow for these small capacitors. But fortunately, we have a better one: The
TL431. This part is called “Precision Programmable
Reference” and it is a very versatile part which can used for many applications. It is also quite cheap: 50 pieces for 1.20,
including shipping. It is even cheaper than a 2.4 volt Zener,
where 50 pieces cost more than 3 dollars. The TL431 is a neat small part in a TO-92
package. Its symbol looks much like a Zener diode,
with the exception that it has three pins instead of only two. The third pin is called REFERENCE. If we look at the block diagram of the chip,
we see, that it consists of a precise 2.5 volt reference and an Opamp used as a comparator. As soon as the reference voltage is above
the 2.5 volt, the transistor switches on. So, if we connect the REFERENCE and the cathode
pins, we get this diagram. Because I mirrored the diagram of the Zener
diode, both are compatible. We have the positive voltage to the right
and positive current up. We immediately see the difference: The bend
of the TL431 is much sharper. At exactly 2.5 volts it starts to conduct
up to its maximum rating of 150 mA. This is nearly the curve we were looking for. The only thing is, that it is at 2.5 instead
of 2.7 volts. So, let’s try out and replace the 2.4 volt
Zener with the TL431. We also start with unevenly charged capacitors
and see, that the current through the TL431 starts to increase much more and much sharper
at 2.5 volts. The voltage across the super cap still goes
above 2.5 volt, because the parts are not ideal. But still, the behavior is much better. At the end, the protected and the unprotected
caps each show around 2.5 volts. If I would now increase the voltage to 5.3
volt, only the unprotected cap’s voltage would increase and would exceed its specified
voltage. Which is obviously not good. So, we could protect both capacitors. Which anyway would be a good idea, because
we do not know, which one has a smaller capacity. Below 5 volt total voltage, this would work
fine. But as soon as we cross these 5 volts, both
TL431 would start to conduct and produce a short cut. They would heat up, and maybe even would destroy
themselves. Maybe you remember the word “programmable”
from the description of the TL431? Programmable means, that we can change the
“Zener voltage” with a simple trick: We connect the reference pin to a voltage divider. Now, the reference measures a lower voltage
and therefore, reaches the 2.5 volts at a higher voltage. If we calculate our resistors with the formula
from the data sheet, we can set the cutoff voltage to 2.7 volt. Problem solved! If you really want to make sure, that nothing
bad happens to your protectors, you can add a small series resistor. Like that, the current through the TL431 is
limited. If you plan to work only with USB voltages,
this is not necessary. So, this is a great concept to protect small
super caps when you are sure, that the two capacities are similar. To show you the limitations, I double the
capacitance of one capacitor by adding a second one. Now, we see, that the current through the
TL431 gets quite high, and the voltage crosses 2.7 volt, even with a cutoff voltage of 2.5
volt. To avoid that, I bought the voltage protectors
shown in the last Mailbag video #137. They can be used for large capacitors up to
500 Farad. I re-engineered its PCB and its diagram is
here. They also use a TL431, but they use a big
transistor to amplify the effect. So, let’s check, if it works. I keep the “tougher” scenario with the
two uneven capacitors and protect the smaller one. Now with a protection board instead of the
TL431. The rest stays the same. Here, the protection kicks in at 2.66 volt
and the voltage never exceeds this value . The other capacitor can easily catch up. And if we would have two such protection boards
in series, our USB voltage could go up to 2 times 2.66 volt=5.32 volt, which is above
USB specs anyway. Summarized:
We wanted to protect our Super capacitors against over voltage
The first concept using simple Zener diodes worked somehow, but with very narrow limitations
The usage of a TL431 Precision Programmable Reference did what we wanted for our small
capacitors, and even at lower cost. With a voltage divider, we were able to set
the protection voltage at the right level. We looked at the diagram of a commercial product
for larger capacitors and discovered, that they also use a TL431
The commercial product worked too, and, because it supports much bigger currents, can also
be used for bigger capacitors. BTW: If we do the calculation of the voltage
divider of the protection board, we find the 2.66 volts we measured before. Not bad! I hope, this video was useful or at least
interesting for you. If true then like. Bye

57 Replies to “#139 Is there a simple and cheap way to protect your super caps? How?”

  1. Excellent! Thank you. I'm using a Pi 3 to run Node Red in a building automation system and I was about to use a battery for clean shutdown but I prefer supercaps for longevity. Protection of those caps was always a sore point.

  2. Nice video. I thought the protection modules would use a specialized IC.
    The TL431 is one of my favourite parts. I guess I could use it with a pass transistor to make a lipo/li-ion balance module.

  3. Thanks for sharing. 🙂 One thing, in the diagram you have inverted the capacitors, so +5V is connected to the MINUS pol of the capacitors.. This happens

  4. I don't know why i am watching this even if my pi an asus eeepc701 is now dead but

    The video was useful
    I will get new pi in next month

  5. This is what I use with my solar NEW! BULLZ AUDIO 2200W 12V BCAP 2.2 Digital Car Power Farad Capacitor | BCAP2.2, with no charge controller or battery, I also use this DC Converter 12V to 5V 3A Double 2 USB to Auto Power Regulator Voltage Step Down with it

  6. Morning Andreas 😀

    I've been working on my solar all week with my super caps. This video is a great addition and has demystified a lot.

    I will be adding this component to my list of purchases right now.

    Wonderful work, always worth the wait and you never disappoint.

  7. I was wondering Mr Spiess. Would it be possible to build an esp, super cap and small solar panel. That will charge the super cap, once it contains enough energy to send off a reading, power up and shuts down, wait for voltage to rise and repeat. In essence it will be self powered.

    As I am looking into wind and solar and neither is a continuous input of power. I did not want to have to deploy a battery. So my thoughts are when there is enough wind/sun it can harvest power and then send my readings to my DietPi 😀😀😀

    Would you mind exploring this in the way that you do Andreas. That would be a great help

    Thank you

  8. At 9:30, Rsup is necessary, even with USB voltage. The whole idea of protection is not for normal situations, but for situations when something goes wrong. Rsup must be there to limit the current for situations when something is malfunctioning, like e.g. a defective TL431 or a shorted supercap.

    I like your videos, thanks and keep them coming, please!

  9. It seems, that schematic of the commercial protector is wrong! The transistor Q2 will be opened until voltage across device will reach 2.66V. At that voltage, transistor will be closed. So this schematic is working in the opposite way as is expected. Either super capacitor connection terminals should be in the Q2 collector current path, either transistor should be pnp type and resistors R4-R6 tied to minus terminal.

  10. Dear Andreas, I like your videos very much. I noticed you reversed cathode (drawn blue) and anode (drawn white) side in the schematic which is shown around 0:56. Keep the good work coming. Kind regards.

  11. I think that the circuit diagram is incorrect
    The transistor should be PNP.

  12. I used power mosfets with Vth at 2.5V to protect my supercap battery. It worked fairly well.

  13. Zener doides almost forgot that curve on the graph. Check out my next video may need a hand mate. What can I say about your video Andreas as alway amazing work.

  14. Hi Andreas, very interesthing video Thanks .About the transistor tester, would you please give me the type you are using as of mine is not giving the right symbol for zener diodes.

  15. Hi Andreas, thanks for this great video. I am currently planning to build a small LED torchlight with supercaps and I was considering to protect them with a TL431. So I am intrigued by the protection circuit that you reverse engineered.

    I suspect that what you have as a zener diode in your circuit is actually a small signal transistor. This would explain why the power transistor is PNP.

  16. I bought those protectors as well, for my 500 F caps. But according to specifications it can only hold less than 1 A (actually the resistors will blow up since they use 0.4 ohm smd resistors, how much Watt can that withstand? They will get up to 6 Amps above 2.65 Volt).
    Can I remove the small smd resistors and use real 20 W 0.5 Ohm external resistor? How much current can the mosfet switch? (I found it: the D1804 is a silicon NPN transistor and not a mosfet, Ucb = 60V, Ic = 8A). So we can switch up to 8 A. Using 2 external resistors of 1 Ohm in parallel we create 0.5 ohm. We could then divert or reroute 5.3 Amps. The resistors would need to be P=i*i*R = 2.65*2.65=7 Watt each (10 Watt each is even better).
    What do you think of this? (I will set up a test bench for this soon as I have the parts needed).

  17. very nice to know that there is basically a programmable zener with such a sharp curve. thanks for testing the different methods

  18. Thanks for another great informative video.
    I have two of the CSDWELL six capacitor pcb's and was very curious about the circuit.
    I'm also interested in this type of circuit as Li-ion balance board since the prebuilt ones are all 4.2V and not adjustable (I want 4.1V)

  19. tl431 rocks! perfect for driving an optocoupler in isolated voltage comparators. much used in power supplies. thanks for another great video andreas

  20. Andreas, thank you very much for the most informative videos!

    What would you think about using 5.5V 4F computer backup supercaps?

    They don't need protection circuitry and 4 in parallel would yield 16F of storage capability. I would believe this should support the RPI3 until shutdown.

    Kind regards,


  21. Genius… And as always, Awesome… 🙌 Had worked with super capacitor banks with inbuilt protection on a project, but never cared what went into it… your make a lot of difference to our knowledge Sir!

  22. Hi Andreas. To make your videos more clear, you should say "charge" instead of load – load gives no meaning in this context. – I know that english is not your main language, but you are doing very well anyway 😀

  23. I don't see how it protects each series capacitor from being reverse charged, since tolerance is 20% some capacitors charge and discharge faster. So the lower value capacitors will reverse charge right after they discharge (in a series circuit)

  24. Having a bag of those TL431 laying around is really useful. They go into pretty much every op amp circuit I build when there's signal to invert, amplify or just generally adjust to max out the resolution of a connected ADC (in a micro controller for example).

  25. Awsome video. If you used multiple zenor diodes would it have a more agresssive/ safer protection effect between the two Capacitors voltage differences? Like keep the higher voltage capacitor from going past the 2.7volts?

  26. Great video! Very informative. In the schematic for the balancer board, what is the role of the zener diode connected to the base of the npn transistor? is the purpose of the 3 1.2ohm resistors to dissipate or bleed current from the capacitor? if so wouldn't the npn be on when the reference is under the trip point?

  27. I really don't understand why are some people criticizing your video and your circuit? You DID explain everything well!

  28. Andreas,
    Thank you. A couple of things:
    Check out the ALD series ALD8100xx for example. Also I don't see a voltage value for the BX84 diode, ex. BX84C2V4.

    14.4k/180k = 2.7V; 12/180 = 2.66 volts

  29. Advice: Check out a company called Amperics, they sell 3.0v ultracapacitors for VERY inexpensive. I spent $80 an scored 40 of the 50F ones! They are tested and proven 3.0v (I personaly determined most don't breakdown until ~3.6-3.63v and the worst was 3.43v, so in series that's at least 6v easy!)

  30. Well done mate 🙂 But one thing you should mention for beginners (or i missed it?) is that in series you lose capacity 1/c total = 1/c1 +1/c2 +1/c3… 🙂

  31. very useful circuit information ! – thank you !! :-b – – Just wondering if Q1 is really a NPN transistor used as a MOSFET driver for Q2 (like a Darlington array) instead of the diode as you have shown. (reference http://pididu.com/wordpress/solarbike/supercapacitor-voltage-limiter/ )This is confirmed in this video : https://www.youtube.com/watch?v=dJklnvxLR5A

  32. But the commercial product, in the end, was unable to comply with the USB specs… it's an "happy end" or not, about them? I'm on my way buying some of these rounded chinese boards!

  33. Many thanks for this informartive video. My question : why do you use a zenerdiode on the transistor-base and which voltage ?
    Best regards from germany

  34. Hallo Andreas
    Regarding the "Gold Caps" there are also versions with 5.5V like this one sold by Conrad..Or are they different ones from your video (lower characteristics?)?

    As for the protection circuit I found this interesting video:


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