Capacitor Question

A note to everyone who reads this particular post asking about capacitors: I know that they are not allowed at FIRST competitions. Please do not tell me that they are not allowed.

Anyway, I am about to compete in a fuel-cell engineering competition, and I need some help incorporating a capacitor into my design.

What I would like to do, is develop a circuit where the fuel cell charges the capacitor prior to the race. At the start of the race, the capacitor is discharged to the motor specifically so I can get a boost in speed. My understanding is that a motor whose voltage and amperage are doubled produces four times the horsepower that a motor would otherwise do. Anyway, the capacitor discharges to the motor dumping as much energy as possible, then the fuel cell provides direct power to the motor for the remainder of the race.

I need to know exactly how I can get this to work, and possibly determine where I can get a capacitor that can reasonably do the job.

My first thought was to find some sort of supercapacitor and develop a setup similar to this. (WARNING: PDF link.)

That would help in the voltage control.

The remaining problems are:

  • find and get the supercapacitor (cameras? or store-bought?)
  • develop a circuit to do what I described above

A 10F (farad) capacitor holds about 36 Joules of energy, which–if dumped instantaneously–can move a 1kg fuel cell car 8.48m/s. (I think) A more reasonable speed would be 5m/s. The fuel cell can then directly drive the motor the rest of the 10m race. Average speed of over 2m/s should be fast enough, as the fastest car to my knowledge last year averaged slightly over 1m/s.

So. How should I accomplish what I want to do? I’m not sure how to design a circuit which will accomplish the starting-line-energy-dump, and I don’t know where else aside from to get a 10F capacitor.

Your help is greatly appreciated.

For the circuitry I would probably look up how the flash on a camera is wired up and setup your system like that, just without the flashbulb :wink:

Flash dumps plenty of volts, not necessarily amps.

The capacitor’s job in my case is to provide a motor with twice its normal voltage and twice the normal amperage in some period of time.

I tried a camera flash thing where the motor was wired up to the bulb terminals…I don’t have a motor anymore. :smiley:

I guess I should have mentioned that you would want a much smaller coil so that you dump maybe 12-14 volts and get more amperage. If I can find where I put that stupid flash for my old camera I will test it and see if it works.

This sounds like a good idea except that a capacitor for the most part will lose it’s energy fairly quickly. You need to remember that the motor is a very low impedance. When you connect the cap to it, a large amount of the charge will be dumped instantaneously while the remaining charge will dump out over time. Check out capacitor discharge curves and time constants in a reference. The ARRL Radio Amateurs Handbook is a great source for this and is available at most libraries. The amount of current available and the time of useful discharge are a function of the load impedance (resistance) and the cap internal resistance. To make this effective, a bank of several capacitors may be needed, which gives diminishing returns on energy vs. weight for your application.


You’re definitely right. The capacitor will lose its energy quickly. That’s what I want.

The capacitor’s job is to dump the energy into the motor in order to give my fuel-cell car 4 times the horsepower to get the car going. It’s sort of like having a head start by going faster than the competition. The fuel cell will be responsible for taking the car the rest of the way. It’s only 10m. A 10F capacitor has roughly 36 Joules. 36 Joules dumped instantaneously will get a 1kg car going somewhere around 8m/s. Given the discharge characteristics of a capacitor, I don’t really expect much more than 5m/s for the first second of the race. That’s plenty fast for me.

Before anyone else responds, please take note that I am a high school junior whose role on a FIRST robotics team was nowhere near electrical engineering. I am completely new to this, and will probably ask the questions of a newbie.

As long as you are aware of these things…know that a 5ms pulse will not get the car to 8M/s due to other physical laws. You can connect capacitors in parallel to increase the capacity. I don’t know what your weight budget is but some of the caps that you can find for car audio applications might be just what you are looking for. The super caps you find listed in many of the electronic catalogs are designed more for memory backup applications. They are not designed for high currents but can keep CMOS memory backup working well for long periods of time. Manufacturers sometimes actually list them as batteries. For best results, you want to keep the series resistance between the caps and the motor(s) as low as possible. To achieve this, use the same techniques as you would on an FRC robot. Use large guage wire (#10 or larger) and make the wire runs as short as possible. I am guessing you are planning on using some kind of switch to connect the caps to the motor so be sure your switch can handle the instantaneous currents you will be running. Please also remember that the motor is not 100% efficient when you are making your calculations. Just to be on the safe side, be sure to short the caps after you run to insure they are not holding a charge when you have completed the run. Normally, I would recommend “bleeder resistors” in your design, but I think they would be counter productive in this application. When choosing a cap, look for one with low ESR (equivalent series resistance) for best results.

Allow me to clarify my previous post.

I meant 5 meters/second, not 5 milliseconds.

My weight budget is theoretically unlimited as long as the final machine fits into a 30cm x 30cm x 60cm box.

The motor voltage is somewhere in the neighborhood of 3volts. So, a final output voltage from the capacitor of 6volts should be doable. The amperage as well. 9A discharge from the supercap that I would like to use should be more than enough.

Choosing the capacitor isn’t too much of a problem for me, unless the supercap battery link I posted earlier has some property which I am not aware of.

You’re right, the motor is nowhere near 100% efficiency, but neither are my competitor’s motors (we all use the same one). So for comparison purposes, I think I’m fine.

So far Al, you have provided me a wealth of information. Thank you for that.

However, I need to design a circuit which performs as follows:

  1. The fuel cell charges the capacitor before a race
  2. At the start of the race, the capacitor dumps its power into the motor, giving me a boost of speed similar to using NOS on a car.
  3. Once the capacitor is depleted, the circuit then directly routs the electricity from the fuel cell to the motor, so no energy is wasted charging the capacitor.

I have very little experience designing these kind of circuits, and that is where I need help. The supercapacitor’s properties seem to be sufficient to provide just the power I need in order to get the car to jump out of the starting gate, and then pretty much coast to the finish line.

I hope my intentions are clearer.

is this the iyfcc?

not quite. It’s a branch of the Department of Energy’s Sciencebowl. Here’s the competition I’ll be attending.

The article you linked to is for a 9 volt source using the Super Cap. notice that the device is designed to supply about 1ma. If you read the sidebar, you will see that the referenced Super Cap can only supply 6 amps max at 2 volts.
The Super Caps, shown in the digikey catalog all have fairly high ESR, some as much as 30 ohms. A value of that magnitude would severely limit the current delivered to your motors.
Are you using any type of controller that would interface to spikes or other relays? If so, you could usually connect a relay to switch the caps between a motor and charge circuit, controlled by the robot controller. You might want to monitor the voltage across the cap and then switch to the internal battery when the cap voltage falls to a particular level. When that occurs just switch the motors to the other power source. Some of the caps on this page might be more of what you are looking for. Values of 100,000 mfd at 6.3 volts for about $7 each. You can parallel these caps as I said before for larger values. They specify an ESR of 0.01 ohms, about the same as our competition batteries.

The other day I was helping the physics teacher at the school and he wanted me to design a project for the class to do concerning capacitors. He brought in these huge capacitors which I believe belonged to an A/C system. It powered a FP motor supplied in the kit for about a second. I’m not sure if the capacitor made the motor stronger or faster, but discharging the capacitor with a wire gave a nice arc, something not characteristic to a 9V battery. Also, the supercap capacitors look very neat, and if I’m right for a school project, your motor is probably between 1.5V to 3.0V. The supercap’s limit is 2.7V, but I think it might take 3V for a situation like this, where you don’t do this over a prolonged period.

For the circuit that disconnects the capacitor it would be great to use a relay even though my mentor calls it ancient arts. as for the capacitor try to look for transmitting capacitors used in amateur radio, not the big antenna tuner ones, but the smaller ones used in the actual radio

this capacitor will probably serve your needs

Fortunately, Motors are inductors. Therefore, simply dumping a capacitor across a motor should be a “good thing”, although I wouldn’t expect anything near the 100% efficiency you were calculating. However, I still think it is a good idea if only for the education.

A simple method would be to simply wire the cap in parallel with the fuel cell. This will give a minor boost, but you know you want something better than that. :slight_smile:

Lets break the circuit into a few key components:

  1. Fuel Cell power source
  2. Really Big Capacitor
  3. Boost Converter to charge 2
  4. Switch to activate the system

It seems you already have 1 taken care of. Can you give us specs? Also, do you need to stop at the end of the race? Do you need to steer?

For 2, be sure that it is rated to handle some voltage. A lot of these super caps are rated for 2-5 Volts. Remember, your boost comes from the difference between the capacitors voltage and the fuel cell’s voltage. If the fuel cell pumps out 3V, you’ll want something rated a bit higher.

For 3, boost regulators are really cool circuits that convert a low voltage, high amperage source into a high voltage low amperage source. Since you apparently have a large start time to charge the cap, this is perfect. Google “Step Up” or “Boost” Regulator. Also, check out and . Alternatively, search for my previous chiefdelphi post on boost regulators. I really love these guys.

For 4 you have a few options. The simplest one would be a single pole, double throw switch. One throw would connect the fuel cell, the other would connect the capacitor. This way, you would flip one switch and everything would come to life. Before this, the boost converter would be busy charging your cap.
Alternatively, you could use a nice MOSFET, a relay, or (my favorite) an SCR. However, they probably aren’t worth the headache.

Good Luck!

Also, you might want to make sure this is legal.



I can use pretty much whatever I want in this particular race. Capacitor and included circuitry included. Just can’t have anything else producing the power.

Also, the DigiKey links you all posted don’t work for me. Could you please put in the part number(s)? That would help me find what you’re talking about.

The fuel cell specs are pretty low at best.
The manufacturer says 1v, @ 500ma. (1 volt 500 miliamps: not sure what the abbreviations are.) So, I shouldn’t have a problem charging the capacitor.

I cannot put any sort of radio-control equipment.

This is a straight 10m drag race-style competition. I can use the barrier to stop the car at the end of the race.

And can someone confirm this: "Doubling the voltage and amperage of the motor means I get 4 times the original horsepower. "

That’s the whole reason why I want to double the voltage between the capacitor and motor, so I can have more power applied to the wheels to give me that extra boost. Eric, from your post I am under the impression that you intend to use the capacitor as a secondary power source, not as a way to dump power into the motor and have the motor in turn give off 4 times its normal horsepower.

Each motor is designed for a specific current based on the strength of the magnet. Automatically dumping in twice the rated current may get you nothing more than additional heat. If your motor was rated for 3-6 volts then you are going to see an increase in power with twice the input. All things being equal, power in equals the square of the current times the load resistance. Since you are doubling the current, the power would go up by a factor of four less the losses.
Based on the fuel cell providing one volt, you can use a voltage boost convertor to charge the caps. The charge voltage curve is the inverse of the discharge curve, but remember that at the start of charge, the cap is taking a lot of current. You could then use a timer to place the cap in series with the fuel cell so you get that extra boost (cap voltage plus fuel cell) to start and then switch to fuel cell alone for the remaining time. I bet that if you looked at the 555 timer circuits and you used the capacitor bank as the timing component you could simplify design somewhat.
You say the rules allow charging before the run? Is so, I think you may be able to get the caps charged up. I was thinking, can you also use mechanical storage? i.e. use a motor to run up a flywheel and let the stored energy in the flywheel drive the base.


I can’t thank you enough for your effort in providing me the help I need to get this project off the ground. I know I can’t say this enough, but thank you for your response.

As for the motor, presumably dumping twice the voltage into the motor would also have the motor draw twice the amps, which is where I get the “4 times the horsepower figure” “When you double the voltage, you also double the amperage that a motor can draw. This means that the power that can be produced goes up by a factor of four.”](

I still don’t quite understand the charge/discharge characteristics of a general capacitor (supercap or otherwise).

Do I need to charge it at the rated voltage? (i.e. 1v source into a 2v cap)
Can I discharge it at a different voltage? (i.e. 50V cap into a 6v motor, or 2v cap into a 6v motor)

As for other forms of energy on the car…As long as its powered by the fuel cell, it’s generally allowed, within reason (No fusion/fission reactors, rubber bands, etc.) As for flywheels, they are definitely easier to implement for me, but engaging them to the drive wheel doesn’t seem too feasible given the size and power limitations. (Weight as you remember is unlimited)

All this circuit talk has my head spinning. The 555 Timer circuit I know is pretty common, but I have no idea how it works. Is there some sort of simulator or circuit-building program that you recommend? That might make things easier.

I just need to design a circuit to incorporate a capacitor into my design so that I get a boost at the start of a run.n

You can charge a capacitor to any voltage. It only becomes a problem when you are trying to charge it to a voltage that is higher than it’s rating. You can use the 1 volt fuel cell to power a voltage multiplier much like the article in your post above. Maxim has a few handy ICs that are designed for these applications. If your motor is rated for 6 volts then charge a cap up to 6 volts and then dump it into the motor. 6 volt capacitors are generally smaller in size than a 16 or 25 volt cap but you can still use the higher rated cap charged to 6 volts.
Capacitors are funny little beasts. When charging, the voltage starts out very small and rises exponentially until max charge is reached. This takes about 6 time constants to achieve, where t=RC. The voltage that appears at the terminals will be V(cap) =Vin(1 - e^{-t/RC}) if you are solving for time. The charge current is the inverse of this. Current is maximum at the start of charge and falls off to zero when a cap is fully charged. In discharge both current and voltage are reversed. The cap will deliver max voltage and current at the start of discharge both falling to zero at the end of discharge, again as an exponential function. this link has a good curve and explanation
The 555 timer is a special circuit that uses the function of RC timing (cap charge/discharge) to perform timing functions. If you were to use the same cap for both motor function and timing, that would be pretty cool.
Once you take a look at the charge discharge curves, you will start to get a feel for the action of the cap in your application. During charge you can have one time constant that is determined by the charging circuit series resistance and chosen so that you do not exceed the fuel cell 500 ma max current. On the discharge side, your time constant is now determined by the series resistance of the motor and wires. The discharge constant will be very short due to the low series resistance of the motor. Say you were using a motor like the Fisher-Price motor. It has a resistance of about 0.12 ohms. With a bank of caps rated at 0.5 farad the bank would be fully discharged in 6 time constants or 6
0.5*0.12=.36 seconds. But it would only produce some useful current for perhaps 1 or 2 time constants. Remember that the using the equation above, one time constant will discharge the cap to 37% of it’s fully charged value. So in 0.06 seconds the voltage in the cap will be down to 2.2 volts but the current will be as high as 50 amps. With all things considered, you might find that in 0.06 seconds, you may have not been able to transfer enough current to get the motor turning. Only experimentation will tell, different motors will have different resistance and power curves. Good luck.
As to the flywheel, all you have to do is have a pulley attached to your drive wheels that pivots against the flywheel. When the race starts, just allow the pulley to fall against the flywheel to conduct the flywheel energy to the drive wheels. A spring would help do that as well as having a grippy surface on the pulley. Then use the fuel cell to drive a motor that spins up the flywheel.