# Charge capacitor using higher voltage

Hi,

I have a backup capacitor rated 1F/5.5V and I want to charge it to 5V with 7V source. How should I do that?

I want the circuit to work the same in the 4-7V range, given that <5V supply will charge the capacitor only to it’s voltage level, not 5V.

Is there some easy way to achieve this? Just lower the voltage before capacitor? Arduino will be used to monitor capacitor voltage and to cut-off the supply (transistor? relay?), when pre-set voltage is reached.

Any idea is welcome

Connect three diodes in series to drop 2.1 volts
7 - 2.1 = 4.9 volts on one farad capacitor.

Use a 15 watt light bulb in series to limit the current. During charging, the light is on. When charging is done, the light is not shining.

Calculate power, current and voltage.

Example :

i = C dv/dt

to charge in one second dt = 1
dv = 4.9
C=1

i = 1*4.9/1 = 4.9 amps

i thought about using diodes to get voltage lower, problem is that when i have 5V supply, I would like all the voltage to be on capacitor, not 2.9V using diodes.

I even though about 5.1V zener, but I don't know, how to isolate it from power supply (so I have 7V on one side of circuit, then some kind of breakage to 5.1V on the capacitor side).

Why not just use a 5v 100mA LDO regulator to charge the capacitor? Check that thw regulator limiting current is lower than the maximum charging current of the capacitor.

I know, I can’t connect it directly, don’t have me for fool.

I only want to charge it to 5V, so it’s somewhere around 90% of max. I haven’t seen higher voltage 1F that small (like 2 or 3 stacked 2032 batteries) and that “cheap” (3-4\$).

Capacitor is charged with solar cells, voltage is 4 to 7V. I want to get the max out of them - considering capacitor max voltage, that’s why I ask 1/2.

When I charge it from Arduino 5V output, it gets from 0V to let’s say 4.5V in no time (well, in minutes). But from 4.5V to 5V, it gets almost infinity (another minutes). I mean - when I reach 80-90% voltage, it gets soooo slow.

My idea is to give it more voltage, say 5.3V to charge it faster, but stop charging at 5V. That’s why I ask 2/2.

I would like to limit voltage just before the capacitor to ie. 5.3V, but just before the cap, not in the rest of circuit (or waste it as heat, ie. by using zener right on the solar cell). If there is lower voltage than 5V on solars, I want to use all that is possible (no diodes then, but the idea was good). I can do something in electronics, but only simple things. I just wanted to ask someone, who could know something that I don’t.

dc42: Wouldn’t there be significant voltage drop if input is under 5V? Anyway, I can switch between using regulator (input >5V) or direct connection (input <5V) with Arduino. Could you please give me the exact part name/type to search for? Something which is really common. I shouldn’t expect it to give me iie. 5.3V, should I? More like 5V, 12V and other common voltages, right?

The voltage drop of a low dropout regulator is very low when the regulator is lightly loaded. Parts you could consider include LP2950 (100mA) and MCP1702 (250mA). However, as the supply is a bank of solar cells that are intrinsically current-limited, why not just connect the capacitor directly to the solar cells, along with a 5.1V Zener diode in parallel?

Not all capacitors are the same. It sounds like you are talking about a carbon layer memory backup capacitor. They have very high internal resistance, and so are not really very good for storing energy for anything requiring more than a few 10s of uA.

polymorph: Yeah, that's probably the one I got. I tried (dis)charging with 100mA and it was good. maybe it will not hold for long, but I will keep it as is.

dc42: I wouldn't like to lose excessive voltage through heat. I don't have problem with regulator though, only part of the energy would be lost there, not all above 5V. Thanks for part numbers.

How should I connect the capacitor with regulator to the circuit so that there are minimal losses? I mean - should I use bipolar transistor or some FET to connect/break it to/from circuit?

All regulators and series's devices incurs loss.
The only way to minimise the loss, note not eliminate it, is to use buck or boost switching regulation. With that you only loose 10 to 20% of the power.

Grumpy_Mike:
All regulators and series's devices incurs loss.
The only way to minimise the loss, note not eliminate it, is to use buck or boost switching regulation. With that you only loose 10 to 20% of the power.

I concur. A linear regulator, a resistor, LEDs, diodes, simple PWM without an inductor, all will lose exactly the same amount of power in the end.

You must use an energy storage device, ie, and inductor in a switching regulator if you want better efficiency.

1ChicagoDave:

Super Capacitor Charge Controller Kit – Kitronik Ltd

FYI, that is a linear regulator.

You said you were going to monitor the capacitor voltage with the Arduino. Why not just use a p-channel MOSFET to turn on the charge when the voltage across the capacitor falls below a certain value and turn it off when it reaches 5V. Oh, and be sure to use a gate resistor on the MOSFET or there are people around here that split their gizzards.

Have a look at the schematic for a possible way to consider doing this.

I would recommend using SPICE to build out a sample circuit. This way, you can model different scenarios and watch the E, I, and power dissipation.

Or, download any of the free ones. Just be certain to model your super-cap to include the internal resistance. Also, a 1F may be off by 20% low... You can use a DVM, accurate voltage source, and 1% resistor(s) to verify the actual capacitance by monitoring the discharge time.

Ray

It is a bad idea to have a potentiometer wired from V+ to Pin 7 without another fixed resistor in series. If (when) you turn that trimmer all the way down, you'll have shorted pin 7 to V+.

BillO:
Have a look at the schematic for a possible way to consider doing this.

This looks interesting. I have some questions though:

1. Can I directly connect the 6,5V from solars to capacitor (provided that I cut it off before reaching capacitor max voltage)? I always thought that I shouldn't charge the capacitor with more than its maximum voltage. Well, my electronics experience is not really good...
2. Why is there resistor on gate (G)? To prevent frying my Arduino because of high current to G? Shouldn't I have one larger (ie. 22k) resistor connected between G and S?
3. Wouldn't there be a problem with zener? I always thought it would "eat up" all the voltage above its rating.

There are more I'm measuring voltage on solars (source) with Arduino too. This is to prevent Arduino from dying if the voltage drops too much (it's powered from solars) = when the Arduino is low on voltage, it cuts capacitor from source, so it can regain power for running itself.

1. How would your circuit behave in the way of measuring source voltage? Wouldn't the source voltage be the same as capacitor voltage when MOSFET is turned on?
2. Wouldn't the zener limit the source voltage if the MOSFET is on and the capacitor is still charging?

I'm sorry, if these questions look dumb, but I don't know much about electronics, only basics.

I will try to write how I understand your schematics - Solars have enough power, Arduino is on and it opens the MOSFET, charging begins. Capacitor's voltage goes up, but solars voltage drops, because the capacitor is "hungry" at the first moments. Arduino keeps itself on, because it closes and reopens the MOSFET repeatedly keeping the solars voltage high enough to power Arduino. MOSFET open state gets prolonged as the capacitor voltage rises and at (ie.) 3V MOSFET stays on (because capacitor isn't that "hungry" and solars voltage doesn't drop that much) until capacitor reaches 5V. Then Arduino closes the MOSFET.

Sorry to write it as a story, but I think it's the most understandable description. Is it how it would work? My main fear is from frying the capacitor with too much voltage and from dropping the solars voltage too much (which should be prevented by software) when MOSFET is opened.

Pavouk106:
This looks interesting. I have some questions though:

1. Can I directly connect the 6,5V from solars to capacitor (provided that I cut it off before reaching capacitor max voltage)? I always thought that I shouldn't charge the capacitor with more than its maximum voltage. Well, my electronics experience is not really good...

The solar cell has an internal resistance that will prevent the open circuit voltage from appearing across the capacitor as it charges. How long that takes depends on the current capability of the solar cells and the total resistance in the circuit, however the Arduino is capable of responding in a few micro-seconds so this should not be a problem. Just to be on the safe side, do you have links to datasheets or specifications on both the capacitor and the solar cell? That would help. It may be that we need to add a resistor coming from the solar cell.

1. Why is there resistor on gate (G)? To prevent frying my Arduino because of high current to G? Shouldn't I have one larger (ie. 22k) resistor connected between G and S?

Theoretically the gate resistor is there to limit the current sourced by the Arduino. Again, I am not sure what size MOSFET you will need, nor how often you will need to turn on and off the MOSFET. I'll have a better idea once you let me know the specs of the capacitor and the solar cell. I do believe it is highly unlikely you will absolutely need one however, I was soundly castigated recently on this site for suggesting that there are cases where they are not needed. So I promised to be a good boy and suggest them so as not to give other people heart trouble. It's only a few cents, and having one certainly will not hurt in this application. Use 75-150 ohms.

1. Wouldn't there be a problem with zener? I always thought it would "eat up" all the voltage above its rating.

It is only there as a safety. If for some reason the Arduino fails to shut off the MOSFET, the zener will prevent the capacitor from being over charged. Since the zener is rated at 5.3V and the Arduio will shut off the MOSFET at 5V, it should never come into use under normal conditions.

There are more I'm measuring voltage on solars (source) with Arduino too. This is to prevent Arduino from dying if the voltage drops too much (it's powered from solars) = when the Arduino is low on voltage, it cuts capacitor from source, so it can regain power for running itself.

1. How would your circuit behave in the way of measuring source voltage? Wouldn't the source voltage be the same as capacitor voltage when MOSFET is turned on?

There is no need to do this. If the Arduino is using all the solar power and all the capacitor power, disconnecting the capacitor will not help. The capacitor is not like a battery and will only charge if there is excess power available and is only then a load on the circuit. If there is only enough power to keep the Arduino going, the capacitor being connected will no hurt the situation. I think you can skip doing this altogether.

1. Wouldn't the zener limit the source voltage if the MOSFET is on and the capacitor is still charging?

Only if the Arduino fails to turn off the MOSFET.

I'm sorry, if these questions look dumb, but I don't know much about electronics, only basics.

Not everyone can be expected to know everything about electronics, and the only way we can learn is to seek knowledge. There are no dumb questions. The only dumb thing is not to ask.

I will try to write how I understand your schematics - Solars have enough power, Arduino is on and it opens the MOSFET, charging begins. Capacitor's voltage goes up, but solars voltage drops, because the capacitor is "hungry" at the first moments. Arduino keeps itself on, because it closes and reopens the MOSFET repeatedly keeping the solars voltage high enough to power Arduino. MOSFET open state gets prolonged as the capacitor voltage rises and at (ie.) 3V MOSFET stays on (because capacitor isn't that "hungry" and solars voltage doesn't drop that much) until capacitor reaches 5V. Then Arduino closes the MOSFET.

Almost. I would power the Arduino from after the MOSFET. From directly across the capacitor. Set the Arduino to turn on the MOSFET when the voltage reaches about 3.5V. This will charge the capacitor and power the Arduino. When the voltage reaches 5V the Arduino switches off the MOSFET and the capacitor runs the Arduino until the voltage drops to 3.5V, then the cycle repeats.

3.5V for the turn on point is only a guess. If that is not appropriate for your application, choose a voltage that is (4V, 4.5V, what ever is best).

Sorry to write it as a story, but I think it's the most understandable description. Is it how it would work? My main fear is from frying the capacitor with too much voltage and from dropping the solars voltage too much (which should be prevented by software) when MOSFET is opened.

I'll need the specs of the capacitor and the solar cell to help much further. It would also help to know the current draw of the Arduno and any other circuitry while it's working. Also helpful would be knowing what other circuitry is being used.

The thing is, that Arduino should be there controlling the (dis)charging of the capacitor, not running from it, so it should be run right from the solars. I have to keep the Arduino alive by disconnecting capacitor when voltage drops, because I need it to hold SW variables.

Other things are really ok as you suggested them.

Solar cells are "china made", came without absolutely anything beside "5,5V/100mA" information, which is, to be honest, not the best info. Four of these in parallel runs Arduino at 6,7V at bright sunlight and above 5V when overshadowed from direct sunlight at around 27mA. I will measure current through and voltage on 56 ohm resistor tomorrow, supplying it with those 4 solars in parallel. It should give us some idea how much power these can do.

Capacitor's datasheet is here (I have 1F version), it's memory backup capacitor, I was running it at 50mA discharge many times, still going well (it is not probably designed for this task I know).

Okay, a few questions from my end.

1. What is the capacitor for?

2. Do you know the load (current draw) of your proposed circuit?

3. What does your whole design do?

BillO:

1. What is the capacitor for?

Driving motor (for a few seconds)

BillO:
2) Do you know the load (current draw) of your proposed circuit?

Shouldn't be more than 50mA for those few seconds

BillO:
3) What does your whole design do?

You didn't want to know that - solar tracker. The capacitor is there to power the turning motor, solars wouldn't have enough power to do it on their own. The main goal is to do it, not to use it for the rest of my life. I would like to make it on my own, not by "copy-paste" style of someone's work.

I still don't have all the parts, I'm missing the motor. But I still have troubles with capacitor, so who cares about missing motor at this moment I know, You would like to know specs of it. "China made on 5V less than 25mA", no info about stall current. I will be pretty surprised, if it all works as it should at last.