Go Down

Topic: Charge capacitor using higher voltage (Read 12283 times) previous topic - next topic

Pavouk106

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

AmbiLobe

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


Pavouk106

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).

dc42

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.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Pavouk106

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?

dc42

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?
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

polymorph

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.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

Pavouk106

#7
Jul 05, 2013, 06:10 pm Last Edit: Jul 05, 2013, 06:11 pm by Pavouk106 Reason: 1
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?

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.

polymorph


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.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts


polymorph

Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

BillO

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.

mrburnette

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.
http://www.falstad.com/circuit/

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

polymorph

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+.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

Go Up