automatic capacitor charging cut off circuit

Thread Starter

Dabu WhiteHacker

Joined Sep 5, 2017
65
hi, today I found a toy which had 3.3F 2.5v capacitor in it. now I want to design a circuit that charges the capacitor to 2v and then cut the supply. I would be using 5v supply(from mobile chargers).
s-l300.jpg
here are some ideas that pop into my mind;
1. Use LM393 comparator with fixed reference on of its input.
2. Use TL431 adjustable shunt regulators to turn of a mosfet on and off.
 

ci139

Joined Jul 11, 2016
777
I am sorry to say that - but your design (options/(specific)requirements) highly depend on - the rest of the circuitry around it

. . . you can put TL431 to cut the source + ? shunt the Super-Cap. (incase of hi speed charging) . . . or -- if you charge it from 10mA light batteries - the low power CMOS comparators and switches are to be preferred as they won't take too high share off your 10mA source . . .
 

Thread Starter

Dabu WhiteHacker

Joined Sep 5, 2017
65
I am sorry to say that - but your design (options/(specific)requirements) highly depend on - the rest of the circuitry around it

. . . you can put TL431 to cut the source + ? shunt the Super-Cap. (incase of hi speed charging) . . . or -- if you charge it from 10mA light batteries - the low power CMOS comparators and switches are to be preferred as they won't take too high share off your 10mA source . . .
there no circuit around it, and power supply can supply upto 1000mA
 

crutschow

Joined Mar 14, 2008
23,500
Here's the circuit using an LM317 regulator powered from the 5V and adjusted to give a 2V output, as AH suggested.
That will charge the cap to 2V and then maintain that voltage.
LTspice simulation below:

1575520526440.png
 

Thread Starter

Dabu WhiteHacker

Joined Sep 5, 2017
65
Here's the circuit using an LM317 regulator powered from the 5V and adjusted to give a 2V output, as AH suggested.
That will charge the cap to 2V and then maintain that voltage.
LTspice simulation below:

View attachment 193767
what is the purpose of R3
this circuit seems promising and i would probably use this circuit
but this circuit waste a lot of power and also charges capacitor slowly and also makes the regulator hot
I was thinking of a circuit that would mosfet for power efficiency and would turn off when it detects 2v at capacitor terminal
but anyways thanks a lot
 
what is the purpose of R3
this circuit seems promising and i would probably use this circuit
but this circuit waste a lot of power and also charges capacitor slowly and also makes the regulator hot
I was thinking of a circuit that would mosfet for power efficiency and would turn off when it detects 2v at capacitor terminal
but anyways thanks a lot
However you do it, from a 5V supply, it will consume the same amount of power unless you go for a switch mode solution.

100 ohms and 3.3uF gives a time constant of 330us. Is that slow?

You could eliminate R3 and rely on the LM317 to limit the current. It will consume the same power but reduce the charge time to less than 6.6us.

[Edit] I just spotted that it is a 3.3F capacitor. That would increase the charge time. In the case without R3 it would be 6.6s. To make it quicker than that you would need a 2V supply with a higher curren rating - up to the maximum current rating of the capacitor.
 

Thread Starter

Dabu WhiteHacker

Joined Sep 5, 2017
65
However you do it, from a 5V supply, it will consume the same amount of power unless you go for a switch mode solution.

100 ohms and 3.3uF gives a time constant of 330us. Is that slow?

You could eliminate R3 and rely on the LM317 to limit the current. It will consume the same power but reduce the charge time to less than 6.6us.

[Edit] I just spotted that it is a 3.3F capacitor. That would increase the charge time. In the case without R3 it would be 6.6s. To make it quicker than that you would need a 2V supply with a higher curren rating - up to the maximum current rating of the capacitor.
yes i wanted to go with switch mode solution, using mosfet as switch
 
You need to find the specification for the capacitor which you have. I looked up a 2.5V 3.3F capacitor and it specified a maximum discharge current of 300mA. It didn't give a maximum charge current.
 

Thread Starter

Dabu WhiteHacker

Joined Sep 5, 2017
65
You need to find the specification for the capacitor which you have. I looked up a 2.5V 3.3F capacitor and it specified a maximum discharge current of 300mA. It didn't give a maximum charge current.
my main concern is not to charging that capacitor faster but charge it efficiently thats why i want to use some kind of switch like mosfet to turn off 5 when capacitor reaches 2v
 

crutschow

Joined Mar 14, 2008
23,500
I just spotted that it is a 3.3F capacitor.
Yup, I obviously missed that also.
my main concern is not to charging that capacitor faster but charge it efficiently
Why are you concerned about efficiency if the voltage source is a mobile adapter (I presume from a vehicle)?
this circuit waste a lot of power and also charges capacitor slowly and also makes the regulator hot
Certainly heating is not a significant factor in just charging up the one capacitor.
Peak regulator power, if the charge current is limited to 300ma, is about 1.1W, which drops to 50mW after the cap is charged.
(The efficiency with the LM317 circuit will be about 20%).
The charge time is about 30s.
 
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