# Charging 35V capacitor from 1,5V AA battery.

Discussion in 'The Projects Forum' started by LukeP, Aug 30, 2012.

1. ### LukeP Thread Starter New Member

Aug 29, 2012
4
0
I need to charge a 35V capacitor 1000μF,
starting from 1,5V AA battery.
The capacitor will be discharged into a solenoid, then the cycle restart.
Capacitor charging time must be around 6 seconds.
How i can do it saving board space and maximizing battery life?
Thanks.

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
The 1.5 V battery will never charge the capacitor to 35 V.

If you multiply 1000e-6 times the charging resistor eg 1KΩ gives a time constant of 1 second. In this amount of time you will get to 63% of the driving voltage, 1.5V or about 0.945 volts. In 5 or 6 time constants you'll get much closer to 1.5 Volts but you'll never get all the way there. Discharging 1.5 Volts into a solenoid will result in a pretty wimpy response unless the solenoid is designed for that voltage.

3. ### LukeP Thread Starter New Member

Aug 29, 2012
4
0
i was thinking about a boost converter to raise the voltage up to 35V... to charge the capacitor.

4. ### JDT Well-Known Member

Feb 12, 2009
658
85
Bit of a sweeping statement, that! Not directly, of course. But with a flyback switching circuit it can be done. Think of a camera flash for example.

He doesn't say there is any limit to the current that can be drawn from the 1.5V supply. Designing an efficient switcher to work at 1.5V is not easy. But, I repeat, it probably can be done!

5. ### LukeP Thread Starter New Member

Aug 29, 2012
4
0
From various datasheet 1,5V AA batteries can take around 200mAh drawing without reducing battery life. i can double the battery pack, 2x1,5V AA, to have a initial range 2-3V to start with.

Disposable camera flash gain much more V from 1,5V AA (around 300V i think), but they have only 27 shots to do!
i want to do 1000 cycle from that 2x1,5V AA, charging 1000 times the capacitor.
A separate trigger circuit actuate the solenoid.
Then the capacitor restart to charge and so on.
I can also extend charging time from 6seconds to 10-12seconds, to limit excessive battery drain, but less time is better.

So fly back it's a good start?

6. ### JDT Well-Known Member

Feb 12, 2009
658
85
According to my calculations (and maths is not my strong point), to charge 1000μF to 35V requires
Q=C/V = 0.035 coulombs.

To do this in 6 seconds is an average power of 0.035/6 = about 5.8mW.

Sounds do-able.

I would adapt a circuit like this:-
http://ludens.cl/Electron/ledlamp/ledlamp.html

To get more voltage I would add extra turns between the collector and the diode. Experimentation needed!

7. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
Possible but very inefficient at such a low input voltage. Could do it cheap with a free running 555C timer driving an NPN with an inductor to boost up to 35V. The 1000uF cap needs an average current of about 6 mA to charge up to 35V in six seconds. That's roughly 200mW. It's possible.

Here is a similar design I made to generate 13V from 1.5V input.

• ###### ScreenHunter_01 Aug. 30 14.15.jpg
File size:
127.2 KB
Views:
55
LukeP and #12 like this.
8. ### #12 Expert

Nov 30, 2010
16,671
7,318
The 13 volt zener will have to go and probably want a disable command (pin 4 to ground) when 35 volts is achieved.

LukeP likes this.
9. ### LukeP Thread Starter New Member

Aug 29, 2012
4
0
Thanks for imputs!
So, to start, i'll double the initial voltage: 2xAA, so initial range it's 2V-3V.
Yes, i need something to stop charging at desidered level.

10. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
http://www.datasheetcatalog.org/datasheets/1150/64052_DS.pdf

The LM3578 will run down to 2V input, it's a very simple and cheap simple boost regulator. You can just set it to regulate at 35V and it will never charge above that.

The 555C/NPN approach I posted will also work, just need a 35V zener to clamp the voltage. It burns more current since it's a shunt regulator. You would need to increase the amount of inductance until you get enough energy to charge the cap in six seconds or less. The inductor shown on the schematic is only 100 uH.

http://www.coilcraft.com/dt1608c.cfm?utm_source=supplyFrame&utm_medium=SEP

Last edited: Aug 31, 2012