charging supercap through transistor

Thread Starter

mikewax

Joined Apr 11, 2016
184
this may seem like a really dumb question, but the transistor dynamics just make my head spin.

i've wired it up like this:

5v-----12ohms------npn------1Fcap------G

the npn is switched from an I/O pin like this:

pin(5v)-------1kohm-------base

according to an online transistor calculator, i can't get more than about 90mA through the transistor (2n2222a) with this circuit. i need like 4 times that much current and i need to be able to switch it on and off from the arduino. does anyone know about this?

thanks, mike
 

wayneh

Joined Sep 9, 2010
17,496
If you use a logic-level MOSFET transistor as a switch, your current is limited by only the 12Ω resistor instead of the base current for the transistor.
 

Thread Starter

mikewax

Joined Apr 11, 2016
184
talk about senior moments, i had a whole senior day. THANK YOU expert and wayneh, i'll order the MOSFET and use the stacked BJTs in the meantime. now i can get back to work. :)))

one more thing though. if i gate the MOSFET with an analog output pin(0-5v), can i thottle the gate-source current up and down? i'll write a routine that will read the capacitor voltage through one pin, calculate dV/dt, and then increment/decrement the voltage on the gate pin untill the dV/dt is equal to some constant value. sound like a plan?
 

Alec_t

Joined Sep 17, 2013
14,280
The FET current changes dramatically with only a small change in gate voltage around the Vgs(threshold) value, so you would need a very fine resolution D/A output to give adequate current control. A bjt might be a better option than a FET for that purpose.
 

Thread Starter

mikewax

Joined Apr 11, 2016
184
The FET current changes dramatically with only a small change in gate voltage around the Vgs(threshold) value, so you would need a very fine resolution D/A output to give adequate current control. A bjt might be a better option than a FET for that purpose.
OK i'll try it thanks. :)
 
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