Author Topic: What MOSFET to use?  (Read 1581 times)

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Offline xorgmc

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What MOSFET to use?
« on: September 25, 2016, 09:46:15 pm »
Hi,

I'm trying to build a LED fairy lights running from a Li-Ion battery (which is charged by a solar cell) with a timer (press a button and the LEDs turn on for about 3h). I want to do it completely analog with a capacitor and "a kind of transistor". First I tried using 2 BJTs in a darlington pair, but it didn't work out really good because either the Collector-Emitter resistance was too high (I guess) and the LEDs weren't bright enough when turning the timer on, or the Collector-Emitter resistance was too low and the LEDs were glowing when the timer was turned off (1mA is not a really good stand-by current on a battery). So a relative told me to use a MOSFET which sounded quite a good idea to me as it isn't controlled by current. But the amount of things that seem important when choosing one seem just massive to me  ::)

The operating voltage is of course about 4.2V - 3.3V, and the LED string lights nicely at about 200mA (40 leds in parallel - 5mA per LED) - I got that with a 3.3 Ohms resistor before the leds.

I thought about a circuit similar to this: http://2.bp.blogspot.com/-U_YWj3oGLgs/Ty3ByRJ7cLI/AAAAAAAAAE4/w106YZFBpwE/s1600/off-after-delay-switch-by-mosfet.jpg

Can anyone recommend a MOSFET that is suitable for such applications? Or should I use a completely different method?
 

Offline bobaruni

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Re: What MOSFET to use?
« Reply #1 on: September 25, 2016, 10:25:41 pm »
It could work OK using that method.
You will need a low gate-source threshold voltage VGS N-CH MOSFET and preferably one with reasonable drain-source on resistance RDS(on).
Keep in mind that as the capacitor discharges, the LEDs will eventually dim and the MOSFET will eventually start to dissipate some heat as the drain-source resistance increases as the channel is no longer saturated.
 

Offline xorgmc

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Re: What MOSFET to use?
« Reply #2 on: September 25, 2016, 10:44:35 pm »
It could work OK using that method.
You will need a low gate-source threshold voltage VGS N-CH MOSFET and preferably one with reasonable drain-source on resistance RDS(on).
Keep in mind that as the capacitor discharges, the LEDs will eventually dim and the MOSFET will eventually start to dissipate some heat as the drain-source resistance increases as the channel is no longer saturated.
Like this? http://www.produktinfo.conrad.com/datenblaetter/550000-574999/563625-da-01-en-MOSFET_N_KA_100V_32_ZVNL110A_TO_92_3_DIN.pdf

Is there a formular to calculate the drain current at a given voltage?
« Last Edit: September 25, 2016, 10:59:26 pm by xorgmc »
 

Offline xorgmc

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Re: What MOSFET to use?
« Reply #3 on: September 26, 2016, 02:32:32 am »
 

Offline Chalcogenide

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Re: What MOSFET to use?
« Reply #4 on: September 26, 2016, 06:22:25 am »
Take a look at the AO3400. It's logic level rated and can handle a couple of amps of current with <50mOhm RDSon. Super-cheap and small (maybe too small if you are new with soldering).
 


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