Driving with a MOS-fet a load at 3V

[Tarsil]

I'm trying to make a economic lamp for camping. I want to use 3X1Ah cells. The led's wil be switched on/of at a freq about 75-150Hz. I need a MOS-fet for driving the led's. Should conduct at only 3V. Got any tip?

 

[kchriste]

Look for a logic level MosFET. Something like an IRLZ44 meets it rated Rds-On spec at 4V and looks like it'll conduct well enough at 3V to drive LEDs.

 

[Analog]

What's wrong with a bjt?

 

[philba]

ok, it's question time...

what kind of cells are you using? I suspect you will have higher than 3V with 3 cells. why do you care about switching? for dimming?

I would think your primary concern is getting the longest life out of your batteries. any voltage over the Vf of the LEDs is wasted unless you use something like a DC-DC converter that outputs at the LEDs Vf (or slightly above). there are lots of switch mode chips made to power LEDs with a varying input voltage and very high effeciency. With one of those, you can probably drive the LEDs until the batteries are sucked dry.... You will get a lot more light that way.

 

[justDIY]

another job for the joule thief!

**broken link removed**

 

[Tarsil]

What's wrong with a bjt?

The BJT was my first solution but if I use a MOS-FET, the voltage drop wil be much lower.

 

[Tarsil]

ok, it's question time...

what kind of cells are you using? I suspect you will have higher than 3V with 3 cells. why do you care about switching? for dimming?

I would think your primary concern is getting the longest life out of your batteries. any voltage over the Vf of the LEDs is wasted unless you use something like a DC-DC converter that outputs at the LEDs Vf (or slightly above). there are lots of switch mode chips made to power LEDs with a varying input voltage and very high effeciency. With one of those, you can probably drive the LEDs until the batteries are sucked dry.... You will get a lot more light that way.

I use 3XNICd @1Ah. The led's I have (10mm @50ma) can be conected without a resistor to 4.3V for short periods. I'l try to keep the circuit as simple&smaller as posible (so no smps chips). The bat will give me a bit more then 3.6V most of the time.

 

[Tarsil]

Look for a logic level MosFET. Something like an IRLZ44 meets it rated Rds-On spec at 4V and looks like it'll conduct well enough at 3V to drive LEDs.

Yup. This seems to be the ideea. But if the voltage drop is to high i'l use a BJT.

 

[Tarsil]

another job for the joule thief!

**broken link removed**

The ideea is to build a eficient lamp, not necesary the smalest.

 

[Tarsil]

I've bought a IRL530..this seems to be the ideea. Th guys!