How to switch 3A with a mircoswitch?

[piesoup]

Hello all

I make LED bicycle lights that draw 3A from the battery. To turn them on and off or to control the different modes, I have been using a 3A latching switch. Now to get one this small, bezel dia is 17mm and for it to be IP67 rated, they cost

[audioguru]

A "logic level" Mosfet switches very well when its gate is 4.5V or more.
Very few Mosfets switch well when their gate is only 3.7V.

[piesoup]

Thanks for the reply Audioguru.
So other than a relay, I should just stick with the big switch? Or is there another way for switching 3A?

Thanks again

[Hero999]

There are plenty of MOSFETs which will switch 3A with a gate voltage of 3.7V. The old IRL540 is has an R_ON of 0.11Ohms at V_GS = 4V and I_D = 14A. With a source voltage of 3.7V it'll be a bit worse but you only want to switch 3A so it should be fine.

What's the lowest battery voltage? It sounds like you're using L-ion? The trouble is it may not be good enough when the battery voltage is low. Going from the datasheet it appears it should be fine down to 3.5V.

http://www.vishay.com/docs/91300/91300.pdf

[piesoup]

Thanks Hero
I noticed that the VGS = 4V. Is that going to be a problem as a fully charged Li-Ion is 4.2v??

Also, how would I connect it to my circuit.
Drain and Source conected in series with my battery and light.
Gate, switched with a latching low current switch, but to Drain or Source? With what size resistor?

Ive found them on Farnell for less than a Pound so these would be great if I can get them to work!

Thank you for answering my questions again

[audioguru]

I noticed that the VGS = 4V. Is that going to be a problem as a fully charged Li-Ion is 4.2v??

Your "4.2V" battery will be only 3.0V when it needs to be re-charged. The datasheet does not list the voltage lost when the gate-source is only 3V but a graph shows that a "typical" IRL540 Mosfet will have a loss of only 0.2V at 3A. A "minimum" one might have a loss of 1V or more.
Where can you buy a "typical" one?

[Hero999]

The old IRL540 will probably do but the V_DS may be too high for what you're doing.

Any of the following transistors will be able to switch 3A with 3V.
http://www.onsemi.com/pub_link/Collateral/2SK4043LS-D.PDF
http://www.irf.com/product-info/datasheets/data/irl1104pbf.pdf
http://www.irf.com/product-info/datasheets/data/irl3103pbf.pdf
http://www.irf.com/product-info/datasheets/data/irl2203npbf.pdf