piesoup Posted November 4, 2011 Report Share Posted November 4, 2011 Hello allI 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 Quote Link to comment Share on other sites More sharing options...
audioguru Posted November 4, 2011 Report Share Posted November 4, 2011 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. Quote Link to comment Share on other sites More sharing options...
piesoup Posted November 4, 2011 Author Report Share Posted November 4, 2011 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 Quote Link to comment Share on other sites More sharing options...
Hero999 Posted November 5, 2011 Report Share Posted November 5, 2011 There are plenty of MOSFETs which will switch 3A with a gate voltage of 3.7V. The old IRL540 is has an RON of 0.11Ohms at VGS = 4V and ID = 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 Quote Link to comment Share on other sites More sharing options...
piesoup Posted November 6, 2011 Author Report Share Posted November 6, 2011 Thanks HeroI 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 Quote Link to comment Share on other sites More sharing options...
audioguru Posted November 6, 2011 Report Share Posted November 6, 2011 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? Quote Link to comment Share on other sites More sharing options...
Hero999 Posted November 6, 2011 Report Share Posted November 6, 2011 The old IRL540 will probably do but the VDS 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.PDFhttp://www.irf.com/product-info/datasheets/data/irl1104pbf.pdfhttp://www.irf.com/product-info/datasheets/data/irl3103pbf.pdfhttp://www.irf.com/product-info/datasheets/data/irl2203npbf.pdf Quote Link to comment Share on other sites More sharing options...
login
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.