Advice on Mosfets

I'm a complete novice when it comes to MOSFET's so I could use some advice about switching some led's with a picaxe 8m2. The power comes from a solar powered 12v battery and each group of leds uses around 20ma. Could I use a suitable Mosfet to directly switch the power or do they have to be used to switch on a relay, I don't envision the total power passing through one to be greater than 400ma.

Thanks

look in manual 3 page 8.

bit overkill for a few led's

Just trying to find out the alternatives and keep it as simple as possible.

Mosfet or Transistor circuit is all you need.

Hi,

Assuming you're running the PICaxe from a 5 volt regulator then almost any "Logic Level" FET will do. But below about 4 volts, rather more care may be needed. The 2N7000 is a simple, general purpose, low-cost FET, perhaps not quite good enough for a full 400 mA alone, but you could put a few in parallel. Others may have their own particular "favourites".

In practice a bipolar (NPN) transistor or a relay can do much the same job. They may appear less efficient (requiring an additional current into the base or coil) but the main loss of useful energy is in the LEDs' (essential) series resistor. For best efficiency you could use a switched-mode constant-current driver, but that's a considerable complication. Also, you may prefer that when/if the 12v battery voltage falls, the LEDs get dimmer (extending the remaining life) as happens with a simple resistor.

Cheers, Alan.

AllyCat said:

Hi,

Assuming you're running the PICaxe from a 5 volt regulator then almost any "Logic Level" FET will do. But below about 4 volts, rather more care may be needed. The 2N7000 is a simple, general purpose, low-cost FET, perhaps not quite good enough for a full 400 mA alone, but you could put a few in parallel. Others may have their own particular "favourites".

In practice a bipolar (NPN) transistor or a relay can do much the same job.

Click to expand...

For a current less than half an amp, I would use a BC337 (NPN, max 800mA). It has a typical current gain of around 100 at around 300mA, with Vce of 1v. The 1v voltage drop can be considered to be part of the series resistance, so "efficiency" is not a problem. The base current would need to be just a few milliamps, so much more efficient than a relay. Each thread of LEDs would need its own current limiting resistor.

A question on the LEDs. If you are taking the LED's current value from a catalogue and not the data sheet, you may not be seeing what you need to know. What is the maximum rated steady forward current? If it is 20mA then de-rate the drive to 15mA. The will be no perceivable reduction in brightness and the LEDs will last a lot longer.

Thanks for all the advice its given me plenty to think about when building future projects. Cheers

TYKE said:

...I could use some advice about switching some led's with a picaxe 8m2... I don't envision the total power passing through one to be greater than 400ma.

Thanks

Click to expand...

Take a look at the ULN2003 which has 7 drivers in a 16 pin dual-in-line chip. These are very cheap (probably about 35pence in UK), will interface to 5V Picaxe and will cope with 400mA at 12V.

Also take a look at my post re: not wasting power with LEDs: http://captainbodgit.blogspot.co.uk/2013/11/dont-waste-power-with-leds.html

Cheers Steve