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science_geek
Registered Member

Date Joined May 2007
Total Posts : 240

Posted 11/6/2009 8:51 PM (GMT -8)

I need to pretty much make an H bridge, i have a prop pin rigged up and switching the led on, but the led is really dim, i have tried switching out the series resistor for the led but it doesnt make any difference. i am only using one fet on the H bridge because i will only ever drive the motor in one direction. the circuit is wired up exactly as this site shows except for the switch is coming from a high signal on the prop pin (http://brunningsoftware.co.uk/FET.htm) any help with getting this to work would be greatly appreciated

Peter Jakacki
thinker tinker

Date Joined Sep 2006
Total Posts : 930

Posted 11/6/2009 9:02 PM (GMT -8)

That's not a H-bridge, the site only shows a simple FET switch. Did you post the correct link?

Anyway, your FET needs to have a low gate threshold, usually referred to as logic level gate drive. Your description also seems at odds with the link so maybe you should just attach an image of the circuit as you have it hooked up.

BTW, use the URL button to start a URL and end it so that it is clickable like this
brunningsoftware.co.uk/FET.htm

*Peter*

science_geek
Registered Member

Date Joined May 2007
Total Posts : 240

Posted 11/6/2009 9:12 PM (GMT -8)

that is the right page, im not making an H bridge really, i just want to do a pwm out of the prop to the gate of a fet, the fet will then allow me to send the amps and voltages needed to the motor. its really only 1 corner of the h bridge with the motor connected from the input voltage to the d side of the fet, the s side of the fet then gets connected to the ground, when you turn "on" the fet it turns on the motor. so an H bridge but only in one direction... the problem is that im not using a motor for this yet becuase i want to get it to work with the led, but the led is really dim but flashes like its supposed to.

Mike Green
Registered Member

Date Joined Oct 2004
Total Posts : 13554

Posted 11/6/2009 9:13 PM (GMT -8)

You can't use that MOSFET. The Propeller provides about 3V for a HIGH output voltage and this isn't enough for many MOSFETs, even those that are designed for logic level switching. You need a gate to source voltage that's well below 3V. Typically the datasheet for the MOSFET will have a graph for the amount of current switched vs. the gate voltage. The gate voltage for the amount of current you want to switch has to be well below 3V.

Here's one example of an MOSFET that will switch up to about 3A with a gate voltage as little as 3V:
www.fairchildsemi.com/ds/FD/FDFS6N754.pdf

Peter Jakacki
thinker tinker

Date Joined Sep 2006
Total Posts : 930

Posted 11/6/2009 9:26 PM (GMT -8)

@Mike I was wondering which MOSFET it was using and I didn't notice the reference to BS270. That's a shocking FET for anyone to use for such purposes, the transfer characteristics are abysmal.
@geek You are better off using a bipolar npn in that circuit. Another MOSFET you can use besides the one Mike mentioned is the IRF3708 which will work well at 3.3V gate driver and handle more than enough current.

*Peter*

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