mosfet help

[Tristan]

I would like to make a mosfet switch using a 555 timer connected to a
4017 counter and then use the signal given to me by the counter to
open and close a MOSFET switch. I would like to switch a reasonably
large load (about 5amps at 10 volts). Can anyone give advice?

In particular I am having problems controlling the MOSFET devices
(philips BUK456-60a/b). They don't respond to the counter signal and
even when I disconnect the counter from the MOSFET - so that no signal
current is supplied – the MOSFET still allows the load current to
flow.

 

[John Fields]

On 6 Jan 2004 11:44:12 -0800, tsterk@artic.edu (Tristan) wrote:

I would like to make a mosfet switch using a 555 timer connected to a
4017 counter and then use the signal given to me by the counter to
open and close a MOSFET switch. I would like to switch a reasonably
large load (about 5amps at 10 volts). Can anyone give advice?

In particular I am having problems controlling the MOSFET devices
(philips BUK456-60a/b). They don't respond to the counter signal and
even when I disconnect the counter from the MOSFET - so that no signal
current is supplied – the MOSFET still allows the load current to
flow.

---
If load current flows flows with the driving source (the counter)
disconnected from the MOSFET's gate, then that means that the gate is
floating in a charged state, allowing the MOSFET to stay turned on.

Connect the gate to ground when you disconnect the counter and, unless
the MOSFET has been damaged, the load current will drop to zero.


Assuming that your circuit looks like this:


10V
|
[RL]
|
D
4017OUT>-----G
S
|
GND


you must drive the gate to +10V to fully enhance the channel (turn on
the MOSFET).

If you're using a pullup resistor from the gate of the MOSFET to your
positive supply it could be that the 4017 can't sink enough current to
pull the gate down to ground to turn off the FET, but you haven't said
anything about what your circuit looks like, so it won't be possible to
zero in on what your real problem is unless you supply some more
details, among which the logic supply voltage would be nice to know.

--
John Fields

 

[Fritz Schlunder]

"Tristan" <tsterk@artic.edu> wrote in message
news:e22618ee.0401061144.2bccd5d0@posting.google.com...

I would like to make a mosfet switch using a 555 timer connected to a
4017 counter and then use the signal given to me by the counter to
open and close a MOSFET switch. I would like to switch a reasonably
large load (about 5amps at 10 volts). Can anyone give advice?

In particular I am having problems controlling the MOSFET devices
(philips BUK456-60a/b). They don't respond to the counter signal and
even when I disconnect the counter from the MOSFET - so that no signal
current is supplied - the MOSFET still allows the load current to
flow.

For those tuning in the MOSFET datasheet can be found here:

http://www.spelektroniikka.fi/kuvat/buk45660.pdf


Have you used MOSFETs before? There are a number of possible causes of the
symptoms you describe.

What does your schematic look like?

Are you sure you don't have the source and drain hooked up backwards? The
naming on those pins seems most counter intuitive (at least until you
understand the semiconductor device physics for the device's behavior) for
N-channel devices. In a normal N-channel MOSFET application current is
supposed to flow into the device through the "drain" pin and out through the
"source" pin. For normal use the source pin is connected to ground as shown
in John Field's little schematic.

What is the voltage drop accross the MOSFET when the device appears to be on
all the time?

 

[Tristan]

Fritz / John,

Thanks for your help – I have corrected the circuit - each peice of
advice was very useful!

I did have the source and drain connected incorrectly, that was the
significant part of the problem. I have also now use a couple of
diodes to protect the mosfet from voltage spikes caused by my load and
also placed a ULN 2003A chip between my circuitry and the mosfet
switches.

Again many thanks and sorry for the delay in getting back to you both.

-- Tristan.