Struggling to choose a mosfet - it's so confusing!

Other Hardware General Electronics
1

Hi everyone,

I've been researching mosfets for a few hours and still don't really understand them fully!

What I'm trying to do is switch the following on or off by using Arduino pins:

Switch on a coil in a contactor. The coil is a 24v coil which takes 200ma of current. (I think that is too much, but I'll work on that another time)

Switch on an inverter monitor. The monitor is 24v and takes about 20ma of current.

The thing is, for the second job, I think I can use a Toshiba C1815 transistor. I have loads of these and I understand the documentation.

But what about the first job? The documentation for mosfets confuses me, and when I try to find a mosfet which is saturated at 5v, I just can't find one. A lot of them seem to saturate at 10v.

Please help!

:slight_smile:

2

Watch this:

.

3

Are you comfortable with SMD?
This one works well from Arduino pins.

Unfortunately the thru hole versions have been discontinued unless you want to buy 3500 of them:

4

The phrase to look for is "logic level". Ignore the Vth rating, that is not relevant. Saturation means
something completely different for FETs, say "on" and "off" when using a MOSFET as a switch.

The key parameters are forward voltage, on-resistance "Rds(on)", and the Vgs quoted alongside
the Rds(on) value. For instance 0.05 ohms Rds(on) at Vgs=4.5V.

5

Thanks for your help everyone!

I understand now (I think)!

I've bought these:

http://www.ebay.co.uk/itm/162474456782?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

VDS I need 30v, this allows 55v.
Drain Current I need 1a, this allows 1.75a at 100c and it won't be running that hot.
RDS on 5v = 150mohm.

Thansk for your help!

6

You probably didn't think of this, but that MOSFET also has Zener diodes attached to the gate, meaning it's far less likely to be damaged by static electricity during handling. A pretty great choice if you don't know the proper procedure for handling an ESD sensitive device.

The only thing to be wary of is that the Absolute Maximum value for the gate voltage (Limiting Values in the datasheet) is +/- 13V. Be very careful if you ever try to use these things with a 12V gate voltage.

7

In the initial post you said 200mA+20mA, now you say 1A drain current. With a large on resistance like 0.15 ohms
your device will run quite warm - not a problem though, but much lower on-resistances are available.

8

MarkT:
In the initial post you said 200mA+20mA, now you say 1A drain current. With a large on resistance like 0.15 ohms
your device will run quite warm - not a problem though, but much lower on-resistances are available.

I do actually only need 200ma. When I said I needed 1a, it was just a generalisation. Antony...