12v open drain circuit

Other Hardware General Electronics
1

I would like to switch 12v to ground at a certain frequency........If I use MOSFET whats the configuration for "open drain"? Does the "gate" voltage have to higher than 12v?

2

Use an N-channel logic level MOSFET like the IRL520 -- it will be fully on with a gate voltage somewhat less than 5V.

3

AOI514 from digikey, 5V will turn the gate on full, the Rds is really low compared to IRL520, and it's much less expensive:

Can be driven from 74HC595 even, here are 32 on a board (AOI516 or AOI518, very similar characteristics), with Arduino driving 4 74HC595 to control the MOSFET gates:
http://www.crossroadsfencing.com/BobuinoRev17/

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4

thevault:
I would like to switch 12v to ground at a certain frequency........If I use MOSFET whats the configuration for "open drain"? Does the "gate" voltage have to higher than 12v?

What current are you looking to switch? That dictates the sort of mosfet you need. With the right mosfet, you don't need 12V gate voltage, 5V or in come cases even 3.3V is enough.

5

MOSFETs are all about gate drive level and on-resistance. Logic-level MOSFETs
will switch with 5V drive, others typically need 10V.

You calculate the power dissipation from the load current and MOSFET on-resistance,
and decide if the MOSFET is good enough (and whether a heatsink is needed).

For instance a 10 milliohm MOSFET will happily switch upto 5A without a heatisnk
(250mW dissipation), a 2 milliohm MOSFET upto 12A without heatsink.

For high frequency PWM you also have to consider switching losses and switching speed.

6

Using an MOSFET N-channel feeding "Gate" 2.5vpp square and switching "Drain" 12v, "Source" with 4.7k resistor to ground. Result is an oscillating square but with .5v and I need a least 8v.

7

I got it......Had the Drain and Source switch and also put the load on drain side instead of source.... :fearful:

8

It's switching 12v DS gnd but it's not going down to 0volts but a 1volt....... :fearful:

9

So connect Source to GND, and take the output from the Drain.

OpenDrain drive.png
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10

That's what I have........11volts at the Drain through 1k, Source GND.....I'm trying switch to GND.....it switches but not to GND only to 1volt...

Adjusted gate voltage and also adjusted offset..........can't get to zero.......

11

What are you using for a transistor? A Logic Level gate, Low Rds, N-channel MOSFET is the best - such as the AOI514.

12

N-channel MOSFET VN0300L

13

Poorly performing part at low Vgs. Need 10V to get it to turn full on.
AOI514 turns full on with <5V Vgs.

Vgs of 4.5V, conducts 20A.

Id of 300mA at Vgs = 5V: