# I need a simple design

Discussion in 'The Projects Forum' started by pyrillix, Jan 31, 2014.

1. ### pyrillix Thread Starter New Member

Jan 31, 2014
25
0
I have a need for a simple circuit to drive a mosfet. The mosfet i have is a high current design (80a). The driver would be operated by a npn proximity switch (6 to 30 volt). The driver would only operate on average around 10 hz, with a max of 25 hz. I am willing to pay a small fee for your assistance.
I have specs for the mosfet and proximity switch I use if needed.
The simpler and more rugged the better, would prefer the driver to be a discrete component..

Jan 31, 2014
25
0
3. ### wayneh Expert

Sep 9, 2010
12,080
3,018
Your application appears to be very simple. Perhaps I am missing something. At that frequency, you don't really need a "driver", you just need a voltage that switches quickly between 0V and >10V. The exact meaning of "quickly" matters. How big a load are you actually switching?

4. ### pyrillix Thread Starter New Member

Jan 31, 2014
25
0
It actually is fairly simple, the load is prob 50 amp max, resistive mostly. The existing circuitry is complex and has other features that are no longer needed, tore all that out.
Right now it literally is a mosfet and a proximity switch, but the voltage the mosfet is switching does vary from 0 to 30 volts.
I thought it might be best to use a driver to get a rapid rise and pull down time as I would like a square a wave as possible..

5. ### pyrillix Thread Starter New Member

Jan 31, 2014
25
0
Oh and I have 5 or 14 volts to drive the proximity switch which is rated at 6 to 30 volt device but seems to work fine on 5 volts at least at this slow cycling rate.

6. ### wayneh Expert

Sep 9, 2010
12,080
3,018
Ah well, now we're learning of a specification. That means we need numbers. What does the input look like, and just how square is square? In other words, what switching time do you need?

7. ### pyrillix Thread Starter New Member

Jan 31, 2014
25
0
Nothing critical, just like as fast a rise time as I can easily get at that slow a switching speed, It really isnt critical. I am a master electrician, can solder down to 1mm, but am an amateur electronics guy..

8. ### pyrillix Thread Starter New Member

Jan 31, 2014
25
0
the power supply to the prox switch is separate from the power supply to the mosfet but is connected on a ground bus

Jan 31, 2014
25
0
10. ### pyrillix Thread Starter New Member

Jan 31, 2014
25
0
the prox switch is a standard 3 wire npn 6 to 30 volt device

11. ### wayneh Expert

Sep 9, 2010
12,080
3,018
Well, I'm wondering if you need a comparator to "square up" the input if it's not already a clean square wave. It sounds to me like the comparator output would be plenty fast for your application. If the input is already pretty good, it may be able to drive the MOSFET directly without the comparator.

Thing is, the gate of a MOSFET is essentially a capacitor. Your ability to turn it on and off quickly depends on how quickly you can charge and discharge that cap thru the gate pin. Switching speed is proportional to current. And the faster you go, the more important the inductances become. You can get ringing. Will you be able to put an oscilloscope on this thing?

12. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,758
1,099
Which way does the output switch when an object is detected? Can you give a link to the spec/datasheet?

13. ### ChateauduChillon New Member

Jan 28, 2014
24
0
+1 the output of even a cheap comparator will have no problem driving the capacitive load of a power mosfet gate, especially at the frequencies you're worried about.

What is currently being used to generate the control signal?

14. ### mcgyvr AAC Fanatic!

Oct 15, 2009
4,769
969
make sure you put a BIG heatsink on that mosfet..

15. ### ronv AAC Fanatic!

Nov 12, 2008
3,281
1,226
Specs would be good or sure as the devil we will turn it on when you wanted it off.

16. ### pyrillix Thread Starter New Member

Jan 31, 2014
25
0
Sorry about posting this twice but really need some help, if so inclined please check out this post..

Last edited by a moderator: Feb 2, 2014
17. ### MrChips Moderator

Oct 2, 2009
12,410
3,353
No need to create a new thread. Simply post a reply to bump thread.

18. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,758
1,099
We're still waiting for the spec/datasheet of the proximity switch.

Jan 31, 2014
25
0
20. ### pyrillix Thread Starter New Member

Jan 31, 2014
25
0
p.s. I do have a massive heat sink, and kickback protection already in place from original design so nothing needed there.