Two Mosfet's in series - Page 1
Forum section: electronics / beginners

author: mrx79
author_rank: Contributor
timestamp: December 02, 2014, 07:59:03 pm
content: Hi,first of all, hallo to all of you as i'am new here.I'am a huge fan of the blog, so i decided to register and to be part of the community.As i'am currently about the move to another house my lab is already packed and i have a short question i hope to get some help with as i can't test them on time by myself.I have a given circuit which switches a solenoid valve which is connected to 12v via a mosfet to ground. So far so good, simple and straight forward.This is also what i can't change, as the hardware is given.Circuit looks like: +12v -> Solenoid -> Mosfet -> GNDWhat i want to do is to insert some kind of "kill circuit" in between the solenoid and the controlling mosfet, which should allow or forbit the circuit to pull the solenoid to ground.So basicaly the total circuit will act like a AND-gate circuit.My basic question now is: can i easily put a second mosfet in between the solenoid and it's triggering mosfet?So: +12v -> Solenoid -> Mosfet #1 -> Mosfet #2 -> GNDThe solenoid is controlled via a PWM and current is aprox. 2amp's @ 100% dutycycle.Hope i made my point clear. I guess not the most complicated question.Hope someone can give me a hint if that work's or for what i have to look out.PS: at least the given mosfet #2 is a N-Channel type.

author: Zero999
author_rank: Super Contributor
timestamp: December 02, 2014, 08:48:50 pm
content: It will work but it isn't a very efficient way of doing it. There will be additional losses in the second MOSFET due to its resistance.It's more efficient to disable the PWM signal, when you want to turn the MOSFET off.

author: mrx79
author_rank: Contributor
timestamp: December 02, 2014, 08:54:24 pm
content: Thanks very much for the fast reply.Yes i know disabling the PWM would be the better solution unfortunatly i can't get access to the control circuit.But i think the additional losses will not effect the operation of the solenoid, so i think i'am fine. Will do some tests in advance of course.Best regards

author: loneoceans
author_rank: Contributor
timestamp: December 02, 2014, 08:57:49 pm
content: Yes at that sort of power (2A 12V) I don't expect to see much losses in your second Mosfet switch, and it will certainly work (a 12V relay would work as well). Like Hero999 said, it will be better if you can disable the PWM signal when you want it to be off. Another method is to put an AND gate in front of the gate of the Mosfet and you'll have one PWM input and one Enable input, if you can modify the control circuitry.

author: jlmoon
author_rank: Unknown
timestamp: December 02, 2014, 09:00:11 pm
content: How about using a CD4066 analog switch device to control the gate of that MosFet. In other words run the gate signal through one of the switches and use the respective enable pin to allow or disallow gate control. I know it is a waste of 3 other analog switches but they are cheap and very effective for what you're trying to do.Another way would be to pull the gate to ground via another small MosFet such as a VN10K series part, this would clamp the pwm and not allow D-S conduction, just be sure to buffer the pwm drive with a appropriate resistor before the clamp

author: mrx79
author_rank: Contributor
timestamp: December 02, 2014, 09:06:58 pm
content: I also thought a minute about a repeater circuit.Simulating the solenoid with a resistor and sense the input with a uC and then decide wether or not to recreate the same PWM on the output. So basicaly isolating the solenoid from it's original control circuit.But i don't like this idea, as it involes a lot of conditions which are unneccessary as i don't want to modify the PWM, i just want to turn it on or off.As saied, unfortunatly i can't get my hands on the gate of the original control circuit :-(If i could i agree with you it would be the way to go...But if there are some ideas which are more elegant which do not involve changing anything in the original control circuit, i'am glad to take them into consiteration.Thanks

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: December 02, 2014, 09:11:45 pm
content: What will be driving the disabling side, electronic/mechanical

author: rob77
author_rank: Super Contributor
timestamp: December 02, 2014, 09:45:06 pm
content: i'm usually doing it as high side N-channel switch (on/off controlled by a latching over-current protection) and a low side N-channel driven by PWM. the big disatvantage is that you need higher voltage for driving the high side N-channel's gate.eventually you could use a P-channel as a high side switch (higher Rdson, but no need for higher volatge).

author: victor
author_rank: Regular Contributor
timestamp: December 02, 2014, 11:19:00 pm
content: Put a AND gate on the gate of the mosfet, feed the pwm in one input and control the othe input as desired

author: jlmoon
author_rank: Unknown
timestamp: December 03, 2014, 12:29:35 am
content: Quote from: victor on December 02, 2014, 11:19:00 pmYes, I agree great idea, cheap & dirty solution!

author: void_error
author_rank: Frequent Contributor
timestamp: December 03, 2014, 08:46:55 pm
content: Well... if you can't access the gate of your N-channel MOSFET you could use (as rob77 mentioned)a P-channel one between +12V & solenoid like this:+12V ---> P-channel MOSFET ---> solenoid ---> N-channel MOSFET ---> GNDIt would be a good idea to have a reversed diode across the solenoid to protect the MOSFETs.

author: SL4P
author_rank: Super Contributor
timestamp: December 03, 2014, 09:15:55 pm
content: OK - I really hate to do this.No contributed value from me, and apologies to the OP.Fake Apostrophe Man stikes again.Two Mosfet's... (what) in series?Two Mosfet's 'pins'?- You could consider the Mosfet's Source and Drain pins in series... as the current flows through them to the load.Excuse: I couldn't avoid it - I'm bored.or you could Be asking about "Two Mosfets in series" in which case I'm completely useless.

author: rob77
author_rank: Super Contributor
timestamp: December 03, 2014, 09:36:10 pm
content: Quote from: SL4P on December 03, 2014, 09:15:55 pmit reminds me of this one

author: SL4P
author_rank: Super Contributor
timestamp: December 03, 2014, 09:41:37 pm
content: Yep - I deserved that - and a f**k as well.