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Using an N-Channel MOSFET instead of an NPN transistor reduces main circuit voltage

ShockTactics

Mar 6, 2017
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Mar 6, 2017
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Hi Everyone,

I'm really stuck with this problem, any help or redirection would be seriously awesome.

I'm trying to make a TENS circuit and controlling it with a 5V signal. The NPN transistor I am using has a Base−Emitter On Voltage of 0.7V, and it gets very hot when I use it in this configuration:


upload_2017-3-6_18-22-54.png


so I used an N-Channel MOSFET with 3.5V - 5.5V gate threshold voltage as in the below configuration:


upload_2017-3-6_18-24-29.png


Now the MOSFET doesn't seem to overheat, but the shock I get from the electrodes when I hook the open poles up to my arm is barely noticeable.

I measured the Collector -> Emitter resistance as 40 ohms on the NPN transistor and the Drain -> source resistance on the N-channel MOSFET as 4 ohms so that made me think there should be less loss. Hence the overall outcome leaves me dumbfounded.


If anyone can help me understand what is going on here that would be amazing! Please let me know if there's any more detail I can give to help.


Cheers for the help,

Will
 

Alec_t

Jul 7, 2015
3,589
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Jul 7, 2015
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Welcome to EP.
The circuit you show is not a TENS generator. Where did you find it?
 

Alec_t

Jul 7, 2015
3,589
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I suggest you do a bit of Googling for TENS unit schematics. Be aware, though, that electric shocks can be FATAL, particularly for anyone with a heart condition or fitted with a pace-maker.
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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I have 3 different models of TENS units. Firstly they're not supposed to "shock" you. Secondly, though I posses the ability to design and build one I see absolutely no reason to do so. Ebay sells them cheaper than you could possibly pay for all the components, PCB, Case, Pads, Leads, etc. The most popular and best bang for the buck is the TENS 7000.

After seeing what you did in your first post do yourself and your family a big favor and.....
STOP WHAT YOU'RE DOING! :eek:

Chris
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
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The and to your direct question is that you are directly applying a voltage to the BE junction of a transistor. This is a very bad thing to do.

The second part is that you are applying an insufficient voltage to a MOSFET gate, and then subsequently not pulling it low.

Perhaps you should have a basic understanding of the components you're attempting to design with.
 

Audioguru

Sep 24, 2016
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In the datasheet of the Mosfet read what the drain-source current is at the gate threshold voltage. Almost nothing!. It is the threshold of it beginning to conduct so it is almost turned off. The threshold voltage has a range so some Mosfets need a much higher gate voltage to barely turn on. Give it 10V for it to be guaranteed to conduct properly.

Your circuit simply turns on the transistor or Mosfet. A high voltage is generated only if the transistor or Mosfet is turned on and off over and over with an oscillator.
 
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