3.3v Low Gate, High Output

Thread Starter

dingo

Joined Mar 11, 2011
53
I'm looking for a way to allow higher voltage flow with a low voltage enabling input.
I have an application that runs on a 1s lipo battery (3.7v-4.2v). I need to allow around 6v through.
I tried 2n7000, 2n3904, RFP30n06lE, and IRLB3034, but the output is not enough. It seems the Gate/Base input needs at least 5v to be "fully" on.
I've also tried low power reed relay and they work, but the poles sticks quite often and I'm not entirely confident on it.

Is there some solid state switch mech out there that will allow me to input a low voltage and allow fully 5-10v through? Battery efficiency is not a concern for me at the moment.
Thanks.
 

dl324

Joined Mar 30, 2015
10,536
I'm looking for a way to allow higher voltage flow with a low voltage enabling input.
I'm not sure what you mean by this. Could you post a schematic or block diagram of what you're trying to accomplish?
I tried 2n7000, 2n3904, RFP30n06lE, and IRLB3034, but the output is not enough. It seems the Gate/Base input needs at least 5v to be "fully" on.
You seem to be confused about how BJTs work. The base-emitter junction only needs to be forward biased by 0.6-0.7V for it to start turning on.

If you're using MOSFETs and have a 3.3V signal available to drive them, but the MOSFET is using a higher voltage supply, you need a logic level MOSFET.
 

dl324

Joined Mar 30, 2015
10,536
The thought is that I need a low voltage application to make some decision and it will decide when to turn power on to a higher voltage application.
Does the second "Power On" mean that the 3.3V circuit is controlling a transistor that switches power for the 5V circuit?
 

dl324

Joined Mar 30, 2015
10,536
Then you should be able to switch power low side with either a BJT or a logic level N channel MOSFET. High side switching would be more complicated because you need a 3.3V signal to be able to turn off a PNP transistor or P channel MOSFET.
 

dl324

Joined Mar 30, 2015
10,536
How are you determining from the datasheet that it can be used in this application?
You need a device that can handle 800mA maximum.

TIP29 can handle 1A and turn on with a 3.3V signal isn't an issue as long as it can provide sufficient current.

AO3400 can handle the current and turn on with 3.3V, but you need to turn it on hard enough so the on resistance will be low. At Vgs=3.3V, on resistance would be in the 20-25 milliohm range, so power dissipation wouldn't be an issue.
1586102128079.png
The current it would require from the 3.3V control signal would be insignificant.
 
Last edited:

crutschow

Joined Mar 14, 2008
24,749
Below is the LTspice simulation of a suggested circuit.
It uses a NPN BJT driving a P-MOSFET as a high-side switch to reduce bias currents.
The MOSFET must be a logic-level type [Vgs(th) should be no more than 2V max] with a maximum ON-resistance of 0.1 ohm.

1586103734514.png
 
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