is there any way to reduce threshold voltage on fets?

[parkerbeder]

I have a switching touchscreen project that is supposed to be done on tuesday which involves switching circuits from a touchscreen. The problem is that the touchscreen switching circuit supplies volts, and I'm using irfz44 fets with a threshold voltage of 2 to 4 volts... well, go figure, they're not fully switching, i have about 1 volt out. is there any way to make them kick on 3.4 volts?

Thanks again for any help!

-Parker

 

[Sceadwian]

Nope. You'd have to use a charge pump circuit to supply the gate with a higher voltage, or something like a photovoltaic isolator, which works just like an optoisolator only there's a lot more photodiodes in series so they can output 8-10 volts at a few hundred ua, enough to charge a FET gate. How much current does the Mosfet have to pass, I'm assuming the Drain source voltage is the same 3.4 volts or is it different?

 

[audioguru]

You didn't read the datasheet properly.
The threshold voltage is the gate-source voltage that barely turns on the Mosfet with a current in the load of only 0.25mA.
The IRFZ44 Mosfet is guaranteed to conduct 31A with a gate-source voltage of 10V but some conduct 10A with a voltage of only 5V.

If your gate signal is 5V Cmos digital logic then you should use a logic-level Mosfet like an IRLZ44.
It is guaranteed to conduct 31A when its gate-source voltage is 5V but some conduct 10A with a voltage of only 3V.

 

[parkerbeder]

Can a guy throw ~6.8 volts to it, such as if that 3.4 went through a lm324 op-amp with a couple of 1K resistors in the voltage divider?

Thanks!

-Parker

 

[audioguru]

Can a guy throw ~6.8 volts to it, such as if that 3.4 went through a lm324 op-amp with a couple of 1K resistors in the voltage divider?

You don't know if your Mosfets need a gate voltage of 6V or 10V unless you measure them because each one is different. They are guaranteed to turn on completely if the gate-source voltage is 10V so give them 10V.

An LM324 quad or LM358 dual opamp are very slow. If the Mosfets use a high frequency pulse-width-modulation then the LM324 or LM358 will not work. If the input is a slow DC voltage that does not change often then for a 10V output these opamps need a supply of about 11.5V or 12V. Use a gain of 3 or more.

 

[parkerbeder]

I must be confused on how mosfets are rated. I had thought that if you supplied as much as the threshold voltage that they would switch... The gain of 2 is working in this application, as I tried it, and they are functioning. This is mainly because of their requirement to drive simple things slowly such as starter solenoids on motors, which switch only once an hour or so, for a few seconds, and care little about the voltage supplied as long as it is around 9-15 and an amp or two. Thanks for your information, I will have to better research how specifications work for fets... just to clarify, how did you figure out that these fets need 12v to output 10v.

Thanks again!

-Parker

 

[Sceadwian]

I must be confused on how mosfets are rated. I had thought that if you supplied as much as the threshold voltage that they would switch

Yep, that would definitly be an incorrect assumption. Mosfets can be a little tricky because you do have to learn to read the PDF. The gate drive required for a low RDS on state is dependent on both Drain/Source voltage and the current it's passing.

 

[audioguru]

just to clarify, how did you figure out that these fets need 12v to output 10v?

You forgot to tell us how much current you are switching.
You are using Mosfets rated at 31A so your current must be almost that high. Then the gate-source voltage must be 10V or more for all passable IRFZ44 Mosfets to turn on fully.

Your supply voltage is 12V. If you want close to 12V at the motor then the source-follower upper Mosfets must have a gate voltage of 22V or more because ... DUH!