DIY Project for Series Mosfet Build

[DRiley]

Hello All,

I am currently working on a project to build my own box mod. I want to build my mod in series and I done a bit of researching to find a set up that works for me. I have a diagram of what it will look like and the components I am using. I just wanna know if there is anything else I should know or if my design could be simplified in anyway. Any help would me much appreciated.

Here is what I have so far. Thanks again!

 

[Bunnykiller]

theres no voltage regulation... running batteries in series without regulation is a situation that could go quite wrong... you are basically still running batteries in series in "mech" style... only limit you have is the pop point of the Mosfet...
and why 2 fuses? only need 1

 

[DRiley]

theres no voltage regulation... running batteries in series without regulation is a situation that could go quite wrong... you are basically still running batteries in series in "mech" style... only limit you have is the pop point of the Mosfet...
and why 2 fuses? only need 1

Is there anything you'd recommended for the voltage regulation? As for the fuses I assumed it need two since I'm still using two batteries as if it were in parallel.

 

[vitchaya89]

good

 

[Suburban]

Been looking into this same idea. What exactly does a mosfet do?

 

[caramel]

Beware that some Mosfets may have very low gate losses, so they will keep firing even after you depress the switch. You might want an additional pull-down resistor.

Additionaly, in the absence of a pull-down resistor, some dirt / moisture / electrostatic charge on the switch may cause the Mosfet to unexpectedly fire.

 

[Podunk Steam]

It's like a magnetically held contact, switches a higher amperage without passing it through the switch itself.

 

[Suburban]

Ok I get it. Thanks for the answer.

 

[DRiley]

Beware that some Mosfets may have very low gate losses, so they will keep firing even after you depress the switch. You might want an additional pull-down resistor.

Additionaly, in the absence of a pull-down resistor, some dirt / moisture / electrostatic charge on the switch may cause the Mosfet to unexpectedly fire.

Did you pull-up resistor? I couldn't find any mention of a pull-down online

 

[DRiley]

So i just ordered all the parts hopefully in the next week or so I can give you all an update on how it's going. Cheers til then!

 

[caramel]

look at R1 here

 

[Bunnykiller]

Is there anything you'd recommended for the voltage regulation? As for the fuses I assumed it need two since I'm still using two batteries as if it were in parallel.

with the setup you have there is no voltage regulation, the mosfet is being used as a switch, so it will see the full voltage supplied by the batteries, and the batteries will see the amp load created by the coil. The real reason people use mosfets is because of the source of hi amp capacity, small sized switches are/was not easily found... there has been an increase of hi amp miniature switches showing up on the market lately for wired application. Normally, a relay would have been used to conduct hi amp currents with a low volt activation method.

 

[Bunnykiller]

as far as the 2 fuses.... just use 1 at 30A... if your batteries are rated for it ( 30+A)

 

[DRiley]

look at R1 here

So I need to add a 10k resistor to the gate and source on the MOSFET?

 

[caramel]

So I need to add a 10k resistor to the gate and source on the MOSFET?

Yes, and make the other resistor (from MOSFET gate to push-button switch) 100 Ohms.

The reasoning is that the two resistors will act as a voltage divider so when pushing the switch you will get a voltage on the gate Vg = Vb*R1/(R1+R3) where Vb is battery voltage. You want to keep Vg above the gate threshold voltage for the particular MOSFET you use (as per datasheet). With R3 100 and R1 10k, you only lose about 1% of Vbat. You can pick other values in a 1:100 ratio too. like 270 and 27k.

 

[DRiley]

Yes, and make the other resistor (from MOSFET gate to push-button switch) 100 Ohms.

The reasoning is that the two resistors will act as a voltage divider so when pushing the switch you will get a voltage on the gate Vg = Vb*R1/(R1+R3) where Vb is battery voltage. You want to keep Vg above the gate threshold voltage for the particular MOSFET you use (as per datasheet). With R3 100 and R1 10k, you only lose about 1% of Vbat. You can pick other values in a 1:100 ratio too. like 270 and 27k.

This is in addition to the 15k resistor on the gate and source yes?

 

[caramel]

It's very hard to understand from your drawing where exactly the 15K resistor is connected.

Assuming that it is between the Gate and Source, then it is indeed the "pull-down" resistor.

And then my recommendation would be to connect the Gate to the Pushbutton through an 150 Ohms resistor not a wire. The purpose is to limit currents in the event that the MOSFET fries and the gate isolation goes bad.

 

[DRiley]

The 15k is connected to gate and source. Yeah the picture isn't super clear.
Also for the 150 ohm resistor the gate is connected to master on/off switch and the goes through the push button.

 

[caramel]

Sounds good.

 

[smaugmra]

Looks like an interesting project. I think I will keep tabs on it to see how it goes.