Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: Martin1512 on April 03, 2010, 01:54:38 PM

Title: MOSFET
Post by: Martin1512 on April 03, 2010, 01:54:38 PM

Hi, I am making motor driver for my DC motor. I found nice schematics on http://schematics.circuitdiagram.net/viewer.php?id=cqs1235544146p.jpg. (http://schematics.circuitdiagram.net/viewer.php?id=cqs1235544146p.jpg.)

Its really good but i want to change it a little . I want to delete 2N3904 and 2N3906 transistors and use BS170 and BS250 MOSFETs. I want to use just 6V(5*1.2) => also change resistor values . Will it be ok without 2N390* transistors ? What are they for ?

Thanks

Title: Re: MOSFET
Post by: cyberfish on April 03, 2010, 05:25:02 PM

I'm GUESSING it's because the microcontroller's 5V output may not be enough to switch the MOSFET, so they added the transistor to amplify the range.

When microcontroller outputs 5V, Q1 will turn on, and G will be close to ground (because of the R2/R3 voltage divider). When it outputs ground, Q1 will turn off, then R2 will pull G up to 12V.

Note that the voltages are inverted, but now we have 0V to 12V.

Title: Re: MOSFET
Post by: Soeren on April 03, 2010, 06:42:34 PM

Hi,

Quote from: Martin1512 on April 03, 2010, 01:54:38 PM

Hi, I am making motor driver for my DC motor. I found nice schematics [...]
Its really good but i want to change it a little . I want to delete 2N3904 and 2N3906 transistors and use BS170 and BS250 MOSFETs. I want to use just 6V(5*1.2) => also change resistor values . Will it be ok without 2N390* transistors ? What are they for ?

First you claim that it's a nice schematic and then you wanna change it - Why?
(I won't go into details about the serious flaw it obviously have).

Do you have an engineering degree that enables you to actually improve the circuit, or is it just the regularly seen misconception of "BjTs are old, so let's change everything to MOSFET to make it better"?

Trust me, you wanna have BjTs there to get reliable switching.

If you want a better H-bridge, try this (http://www.bobblick.com/techref/projects/hbridge/hbridge.html).

Title: Re: MOSFET
Post by: Admin on April 03, 2010, 09:39:37 PM

My recommendation is just buy a motor controller. It'll save you tons in time, frustration, and reliability. They don't cost much these days, anyway. ;D

Title: Re: MOSFET
Post by: Martin1512 on April 04, 2010, 03:43:47 AM

Well before, i wanted to make BJT. But then i found BS170 and BS250 in shelves so i tried to make MOSFET H-Bridget with them. My Atmega32 turned them on. And motors were working pretty well. So i just wanted to know why are there these 2N390*, when everything works without them. I am total beginner with electronics, thats the reason I am asking...

Title: Re: MOSFET
Post by: Soeren on April 04, 2010, 09:17:05 AM

Hi,

Quote from: Martin1512 on April 04, 2010, 03:43:47 AM

[...] My Atmega32 turned them on. And motors were working pretty well.

It might seem so at first, but had you measured them up (eg. the R_ds_on and the time in the linear region), I am sure you would have seen them as inferior to the BjTs (in this application).

Each device have its place and it's all about choosing the right ones for a given application.

Title: Re: MOSFET
Post by: Martin1512 on April 05, 2010, 11:34:44 AM

Ok thanks, i will try H-Bridge you sent me. :)