Current Supply to DC motor

[kahlveen]

Hi,

My problem is like this: I am using the PWM output from a PIC16F73 to control the speed of a DC Motor. The motor is connected to a 12V supply and when loaded draws up to 8A of current. Is there an IC I use to control the motor? I've heard of the L298 that some use to drive small loads (2A) but its not enough.

Can anyone help?

Thanks

[Comatose]

What are you trying to control the motor to do? A simple logic level FET low side and a diode will drive it in one direction at varying speed off of a pic. If you need multiple directions, then you'll need an H bridge, which at the minimum is four transistors and two drivers.

[kahlveen]

What are you trying to control the motor to do? A simple logic level FET low side and a diode will drive it in one direction at varying speed off of a pic. If you need multiple directions, then you'll need an H bridge, which at the minimum is four transistors and two drivers.

Just a simple speed control via the PWM. I do not need to vary the direction so a switch that can be turned on and off by the PWM will do. But the switch needs to be able to take up to 8A right? if not it overheats and dies?

This logic level FET low side, do you have the part number I can buy off the shelf or from farnell?

Thanks

[slipstick]

There are many published designs for speed controllers and nearly all use logic level FETs driven by a PIC. Have a look at the links thread at the top of the forum for some pointers.

There are hundreds of appropriate FETS. Look for one with at least 30A rating because you don't want to use them right up to their limit unless you have a huge heatsink and very good cooling. I'd probably use something like an STP40NF03L (40A 30V) or STP45NF06L (45A 60V) but there are many others.

Steve

[TugBoat]

I would suggest you start with this thread that lists brushed motor ESC designs (these are all controlling a DC motor using PWM at currents of around 12A for a S400 motor):

http://www.rcgroups.com/forums/showthread.php?s=&threadid=157193

You will see that most of these use an N-channel FET to control the motor and you need a Schottky diode across the motor.

The 8A is not a problem. But you need to select a FET with a very low Rds figure at 5 volts Vgs. There are lots of FETs that will do the job: IRF2203N, IRF3706, IRL1404, IRF7456, 2SK3355, etc. If you look at the ESC designs in that thread you will find they use all sorts of FETs. The problem is normally finding one that is in stock at a reasonable price (in small quantities). So find out what is available and then check the parameters for the part.


Tim

[kahlveen]

There are many published designs for speed controllers and nearly all use logic level FETs driven by a PIC. Have a look at the links thread at the top of the forum for some pointers.

There are hundreds of appropriate FETS. Look for one with at least 30A rating because you don't want to use them right up to their limit unless you have a huge heatsink and very good cooling. I'd probably use something like an STP40NF03L (40A 30V) or STP45NF06L (45A 60V) but there are many others.

Steve

Eureka... thanks!

[kahlveen]

I would suggest you start with this thread that lists brushed motor ESC designs (these are all controlling a DC motor using PWM at currents of around 12A for a S400 motor):

http://www.rcgroups.com/forums/showthread.php?s=&threadid=157193

You will see that most of these use an N-channel FET to control the motor and you need a Schottky diode across the motor.

The 8A is not a problem. But you need to select a FET with a very low Rds figure at 5 volts Vgs. There are lots of FETs that will do the job: IRF2203N, IRF3706, IRL1404, IRF7456, 2SK3355, etc. If you look at the ESC designs in that thread you will find they use all sorts of FETs. The problem is normally finding one that is in stock at a reasonable price (in small quantities). So find out what is available and then check the parameters for the part.


Tim

I don't understand. What is very low Rds at 5Volts Vgs?

[vintage1]

A fet that goes hard on with ONLY 5v on the gate. Many FETS expect 12-15v on the gate to turn hard on.

Arranging that adds complexity so buy the 'high gain' FET.

[kahlveen]

A fet that goes hard on with ONLY 5v on the gate. Many FETS expect 12-15v on the gate to turn hard on.

Arranging that adds complexity so buy the 'high gain' FET.

ok i get it... so for example the IRL1404 suggested by tugboat goes on with Vgs = 4.3V and Id = 40A and rds(on) at 5.9mohms...

so we want FETS that gives greatest Id at lowest Vgs...yes? and the lower the rds(on) the lower the heat energy wasted?

sorry..i m new to this kinda stuff...

thanks

[Comatose]

Well, sorta. ID is almost a red herring, and very package dependent. A die in to-220 might be rated at 60A and only 10A in so-8, but for our purposes they're the same. Rds is all you really care about. (Okay, you also care about the capacitances, but not enough for you to worry about them for this, just keep the switching rate in the few tens of kHz max). Your power dissipation is then simply calculated by I^2*Rds. Unless you can slap a heatsink onto the transistor, figure 1.5W for to-220 and 1W for so-8.

The reason that ID doesn't really matter is that that is the rating if you can cool the transistor perfectly. The only way you can do that realistically is with a water cooling setup.

I think many of us here have made the "well, I only need 10A, so I'll buy a 10A mosfet" mistake when we were just starting out. Thats usually followed by the "Holy mother of GOD that thing is hot" discovery.

[kahlveen]

Well, sorta. ID is almost a red herring, and very package dependent. A die in to-220 might be rated at 60A and only 10A in so-8, but for our purposes they're the same. Rds is all you really care about. (Okay, you also care about the capacitances, but not enough for you to worry about them for this, just keep the switching rate in the few tens of kHz max). Your power dissipation is then simply calculated by I^2*Rds. Unless you can slap a heatsink onto the transistor, figure 1.5W for to-220 and 1W for so-8.

The reason that ID doesn't really matter is that that is the rating if you can cool the transistor perfectly. The only way you can do that realistically is with a water cooling setup.

I think many of us here have made the "well, I only need 10A, so I'll buy a 10A mosfet" mistake when we were just starting out. Thats usually followed by the "Holy mother of GOD that thing is hot" discovery.

hahaha...yah.. i would know... the L298 melted my breadboard... and i was sniffing around thinking who melted theirs...haha...

i think that IRL1404 should be fine... with the PWM output signal at 5V from the PIC uP (1.22kHz.... too low perhaps?).. i would get a Id ~ 40A.. should be sufficient for max amp draw of 8A - 10A...

thanks

[Comatose]

1.22kHz is technically no problem. You'll get a bit of motor noise at that frequency, which might be annoying, but how much and whether that is acceptable is a function of the motor and whatever you have attached.

IRL1404 will work fine in that application. A bit on the spendy side, but if you only need a couple its probably prudent to overdesign anyway.