R/C simple motor speed control (two different voltages)

[eimix]

Hi,

i have TX-2B/RX-2B transmitter/encoder and reciever/decoder.
http://www.silan.com.cn/english/products/pdf%5CTX-2B(RX-2B)AY.pdf

it has only few functions: forward, backward, left, right and turbo forward.
so there is no way to make "real speed control" using it, but i'm not going to throw away it

I want to make "some kind of speed control" that has two speed: low and normal.

I have battery pack 6V, and recever/decoder has ~3V in outputs - so one pin is 3V then forward, and other one is 3V then turbo forward.

What kind of simple circuit should i use to send 3V and 6V to DC motor?

My first guess is:
**broken link removed**
Are any other ideas?
Thanks

 

[eimix]

I'm already a week googling about MOSFET's and circuits with it. And i cant find circuit there linear mode of MOSFET is used (lower Gate/Source voltage - lower Source/Drain current, and higher Gate/Source voltage results in higher Source/Drain current).

So why MOSFET has such property and nobody is using it?

 

[audioguru]

Maybe you have the drain and source of the Mosfet connected backwards?
The IRF540 Mosfet fully turns on with a gate voltage that is 10V higher than its source voltage. You have the Mosfet wired as a source-follower so the gate voltage must be close to +16V for the source to be +6V.

The receiver IC has an abosolute max allowed supply voltage of 5V so your 6V is too high.

Did you try your motor on only 3V? Maybe it won't start and it might stall if it is loaded.
Pulse-Width-Modulation is used to slow a motor while retaining torque.

 

[eimix]

IRF540 is N power MOSFET, so connection is good (at least it works in PC simulation).

The point is i do not want to turn on MOSFET fullyt - to use it in linear mode.

receiver IC originaly was supplied with 6V,
old battery had two outputs (6V and 8V) - 6V for receiver and servo motor, 6V and 8V for old small motor (low and high speed).

now i have Speed 400 motor (6V) and it runs normaly from 3cell battery (3.6V) in motor datasheet 3V is minimum.

But worst thing i found out - MOSFET in linear mode behaves as resistor - so it is the same as connecting resistor in series with motor - wasting energy or am i wrong?

hmmmz, i will start desinging controller with PWM
i know alreadey how to controll duty cycle of 555 timer, H-bridge.
(at this time i do not want programable IC for full control of it)
is it possible to add start-soft-ramp for PWM generated with 555?

 

[audioguru]

Of course a Mosfet in the linear mode is an energy-wasting resistor. Its wasted power makes it hot.

Mosfets have a wide range of gate voltages, even if they have one part number.
At a motor current of only 2A maybe the gate voltage needs to be from 3V to 8V above the motor's voltage. Where will up to 14V come from?

Your Mosfet is a source-follower that has some voltage regulation. When the motor tries to draw more current then the source voltage drops which creates more gate to source voltage so the mosfet turns on more.

 

[eimix]

I did it with PWM.

**broken link removed**

capacitor in RC should be ~0.1uF (for simulation purpose it was set to 100uF).

How does this circuit looks like? possible to work?

 

[audioguru]

Now you have the source of the Mosfet grounded so its gate needs a voltage of only 10V to fully turn on.
The supply for the 555 oscillator is 5.3V so the gate of the Mosfet goes high to only 4.1V and will be just barely turned on.

 

[eimix]

according to **broken link removed**
if Vgs is > treshhold MOSFET allows >10A drain-source current, this is enough for my application.

Thanks for suggestion, i will try to find something with lower Vgs for fully turn on.
(i dismantled some coffee machine (broken) it has some MOSFET's in it i'l check specs)

Any ideas about soft-start?

 

[audioguru]

In your circuit, the dides will reduce the supply voltage to the 555 to 5.3V and the output high voltage from the 555 to the gate of the Mosfet will be 4.1V.
The datasheet for the IRF540 and most other Mosfets shows that some have an output current of only 250uA with a gate voltage of 4.0V.

The curves for a typical Mosfet shows that with a gate voltage of only 4.5V then there is an output voltage loss of 2V when the load current is 10A.
You don't know if your Mosfet is minimum or typical.

An IRF3711Z mosfet is "logic level" so it turns on very well with a gate voltage of only 4.5V.

I think your Soft Start and Turbo will work in your circuit. But the Turbo will not be at max power.