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

Discussion in 'The Projects Forum' started by eimix, Sep 2, 2008.

1. eimix Thread Starter Active Member

Sep 2, 2008
32
0
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?

First of all ideas was to use MOSFET in linear mode (first guess was):

but this is wrong becouse in this case I want to work MOSFET in linear mode, but then it behaves as resistor and is wasting energy. (ofcourse circuit has some problems: MOSFET should be in low side)

So i did it with PWM (second guess):

capacitor in RC should be ~0.1uF (for simulation purpose it was set to 100uF).
some sort of soft-start is made, but does it work in practice?

How does this circuit looks like? possible to work?
Are any other ideas?
Thanks

2. Audioguru Expert

Dec 20, 2007
11,251
1,350
Hello again,
As I said in the other forum the IRF540 Mosfet needs 10V on its gate to fully turn on. Your circuit gives it a gate voltage of only 4.1V.
An IRF3711Z Mosfet turns on much better with a gate voltage of only 4.5V.

3. SgtWookie Expert

Jul 17, 2007
22,201
1,809
You also might look at IRLZ14, IRLZ24, and IRLZ34 devices. They are specifically designed to work with logic-level voltages on the gates.

Consider using Schottky diodes instead of standard diodes. For example, look at 1N5817's; with 1A current the Vf is 0.32v; a standard rectifier would have a much higher Vf.

4. eimix Thread Starter Active Member

Sep 2, 2008
32
0
in local stores i can find only IRLZ34N (IRLZ14, IRLZ24 not available, IRF3711Z is available after month)

and according to www.octopart.com IRLZ34N is not IRLZ34
and has high on voltage: http://alpha.octopart.com/International_Rectifier__IRLZ34N.pdf

but i found another logical level MOSFET IRL540N with lower on voltage
http://alpha.octopart.com/International_Rectifier__IRL540N.pdf
or IRLR2905
http://beta.octopart.com/International_Rectifier__IRLR2905.pdf
or IRL3705N
http://beta.octopart.com/International_Rectifier__IRL3705N.pdf

Are any of these sutable for me?

5. Audioguru Expert

Dec 20, 2007
11,251
1,350
The very good Mosfets will work fine in your circuit.

6. eimix Thread Starter Active Member

Sep 2, 2008
32
0
Thanks
i will ask for very good MOSFET's in store

7. eimix Thread Starter Active Member

Sep 2, 2008
32
0
i connected everything, and it worked (forward and turbo) but after that o connected relay for reverse functionality. for short time it worked - and stopped working

As i see - timer 555 control voltage is always 0V (even then disconnected).
So output pin is also 0. (but they are not the ground)

Vcc 6,3V (i changed a little bit) nad reset pin is fed from inputs.
Reset voltage ~1.5V (it was the same before)
Trigger and treshhold ~2V(capacitor)
Discharge pin 1,6V
Ground - 0V

Could it be becouse of relay? switching on/off?

8. eimix Thread Starter Active Member

Sep 2, 2008
32
0
Finnaly it works
some small modifications to last circuit, and it works
soft start / two speeds / reverse.

But i still looking for normal H bridge driver with soft start and non-digital input for speed control (voltage control). does any body knows such a one?

Thanks for help everybody

9. Andrew Leigh Well-Known Member

Sep 8, 2008
92
2
Hi,

I have a photocopied article from Electronics Today International which features a purpose designed RC Controller to give smooth acceleration as well as good crawl speed. The article does point out however that the less poles the motor has the harder it is to control.

I don't know how well it will scan but if you are interested let me have your e-mail address and I will try get a readable copy to you.

Cheers
Andrew

I was going to build one for my sons RC car. His has has a servo motor which drives an arme connected to a a variable resitor. The existing mode is either "wheelspin and go" or "dont go".