# Help to understand a "driver circuit"

Discussion in 'Homework Help' started by r_baskaran, Dec 11, 2012.

Dec 11, 2012
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Hi guys

I just got a driver curcuit to control a step-motor, but I don't understand how the curcuit is working.

Can somebody please explain, what the different components are doing and how I calculate and choose the right components, if I had to make the curcuit from the beginning.

I'm controlling the curcuit with a PSoC5 and M1+ is connected to a voltage source.

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Last edited: Dec 11, 2012
2. ### Audioguru Expert

Dec 20, 2007
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When the gate of a Mosfet goes high then the drain goes low which applies M1+ to a coil in the motor. The diodes arrest the high voltage spike caused by the inductance of a motor coil when the Mosfet turns off.

Look up the datasheets for the Mosfets and diodes to see their spec's.
The Mosfet must be a "logic-level" one and have a max current rating more than the stalled current of the motor.

If the motor is pulsed at a high frequency then the diodes must be very fast and have a max current rating more than the stalled current of the motor.

3. ### ScottWang Moderator

Aug 23, 2012
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Dec 11, 2012
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Thank you very much

1. What about the resistance? How do I choose the size of that?
2. I'm not sure, I understand what the diode is doing. What happens in the connection between "D" and the diode + the terminal above M1+, when "G" goes high?

5. ### Audioguru Expert

Dec 20, 2007
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Dec 11, 2012
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So what if I want to use this curcuit to turn on/off a LED and a light bulb? - Do I need the diodes then, or can I just remove them, and make a "open-curcuit" where the diodes are placed?

Is the only purpose of the diodes to avoid that the inductance of the motor disturbs the rest of the curcuit?

And is there a specific formula for calculating the size of the resistance?

Last edited: Dec 15, 2012
7. ### Audioguru Expert

Dec 20, 2007
10,668
1,190
Yes, do not use the diodes.

No.
The diodes arrest the high voltage spike from the inductance that occurs when the Mosfet turns off. The high voltage might destroy the Mosfet and destroy other parts.

The resistance in series with an LED is determined by the typical operating current of the LED and its forward voltage as they are shown on its datasheet, the supply voltage and Ohm's Law. The supply voltage minus the forward voltage of the LED determines the voltage across the resistor. Then the resistor value is Vr/current. For example a 3.2V white LED at 20mA and a 5V supply calculates the resistor to be (5V - 3.2V)/20mA= 90 ohms. Use 91 ohms which is the closest standard value.

Dec 11, 2012
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Oh sorry, that's not what I meant. I just want to know why they have chosen a 10K ohm resistor and not for example a 1K ohm? Do I even need the 10K ohm resistor between the input and the gate, or can I just remove it?

9. ### ScottWang Moderator

Aug 23, 2012
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The Rgs is designed to avoid that when the input is in the floating, when that situation was happening, the Rgs could pull down the G to GND, and turn off the MOSFET.

The value of Rgs is related with frequency of input signal and noise, so the Rgs can be 1K~1M.

If you put a R between input and G, when the frequency is low, then it didn't affect anything, but if in the high frequency that could cut off the input signal, because there is a capacitor between G and GND when the part was made, it will became a RC filter.

Sometimes, when you add any resistor into the circuit, you should be careful, because it could affect the voltage by voltage dividing.

Last edited: Dec 15, 2012

Dec 11, 2012
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So the purpose of the resistor, is to turn off the transistor, when there is no input signal, by pulling down "G" to ground?

"The value of Rgs is related with frequency of input signal and noise"
I'm sending a 5V signal from a PSoC5, as an input signal. Do you know what specs i need from the PSoC5 datasheet, to calculate the size of the resistor?

11. ### ScottWang Moderator

Aug 23, 2012
6,459
1,001
Yes.
There is no input signal meant that pin is floating.

Normally the Rgs using 10K is OK.