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Topic: Kickback Diode On Motor - question (Read 5 times) previous topic - next topic

JPgs

Hi all, I've search a lot of websites, a freewheeling/kickback diode is needed to protect transistor from back voltage/voltage spike generated by motor, but the diode was placed on a different location on different websites.

Question 1) if i drive a motor using an Arduino I/O pin with circuit diagram as attached, then where should i put the diode to protect my circuit? Point A or B ? Actually I'm not really understand on that (how the circuit work with the back EMF of motor using diode)

Question 2) Is the capacitor at point C really needed ? why do i need it ?

Please help, Thanks. :~

Grumpy_Mike

1) Point A
2) A cap here will reduce electrical noise from the motor.

JPgs

thanks...
1) then what about point B ? is it point B diode was wrong or useless ?
2) if the motor is replaced by a relay coil, then is the capacitor still suitable to be there?

Grumpy_Mike

Point B is seen in some diagrams but protects the transistor rather than the rest of the circuit.
A capacitor will still reduce the noise from a relay but it dosn't generat as much so it is less important and I would leave it out.

JPgs

Thanks... got it all...
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rather than the rest of the circuit.

BTW.. last question here... i thought point A diode is to protect only the transistor ? Or the Arduino pin as well ? if its protecting Arduino as well... how? not the transistor already separate between the Arduino pin and relay coil ?
Thanks for your time Grumpy_Mike...

Nick Gammon

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DSC_0136.JPG (595.32 KB, 2592x1944 - viewed 6 times.)


Half a megabyte for a sketch? Maybe reduce the resolution next time?
http://www.gammon.com.au/electronics

Grumpy_Mike

When a motor or coil has the power removed the magnetic field in it collapses. This induces a voltage on the terminals of the coil that is opposite in direction to that which caused the magnetic field. Hence a reverse biased diode across that coil will short out this reverse voltage and prevent any damage to other components.
If this diode is placed across the switching transistor this will still short out reverse voltages from the coil but it relies on the fact that the two terminals of a power supply present a very low impedance. This is not always the case and it forces the reverse current through the power supply. While this can be sufficient some times I always think it is better to tackle a problem at source rather than down stream so to speak.

MarkT

Another way to think about it is that inductances don't like current to change - it you try to reduce the current suddenly by switching off the transistor the coil will try to keep the same current flowing (generating whatever voltage is necessary).  Placing the reversed diode there means that the current will simply divert to the diode and the coil only needs to generate the small voltage necessary to forward bias the diode.

In reality the _rate_ in fall of current depends on the voltage, so the diode approach means the current falls quite slowly - sometimes this is a problem (say you are driving a high-speed solenoid) - in which case a Zener diode can be added in series with the diode to increase the voltage (and speed the fall in current).  The maths is  L.dI = V.dt  ( put another way   V = L dI/dt,  voltage is inductance times rate of change of current )
[ I won't respond to messages, use the forum please ]

Grumpy_Mike


dc42


...
If this diode is placed across the switching transistor this will still short out reverse voltages from the coil but it relies on the fact that the two terminals of a power supply present a very low impedance.
...


I don't think so. The diode connected across the transistor is the wrong way round to conduct reverse voltage from the coil when the current through it is interrupted, unless it is a Zener diode.

However, in an H-bridge with diodes across all the transistors (or the body diodes of mosfets), you're right, those diodes contain the back EMF by feeding it into the power supply.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

kf2qd



I don't think so. The diode connected across the transistor is the wrong way round to conduct reverse voltage from the coil when the current through it is interrupted, unless it is a Zener diode.

However, in an H-bridge with diodes across all the transistors (or the body diodes of mosfets), you're right, those diodes contain the back EMF by feeding it into the power supply.


The diodes are doing the same job in either position. the diode across the transistor is there specifically to protect the transistor from reverse transients from switching off an inductive load. The Diode across the motor would short the voltage spikes through just the motor and would stress other elements of the circuit less. The capacitor would be a good addition for either a motor or a coil, just make sure it has a high enough voltage rating for the spike.

dc42


The diodes are doing the same job in either position. the diode across the transistor is there specifically to protect the transistor from reverse transients from switching off an inductive load.


No, that's not right. Let me explain in more detail. The diode across the transistor protects it from reverse transients (collector voltage going negative with respect to the emitter). When the motor switches off, its inductance produces a positive transient (collector voltage going large and positive). A diode across the transistor doesn't do anything to prevent this transient.

If you're expecting both positive and negative transients, you need both diodes.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

JPgs

Thanks Everybody... I got it all... thanks...

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