bidirectional control DC motor by using relays

[bananasiong]

hi all,
i've built this circuit to control the DC motor in both direction. It works well. can anyone tell me is there anything need to be modified?
i've tried using other transistor, 2N3904. but it can't switch. only this TIP41 can switch the relays.
The base of the transistors go to the uC.
00--> forward
11-->reverse
10/01-->stop

 

[Nigel Goodwin]

hi all,
i've built this circuit to control the DC motor in both direction. It works well. can anyone tell me is there anything need to be modified?
i've tried using other transistor, 2N3904. but it can't switch. only this TIP41 can switch the relays.
The base of the transistors go to the uC.
00--> forward
11-->reverse
10/01-->stop

Depends on the spec of the relays, however you don't show diodes across the relay coils?, these are ESSENTIAL!.

 

[bananasiong]

Depends on the spec of the relays, however you don't show diodes across the relay coils?, these are ESSENTIAL!.

essential!?!? i didn't know that? why is it needed? can i use 1N4148?
then how about the transistor? can i use other value since the TIP41 is very big.

 

[Dr.EM]

A 3904 might be able to drive the relay coil, look at thier resistance and use ohms law to find how much current they need, then see if its within the limits of the 3904 (which I think is around 100ma ?). 4148 diodes are signal diodes and arn't really the most suitable. 4001 (or 4002, etc) are cheap, common and are much more suitable. They need to be there to absorb back emf from the relay coil when it is de-energised, this emf can otherwise damage the transistors (check the 3904s you used for damage).

 

[bananasiong]

They need to be there to absorb back emf from the relay coil when it is de-energised, this emf can otherwise damage the transistors

what is absorb back emf?

 

[Nigel Goodwin]

what is absorb back emf?

When the current through an inductor (like a relay coil) is turned off, you get a large (100's of volts) reverse spike as the magnetic field collapses - this is called the 'back EMF'. This spike will destroy the transistor feeding the relay, so you place a reverse connected diode across the coil, and this prevents the spike causing damage - a 1N4148 should be fine for your relays.

As suggested, if you've tried smaller transistors they have probably being damaged - the much larger TIP41 has more chance of surviving.

 

[bananasiong]

When the current through an inductor (like a relay coil) is turned off, you get a large (100's of volts) reverse spike as the magnetic field collapses - this is called the 'back EMF'. This spike will destroy the transistor feeding the relay, so you place a reverse connected diode across the coil, and this prevents the spike causing damage - a 1N4148 should be fine for your relays.

the reverse spike comes from which part of the coil? both part?
then the diode will direct the spike back to the coil. right?

As suggested, if you've tried smaller transistors they have probably being damaged - the much larger TIP41 has more chance of surviving.

so if i use tip41, do i still need to use diode? or i need it for safety.

 

[Nigel Goodwin]

the reverse spike comes from which part of the coil? both part?
then the diode will direct the spike back to the coil. right?

Each relay only has one coil?. Check my PIC tutorials, the hardware extras section shows how to wire a relay from a PIC.

so if i use tip41, do i still need to use diode? or i need it for safety.

You ALWAYS need a diode! - there are other methods, but for your purposes ALWAYS USE A DIODE.

 

[bananasiong]

Each relay only has one coil?. Check my PIC tutorials, the hardware extras section shows how to wire a relay from a PIC.

yes.. i just checked it here
but i don't understand this :this back EMF will be safely 'dissipated'.
disipated? how??

You ALWAYS need a diode! - there are other methods, but for your purposes ALWAYS USE A DIODE.

okay, okay... i will use the diode.. don't get angry.. ^_^

 

[Nigel Goodwin]

yes.. i just checked it here
but i don't understand this :this back EMF will be safely 'dissipated'.
disipated? how??

Stand a wineglass on a table, tap it gently with a spoon, notice that it makes a long ringing sound - now touch the glass with your finger and tap the glass again, it no longer makes the ringing noise.

Exactly the same reason!.

 

[JimB]

Look here:

https://www.electro-tech-online.com/threads/counter-emf.17418/

for a discussion of big transistor destroying voltage spikes from relays.

In the last post in the thread, I have some 'scope traces from a real relay.

JimB

 

[bananasiong]

Stand a wineglass on a table, tap it gently with a spoon, notice that it makes a long ringing sound - now touch the glass with your finger and tap the glass again, it no longer makes the ringing noise.

Exactly the same reason!.

oo... i understand this now... the term used should be.. "the back emf is dissipated"? or "the back emf is absorbed"?


thanks JimB too for telling me the url.
from your graph, if there is no diode for protection, it goes to very low.. but i thought the spike is a few hunderd volts and will distroy the transistor?

 

[Hero999]

Here's my basic explaination of back EMF.

When you expose a coil to a sudden change in magnetic flux (feild) a voltage proportional to the rate of change in flux is induced in the coil. This is exactly what happens when you turn your relay off, the magnetic flux surrounding the coil suddenly disappears inducing a huge voltage spike in the coil which can damage any semiconductiors driving it. An easier way of understanding is that the relay coil is and inductor and inductors try to resist change in current flow, so when you disconnect the power it tries to keep the current flowing therefore generating a high voltage.

How do we solve this problem?

We add a diode in reverse parallel to short circuit it.

 

[bananasiong]

How do we solve this problem?

i need some time to digest...
back EMF, is it back electro...... force? what is it called?

the voltage spike induced come from which part of the coil? For example, the coil is connected one end to the Vcc and the other to the collector, the spike goes towards the collector? Then the reversed parallel diode will direct it back? or absorb it?

inductors try to resist change in current flow, so when you disconnect the power it tries to keep the current flowing therefore generating a high voltage.

try to resist change in current flow? what does it mean? it means it tries to keep the current flowing?
How come when it tries to keep the current flowing, the voltage spike will induced??

 

[bananasiong]

can anyone answer me?

 

[akg]

i need some time to digest...
back EMF, is it back electro...... force? what is it called?

the voltage spike induced come from which part of the coil? For example, the coil is connected one end to the Vcc and the other to the collector, the spike goes towards the collector? Then the reversed parallel diode will direct it back? or absorb it?


try to resist change in current flow? what does it mean? it means it tries to keep the current flowing?
How come when it tries to keep the current flowing, the voltage spike will induced??

yes it is electro motive force .
as current flows through an inductor , it create a magnetic field around it, when the current stops , the magnetic flux should also stop . so it starts reducing its strength , and u may remeber a conductor in changing magnetic flux causes electric current which depends on the rate of change . so a sudden collapse of magnetic flux creats a high voltage , in this case towards the collector. the reverse parallel diode starts conducting once the voltage across the the coil reaches say 0.7V and never allows to rise above that , protecting the collector.

 

[Hero999]

try to resist change in current flow? what does it mean? it means it tries to keep the current flowing?

Yes, the magnetic feild surronding the inductor is stored energy, it tries to keep the current flowing.

How come when it tries to keep the current flowing, the voltage spike will induced??

Because it's trying to push the current through an open circuit, this causes in theory an infinite voltage to be generated, in practice it's between 200V and 2kV and depends on the nature of the coil.

 

[bananasiong]

the reverse parallel diode starts conducting once the voltage across the the coil reaches say 0.7V and never allows to rise above that , protecting the collector.

so, u mean the spike will go up to only 0.7V because of the diode right? then the 0.7V won't affect the transistor, right? can a capacitor do this? touch a capacitor between one end of the coil and the collector to the GND.

Because it's trying to push the current through an open circuit, this causes in theory an infinite voltage to be generated, in practice it's between 200V and 2kV and depends on the nature of the coil.

thanks..

 

[Hero999]

so, u mean the spike will go up to only 0.7V because of the diode right? then the 0.7V won't affect the transistor, right?

Yes, 0.7V is far too small to cause any damage.

can a capacitor do this? touch a capacitor between one end of the coil and the collector to the GND.

It depends on the size of capacitor and inductance of the coil, capacitor in paralell with form a tuned circuit, the curent in the inductor will charge the capacitor, when it's fully charged it with then discharge back into the coil, this oscillation will cary on until the energy is disipated by the resistance in the circuit. It will only help the capacitor is large enough to absorb enough current without charging up to a dangerous voltage, this might not be practical since it might need to be so large that only an electrolytic will do which is no good since it's polarised and cannot safely charge up in reverse. A capacitor is only normally used when the relay is powered from AC and a resistor is often added in series with it to help disipate the excess energy enabling a smaller capacitor to be used, this arrangement is often called a snubber network.

Some helpful links:
https://en.wikipedia.org/wiki/Tuned_circuit
https://en.wikipedia.org/wiki/Snubber

 

[bananasiong]

how if i do it in this way using the capacitor?