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DC Motor and diodes

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omerysmi

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I connected my motor in this way:
SCHEME.PNG

The relays control the direction of the motor and my question is if the diodes are connected in the right form?
I have tried it in real and it worked perfect but i don't understand how...in any way one of the diodes will be off and there will be an open circuit between the motor and the diodes..am i right? so how it worked without no problems?
 

Diode branch is OFF (Not working)
why you need Diode?
 

Hi,

If you say it works, then that is good. I don't understand how the circuit operates.

What does each Arduino PWM pin do?

What does each relay pin do at forwards and at backwards?

How do the diodes protect the Arduino and the relays, placed where they are cathode to cathode around the motor?

Eventually will one diode or the other diode break down with the reverse motor current it must have to block every time the motor changes direction?
 

d123 said:
Hi,

If you say it works, then that is good. I don't understand how the circuit operates.

What does each Arduino PWM pin do?

What does each relay pin do at forwards and at backwards?

How do the diodes protect the Arduino and the relays, placed where they are cathode to cathode around the motor?

Eventually will one diode or the other diode break down with the reverse motor current it must have to block every time the motor changes direction?
Click to expand...
- Don't pay attention to the pwm, consider it as a standard I/O ('1' or '0').
- Assume that '1' in the first relay will turn the motor foward and '1' in the second will turn the motor backward ok?

I also don't really understand how it worked for me because there is no any discharging path for the reverse current...

One more thing that i forgot to mention is that between the arduino to the relays there is some buffer(uln2003-it has an internal diodes so maybe that's why my arduino and other components are not damaged?)
 
Last edited:

Now you mention the buffer... :)

So then does the motor stop spinning slowly as the reverse current has nowhere to go except into a diode blocked by another diode?

Maybe the diodes burn the reverse current up as dissipated energy as well.
 

The two diodes you have do nothing since they are connected back-to-back and cannot conduct in either direction unless one of them breaks down. It would work if the two diodes were Zener types.

To protect the relays from back EMF for a reversing motor using standard diodes, you need two pairs of diodes in series.
Connect each pair between the supply V+ and V- (anode of the bottom diode to V-).
Connect the junction of one pair to the top of the motor and the junction of the other pair to the bottom of the motor.
 

crutschow said:
The two diodes you have do nothing since they are connected back-to-back and cannot conduct in either direction unless one of them breaks down. It would work if the two diodes were Zener types.

To protect the relays from back EMF for a reversing motor using standard diodes, you need two pairs of diodes in series.
Connect each pair between the supply V+ and V- (anode of the bottom diode to V-).
Connect the junction of one pair to the top of the motor and the junction of the other pair to the bottom of the motor.
Click to expand...

but the motor is not connected directly to V+ and V-..i think that i didn't understand what you mean very clearly..could you draw it for me on the circuit above?
 

A relay coil has inductance. When current in an inductor stops, it creates a "back EMF" voltage spike at the part that is driving it (your ULN2003 output). The ULN2003 already has a diode at each output connected to the COM pin #9 that should be used to arrest the voltage spike as shown in the datasheet.
 

The controller & the motor is isolated by relay

controller load(s) is ULN inputs
ULN load(s) is relay
relay load is motor

when controller's pin =1 ULN works , it sinks current from the relay' coil and relay works
then when Pin = 0 the FWD (diode of uln) works and protect the uln
when relay ( DPDT ) works one motor wire connected to Pos. & the other connected to Neg.
so your loop now pos >> relay1 >> wire A >> motor >> wire B >> relay1 >> neg.
if the second relay works
pos >> relay1 >> wire B >> motor >> wire A >> relay2 >> neg.
 

are you sure that these are not TVS diodes , they look the same as normal diodes . what is their part number ?
 

Motor can work without any diode
there is no problem

if you want to protect relay (not the controller) use 4 diodes
connect motor terminal A to anode of diode 1 and connect its cathode to Pos.
connect motor terminal A to cathode of diode 2 and connect its anode to Neg.

do the same thing for terminal B & diode 3 diode 4
 

A question:
What is the diode's part number?

I agree 100% with Crutschow: if they are normal rectifier diodes, they don't do anything. Ever. They are connected back to back and one will always block the opposite one.

Now, if they are breakdown diodes like zeners or Transorbs, that is another story. But we need to know the diode's part number.
 

Electro nS said:
are you sure that these are not TVS diodes , they look the same as normal diodes . what is their part number ?
Click to expand...

schmitt trigger said:
A question:
What is the diode's part number?

I agree 100% with Crutschow: if they are normal rectifier diodes, they don't do anything. Ever. They are connected back to back and one will always block the opposite one.

Now, if they are breakdown diodes like zeners or Transorbs, that is another story. But we need to know the diode's part number.
Click to expand...

1N4001 diodes
 

now i noticed something , there is no mension whether there are diodes on the coil of the relay ? these will affact the circuit more than those across the motor . provide a schematic or photo or the circuit , not just a block diagram
 

omerysmi said:
but the motor is not connected directly to V+ and V-..i think that i didn't understand what you mean very clearly..could you draw it for me on the circuit above?
Click to expand...
I'm referring to whatever power supply is powering the motor through the relay.
That way any inductive spike voltage will have a path back to the supply when a switch or relay is opened that powers the motor.
ahmed saad described the same diode connection in a different manner.
For that you can use 4 of the 1N4001 diodes you have.
 
Electro nS said:
now i noticed something , there is no mension whether there are diodes on the coil of the relay ? these will affact the circuit more than those across the motor . provide a schematic or photo or the circuit , not just a block diagram
Click to expand...
In Post #5 he said he has a ULN2003 driver IC between the Arduino and the relay coils and it has suppression diodes in it.
 
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