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[SOLVED] Problem with lcd and motor

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Full Member level 2
Oct 27, 2010
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I am designing a project where a c8051f340 uc kit is connected to lcd and several push buttons and couple of fans, eeprom and some other components. The power supply of lcd, eeprom and the rest of electronic components is from a transformer different from the transformer feeding the 2 fans. when i tried to add a motor which is controlled through a push button(ON - OFF) , and the power of the motor is from the same power supply of the 2 fans, the lcd display funny characters when the motor is switched on. I think something is wrong with the power but I don't know what. ofcourse the ground is common between all circuits.

Without a lot more circuit details I can only suggest one thing. Put a 0.1uF cap across the back motor where the leads come out of the motor. It could be noise from the motor getting into the other circuits. That's my shooting from the hip guess.

ok i will give it a try and get back 2 u, thanx

this is the last warning to you for using sms typing format or using short hand typing ... i will delete all your threads and post... and infract you....

if you continue this you might also be banned...

Agree, it might be noise or surge problems, or possibly your power supply can't supply enough current to power everything. You might need to add caps and possibly inductor filter. If possible, you should always attempt to isolate digital circuits from fans and motors.
what do you mean by isolating the fans and motors from the digital circuit??, the fans and the motor are fed from another transformer than the digital circuit.
Another point: from the datasheet of the lcd the voltage needed to use the lcd is 5v but the silicon lab uc generates 3.3v and i am using it in pushpull mode which means no pullup resistors are used. The lcd has been working for 4-5 months without any problem could this be the problem??

...what do you mean by isolating the fans and motors from the digital circuit??, the fans and the motor are fed from another transformer than the digital circuit...

This is not guarantee that no spike will be induced.

I suffered with the same problem using solenoid ( that is a high inductive component ) wich propagated noise from relay-contact to relay-coil. That noise still was high enough to disturb 5V bus close to the 12V wich supplied above realy.

There are not some rule to calculate it; once noise behaviour depends on : wiring / distances / motor characteristics / etc...
I usualy place Polyester values from 0.22uF to 1uF ( 400V rated or more ) close to inductive devices.


Your motors at not as inductive as a solenoid. They tend to generate what is commonly called hash, a broadband RF. For the most part the hash is created differentially across motor leads and will radiate based on the area of the loop formed by the motor leads. By placing a cap 0.01uF to 0.1uF at the back of the motor you are keeping the loop small, really just the terminals and internal brush connections. Keep the wires to the motor twisted will also help the loop small but you need the cap right at the board where the leads are connected.

But if the LCD is acting up it also means it's susceptible to being interfered with. You can improve its immunity by keep wires to it short and make sure it's power supply is bypassed well.

One last though is that you could have a ground problem. The fans and motor are run off a separate power supply but they share the same ground. You need to make sure the fans and motor ground is connected to the logic ground at one point close to the logic power supply ground. Otherwise the ground currents from the motor could be affecting the logic ground.


Sorry, I assumed your motors were DC. If not they my post doesn't really apply.
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why 400V???? isn't this value too high?

Maybe no enough !

You did not say if the motor is supplyed by 220Vac or 110Vac.
If the case of 220Vac the peak value is ~310V.
Considering common peak spikes of 50% gives ~450V.

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