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Pls help me against SURGES !

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najeeb

New Member
Hi ,

I want to know about protecting digital circuits from surges. Especially the counters. The surges leads to miss-counting.

Pls read my situation:

I am now doing the modification work of a paper cutting machine. What my plan is to computerize the machine.
There is a moving gauge (called backgauge) in this mahine whose displacement determines the cutting length of the paper. What i want to do is, making computer control for the movement. So that we can enter the cutting length in Millimeter through the computer keyboard and the back gauge will move that much distance.
The back gauge is driven by a 3-phase motor and contactors are used for switching direction of movement (Increase or Decrese the cutting length).
I have designed digital interfaces. It consists of 12 bit digital counter, which is used to count the distance in 'mm' as the backgauge moves. An optical encoder is used for producing clock (1 CLK/ mm) The 12-bit data from the PC is latched to the circuit and continuously compared with the counter value as on move. When the counter value becomes equal to the latched value, back gauge is stopped. I am using computer parallel port for interfacing.
Now my problem is that when we switching the motor (hence contactors and relays. Also there is one electro-magnetic brake for the motor) , counter out puts as well as latched data are changing.Thus the computer shows wrong value and distance moved is not exact. Some time the value and distance are accurate.
What I can have as a solution for this? I have tested differnt power supplies like SMPS, but no appreciable result.
All digital ICs are of 74LS series. 3 cascaded 74LS193 is used for up/down couting. (12-bit)

So pls help me!

Thank You
Najeeb
 

moki

New Member
Najeeb, try putting some 0.1uf capacitors across the supplys of all IC's as well as inputs. I had similar problems with a 555 timer circuit which I was using in a switching circuit with a foot switch. The circuit would even trigger if I touched the input with my finger. However after installing the 0.1uf caps this disappeared.
 

john1

Active Member
Hi najeeb,

You could also check that the earthing of the electronic
circuit-work, and any of its screening is conductive to a
good known earth.

Poor earthing can be a source of many problems.

Regards, John :)
 

panic mode

Well-Known Member
How long is the cable from encoder to counters? Is it shielded?
Is the shield connected to ground on ONE end only?
 

Optikon

New Member
Ground loop problems, power supply noise, ground "bounce", reflections on high speed lines can cause double-clocking. try slowing the clocking edge down alittle an of course properly bypass the components first.
 

Sebi

Active Member
For noiseless switching change the contactors to thyristor switch. It can switching on zero-cross, to minimize spikes.
 

jem

Member
Could it be that you are chasing the wrong issue? From your description, you mentioned that you stop the motor when the encoder count matches the preloaded count. If that is the case, more often than not you will overshoot your target. Contactors take a bit of time to activate. So do brakes. If that is the case, you will need to modify your control algorithm.
 
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