basic electric theory-#2

Thread Starter

xbox112005

Joined Oct 30, 2011
9
You are an electrician working in a plant that uses programmable logic controllers to perform most of the logic for the motor controls.The plant produces highly flammable chemicals so the plcs use a 24 volt vdc output that is intrinsically safe in a hazardous area.You are having trouble with output modules of the plcs that are connected to dc control relays.The outputs are going bad at an unsually high rate and you suspect that spikes voltages produced by the relay coils are responsible.To test your theory,you connect an oscilloscope to an output that is operating a dc relay coil and watch the display when the plc turns the relay off.You discover that there is high voltage spike produced by the relay.What device would you use to correct this problem,and how would you install it?
 

t_n_k

Joined Mar 6, 2009
5,455
Perhaps ask yourself the question as to what techniques are used to suppress back emf on DC relays.

You make no attempt to propose an answer - correct or not. Have you read the sticky concerning the expectations for posting in the homework forum?
 

Thread Starter

xbox112005

Joined Oct 30, 2011
9
Perhaps ask yourself the question as to what techniques are used to suppress back emf on DC relays.

You make no attempt to propose an answer - correct or not. Have you read the sticky concerning the expectations for posting in the homework forum?
I dont want the answer i just need a push in the right direction!
 

t_n_k

Joined Mar 6, 2009
5,455
A typical technique for back emf suppression with DC relays includes using either an RC snubber network or a correctly orientated diode in parallel with the relay coil.
 

strantor

Joined Oct 3, 2010
5,289
+1 t_n_k
I am an electrician working in a plant that uses programmable logic controllers to perform most of the logic for the motor controls. Everything I have seen has been a diode in parallel with the relay. google "flyback diode".
 
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