AC or DC detector swintch ?

Join Date
Jun 2002
Posts
24
I´m working on a system that use 30 proximity inductive detector (metal detectors).
I want to know what is the best solution , 24 VDC or 220 VAC detector swich, in terms of interference noise and fiability.
I´ll use Smerchall detectors.

Thank you
 
Agostinho,

I'll answer your question as it was posed.

"I want to know what is the best solution , 24 VDC or 220 VAC detector swich, in terms of interference noise and fiability."

If you are NOT using shielded wiring for your sensors, then they are subject to noise. Noise is usually in the form of low, stray voltages that have some effect on the measured effect.

Six-Volts of noise on a 24-Volt system presents 25%-Noise. That could cause a problem. Six-Volts of noise on a 220 VAC system presents 2.7%-Noise. That will NOT cause a problem.

I think the correct answer, in terms of your posed question, is clear.

In terms of your question, as it was posed, the answer is... "Use the 220 VAC".

Since we are working with electricity, which, in some forms, can KILL, there is no reason to make anyone feel comfortable with it. I am an Electrician as well as a Process Control Engineer - I don't want to, nor will I ever, find myself too comfortable with that KILLER!

Using 24-VDC certainly is safer than 220-VAC, or 440-VAC, and much more certainly safer than 15,700-VAC, but no one should ever get the idea that "Hey! It's only 24-VDC Electricity, it can't hurt me!" Everyone that works with Electricity should have the "Electrician's-Shake".

If the world was based on 24-VDC, that would be one thing... but, I guarantee you, the world ain't based on 24-VDC!
 
Terry's answer re noise may be true in theory but there are other factors to consider. Most AC prox. switches are 2 wire (switch in series with the load). If the prox. is connected to a PLC input, not a relay or some other device that draws more current than a PLC input, then I have found that the 2 wire prox. wiring is very susceptible to noise. I remember an installation where the 2 wire prox. switch wiring was put in the same conduit as the wiring to some air valve solenoids. This caused no end of problems until the wiring was separated. I would always pick a 3 wire type over a 2 wire type, regardless of the voltage.
 
Vic,

Regarding the 2-Wire Prox...

2-Wire Prox's are designed with leakage current built-in. That is, when they are "OFF", they are still kinda "ON". They are "ON" to the extent that they need to have some minimum amount of current flowing just to keep the sensor awake. Then, when the sensor detects something, it turns "ON" the output. The sensor opens the flood-gates and passes as much current as the resistance of the down-line load allows.

If the down-line load is a PLC Input, then you must be mindful of how sensitive the PLC Input is. That is, How much current does it take to turn ON the Input?

If the leakage current in the Prox is anywhere near the value of the "Turn-On Current Value" for the PLC Input, you will ALWAYS have problems.

In that case, you can use an interposing relay. But you should also check to see what kind of current that relay needs to operate and how much current the sensor can support.

A miniature relay won't need much current to operate. But if the leakage current is substantial compared to the operating current of the relay, then you're gonna have a warm relay that might die prematurely.

A larger relay might draw more current than the sensor can support.

Nothing's ever easy.
 
please use three wire 24 volt types as the story about emc is less on 220 volt systems is ****. It is not a volts problem but a current problem.
never had problems even 200 meters is working fine. I use Pt100 sensors that long with simple filter.
 

Similar Topics

Hello all- I have a unique challenge using a customers Direct logic 06 PLC. This customer has a DC motor operating at 10 RPM which is turning a...
Replies
1
Views
90
This is not a PLC question, but maybe one of you have seen a product like I am looking for. I have a portable pneumatic misting system that is...
Replies
7
Views
558
Good Evening , We had a Mettler Toledo Safeline Metal Detector acting unstable last week . We brought Mettler Toledo in to look at it ...
Replies
3
Views
809
Currently my company has a MT metal detector model V4 that has been down for the better part of a month. The metal detector is giving a F06-5 IO...
Replies
8
Views
2,985
Hi All Does anyone know what is a root cause of such an error? How to avoid it?
Replies
0
Views
954
Back
Top Bottom