Inductive Prox Problem 1

[Teflon]

Hi all,

I just finished a phone call with the manufacturer of an inductive prox that I'm using. The light on the prox switch indicates it is always on, however there is no output, other than the spec'd leakage current. I have moved the sensor away from all possible trigger material, but it is still on. I have also attempted to trigger the sensor in case it is reverse logic.

The constant LED would appear to indicate a short-circuited sensor (as confirmed by the manufacturer). However, the strange part is that I should see a full load current or none at all with a blown device. I am seeing 1.25mA of load current (device leakage current) at all times.

These tests were done with a 12K and then a 6K resistor as load after attempting to run my application. I must note, the Solid State Relay is loading the prox by approx 4mA and will require additional load (i.e. resistor in parallel) to bring the load above the prox's 5mA requirement.

The prox has the following specs:

Wiring: 2 Wire
Type: Inductive
Range: 18mm
Shielding: Unshielded
Voltage: 20-250VAC
Voltage Drop: 7V max
Max Load: 500mA
Leakage Current: 1.7mA Max
Holding Current: 5mA Min
Includes Output Indicator LED

Source Voltage: 120VAC

My application has the sensor switching one of two loads. Each have the following specs:

Solid State Relay:
9-290VAC/DC, 1/2Wmax
Approx. 4mA Load Current
Solenoid:
120VAC, 6.8Wmax

Any ideas? The manufacturer is sending me a new prox, but I'm just curious about this problem.

Tef

 

[buzzp]

Is the prox a sink or source (NPN or PNP) device? Also, it seems your switching the SS relay which in turn drives the solenoid. But you say the prox is switching one of two device, SSR or Solenoid. If the prox is switching the solenoid directly then the prox probably puked the first time you tried to switch it due to the high initial current of the solenoid (just a guess).
Also if the minimum current of the prox is 5mA and the load current of the SSR is 4mA then you better increase this current some so the prox functions properly, all the time.

 

[Teflon]

Prox is NPN. I switch the solenoid directly with the prox and use a mechanical switch to have the prox change to the SSR. I confirmed the load prior to use. The solenoid is 7W and the SSR is capable of 30W load. The manufacturer verified that the load should be ok.

As noted above, I am adding a load resistor to meet the prox switch's minimum load requirements.

The switch use does bring up one thing. I wonder if there are any issues with breaking the solenoid load from the prox... Perhaps I spiked the prox? No, wait, the prox was having problems before the switch was activated.

 

[buzzp]

Hmm.
If the prox switched the solenoid then this might have caused some problems. DC solenoids can draw up to 20 times the 'nameplate' current when they are energized. I believe AC solenoids are around 10 time normal current (not for sure on the AC solenoids).
I would seriously consider using the SSR in between the solenoid and the prox switch (let the prox switch the SSR instead of the solenoid directly). I think you will find the reliability to be much better as well as the longevity.
Breaking the load might be an issue too (might not have been up to now). Consider putting some suppression across the coil of the solenoid. Although, this might affect the energize and de-energize times depending on what you use for suppression.
I am trying to think here about AC switches instead (all of my prox switch experience is with DC). Ill have to review some specs when using AC. Seems like this might have issues. I hope some of this discussion helps.

 

[Teflon]

DOH! Of all the... I can't believe I left suppression of the solenoid off the design. SOP is to add quencharcs, etc. for systems with PLC's and other noise sensitive equipment. I hadn't considered the need for suppression to protect the switch. Thanks for the reminder! I'll be placing that on the solenoid when the new sensor comes in.

The SSR itself is a latching version. The circuit alternates the solenoid on every other prox activation. The switch bypasses the SSR to turn the solenoid on during every prox activation.

<Slaps head> Thanks again for the reminder.

 

[jbartos]

Suggestions:
1. Visit

http://oeiwcsnts1.omron.com/pdfcata...le/SensorsParallelConnections.pdf?OpenElement

http://www.servida.com/web/input_output.htm

etc. for an application note (sample)
2. The electrical equivalent circuit and its modeling will make the application more accurate.

 

[buzzp]

Well let me know what the results are when you try the new switch. Good luck.

 

[Teflon]

The new switch works well. I added a 12K, 2W resistor across the SSR control to ensure enough load on the prox. That should give a total of 15mA load on the prox (the prox requires 5mA min to 300mA max).

In addition, I was told the system does not have to switch the solenoid during every prox activation, it simply needs to stay on with the switch. I've moved the solenoid off the prox and onto the switch. That mitigates any potential problems with the prox being overloaded by the solenoid, even though the mfg was ok with it.

System looks good and is on it's way to Egypt. To top it all off, our distributor double-ordered the prox by accident, so they told us to just keep the extra when it comes in.

Thanks for the feedback all! Case closed.

 

[buzzp]

Thanks for the update. Glad everything worked well for you.

 

[jbartos]

I beg your pardon for the improperly working link.
Suggestion: Visit

http://www.omron.com

for: sensors parallel connections
Also, cut and paste the string below into the browser:

oeiwcsnts1.omron.com/pdfcatal.nsf/0/A400B83B054729AD86256BD40058B014/$File/SensorsParallelConnections.pdf?OpenElement

 

[Teflon]

Thanks for the info Jbartos, but this application does not use parallel sensors. Good reference material, though.

Tef

 

[jbartos]

Suggestion to the previous posting: It depends what is meant by &quot;parallel sensors&quot;. The applicatios subject reads:
SUBJECT: SENSOR: PARALLEL CONNECTION

 

[Teflon]

There may be a problem on this site or, most likely, I'm misunderstanding your prior post. I do not see &quot;SUBJECT: SENSOR: PARALLEL CONNECTION&quot; in the topic heading of this thread. Maybe there's an error in this web page?

My definition of parallel sensors would mean wiring the outputs of 2 sensors together. My application only uses 1 sensor.

I think I see what your referring to. I mentioned wiring a resistor in parallel. I should rephrase that. The resistor is in parallel with the solid state relay control input (as opposed to the prox itself).

 

[jbartos]

Clarification: I am sorry, I interpreted &quot;Thanks for the info Jbartos, but this application does not use parallel sensors.&quot; as if my posting was expected to use or describe parallel sensors.