Speed Transducer

Thread Starter

odm4286

Joined Sep 20, 2009
265
Hello everyone, was just looking for some constructive criticisms for this circuit. Its pretty simple, just a speed transducer. Is there a better way to do this or am I on the right track. Thanks ahead of time.

 

#12

Joined Nov 30, 2010
18,224
There seems to be a basic fault centered around D1. There is no current path for the base of Q1 because the diode opposes that flow. Perhaps you can tell us the intent and how the transducer works.
 

Thread Starter

odm4286

Joined Sep 20, 2009
265
There seems to be a basic fault centered around D1. There is no current path for the base of Q1 because the diode opposes that flow. Perhaps you can tell us the intent and how the transducer works.
D1 it's there to prevent back feeding t1, my hall effect sensor. Did i not set this up correctly? It works great in the simulator.
 

Dodgydave

Joined Jun 22, 2012
11,285
Your transistor is on all the time, because the base voltage goes to 5V,and the emitter is at 24V, you need a NPN type
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
What is the nature of the transducer, most 3 wire devices have a open collector output or similar, if so, why not input directly into the PLC?
Also if using a PLC, why not input the defeat switch (1 N.O. contact) into the PLC and do the logic internally?
And if the output of the Transducer is O.C. NPN, you could be loading it?
Use an Opto if isolation or voltage transition is needed.
Max.
 

crutschow

Joined Mar 14, 2008
34,285
Don't know what simulator you are using but it's lying to you. As #12 noted, the diode blocks the base current through the transistor needed to turn it on (current flow from emitter to base) so it will always be off. And if you reverse the diode, then the transistor cannot be turned off unless the voltage from the Hall sensor output goes to +24V.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
If going with the small ones that #12 showed, Honeywell make a 4000 series that go to +24v.
If your sensors are NPN, then if the PLC has the option, set it for source input (sink device).

Max.
 
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Thread Starter

odm4286

Joined Sep 20, 2009
265
hey guys thanks for the replies and sorry for the lack of information. The hall effect sensor im using is a OH090U

Datasheet:
http://pdf.datasheetcatalog.com/datasheets2/29/297921_1.pdf

This sensor has an output voltage equal to input voltage when no magnetic field is detected. Anything above 90 gauss triggers the sensor and the output is then 0v.

So my logic here was to have it high all the time unless a magnetic field was detected. Then it would go low, "closing" the pnp transistor and allowing current to flow from the emitter to the collector. So in the end you'd end up with pulsed DC which could be used to determine speed.

The 6.8k resistor represents the input resistance of my PLC input card. Thanks again for the help, I hope this clears some things up. Let me know if I'm missing some basic transistor theory here, I'm still learning.
 

#12

Joined Nov 30, 2010
18,224
Pretty much the same thing as what I posted. Open collector, 50 ma current limitation.

One, you don't add 20 pf just to make it look like the speed test circuit.
Two, I don't think the 6.8 k resistor that Alec drew was intended to resemble the inside of your PLC.
Three, If you're going to do the PNP circuit, both of the power supplies have to be the same voltage. Still, you're better off with Alec's circuit.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
The component you posted takes 24vdc? So why the 5v?
Also if you can configure the PLC for a sink input device, just input direct to the PLC.
With a PLC it does not matter what the logic level is, it can be reversed in the PLC.
Also, if trying to capture a high speed pulse, the freq has to be lower than the PLC refresh rate, otherwise you miss pulses, the upper end PLC's have a Encoder or pulse module for capturing such events.
Max.
 
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#12

Joined Nov 30, 2010
18,224
The component he posted works on 5 volts, so why the 24 volts?

Probably just showing my ignorance of PLC's.:rolleyes:
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
OH090U shows 24v VCC, unless I am missing something?
If so, It could be fed from the PLC input supply and the 5v supply not needed?
The make/model of PLC would also help.
Max.
 

Thread Starter

odm4286

Joined Sep 20, 2009
265
its a modicon m340. The 5V signal is to switch the 24V and yes I am trying to count pulses and yes Ive done the math and the PLC can handle the pulse rate. By the way this is just a learning project so it does not have to be perfect and I appreciate all the help, thanks guys
 
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