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#21
09-30-2008, 02:45 AM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

If your PLC can measure pulse amplitudes, you can save a lot of heat and power by pulsing the current source. I am unfamiliar with PLCs, so I have no idea whether this is possible.
#22
09-30-2008, 04:31 PM
 viju Senior Member Join Date: Sep 2008 Location: India Posts: 111

Yes I can control my measuring time in milliseconds through PLC.If so, will I be able to measure the resistance?
#23
09-30-2008, 09:30 PM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

Quote:
 Originally Posted by viju Yes I can control my measuring time in milliseconds through PLC.If so, will I be able to measure the resistance?
To measure pulse amplitude, the PLC has to know when the pulse is present, so it can sample it. Is that something it can do?
If you can read voltage, the conversion to ohms is simple.
#24
10-01-2008, 12:47 AM
 viju Senior Member Join Date: Sep 2008 Location: India Posts: 111

Yes.You are right.I will design the fixture with a pneumatic pin plunged into the measuring point with source.After plunging the pins I will switch the source to ON.Through the sense pins I will read the voltage( well inside the source pins) and amplify it.I am right.
#25
10-01-2008, 02:17 AM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

Quote:
 Originally Posted by viju Yes.You are right.I will design the fixture with a pneumatic pin plunged into the measuring point with source.After plunging the pins I will switch the source to ON.Through the sense pins I will read the voltage( well inside the source pins) and amplify it.I am right.
That sounds good. Measure the voltage as close to the weld as possible.
#26
10-02-2008, 01:50 AM
 viju Senior Member Join Date: Sep 2008 Location: India Posts: 111

Hi Bill,

Awaiting your transistor based constant current circuit.

Wookie and Steve:

I have just built the circuit and tested it.It works fine but with some heat in the regulator IC. I have a SMPS meanwell ( 24V,2.1amps) connected to 7805 regulator IC which feeds to LM 317.And to remind you I am building this for a automated production line where the cycle time for checking the subject is of less than 2 seconds.Our company works 16 hours a day.SO assume how many times the measurements are made.Please help me.
#27
10-02-2008, 01:53 AM
 hgmjr Super Moderator Join Date: Jan 2005 Location: Tennessee, USA (GMT-6) Posts: 9,030 Blog Entries: 11

One way to reduce the dissipation in the 7805 is to lower the voltage at its input. If you drop it to 10 volts, that will cut down significantly on the wasted power that is heating the 7805.

hgmjr
#28
10-02-2008, 02:58 PM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

You said your cycle time is less than 2 seconds. What is the minimum pulse width that will still allow you to measure the voltage? If it is short relative to ≈2 seconds, then the duty cycle can be low, minimizing average power.
#29
10-02-2008, 06:16 PM
 Bernard Senior Member Join Date: Aug 2008 Location: Tucson AZ USA Posts: 3,152

Why not just use a 24Ω 10 W resistor and 12 V in series with work piece ??
#30
10-03-2008, 04:04 AM
 Bill_Marsden Super Moderator Join Date: Mar 2008 Location: Dallas, TX (GMT-5 w/ DST) Posts: 19,022 Blog Entries: 5

A constant current source has some interesting properties. Basically you are making an ohm meter (a calibrated one). If you try to use a fixed resistance then the resistance in the device under measurement (in this case the pipe) significantly affects the results, with a constant current source it doesn't.
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 Tags constant, current, source

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