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.

 

Yes I can control my measuring time in milliseconds through PLC.If so, will I be able to measure the resistance?

 

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.

 

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.

 

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.

 

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.

 

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

 

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.

 

Why not just use a 24Ω 10 W resistor and 12 V in series with work piece ??

 

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.

 

There is a picture of a constant current sink in the electronic load section at

http://www.luciani.org/geda/util/matrix/index.html

To use this circuit connect a low voltage source to one side of your weld and the other side of weld to the drain of the current sink. The lower your voltage is the less power dissipated in the FET. The amount of current is controlled by the voltage into the +IN of the op-amp and the current sense resistor. You

If you keep the case of the MOSFET at room temperature (heatsink, FAN) you can
dissipate a fair amount of power.

(* jcl *)

 

OK, I hadn't uploaded to the albums yet. This schematic presumes a regulated power supply, and will get hot for all the same reasons the LM317 will.

This schematic is a source, that is, it outputs the current you set it for. The other variation is a sink, which is a programable load, and looks a lot like this...

These designs work because a BJT transistor is basically a constant current device.

If you check out my blog you can get the rough library I've been working on, I like to draw concept circuits (but sometimes my concepts are flawed). Just click on the blue number next to blog entries on my post.

 

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.

OK, so your 7805 regulator has to dissipate 24v-5v=19v x 0.5A = 9.5 Watts of power as heat.

Why don't you get a 5v switching supply like has already been suggested?

Here's one: http://www.mpja.com/prodinfo.asp?number=16000+PS
Cheap, under $23.

If that's not cheap enough for you, use +5v from a computer power supply. On the 4-pin Molex connectors, red is +5v, yellow is +12v, and black is ground. I suggest you not use the +12v, as all you really need is the +5v.

 

Wookie,

I tried with power supply you mentioned.It works as expected.Thanks.

 

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.

12V/24 ohms looks like a constant current source when your load is milliohms. If the weld is massively defective, it probably won't matter that the current is not constant.

 

Bill, You forgot to indicate where the current sink output is on this one.

 

Bill, You forgot to indicate where the current sink output is on this one.

Try the power supply leads, it's an electronic source.


***************

Opps, I meant load.

 

OK, I hadn't uploaded to the albums yet. This schematic presumes a regulated power supply, and will get hot for all the same reasons the LM317 will.

This schematic is a source, that is, it outputs the current you set it for. The other variation is a sink, which is a programable load, and looks a lot like this...

These designs work because a BJT transistor is basically a constant current device.

If you check out my blog you can get the rough library I've been working on, I like to draw concept circuits (but sometimes my concepts are flawed). Just click on the blue number next to blog entries on my post.

Bill:
Can the supply voltage be 12V instead of 18V?What range can I adjust the current Source?

 

The current source can be any voltage, you tweak the design to whatever specs you set yourself. One problem with higher voltages though, power (in watts) = voltage X current. Increase the source voltage and heating on components goes way up. If the power supply isn't regulated then you have to modify this circuit a bit. I'd be happy to show the tweaks needed. The LM317 is probably the best design though, while a transistor works the 317 works better, and the heating issues are the same in both cases. The LM317 is actually cheaper in many cases too.

The transistor version can be extremely variable, but you may need to add a second transistor to increase the range.

If designed properly, the current regulator doesn't care what the input voltage is. The input voltage just sets what the output voltage of the current regulator when there is no load.

What are you thinking? Are you wanting to play around with some concept ideas, or are you aiming for something more practical?

 

Bill:
Though I am an Electrical Engineer and Automation Engineer.While I was asked by my management to measure the resistance of the welded joint this current source idea clicked to me.I didn't find the one in the market.Adding to that I know very little in Electronics.Hope to learn concepts and ideas from you members.Don't mistake me.