[SOLVED] Constant Current power Supply

[KlausST]

Hi,

Google has a lot of them ;-)

Klaus

 

[Ashkar]

Hello Klaus,
Surely google have a lot of them so to be correct I just need to know that what differential configuration would be best to apply.
As we need a differential op also that is to be feed into the bipolar adc.
That's why I am asking to send a schematic for that.
And also look at the Adc CS5530 is it worthy to instal just this adc insted of a front end opamp and a adc.

 

[KlausST]

Hi,

A standard differential opamp circuit has two resistors of the one value, two resistors of another value and an opamp.
5 devices.

The valus depend on many paeamerers. Gain is the one, the others depend on the osed devices and your needs.
First you have to read the datasheets carefully, then specify your needs, try to calculate. If something is not clear, then ask.

Klaus

 

[Ashkar]

Look at this circuit .This is used for stable voltage and current sourceto measure the required values of resistance from 0.001ohm till 1k
the shunt is a precised 0.47 ohm 1 watt resistance.
What do u say Klaus?

 

[KlausST]

Hi,

sorry i don´t understand the circuit.

Where is the DUT? and where is the 4 wires connection to the dut?

There is a box "LDO". An ldo needs at least 3 connections (in, out, gnd)

Please explain.

Klaus

 

[Ashkar]

The right hand side opamp is showing a dur with an opamp connected.
The ldo is shown as a block diagram .with the moafet connected in series to it.

 

[KlausST]

Hi,

OK, right amplifier is for voltage. What is the output referenced to?
left amplifier is for current. What is the output referenced to?

The function is OK so far.

How does R1 ...R5 work? I can´t see the function.


Klaus

 

[Ashkar]

This is a clear image of this.

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R4 feedback
R3 current limiter

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Reading current from u4 a
Voltage reading from u1 b
Constant is Supply 1.00v to which I would be reading the values to calculate the resistance.

 

[KlausST]

Hi,

OK, right amplifier is for voltage. What is the output referenced to?
left amplifier is for current. What is the output referenced to?

The function is OK so far.

How does R1 ...R5 work? I can´t see the function.
edit: sorry, i meant R5..R8.


Klaus

 

[Ashkar]

Could u please refer me a good design Klaus.

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For variable resistance that is to be places in DUT
Ranging from 1milli ohm to 10k

 

[KlausST]

Hi,


this is from post #10

the minimum recording of resistance are 0~2000 Micro Ohms.

this is from post #50

Ranging from 1milli ohm to 10k

two totally different tasks

Please sit back and decide what you really want for Range, Resolution and accuracy
Then write it down:
* resistance range
* AC or DC measurement
* what excitation current/voltage

read the whole thread again

then go step by step:
* develop the voltage/current source
* then develop ac excitation, if needed
* then develop the measurement HW
* then develop the measurement SW

only on some particular situations you may need help. Paint picture, show values. Write what is clear and write exactely where you need help.

Could u please refer me a good design

there are millions of good designs... but what exactely do you mean?

Klaus

 

[Ashkar]

Appologies for confusion.
I need a range of 0 milli ohms to 10k using a selector switch or may be a auto ranging would be a good idea.
Secondly a dc exited source is to he used.
Current in the circuit has to be 1,amp max
Adc is 16 bit

 

[D.A.(Tony)Stewart]

You cannot design anything without a good spec. try again.

Range, Resolution, accuracy, excitation,
min or max as required.


10kOhm max , 1A CC, 16 bit is NOT a design spec. or even a well defined problem.

 

[Ashkar]

I should try again.

Range o milli ohm to 200milli ohm ,200milli ohms to 1ohms ,1 ohm to 200 ohm,100 0hms to 10 k

Resolution - after decimal 5 digits in mili ohms range ,after decimal 4 digits in 1 ohms to 200 ohms, after decimal 3 digits in 200 phms to 10k range.

Current 1 amps ,100ma,10 ma resectively

Conversion via 16 bit adc

Exitation source - DC

 

[KlausST]

Hi,

four ranges of ohms --> manual selecting or automatically... hmmmm OK

three ranges of current --> it doesn´t match to the ohms ranges. So, how this?


1A for 200Ohms max => 200mV max => 200mW max. .. this is possible.
..
...
10mA lowest current.. 10Kohms max resistance. ==> this needs 100V and gives 1W of power. maybe too much for some 10k resistors.

Is your device battery powered?
Is it portable? if yes, what temperature range do you need?
What display do you have?

********
Please decide if you want to build a 4 wire milliohm meter or a (2 wire) kOhm meter.

I reccomend do start with a single range 0...200mOhms meter with 1 mOhm of resolution.
If all that is finishe you can upgrade it step by step.

What power supply do you have for the complete ohmmeter?

Klaus

 

[Ashkar]

Yes the device is battery powered.
0 to 50 Degrees temperature.
16x2 Lcd display is used

 

[KlausST]

Hi,

battery powered. Volts? Ah?

did you decide how many hours your meter should work before recharge?
******

I´m asking because a linear power supply draws more current from the battery than you need for measurement. So measuring with 1A you draw 1A+ from your battery.
With a good designed switching current supply maybe you draw 1/10 of it.
Do you think it is worth to go for a switching current supply?

On the other hand you have a controller to manage all the timing. so maybe you need a display update every 300ms. And measuring U and I (with multiple measurements and delay) needs 3ms, then you can switch oof current supply for 297 ms. Averaged now you draw only 1/100 of the previous battery current.
With switching supply even 1/1000.

Wich way to go? Give us all your information and ideas.

Klaus

 

[Ashkar]

Ok , to sum up I am giving the basic layout.
Internal battery of 3.6v lithium ion battery with 1000mah capacity.
I think a switching regulator is a good idea to save the battery charge and keep up the system running.
What do you think on that.

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And I hope to get a backup capacity to be about 100 times on a single charge

 

[KlausST]

Hi,

What do you think on that.

I am on the high quality side and high precision and high reliability....

But YOU decide what to do. It´s your measurement device. And if you want to build a low cost one it is OK for me. I´ll help you to get it working.


switching current source:
I recently saw switching current source LED drivers. With very low V-FB. For low power loss they should be synchronous.
Look if you find something that fits your needs. (input voltage, output current, low feedback voltage, no need for high precision, no need for extremely low ripple, no PWM outout)

Also have a look on LTC3600. I never worked with it, but seems to be good. Check price and availability.

Good luck

Klaus

 

[Ashkar]

Hello Klaus,
My first point is on high reliablity,great precision and good quality.
But the cost of this instruments has to be kept low.

I gone through th LTC3900,it seems a good option for us.
But the price is very high.
So I think a self designed linear type current source would be a good Idea.
I would used a 12v lead acid battery instead of a li ion battery.
Weight would increase by 300gms ,but its acceptable .