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[SOLVED] Constant Current power Supply

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Ashkar

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Some one kindly Suggest me a 5 Amperes Constant current power supply for Micro Ohm Meter.
 

hi Ashkar,
What compliance range do you require for the CC supply.?

Basically what is the highest voltage that will expected across the 'load'.

E
 

Thank you for your replay,actually I am little confused about the voltage at which the micro ohm meter might measure the required resistance.
Need some help on this also.
 

A 5v dc supply is required for this purpose
 

Hi,

you want to build a 4 wire microohms meter?

If you use a microcontroller to do the calculations, then i reccomend:
* Use a stepdown switching regulator with current limit set to 5A.
* use the enable/shutdown to control power with uC
* measure current
* measure voltage
* calculate resistance.

With that there is no need for a precise "5.000 A" current. The calculation will do it right.

Klaus
 

All your above stated facts are true.
But why isn't a constant current source is required?
 

Hi,

theroetically you can also work with a voltage source.

But it´s value has to be in the microvolts. This is not easy to do.

Or you can use a resistor. 5V, 5A = 1 Ohms.
The disadvantage compared with switching regulator is the power loss.

While a linear regulator or a resistor dissipates max. 25W the switching regulator dissipates maybe 2W max.

Klaus
 

So to measure a DUT with resistance value of 0.001 ohm what value of current might be required ??
 

Hi,

To measure 1mohms 5A are good.

It gives a voltage drop of 5mV and with a good low offset (chopper) opamp you can get a resolution to 1% of it.

Klaus
 

I need to re correct my specs.
the minimum recording of resistance are 0~2000 Micro Ohms.
 

Hi,

0 is not possible.

Or do you mean 0 to 2000 microohms with resolution of 1 microohmm?
With DC stimulation you may see errors caused by thermocouple effects. If you need high precision you might think of some kind of AC stimulation.

Klaus
 
Hi,
You are right on this,I need a resolution of 1mOhm.
And I know about that,effect of dc will cause drifts on the readings.So a calibrated shunt across the current line will determin the current via ohms law. We get current via shunt
Plus the voltage drop across the DUT in mV.
Am I right!

- - - Updated - - -

Ac exitation will be good to our measurment needs,but doesn't a constant current regulation might be hard to deal with?
 

Hi,

just to be sure: resolution is 1 microohm or 1 milliohm?

The current shunt is not that big problem, because you have equal temperature at he joints, so ideally the thermocouple effects will cancle itself.

The bigger problem is the voltage measurement at the DUT. Here the voltages might be much smaller than at the shunt and the temperatures are not predictable.

AC excitation is not that problem.
I´d use some H-bride configuration with MOSFETs. the gate currents are negligible and the voltage drop is no problem. Low voltage types (<20V) with Rds-on of a few mohms are easy to find.
Switching speed is no problem, power loss is also no problem.

If you need precision, then do it with AC.

software is important, too. But that after hardware is finished.


Klaus
 

Resolution is 1 micro ohm!
But I want to know about how to limit current in the circuit at minimum range of micro ohm.
And a precision opamp will read a voltage range in milivolts!
I think we would require large current to measure the DUT
 

Hi,

You can use almot any switching buck controller with some external circuitry.

Or you can use LT3741. It is a efficient way to get voltage limit and current limit with low power dissipation.

If you want a resolution of 1microohm at a current of 5A then your voltage reolution must be 5microvolts.

Resolution is one thing, precision is the other.
If you want a repeatable precision of 1 microohms, then (in my eyes) you must go the AC way.

5A AC and a chopper opamp and some software and it will run.

Klaus
 

Lt 3741 is a pretty good current driver ,I will totally look into it.
And being on the signal conditioning circuit on DUT,the concern are towards measuring the micro volts over to adc acceptable range.
If we would use AC signal instead then the need of AC TO DC RMS converter will increase the Circuit Complexity and Error rates. LTC1966 is a substitute to do so.
What do you think.
 

Hi,

No rms calculations, just bipolar adc input.
Simple and precise.

Klaus
 

Hey,
Its a good idea to choose a bipolar Adc .
So we now require the followings:-
Opamp running in chopper mode
Signal converted via bipolar adv
The a MCU with LCD display
And a current driver using H-bridge to generate AC output with current limiting.

and could you please share your h-bridge
 

For the current generator, I was thinking of a single ended transformer coupled set up. Run it at 22 HZ (to avoid inductive effects and the mains frequency, also the op-amps will have a higher gain). Put a hall effect current sensor on the output line and use it to control the current by modulating the Vcc to the output transistor.
Frank
 
Thanks Frank for joining.
Could you please show a schematic for the same.That way we would get a clear view on this idea.
 
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