Electronics Ohm Source

[pany2008]

Hi all,

I want make microcontroller based digital milli ohm source, where i can use for rtd calibration or can be used as ohm source for many field testing application.

Can any one suggest me guide to make the source.

Thanking You.

Pankaj

 

[chuckey]

What currents are going to flow through this virtual resistor? at what frequency?. Milliohm resistance boxes have very expensive multileaf switch contacts.
I would think the way to go is to use a 10W class B push pull audio amplifier and reduce its gain via the NFB loop to give you the output impedance you require. Your main enemy would be DC drift with temperature and supply variations. It would give you a VR with one side connected to earth.
Frank

 

[pany2008]

thanks for reply,

The current to the resistor will be max 10 ma, and no frequency its going to pure DC max voltage will be 12, ideal volt are 5 volt DC with 1 mA.

Can you suggest any idea.

 

[chuckey]

Thinking aloud, (lets pretend op amps can deal with amps of current- it makes the maths easier). If you use the common terminal of a +- supply as one terminal of the resistor. Then the other terminal goes to the op amps output via a resistor. the actual input point now monitors the voltage. So if its set to 1milli ohm, if 1mV is applied to the input, the opamp output must "****" 1A out of the applied voltage source. You can see where this is going. . if 2mV is applied, the amp must **** out 2A. . . Now if we reduce the circuits gain so that 2mV "sucks" out 1A, it will look like 2 milli ohms.
So now design the circuit, I think you will have to use instrumentation amplifiers, as 10 mA X 1 milli ohm = 10 micro volts. So suppression of noise is paramount.
Frank

**** = s u c k

 

[FvM]

The current to the resistor will be max 10 ma, and no frequency its going to pure DC max voltage will be 12, ideal volt are 5 volt DC with 1 mA.

The term "milli ohm source" doesn't well fit the said current and voltage numbers. They talk of kOhm range, RTD calibration would involve around 100 ohm with 40 mohm resolution corresponding to 0.1 K. Precision OPs with a 16 bit multplying DAC can do.