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Making an autoranging multimeter

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Richard S

New Member
Hi everyone.

I have an expensive multimeter which I use for precise readings. However, I would like a bench multimeter for very quick rough tests, to save getting out the expensive one.

My son gave me an old case which looks ideal (a nice 12" wide rack mount aluminum case), and I want to do the electronics.

The readout is no problem - I can drive 7-segment LEDs with no problem, probably having four or five digits. I will use a PIC for the brains.

My query is how to do the input:

The specification:
Voltage: Up to 800v
Current: Up to 50A
Resistance: Up to 20M

Voltage:
The PIC has a maximum ADC input of 5v. Using a 200:1 voltage divider would give you very poor resolution at low voltages. Instead, could I use a 200:1 voltage divider, and then an op-amp to multiply the voltage? The multimeter would need to automatically decide on the gain.

An 800v input would give 4v after the divider - fine to be inputted. A 5v input would give a 0.025v signal. This is too small to give good resultion when using 10 bit adc, so it needs multiplying (100 times?)

Of course another option is to use several voltage dividers, switched by relays. The PIC would activate the correct one. The only part I am not sure about, is how to stop the voltage being to high on the input pin, if the wrong divider is connected. Someone told me you can put any voltage into a PICs input, as long as the current is limited. The internal diode will stop damage occuring. Microchip show a circuit where mains is fed through a 100k resistor into the PIC, and it is fine. Could all voltage dividers be connected at all times?

I have no idea how to measure AC voltage - this isn't that needed, so I think I will miss it out (unless anyone has any ideas).

Current:
I have some very low resistance shunt resistors (0.001 ohms), so I can use an op-amp to multiply the voltage. Again, different multiplication is needed depending on the range (50A gives a 0.005v drop, while 20mA gives a 0.00002v drop). Prehaps several different shunt resistors are needed?

Resistance:
I assume I put a very small voltage (0.1v or so) across the resistor and measure the current? Again, this sounds like it will want multiplying.

How does a normal multimeter measure resistance?

Many thanks for any help, input or views.

Richard
 
Last edited:

cervy_012

New Member
a reply: Have you finished your project?

it' s been a long time.. i am having the same activity as yours. i am making autoranging not DMM but Ohmmeter only.. can you help me brother on this. my email is [email protected]
hoping for your reply,
Cervantes E. Nicodemus, ECE Student
 

MrAl

Well-Known Member
Most Helpful Member
Hello,


You are making an ohm meter only? What range do you want to measure?
 

cervy_012

New Member
Sir,,an autoranging ohmmeter,, for Range of 1 OHM to 2MegaOhms,, is it possible sir? Do you have circuits or any modules.. I have here a non autorange Multimeter,, my plan B. is to modify it to an autorange.. but i have still no idea how to do it,, Thank you
 

MrAl

Well-Known Member
Most Helpful Member
Well i guess you solved that problem then :)
 

MikeMl

Well-Known Member
Most Helpful Member
Last edited:

MikeMl

Well-Known Member
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