Measuring Volts and Amps digitally

[The Master]

I dont know about a voltage reference chip. I was thinking to have a variable resistor to get it more or less calibrated properly then have an offset value that can be set by the PC so i can fine tune it without opening everything up to get to the PCB. If a voltage reference chip does what i think it does then it sounds like a good idea and i might use one of those instead.

Ahh, ive always know V=IR as ohms law. Ive never seen E=IR before. Could that be a new / old or english / american thing?

I think if i calculate the wattage of resistor i need then double it i should be alright. Im going to put a safety cut out on too so if either AC or DC tries to draw too much current it will warn you on the PC screen and if it goes over a certain value it will cut the power to that transformer. A PIC chip will control cutting it off (the PC just sets the values) so it should be pretty quick to cut the power incase of a short etc.

I think i understand that equation now.
P = IIR
ResistorWattage = Amps X Amps X ResistorOhms

Rapid sells some really low value resistors but they are a bit expensive. The only ones i see that are 0.1ohms or less are 50p each! I need 8 so thats £4.00 already (plus a bit for accidents). They are 4W though. Ill have to keep looking for some cheaper ones.

Thanx for that formula. I think it will help with other circuits too. I always use 0.5W resistors but now i could use smaller ones in certain places depending on the amps and ohms. I should start making a list of usefull formulas.

What is the voltage range of a PIC chips ADC input pins? Im sure i could find the lowest value out by trying it but i want to find out the highest value too without releasing the mysterious blue smoke

[skimask]

If a voltage reference chip does what i think it does then it sounds like a good idea and i might use one of those instead.

It probably does. But if you use high tolerance resistors, you should only need to calibrate once.

Ahh, ive always know V=IR as ohms law. Ive never seen E=IR before. Could that be a new / old or english / american thing?

2 peoples separated by a common language!

I think i understand that equation now.
P = IIR
ResistorWattage = Amps X Amps X ResistorOhms
Rapid sells some really low value resistors but they are a bit expensive. The only ones i see that are 0.1ohms or less are 50p each! I need 8 so thats £4.00 already (plus a bit for accidents). They are 4W though. Ill have to keep looking for some cheaper ones.

I think they're expensive because it takes soo much 'energy' (manufacturing energy, R&D, etc) to get them to those low low values and keep decent tolerance over a wide voltage/current/temperature range.

I always use 0.5W resistors but now i could use smaller ones in certain places depending on the amps and ohms.

1/4w are generally the cheapest, whether SMT or thru-hole.

I should start making a list of usefull formulas.

wikipedia...for instance
http://en.wikipedia.org/wiki/Ohm_law
http://en.wikipedia.org/wiki/Voltage
http://en.wikipedia.org/wiki/Electric_current
or my personal favorite...
http://en.wikibooks.org/wiki/Circuit...Resistive_Film
and so on and so on...

What is the voltage range of a PIC chips ADC input pins?

Vdd to Vss... It's all in the A/D section of the datasheets