comparator for current

[georgetwo]

Hi
im new to pic programming, I have done some practice with comparators. I want to know if it is possible to write a program for over-load or high current indication on pic16f628.
how can I code that? Do i need n external cct?

One more thing, how do i set an oscillator for 50Hz forever?

Thank You

 

[Sceadwian]

Comparing a current is typically done with a current sense resistor. A VERY low value resistor that develops only a slight voltage, often fed through an amplifier into a voltage comparator. The voltage is directly proportional to the current in the sense resistor as long as the power is very low in relation to the size of the sense resistor. Suggestions on how to make a circuit that would work for you would require information about the amount of current you need to source, and at what voltage. There's a big difference on how you value a sense resistor and circuitry for 1ma 10ma, 100ma, 1amp, 10amps, 100amps etc... Purchasing appropriate shunts and having them calibratied can allow you to measure any practical current level.

 

[georgetwo]

Thank you very mush for that idea,
I want to measure the dc load on 12V,200AH
I tried googling current sensors and i saw some ring-like sensors, and some chips, which one will be best 4 me?
Is it possible to make one?

 

[Sceadwian]

You mean 200amps? AH is not a current rating it's a capacity rating for a battery. You're using the wrong search terms on Google. The two that will get you the results you want are current sense resistor and shunt resistor. It is possible to home make shunts, but you'll have to calibrate them. It's pretty simple, a shunt is just a resistor with a VERY low known resistance, in a circuit when you can measure the voltage dropped across the resistor and you know the resistance you can calculate the current that is going through it.

Wire resistances Copper wire figures AWG Table Gauge Resistance per foot Current ratings Resistivities at room temp Typical current for mains wiring index back site search acronyms discussion mail to a friend post message

Shows the amount of resistance per foot of various common AWG types.
If you really want to measure 200amps you're probably going to want to buy a shunt. They're not that expensive.

200 AMP SHUNT, 50MV=200 AMPS | AllElectronics.com
is one example. Most shunts are rated for 50mv full scale meaning at 200 amps of current draw the voltage drop is 50mv across the shunt. You then use an opamp that's set to amplify a differential voltage to whatever scale you need.