Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
can any one tell me the proper definition of a current source! cause a source has to have current as well as voltage so how can you call a source a current source only?
A VOLTAGE source (say 5 volts) will output 5 volts at 0 current, 1 amp or any current. Voltage = 5, current = 0 to infinity
A CURRENT source (say 1 amp) will output 1 amp at any voltage. If you short out the terminals it will output 1 amp at 0 volts. If the load is 5 ohms then it takes 5 volts to use up the 1 amp. If the terminals are “open” then the voltage goes to infinity.
and now the practical use of a current source: Assume a big building with many sensors for the building management system (BMS), e.g. pressure transducers for the air control system. Dealing with long distances there is always a voltage drop on the way from the sensor output to the control center using voltage.
Using a current source (commonly 4 to 20mA) the losses will be compensated by driving current through the wire. The receiver has to convert the current to voltage to feed into the A/D converter for further signal treatment. The conversion to voltage is standardized in Germany to a burden resistor of 500Ohms. Converting 4mA to voltage using 500Ohms the voltage will be correct and precisely 2.00V (4mA*500Ohms). At maximum current the resulting voltage is 10.00V, no matter what the voltage drop on signal line is.
A large load for a current source is a small resistor (less output voltage to drive a given current in the resistor) but a small resistor is a large load for a voltage source! (more current needed to produce a given output voltage across the resistor, which is the definition typically used)
A current source tries to maintain constant current by modifying the output voltage as the load changes.
A voltage source tries to maintain constant voltage by modifying the current as the load changes (or at least the closest two definitions I can get that are opposites of each other).
Grab your digital multimeter if you want to see the world's most common use of a constant current source. The ohmmeter function for measuring resistance uses a constant current source to force an accurate, known current though the unknown resistance and then measures the IR drop created, displaying the result as a resistance reading.
A typical DMM might use a 2ma current on the 2K ohm range, 200nA on the 20K range, 20na on the 200K range, etc. A constant current source has to limit its open-circuit voltage so that it won't be dangerous. After all, trying to drive 200mA through 10K ohms would develop a lethal 2000 volts (at 200ma) and the current source would have to be capable of generating 400 watts of power.
If you know what your DMM uses for its constant current, it can be used to check the accuracy of ammeters. The DMMs constant current is likely accurate to within at least 1% even for a cheap instrument, and 0.01% for a really high-quality instrument made by by Agilent. Just connect the ammeter to the terminals of the DMM, set the DMM for it's lowest resistance range and the ammeter for a range that is compatible.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
