# What are the uses of current sources?

1. Aug 12, 2012

### Chander G

Hi everyone,

In our undergraduate electrical course,we came across current sources as theoretical concepts.
Are current sources used in real life(such as in electronics)?How important are they?

Chander

2. Aug 12, 2012

### marcusl

Yes, current sources are used "for real" though not nearly as often as voltage sources. Here are two examples: 1) A current source is used to measure the I-V curve of a semiconductor device. 2) Current mirrors, which are a form of current source, are widely used inside analog integrated circuits to bias transistors, especially when devices must be matched (op amps, e.g.).

3. Aug 12, 2012

### Chander G

Thanks.

4. Aug 12, 2012

### yungman

Yes, there are use of current source. One example used is for electromagnet as

$$\int_s \nabla \times \vec B \cdot d\vec S= \mu I$$

So the magnetic flux density is controlled by the current through the electromagnet.

Driving an electromagnet with voltage source is not as accurate as the resistance of the winding can change with temperature. But using a constant current source, change of resistance only cause change of voltage across the electromagnet, the current remain constant and flux density theoretically remain constant. In most part it is true. I designed a magnetic controller using true constant current source to get accuracy into parts per million.

And yes, constant current source is used inside IC as active load. The definition of constant current source is having very high output impedance. It is used as active load for collector of BJT or drain for MOSFET in common emitter/common source stage to get maximum gain out of the stage.

I am not familiar with the latest LED light stuff, but LED is better run by constant current. If you drive LED with voltage source, you can burn the LED easily as once the LED turn on, the current increase exponentially with increase voltage across the LED. Also the LED turn on voltage change with temperature in a negative way, you can get into run off situation and burn the LED. Using constant current source, the current remain constant even the turn on voltage of the LED change.

5. Aug 12, 2012

### Jobrag

I use them all the time for calibrating 4-20 Ma input cards.

6. Aug 12, 2012

### gnurf

7. Aug 12, 2012

### nsaspook

Battery chargers. Most multi-stage chargers use a constant current source in at least part of the charging process. For a Lead-Acid battery it's usually during the bulk charge mode.

8. Aug 12, 2012

### yungman

People have be very careful about constant current sources. The definition of current source implies the output impedance is infinite or at least very high. The current breed of so called current supply using switching design is not true current source. They are voltage source with a current sensing feedback to control the voltage. There are application that this will absolutely not work. Case in point for magnet control, using this kind of supply will slow down the speed tremendously. This is because unlike capacitor load where the lower the output impedance, the higher the speed. For inductor, the lower the impedance, the slower the speed. Remember for current time constant:

$$\tau=\frac L R$$

We contracted out the current supply design out, and we got the switch mode supply and it absolutely failed. I had do design an analog supply with big MOSFET as current source to get the high impedance. Also remember the big capacitors of the switching supply would be in parallel with the electromagnet? That would be a parallel tank circuit!!!! And it can sing!!!

Last edited: Aug 12, 2012
9. Aug 12, 2012

### Averagesupernova

One place I have used a current source is to drive the LED backlighting on a graphics display panel. If you don't want the backlighting changing brightness with varying power supply voltage (mobile 12 volt applications) this is what you do. I thought it was a better solution than regulating down to 8 volts or so and using a series resistor. The tolerance of the voltage on the LEDs varied enough so I didn't want to go with a voltage regulator and series resistor. I could have gone with a low dropout regulator and larger resistor for tighter current regulation but the cost went up. So, if configured a 5 volt regulator with a resistor to form a current source.