Hi All
For what it is worth and for those interested here are a couple of simple constant current source circuits i have used very successfully in the past.
Very simple.
A reference voltage is provided for transistor base. This could be a Zener (shown), a few diodes, a dedicated reference or even a LED (as an indicator as well). It does not matter as long as it is stable.
RReference is calculated to suit reference device.
RSence is calculated for a voltage drop equal to VReference minus 0.6V at the required current.
This circuit is not terribly elegant but is very simple and is good for a quick and cheerful constant current source and what is important it works.
RSense should be kept as low as practical to reduce wattage dissipation requirements in the resistor.
For larger currents transistors like the 3055 or PNP equivalent can be used. For larger currents again a heatsink might be needed.
Once again RReference is calculated to suit the reference device.
The output voltage will be VReferebce minus 0.6V
Transistor 3055 is a popular transistor where a healthy current is required.
If a larger current required more transistors can simply be added in parallel. A common reference is used for all but insert about 10Ω in series with each base to help stabilise the whole thing.
Once again these are simple cheerful circuits that can be used for experiment or when a quick solution is required.
Hope this helps a bit.
Cheers Bob
My understanding is that a circuit draws the current it needs.
Can you give an example application for when you might need to fix the current going into a circuit?
Thanks for sharing those circuits, really clear and practical! The note about adding transistors in parallel for higher current (with series resistors on each base for stability) is a nice touch too.
@Pixmusix, with a normal constant voltage supply, the load indeed takes the current it “wants.” But sometimes, you specifically need to limit or stabilise current, because not all loads behave nicely with a simple voltage supply. A few examples that come to mind are LED Driving, Battery charging, and 4-20mA loops.
Hi Pix
Ryan has provided some common uses.
one I personally had was I needed some stable switched voltages to control something else at preset switched conditions. I arranged a constant 1mA supply which was necessary as the source supply was not particularly regulated or stable enough. Put this 1mA through a resistor string which gave me a convenient 1V per ikΩ resistance. result a series of stable voltages I could switch completely divorced from any power supply fluctuations.
Lots of uses actually. Quick and easy.
Cheers Bob
OH I forgot. The main use is to take care of load fluctuations like adding more LEDs to a series string. This sort of circuit will keep the load (and thus the individual LED current) current constant as long as the supply voltage is high enough to accommodate the additional voltage drop.
What I described is a simple series or cathode follower linear regulator. The technique of adding power transistors for extra current is given some attention in the application notes for most linear regulators such as the 78XX or 79XX series. This is a fairly common practice with this type of regulator.
Cheers Bob
