I am searching for a cheap constant current driver/circuit. I have four of this kind that takes 12V input voltage and gives 900mA at 12V (unfortunately it is not working with 5V input).

I need a circuit that takes 5V input (from a 5V/20A power supply) and produces around 1A constant current.
Is there such a device out there? Which parts would I use to build one on my own? Thanks.

paigioph:
I am searching for a cheap constant current driver/circuit. I have four of this kind that takes 12V input voltage and gives 900mA at 12V (unfortunately it is not working with 5V input).

I need a circuit that takes 5V input (from a 5V/20A power supply) and produces around 1A constant current.
Is there such a device out there? Which parts would I use to build one on my own? Thanks.

You will need to step up the 5 volts to 12 (or more) volts. Then, use this classic circuit:

The constant current is defined by R and the formula is [b]R = (1.2 / I)[/b]

So, for 900 mA, you would do R = 1.2 / 0.9 = 1.33 ohms.

Of course, for 12 volt compliance voltage, you need at least 2 to 3 volts more on the input, so your 5 volt up-converter should provide 15 volts DC minimum.

Downside of that ancient circuit is that it has a dropout voltage of ~3.25volt@1Amp
2volt across the regulator and 1.25volt across the resistor.
Not possible with this circuit to power a ~3.3volt LED from 5volt.
Leo..

I've used this circuit to hold 1.5 amps through an E-cig heater, worked very well, if load is non inductive (resistance or LED) you won't need D1, though you may need to heat sink Q1.