This is a FET Constant current drivers circuit for drive LED display which can use FET-BF256 instead resistor so well.
Normally when you use LED display in any circuits often use a resistor for limit LED current. Because easy and cheap.
But it is not best, this way is ideal for the stable voltage source only. When we change voltage source, the current that flow through the LED will also changes, causes LED not stable brightness. It may be damaged, it must be a constant current flowing through it.
Such as in digital logic probe circuit, which we need to test to TTL type that use 5-volts only, and a CMOS type that wide voltage of 3-volts to 16-volts. When we need to have the LED that stable brightness all voltage source.
I have a good way. A “ FET” is requirement because when we connect Gate and Source together, then put it instead the resistor. They are similar as Figure below.
I use number : BF256 normally it is used as N-channel RF amplifier (in VHF/UHF frequency) , it is small size FET to-92 type. Use in voltage under 30-volts. And see a position lead(Gate,Source and Drain lead) of BF256 in Figure.
And I test it on a breadboard as video below. I use the power supply is … Adjustable dc voltage regulator circuit using ic-7805. which have voltage output of 5V to 22V as we need. (TTL and CMOS voltage)
Firstly I adjust voltage at 5V (see on meter above) At the same time, I measure current that flow through LED have 4.22mA only.(see on meter right) But LED is normal brightness. Normally LED has current required of 15mA.
Then, I adjust voltage up, while the current is stable about 5mA only and the LED also stable brightness as we need…happy circuits.
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