Electronic Circuits and Projects Forum

If you add another transistor like Q1 connected from Q2's gate to ground you can use it to turn Q2 on and off. Put a resistor in series with the base of the added resistor (no greater than 10 times the value of R1). Connect this resistor to the filter/amp circuit. A high voltage at this resistor will turn off the LEDs. Less than 0.7V will turn the LEDs on.

The spikes it can tolerate are determined by the voltage ratings of Q1 and Q2. To be on the safe side add a small resistor in series with the + power and add a large capacitor (100µF or larger) from the circuit side of the resistor to ground. Select the resistor size to generate a 1-2V drop at the LED current level (R = V/I).

You don't concern yourself with the voltage drop across the LEDs. They will drop whatever they need. You are concerned about the current and that is what this circuit regulates.

The two transistors are configured as a constant-current source (current independent of voltage). Initially Q2 turns on due to the positive bias voltage from R1. When the voltage across R3 due to the current through the transistor and R3 reaches about 0.7V, Q2 turns on and reduces the voltage at the gate of Q1 to maintain 0.7V across R3. Thus the current through the LEDs stays a value of about 0.7V/R3.

You select R3 to give the desired current. In this case you want 0.7V/1A = 0.7 ohms. If you can't find that value you can parallel two or more resistors to get the desired value.

Be aware that Q2 will be dissipating over 8W when operating and will need to be on a heat sink.

You need to be concerned about the transistors blowing because if they blow, that can cause the LED current to increase and blow them also.

But as an alternative circuit I recommend using an LM317 regulator in a current limit configuration (see this). It requires only one resistor and the regulator automatically protects itself against over-current and over-temperature. It also will require a heat sink. A resistor capacitor filter at the power input as I previously described is also recommended.

Originally Posted by stl_jones@yahoo.com

Ok thanks for helping me with all my newb questions.

How can I test this circuit before using the expensive LED's on it? Thanks

Disconnect the LEDs, and put an ammeter in their place. It should show around 1 amp. Q1 will produce more heat during that test than in normal running so you should keep the test time short.