# using high and low switch together on a load

in the below picture is a test method im trying to make it work

• if succeeded, I will implement it in my LED clock project
• I'm making a four 7 segment LED using 12V led strips as 3x3 bits for every segment
• The method to control the the segments is by using the 74595 shift register to call each LED
• and using the "high side" "low side" transistors as switch to call the LEDs
• by making a common among the LEDs (common cathode or anode)

Anyway, I have very little information to rely on and need an expert opinion. Is the circuit in the diagram correct? (original)

If yes, how can I calculate the resistance value for R1, R2 , R3?

what are the power rating should I be using?

I used for the low side switch R3 = 300 Ω and for the high side switch R1 = R2 = 1 kΩ

But when I tried it, the Q2 transistor was getting really hot.

As for the Arduino code, a simple blink for the digital pin high and low states.

the Q2 transistor was getting really hot.

Because the collector was held at roughly 11 V by the b-e junction of Q1, and nothing was limiting the current from 12 V, through the Q1 b-e junction, to the collector of Q2.

You can add another resistor (or simply move R1) between the collector of Q2 and the base of Q1 to limit Q1's base current and reduce the power consumed by Q2.

how can I calculate the resistance value for R1, R2 , R3?

You want to allow enough base current for the transistors to be fully saturated. That means about 1/20 to 1/10 of the collector current.

The collector current should be limited by the circuit attached to the collector, which your circuit doesn't do, unless your LED strip provides a current limiting mechanism. If not, you can add a resistor in series with your LEDs to limit the current through the Q1 and Q3 collectors.

what are the power rating should I be using?

Once you've finalized your circuit, figure out how much power each transistor will be consuming, and use a rating at least 2 or 3x that value.

• I think we can assume that the three LEDs represent the "12V LED strip" that the OP is talking about, with an implicit resistor. Nov 1 '19 at 18:40
• thank you for the reply it was very informative as for the current limiter as the Dave said it has one built in as i was too lazy to put it in the circuit diagram Nov 2 '19 at 16:50