# Compensating for transistor resistance in circuit.

#### Patrick Parkhill

Joined Apr 19, 2016
4
I am extremely new to electronics. I am making a project where 5 groups of 6 LEDs are controlled by an Arduino. I have the LEDs in parallel with 470ohm resistors on each cathode. Each anode is collector of the transistor. The emitter is connected to ground. I have the Arduino's 3.5v pin running to the base of the transistor with a 1000ohm resistor. The positive terminal of the 12v lead acid battery is connected to the Arduino voltage in and each LED resistor. All grounds go to the negative battery terminal.

When connected the LEDs glow, but quite a bit less than without the transistor.

I have made one group so far. So when I connect the ground to the led anodes it glows much more than if using the transistor. The Arduino is on a separate circuit so I don't think it would affect the lights.

My guess is that a lower ohm resistor for each light would compensate, but I don't know where to get the information to calculate that properly.

Transistor is NPN c1740. Emitter at top, collector, then base.

My other guess is I'm a newb and doing it wrong.

#### #12

Joined Nov 30, 2010
18,224
I think the limit is the arduino 3.5 volt pin. To make a transistor slam hard, you need 1/10th of the collector current going to the base. Figure up your total LED current, divide by 10 and calculate a resistor for 3.5v - 0. 7 = 2.8v and the resistor is 2.8/(Ic/10)
If the arduino can't get there, you will have to use more than one transistor to boost the current.

Hang on. You have 470 ohm resistors in parallel with the LEDs? That isn't right. The resistors should be in series with the LEDs.
Another hint: You can put maybe 3 LEDs in each series string before you use up your 12 volts.

#### dannyf

Joined Sep 13, 2015
2,197
When connected the LEDs glow, but quite a bit less than without the transistor.
Sounds like those leds are running off 3.3v/3.5v. If they are white / blue leds, that doesn't leave much current through those leds.

If I were you, I would run the leds off of the 12v battery directly, and hopefully in serial.

#### Patrick Parkhill

Joined Apr 19, 2016
4

Resistors are in series with the LEDs. Power -> resistor -> LED -> ground. LEDs are in parallel with eachother.

The concept is to have the Arduino switch only. The battery is supplying the voltage to the LEDs. So it's 12v to the resistors then the ground is switched by the transistor.

Was I wrong in putting a 1000ohm resistor between the 3.5v from the Arduino and the base of the transistor?

#### Patrick Parkhill

Joined Apr 19, 2016
4
Sounds like those leds are running off 3.3v/3.5v. If they are white / blue leds, that doesn't leave much current through those leds.

If I were you, I would run the leds off of the 12v battery directly, and hopefully in serial.

3.2v-3.4 forward voltage LEDs white 3mm. The hope is to connect to the programmed pin of the Arduino and program a randomized twinkling of the LEDs in their group.

#### #12

Joined Nov 30, 2010
18,224
Was I wrong in putting a 1000ohm resistor between the 3.5v from the Arduino and the base of the transistor?
and the resistor is 2.8/(Ic/10)

#### Patrick Parkhill

Joined Apr 19, 2016
4
Ah, thank you. So: 2.8/((0.020 * 6)/10) or 233ish? Each LED had a 20mA current, if I'm not mistaken.

Worth note:
I am literally 2 weeks removed from wiring on a children's electronics kit to this. I am a computer programmer by trade. The brightness of the LEDs is more of a curiosity. They aren't dim. If this is a functional and correct, most importantly safe circuit. I can deal with the light dimming. I'm looking for the mystery more than anything. I didn't know about needing 1/10 the current to the base. That is very helpful. Thank you.

The project in whole is a 12v motor rotating the 3rd tier of a 4 tier cake the bottom will be a box containing my circuits and the gears and motor to operate the rotation. My LEDs will be mounted in holes in the base and illuminating sugar.

I chose parallel because if one fails, I believe the rest stay lit. In serial, if one fails the whole group goes dead. My understanding is that parallel is less efficient but able to withstand failure.