# Transistors and LED's drop in brightness

Hi,

I have an Arduino pro mini that is driving a power led (9v) and two power resistors (to generate heat) each with it's own transistor.

All is well, except that when the transistor for the heater turns on there is a significant drop in the brightness of the LED.

Is there a way to wire the circuit so there is no significant drop?

Currently the LED and two power resistors are all connected to the + of my supply. With ground being connected to the base or middle pin of the transistor. The Arduino is setting the pin high to open the transistor.

I've never used more than one transistor before but would wiring them in parallel sort that and how do you go about wiring transistors in parallel.

Cheers in advance for any help.

Is there a way to wire the circuit so there is no significant drop?

Get a power supply with a higher current rating. The power supply might be supplying 9V with the proper load, but when you turn-on the transistors and try to get too much current, it's not 9V anymore. And, the power supply may die. (If you're using a battery, the battery will drain quickly... I assume you are not trying to run a high-power LED and heaters from a regular 9V battery.) Do you have a multimeter to measure the voltage?

Please show us a schematic. Without a schematic, we are guessing.

What's the current rating on the LED? How about a link to the LED specs? There's no such thing as a "9V LED", but it could be an LED and resistor (or some other circuit).

What's the resistor value? (Ohm's).

The relationship between voltage, current, and resistance is defined by [u]Ohm's Law[/u] (Current = Voltage / Resistance). Two resistors in parallel is twice the current (half the resistance), plus you need to add the current for the LED.

With ground being connected to the base or middle pin of the transistor.

That's wrong! (There are some circuits where the base is grounded, but I doubt this is one of them.) "Middle" might be OK depending on the pin-out, but if the base is the middle pin, it's wrong. The base is always the middle on the schematic symbol, but not always on the part.

I've never used more than one transistor before but would wiring them in parallel sort that and how do you go about wiring transistors in parallel.

You'd normally wire them just as-if the other one wasn't there.

I have the LED and heaters connected to a 1.5 amp desk supply.

The LED is an array of 3 at 10v / 400ma. I have it in series with a 1R resistor (3W).
Power resistors are 68ohm.

The transistors are Power N-channel mosfests.

I'm sorry I don't have a schematic but I've put together a Fritzing of my circuit. The second breadboard is the external supply.

I am quite new to electronics - do I just add the current across each component at 9v to work out the amount of current being drawn?