# Trying to Reduce current draw on LM7808 to reduce heat

Hi,
I'm trying to integrate the OEM dimmer into my LED project for an antique vehicle and I'm looking for some help. My prototype works great except the 7808 gets to hot I believe to be put in a small enclosure to fit the space I have available.
I'm using the 7808 to ensure a stable voltage is available to my Arduino Micro.

Most of the current draw, I believe, is going through my voltage divider that I'm using to measure what brightness the LED's should be.

I've tried increasing the resistance of R1 but I lose resolution and the ability for the Arduino to measure it.

I would love any feedback.

jdflyer1999:
Most of the current draw, I believe, is going through my voltage divider that I’m using to measure what brightness the LED’s should be.

I’ve tried increasing the resistance of R1 but I lose resolution and the ability for the Arduino to measure it.

I would love any feedback.

What do you calculate the current draw of the voltage diveder as ?

srnet: What do you calculate the current draw of the voltage diveder as ?

I came up with a max of 165mA and a min of 110mA. The temperature of the 7808 is measuring 60 deg C on a breadboard.

You are gonna fry everything! ok here what I would do: option 1:

Get rid of the 7808 and use 12v from your cigarrete lighter so you can use VIN, and maybe adding a heat sink to the builtin 5V regulator IC.

definitely increase the value of R1! (max current for the 3.3v output is 50 mA)

option 2:

Use an external cheap usb charger and feed the device through the usb port,and increase the value of R1

option 3(best one I think): provide the voltage for the divider from outside the arduino,not using 3.3V maybe 5v from your cheap usb charger, you can do the math for the values of R1, once you are sure the range of voltage over your rheostat is ok for your arduino,adapt your code to those values.

hope it make sense.

The 7808 is probably just part of your problem. The regulator of the Arduino is bound to get at least as hot this way! You're using 3.3V so I assume that's a Pro Mini. Surprising it actually works in the first place, that regulator is too small to handle that much current (too much heat).

Get a buck converter that produces 3.3V directly from your 12-14V car supply, connect that 3.3V to the Vcc pin of your Arduino. Problem solved. No more heat.

That just leaves the problem of your Rheostat. That is a very small resistance, I'd suggest you lower the current going through it and then use an OpAmp to increase the voltage to 0-1V range, reading that with the internal reference of the Arduino

You could create a constant current driver using two BJT transistors, 5 mA would be enough Then you get up to 50 mV across the 0-10Ω resistor. A 20x amplification gives you a 0-1V output.

You can reduce the regulator load significantly by simply:

1) connect the 20 ohms to the Electrical system. Assuming the 20 ohm and pot were originally connected this way and they can deal with the current load.

2) Divide down the output of the 0 to 10 ohm resistor so if falls into the pro mini Ain range.

3) Create another divider from the vehicle electrical system to another analog input on the Pro mini.

Read both Ain's and calculate the parameter you need.

Johbn

JohnRob: You can reduce the regulator load significantly by simply:

1) connect the 20 ohms to the Electrical system. Assuming the 20 ohm and pot were originally connected this way and they can deal with the current load.

2) Divide down the output of the 0 to 10 ohm resistor so if falls into the pro mini Ain range.

3) Create another divider from the vehicle electrical system to another analog input on the Pro mini.

Read both Ain's and calculate the parameter you need.

Johbn

Hey John!

I thought about that originally, but one of the design goals is to reduce the current through the Rheostat to prolong its longevity. They are no longer available some are trying to save what is left.

Maybe you can use a different regulator instead of 7808, such as a small switching regulator power supply module. Depends on how small this enclosure is.

+1 for using a 3V3 switching regulator as already suggested. Powering the reostat by any other means will cause a loss in the ratiometric behavior of the ADC.

Hey John!

I thought about that originally, but one of the design goals is to reduce the current through the Rheostat to prolong its longevity. They are no longer available some are trying to save what is left.

Then you simply increase the 20 ohm resistor so the rheostat has approx the same current that would be drawn if the rheostat leg was on 5V. Or is the 20 ohms part of the rheostat assy?

Another thought would be to control the power to the rheostat assy (with a N-fet and P-Fet) turning on the power to the rheostat only when you wish to take a reading.

One other thought I had when looking at your schematic. You draw the circuit ground and the rheostat ground with different symbols. If these ground are not the same your readings will have a lot of variation.