I am working with a baseboard thermostat to replace the low voltage control side with some custom controls. The thermostat is divided into two sides, a high voltage board and a low voltage board. There is a 4 pin connector(with only 3 used) between the two that is labeled to have ground, power and control(triggers the load when high). The chip on the low voltage board is an ATmega169P.
Right now, I can control the load(in my test setup, not an actual baseboard) from an external controller if I power it from an external power source(ie USB) and connect the 3 wires to the controller as well. Without the external power, the controller I'm using(just a Adafruit Trinket for now, not my final plan) does not boot up – it's power light blinks dimly and that's it.
I don't have any great scope or similar for checking things, only a multimeter and probes. When I check the voltage across power-ground I get 6.8V, probing at the original ATmega169 power lines get's me 3.3V(as it should) after a on circuit power regulator. If I run the 6.8V through a LV7805 I get a solid 5V there. All this makes sense, but once I put a load on the power coming from that circuit, it fails to keep the voltage up.
Connected to the Trinket, and running through the onboard regulator, I only see 3.0V on the other side of the regulator. If I build a quick and dirty voltage divider and measure it's output(setup to divide in half), I get 1.43V.
At this point I'm well past my knowledge level. I did not expect this part of my project to be what messed with me. A couple of theories that I don't know how to test:
Is the output really low amperage? How can I see what the max output on this power source is?
Is the voltage changing heavily? I don't see any fluctuation on the meter.
Is there something odd with the power source coming from the high voltage side due to it only being a two wire thermostat? The design works with the existing board, but I don't know how it deals with the fact that it only connects inline on the hot wire, no connection at all to neutral or ground. I assume it's allowing some current through all the time to get power for the main processor.
As I try to figure this out, I'm looking for pointers to help move it forward, and I've either not found what I want online, or I don't know the proper stuff to search for. Suggestions on what to try are appreciated, I do want to understand what I'm working with better over time.
Best Answer
The ATmega169P runs on 330µA (that's micro amperes.) So, 0.33mA.
The Trinket uses a ATtiny85 that runs on 300µA. It also, however, has an LED that turns on when it is running. The Trinket schematic shows the LED connected through a 470Ohm resistor. Given a 2Volt forward voltage for the green LED, and powered from 3.3Volts, you get about 3mA for just the LED. That's a whole order of magnitude more than the original processor (ATmega169P.)
It is entirely possible that the high voltage side supplies very little current to the low voltage side since the original processor didn't need much current. Your Trinket pulls 10 times as much current, and so may be just too much for the power supply.
You might try removing the series resistor from the LED on the Trinket (open the circuit to the LED so it doesn't light up.) That should greatly reduce the current, and may get things running.
I've answered what I think is the meat of your question. Let me know if I've guessed wrong, and please do consider simplifying the question.