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Part I. Introduction
Part II. Theory for Failure
Part III. Testing Equipment Used
Part IV. Software Used
Part V. Parts Used
Part VI. LTspice Simulation
Part VII. KiCAD Schematic
Since this Controller adjusts the blower's speed on the fireplace and this motor is rated for 1.8 [A], let's design the PCB for double the max operating current because we can :).
The only added components from the original design:
We will reuse:
We will replace:
A Bourns 91A1A-B28-A22L Knob Potentiometer (2[W], 0.2 to 250k [Ohms], linear) will repalce the original slide potentiometer (100[mW], 30 to 300k [Ohms], log). Since this potentiometer is in parallel with two other resistors, a 240k and 296k [ohms], the max resistance of the potentiometer doesn't matter that much. However, the minimum reistance does matter because it greatly affects the max speed (ie, the potentiometer controls the rate that the capacitors charge, which controls how the 120 [Vac] sinewave is chopped; thereby, changing the fan speed. marvelous).
Since the potentiometer goes down to 0.2 [Ohms], an excessive amount of current is drawn across the potentiometer, so a 24 Ohm resistor will be added in series with the potentiometer to approximately match the original slide potentiometer's minimum resistance.
Part VIII. KiCAD PCB Design
Part IX. Disassembly of Original Boar & Removal of Components
Part X. Results
When removing the capacitors from the original board, the 47 [nF] Film Capacitor was measured at 1.2 [nF]. The 100 [nF] film capacitors were still in 10% spec.
After simulating the LTspice model with such a value (1.0nF) it became clear that the reason the potentiometer failed was because the 47 [nF] capacitor failed. This 47 [nF] capacitor is marked C1 on the original board (YLFC700).
Reason for failure: the capacitor is configured as a snubber and there is likely overvoltage spikes due to the motor.
Solution: Use a cermaic capacitor that has a higher maximum voltage rating that can withstand spikes resulting from the TRIAC switching.
Successfully operating as of March 2021.
Part XI. Legal Notice
Under the MIT license to this project, you may use this design freely. An email to alledgedlyengineering @ gmail stating that you used this design would be nice to know. By using this project you are agreeing to the risks involved with AC current and altering a UL listed product to fit your needs. In addition, you agree to not hold me responsible for any injury or result of injury incurred by modifying the YLFC700 Fan Speed Controller. This project has no affliations with any of the manufacturers used in this design or the PCB fabrication house used.
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