Here is a circuit of electronic fan speed controller:

Fan regulator

Speed of the fan can be controlled by the following logic levels on various pins:

Truth table

This circuit is meant to be used with a 220 VAC power line and a ceiling fan which looks like this:

ceiling fan

Typical wattage is 60-80 watts for these fans.

I want to optimize the circuit and I feel that I can get rid of R7, R8, R17 and R19. R7 and R8 appear to be inrush current limiting resistors (am I right?) whereas R17 and R19 are discharge resistors for the capacitors.

I tested the concept and it worked fine without those resistors. However I am not sure about the long term effects.

My concerns and questions are follows:

1) Can I remove R7 and R8 without having an adverse effect on life of the circuit and the fan connected to it?

2) With 200K discharge resistor in place, the capacitor discharges from 220 VAC to under 5 VAC within 2 seconds. Without those resistors, it takes around 8 seconds. Complete circuit will be inside a plastic casing. Is this discharge time acceptable? Are there any safety regulations related to this? Will removing those discharge capacitors have any adverse effect?

  • \$\begingroup\$ you left out information ... you want to remove resistors from a commercial product? \$\endgroup\$
    – jsotola
    Nov 29, 2017 at 7:38
  • \$\begingroup\$ @jso - yes. and my company manufactures this product. \$\endgroup\$ Nov 29, 2017 at 8:22
  • 2
    \$\begingroup\$ if you think a capacitor discharges from 220VAC to 5VAC, then you're not the right person to be thinking about hacking components out of this design. Have you looked at the cost of resistors versus the cost of fixing marginal EMI (R7) or safety (R17) tests? \$\endgroup\$
    – Neil_UK
    Nov 29, 2017 at 8:29
  • \$\begingroup\$ I am here wondering if removing two resistors is really worth it. I mean, if you are selling milions of units it probably is... \$\endgroup\$ Nov 29, 2017 at 9:59
  • \$\begingroup\$ Don’t remove R17 and R19. The capacitors will discharge in a few seconds with a meter connected, but will take several minutes at least without, and this voltage will be exposed on the controller’s terminals. \$\endgroup\$
    – Frog
    Jun 18, 2021 at 5:20

2 Answers 2

  1. You'd have to do long-term testing to find out. You won't save any money if you skimp a few cents on resistors, but then have a rush of warranty claims in a few months time because the semiconductors keep blowing.
  2. This is a safety-related component. Touching a charged capacitor while standing on a step latter to fault-find a bit of kit (which probably has a blown semiconductor), and touching a capacitor charged to 300V DC could be nasty. When measuring the remaining voltage, are you allowing for the resistance of the voltmeter?

A capacitor working in an AC circuit will hold the charge that the AC waveform is at the exact moment of disconnection. When there is a 220V AC sine, the instanteanous voltage will peak from a -311V to +311V, crossing 0 in between. Just imagine you left one of the capacitors still charged at -311V from a previous operation, and the triac in the cricuit is going to switch on at the moment the sine is peaking at +311V, the inrush current will be instantaneously very high, probably not blowing up anything at the moment but may degrade performance of the semiconductor over time. So, yes, I'd keep the resistors to discharge the capacitors when not in use.


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