HAPCAN FORUM

Hi,

I'm trying to figure out what R17, R18 and C16 are used for in the circuit of the dimmer module (http://hapcan.com/devices/universal/uni ... -0-x_b.pdf). If C16 is used for RFI noise suppression (like C17), then why are the resistors needed?

Can anyone comment on this?

Cheers!

Hi,
the voltage on C16 changes rapidly due to the nature of the phase control. It means that without resistors the capacitor C16 would discharge with high current. To reduce this current resistors R17, R18 are used.
Unlike on C16 the voltage on C17 changes slowly (from 0 to 230V within 5ms - like mains).

Aha, I see. When the dimmer interrupts the circuit, the voltage between neutral and load drops off very quickly, leading to a large current through C16 if the resistors wouldn't be there.

However, wouldn't the current through C16 be limited by the load? Then again, when designing this circuit I suppose it is best not to make any assumptions about the attached load and just make sure you limit the current internally...

Also, I still wonder what the purpose is of C16. Why not leave it (and R17, R18) out entirely? Is it's goal to reduce the drop off time of the voltage over the load, in order to reduce RFI?

Yes, usually RC blocks are used to reduce RFI.