Relay energy saver schematic, Need help on the R & C val

In Elektor March 2006 pg 73 an Energy saver for relays schematic was publised as a design tip. Although it specified a green 20ma LED the other values were a little broad and no theory or calculation data was given.

It would be easy to add this to the schematic and I don't need to use RB3 as a PWM (I use TIMER2 for a 1/256 RTC & display refresh on the Cricket thermostat) so this would be a rev H change.

Any suggestions for the following schematic? C20 & R8?

If you have a spare PWM channel, using PWM will be simpler and take even less current.

Basically once the relay is latched, you are turning on the transistor and letting coil current rise, then turn off and let the current flow through the diode (which should ideally be a Schottky). Current will decay sort of slowly through a near short like that. So no resistor, caps, or LED and it's quite efficient.

I realise the PWM is the most efficent way to run the relay, But PWM uses timer2 and I'm using that as a free running timebase.

Also in my other schematics (Ladybug as an example) I have 6 relays and it would be nice to add the cap to the design. (same relays)

Oznog said

If you have a spare PWM channel, using PWM will be simpler and take even less current.

Click to expand...

Actually I might just do that for the Cricket. I can use TMR0 /4/16/256 = 225hz IRQ for display refresh and RTC update.

I will use the non PWM version for my Ladybug.