GEO Relay Question

[kurtlives]

How are R1 and R2 selected? I am thinking they just have to be the same value. I have seen the same schem before (minus D1) where R1 and R2 were 470R.



Thanks...

[composition4]

The combined value of R1 and R2 should be what you want for the base resistance for Q1... the total value you need will depend on the transistor HFE and the current that the relay and LED require.  At 12V as the schematic says, 470R for both resistors provides quite a lot of base current for what would probably be a pretty small load.  Using 4k7 @ 12V instead provides just over 1ma base current, which assuming HFE = 100 is fine for about 250ma according to my calcs.... way more than enough for any relay and LED that you'll use in a stompbox

[kurtlives]

Ok thanks. Makes sense this being a current amplifier.

The relay is only drawing 18mA so I guess I'm good!

[PRR]

> I am thinking they just have to be the same value.

Not at all.

Similar values (say 6K and 3K) give the best filtering; way-different gives less filtering.

In MASS production, same-value may be cheapest.

What composition4 said. You find your load current. You divide by hFE to find minimum base current. You multiply by 5 or 10 because you want to SOCK the transistor to be sure it turns fully ON. You find out your control voltage, subtract 0.7V for base-emitter drop, and consult Ohm's Law to get total resistance. Split that for a smoothing cap, if you wish. Unless your external control is very nasty, you may not need smoothing.... seems a frill to me.

Assuming the external control is the full 12V supply: (12V-0.7V)/(4K7+4K7)= 1.2mA. Assuming hFE is _known_ to be 100, then it will just-barely turn-on a 120mA load. That's bad practice. If it almost turns-on you may have only 6V across the relay (it may not switch) and 6V 60mA in the transistor. The resulting 6V+60mA= 360mW of power and heat is at the upper end of what older TO-92 jellybean parts can stand, life may be short. So poor relay action and wastefully hot part: no darn good. This plan should not be tasked with more than 60mA, 24mA is better, 12mA is for-sure rock-solid.

> The relay is only drawing 18mA

And what is the LED eating? Like 10V/4K7 or 2mA. In this case not much different, but always quick-check such things. Be sure each "minor added load" is truly minor and that you don't have so many "minor loads" that it becomes a burden.

1mA base current, 20mA load, any high-hFE transistor, this will be fine.

[kurtlives]

Thanks, with LED added its 20mA total.

I'll go with how the schem has things listed. Seems like it will be more than fine.