Calculating a darlington pair bypass resistor value

[boylesg]

The base-emitter resistor is optional and can be use to improve turn-off time. This resistance must be small in the range of 1Ω to 1kΩ to ensure that any leakage current from TR1 does not activate transistor TR2.

Where would people recommend I start? 10R, 100R, 500R? Is there some way of calculating this to get a ball park figure.

I have a heap of TV HR output transistors and a number of Ic=3A amplifiing transistors that I would like to combine into a darlington pair and drive this from a 555 + BC327 or BC337. As an alternative to big power mosfets which are harder to come by in TVs.

 

[godfreyl]

100R is a good ball-park figure with power transistors. For some applications you might want lower e.g. 47R or maybe 22R.

There isn't an exact right answer - it's a bit like how much salt to put on food.