29-05-2011, 11:26 AM
Hi Mark,
Thanks for your comments.
I did contemplate fitting protective diodes across the series-pass Darlingtons - until I realised that these are already included in the TO-3 package! As for diodes across the O/P terminals - yes, I thought about that too - but somehow they failed to get installed
- easily retrofitted.
I like the idea of a LED as a replacement for the Zener, but I do not know the temperature co-efficient of the LED voltage. Do you? Does the co-efficient vary with different colours of LEDs? Usually I use a 4v7 Zener, since this voltage has an almost zero temperature co-efficient, but at the time, all I had in stock were 5v6 Zeners.
The capacitor, C2, I must admit, was found a worthwhile addition by accident! Basically it significantly reduces the ripple on the O/P when the O/P voltage approaches the maximum available. Of course, there is a price to pay for this: at the instant of current overload, the rate of decay of the O/P voltage is reduced on account of this capacitor. If its value is too large, the delay is long enough to cuase serious distress to the current-sensing resistors. Hence, the final value chosen for C2 was a trade-off. I did think about introducing some additional circuitry in this area so that at the onset of the current overload condition, the capacitor would be electronically switched out of circut. But you know how these design jobs can go - there comes a point where you have to say "Enough! Time to finalise & build!"
As for documentation, well, that is one of those topics that I have a 'thing' about. I learnt long, long time ago that if I make something (and not necessarily electronic / electrical) and say to myself "Done: I'll never need to go there again! Tidy up - and throw those bits of scribble away!" I will regret that decision one day - even if it's a case of "Once upon a time, I made xxx; I now need something similar: how did I do that?" As is frequently heard, "The job's not done until the paperwork is finalised."
Cheers,
Al.
Thanks for your comments.
I did contemplate fitting protective diodes across the series-pass Darlingtons - until I realised that these are already included in the TO-3 package! As for diodes across the O/P terminals - yes, I thought about that too - but somehow they failed to get installed
- easily retrofitted.I like the idea of a LED as a replacement for the Zener, but I do not know the temperature co-efficient of the LED voltage. Do you? Does the co-efficient vary with different colours of LEDs? Usually I use a 4v7 Zener, since this voltage has an almost zero temperature co-efficient, but at the time, all I had in stock were 5v6 Zeners.
The capacitor, C2, I must admit, was found a worthwhile addition by accident! Basically it significantly reduces the ripple on the O/P when the O/P voltage approaches the maximum available. Of course, there is a price to pay for this: at the instant of current overload, the rate of decay of the O/P voltage is reduced on account of this capacitor. If its value is too large, the delay is long enough to cuase serious distress to the current-sensing resistors. Hence, the final value chosen for C2 was a trade-off. I did think about introducing some additional circuitry in this area so that at the onset of the current overload condition, the capacitor would be electronically switched out of circut. But you know how these design jobs can go - there comes a point where you have to say "Enough! Time to finalise & build!"
As for documentation, well, that is one of those topics that I have a 'thing' about. I learnt long, long time ago that if I make something (and not necessarily electronic / electrical) and say to myself "Done: I'll never need to go there again! Tidy up - and throw those bits of scribble away!" I will regret that decision one day - even if it's a case of "Once upon a time, I made xxx; I now need something similar: how did I do that?" As is frequently heard, "The job's not done until the paperwork is finalised."
Cheers,
Al.
