Re: 465/475 Volts/Div range LED swap problem.

Dave Peterson

There are many facets to this experience for me:

One is that I want this info in the groups messages in the hope that it might help others in the future. So pardon if this is rudimentary for the more experienced of the group.

Another is that I wanted to reeducate myself in BJT circuits. I haven't dealt with them since school. This has been a great experience.

It was actually the thought of adding a parallel resistor to the 10X led/resistor to provide an alternate current path that led me to using a NPN. I wasn't sure what was going to happen when Q382 tried to drive the 10X LED. Wouldn't the low R win over the LED and keep it weak/off? I didn't have LTSpice setup yet. So I thought, since I'd just gotten this load of NPNs, why not? And down the Rube Goldberg path I went.

The NPN does end up with an elevated base voltage when 10X is on and 1X is off. It doesn't effect the 1X diode, but it is dangerously close to turning on.

Now that I have the simulation set up I was able to test the 470 ohm resistor, and sure enough, it works great. See, I knew I was making it more complicated than need be. Simplicity is definitely what I was trying to accomplish.

I spent a good amount of time yesterday and this morning coming up to speed on LTSpice. For example, I needed to figure out how to add a Darlington pair for Q382. As well as the simple things too. In fact I started with two 2N2907's as my Q382 at first. Worked pretty well, and got me up to speed enough to tackle the next level exercise of adding a third party model (and make it portable too).

I'm not very good at learning a new tool in a purely academic way - even if it is very similar to what I've used before. I need something practical to apply it to. This was a perfect circuit to cut my teeth on. Thanks to all who suggested it before. Was that you Michael? It has now also been a good exercise in finding minimal solutions. I also now have a better model to test on than a bread board with too few components.

I don't think I'll ever be up to speed on these scopes and circuits as so many of you. But I am learning, and very much enjoying myself.

Thanks to all who tolerate me and help.

On Saturday, February 20, 2021, 11:14:07 AM PST, Ozan <> wrote:

decided to wire one parallel to the 10X LED. I was really trying to minimize
the complexity of this solution. This was the best I could come up with.
See my reply
A 470-ohm from collector of the Darlington to ground (across the LED+current limiting resistor) is all you need. No need for ten LEDs in parallel, and it is not the most robust solution either.


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