Re: Circuit is terrible at maintaining constant current through LED - PROBLEM SOLVED - circuit.png (0/1)

In article , snipped-for-privacy@hotmail.com mentioned...

> > > Second, and perhaps as important, the current-mirror has serious > > problems with Early-effect and with base-current errors. A much > > better circuit is possible with the same or less parts count, as > > I've shown several times here in s.e.d. in years past. > > > > Thanks, > > - Win > > OK here is the fixed circuit using Pulse Frequency Modulation supply > based on Hysterisys feedback. > >
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> > It goes into full regulation when Vbat exceeds LED Vf by 100mV. > > That stupid analog current mirror circuit of mine actually has a good > minimum voltage in the upper current source, but terrible in the lower > mirror part. That is unavoidable is it not? I think this is because > you can't get any gain out of a transistor with a mere 100mV bias > accross the C-E "junction". The reason for the base current errors is > because I needed to stay with NPN transistors because I was using a > transistor array IC. What a pile of rubble.

Why come you didn't throw in a 100 uF bypass cap across the battery? Might help a bit.

I got on Hosfelt Electronics web site and ordered a couple c-notes worth of parts. Most of that was in the submini toggle switches, which were half the price I could find them at other places. The DPDTs were only $.65. Cheap. For some odd reason, the SPDT were more, but still cheap, $.89. I got 4 of the Mueller Kelvin Clips, and those were $7.50 each, so that added up, too. But I think I nickled and dimed too many pretty colored ultrabright LEDs. Slap my hand. But hey, it's my moolah.

But the best deal I found was the .22F 5V memory caps for only $.50 each, so I got twenty of those. I'll be on my way to completing that Forever Flasher that I ranted about earlier. Five of those only cost $2.50, which was less than the 1 farad for almost $3 at Mouser - even more at Digi-Key. :o)

(Gee, at that price, I hope they're not used! They didn't say..)

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