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Old 20th June 2012, 04:12 PM   #1481
AndrewT is offline AndrewT  Scotland
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1 red LED @ ~1.9V & a 1k trimmer should be about right for a maximum of ~4V output.
I think you need to measure your voltages again and post the results.
You probably need two red LEDs & 1k for ~5V output (1.9+1.9+[0 to 2V]+0.6V = 4.4V to 6.4V).
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Old 20th June 2012, 04:15 PM   #1482
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Not for BJT
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Old 20th June 2012, 04:18 PM   #1483
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The output voltage is determined by the Vref string & the Vbe of the control transistor.

It has nothing to do with the choice of FET or BJT for the Shunt transistor.
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Old 20th June 2012, 04:20 PM   #1484
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Read BiB guide.
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Old 20th June 2012, 04:33 PM   #1485
AndrewT is offline AndrewT  Scotland
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Which bit of the BIB guide do you want me to read again?

Just in case you didn't read my previous posts, they are quoted here
Quote:
The output voltage is determined by the Vref string & the Vbe of the control transistor.
Quote:
1 red LED @ ~1.9V & a 1k trimmer should be about right for a maximum of ~4V output.
I think you need to measure your voltages again and post the results.
You probably need two red LEDs & 1k for ~5V output (1.9+1.9+[0 to 2V]+0.6V = 4.4V to 6.4V).
Quote:
From there you get the actual current passing and can use that to substitute for the 0.002 value that I guessed at for a 4.3mA Idss running @ 600mVds.
What is the actual diode Vdrop (Vf)?
Quote:
I think you need to measure your voltages again and post the results.
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Old 20th June 2012, 04:40 PM   #1486
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Appendix 1
Examples for 3mA (in circuit) Q303 CCS current. R303=R103,203 also. Q303=Q103,203.
a. 2.5-5.5V Vout BJT output reg. 1 red 1.9V LED, 1K trimmer. Other parts jumper.
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Old 20th June 2012, 04:45 PM   #1487
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by merlin el mago View Post
Appendix 1
Examples for 3mA (in circuit) Q303 CCS current. R303=R103,203 also. Q303=Q103,203.
a. 2.5-5.5V Vout BJT output reg. 1 red 1.9V LED, 1K trimmer. Other parts jumper.
one red LED @~1.9V + one 1k trimmer set to 1k0 @ 3mA + Vbe @~0.6V comes to 5.5V for the output and setting the trimmer lower allows adjustment down to ~2.5Vdc.

You measured and posted a 2mA result for your Id. That changes the range of output voltage to ~2.5Vdc to ~4.5Vdc.

I have asked you to measure and post your results again.

Once you have done that we can maybe make some progress.
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Old 20th June 2012, 05:30 PM   #1488
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After 1 hour of warm up I measured 5.25Vout without ripple.
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Old 20th June 2012, 05:40 PM   #1489
AndrewT is offline AndrewT  Scotland
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good.
That means your earlier measurements have changed since you corrected the errors.
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Old 20th June 2012, 06:13 PM   #1490
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Yes, thanks Andrew for support.
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