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Dennis Feucht

Seemingly Simple Circuits, User-Proof External Supplies: Circuit 2, The Improved One-BJT Current Limiter

Dennis Feucht
ziggle314
ziggle314
3/9/2016 4:42:12 PM
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Re: Question on One BJT Current Limiter
Thanks for mentioning the Vo omission. I was not getting your table results and this explains the discrepancy. I have an application for the circuit and I will be building it in the near future. I will post my results as a comment to this blog post when I complete my work.


I am a big fan of your 4-volume analog design library. Keep writing up your analog work.

Mark Biegert

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D Feucht
D Feucht
3/9/2016 4:07:42 PM
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Re: Question on One BJT Current Limiter
Mark,

You're right again. My notebook date for the derivation was 23MAR09 - evidently a "bad-algebra day". Editor Steve Taranovich will be updating the text.

There are a couple of other minor errors and an omission following these formulas. The omission is that the circuit values were solved using VO = 4.0 V. The VBE open- and short-circuit values given in the derivation of the parts values are negative for a PNP and should be VEB instead.

Thanks again for your feedback. It is always gratifying to see a reader actually work out the math! If you use this circuit, let us know if the measurements agree with the calculated values.

 

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hamiltonraza1
hamiltonraza1
3/9/2016 3:16:08 PM
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Re: Question on One BJT Current Limiter
great post you have poste here

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ziggle314
ziggle314
3/8/2016 9:05:37 AM
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Re: Question on One BJT Current Limiter
Sorry to bother you again. Could you look at your expression for RE? You have a term, VO-VEB(SC), that my derivation shows should be V. I get all of your other expressions.

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ziggle314
ziggle314
3/5/2016 7:23:07 PM
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Re: Question on One BJT Current Limiter
Thanks Dennis.
You are correct about the inequality – I just typed it wrong. I am not used to typing equations using ASCII, and I should have checked my input closer.

Thanks for the interesting article.

 

Mark

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D Feucht
D Feucht
3/5/2016 2:09:13 PM
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Re: Question on One BJT Current Limiter
I rechecked my math from the original notebook derivation and you are right that it needs correction. Your result is slightly different that what I ended up with, though closer than my original equation. My result retains the inverted inequality sign (R sub B <=) because if you substitute for R sub i and then solve for R sub B, the R sub E term is subtracted. Consequently, if R sub E > 0 ohms, then R sub B is made smaller so that for R sub E = 0 ohms, R sub B is maximum.

Because the comment editor is not conducive to equation-writing, I have asked the editor to correct the original article. The correction also clarifies how the R sub B equation is derived.

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ziggle314
ziggle314
3/4/2016 6:30:40 PM
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Question on One BJT Current Limiter
Hi Dennis,


I was able to almost derive your results, but I have a factor in my results that is different than what you obtained and I wonder if you could take a look. For example, when I try to derive the constraint on RB, I obtain the following result.

RB≤(β/IO)*(V-VEB(sc)/(V-VO-VEB(oc/V))

Your result is similar, but I obtain a V instead of VO-VEB(oc) in my denominator.

RB≤(β/IO)*(V-VEB(sc)/(V-VO-VEB(oc)/(VO-VEB(oc)))

I was wondering if you could give me any insight on where to look for the problem.

Thanks

 

Note: Edited to correct an inequality error.

 

 

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