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Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
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Posts: 102
I found the following circuit fascinating and became involved in a kind of obsessive attempt to analyze it.. Go to and click on S.E.D./Schematics and CurrentSense.pdf
I assigned myself the task of coming up with analytical expressions for the voltages at the transitions.
First, I considered that with logicout high and load current falling, just when the circuit is about to trip R3 and R4 see equal voltage and R5 has no current. So I redrew the circuit without R5 and wrote this expression for the voltage across R1: [(3.3-Vbe)(R2/(R2+R3))] + [(R3/(R2+R3)) .026 ln (R3/R4)] The term on the left expresses the voltage across R2. The term on the right accounts for the voltage offset across Q1 and Q2, which have collector currents in the ratio of R3/R4. Now, using Vbe=.6 gives sense voltage .14 volts, load current .00154 amps, which agrees with the graph on the second page. So far so good.
In the other case, with logicout low and rising load current, I wrote (3.3-Vbe)(R2/(R2+R3||(R4+R5))) to express the sense voltage. But using Vbe=.6, it doesn't agree with the graph. I have to use a Vbe of about one volt to agree with Jim Thompson's graph. And he's a heavyweight, so I'm wondering if there's something wrong with my understanding of the rising transition.
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simple current sense circuit
