On 30/05/2011 08:08, cdb wrote:
> Bob Blick has a circuit on an SX forum, I happened across it the other da=
y.
>
> 7 resistors and one op-amp (rail to rail) high side sensing.
>
> V+in -    sense_R ---------- V+out
>
> R_div_1a           R_div_2a
> R_div_1b           R_div_2b
>
> 0v-------------------------------
>
> Op-amp V+ connects to R_div_1 centre tap
> Op-amp V- connects to R_div_2 centre tap
>
> Op-amp feedback resistor from op_v_out to op_V-
> Resistor from op_V+ to 0v
>
> Nominal values:
> R1a,R2a 1M
> R1b,R2b 100K (input voltage dived by 11)
> Feedback R and R to 0v  10M
> Sense resistor 5v/current to be sensed.

People used to use LM3900 to do high side current sense  and PSU input=20
volts as the inputs are essentially current driven sinks there is=20
apparently no + rail limit. It can exceed amp rail.
4 x "Norton" input opamps. AKA LM2900

--=20
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