Im sorry Becarso, i wrote completely wrong, im
not shorting
the current sensor output, im shorting the
regulator output!
R1 right hand side, supply to load point.
Geeee i can really screw things up!
>Need more info. Whose datasheet? What is/are the rail voltage(s)? >What's being shorted? What is the highside voltage you are sensing >current near?
National,30V,the regulators output, (im doing a uC controlled supply,
NPN pass, Op amp regulator loop) Voltages are between 0 to 30V DC.
>If it is the circuit on page 13 of the National Semiconductor datasheet, >with a transistor and the voltage proportional to current being provided >at the transistor emitter, then you will pull momentarily a large current
>if the output is shorted,
Wich output do you mean, the regulator
or the op amp current sensor circuit?
>but the op amp output should go to about Vbe above ground after >that. However this may not be a stable state from the standpoint of >oscillations and during the swings may well then destroy the transistor.
The sensor circuit are never shorted, the op amp output blasts to
+V ie, 28V or 32V (havent decided yet wich supply) but emitter has
1k ohm to GND so current are +V -Vce about 30mA +Vbe ies nearly
+30V on base! perhaps not so good, whatif i put in a ohm and a zener between opamp output and transistor base? The accuracy is not a very
important matter i have a lookup table in the MCU to map the current values anyway im just after a rough voltage representing the current.
This data sheet:
http://www.national.com/pf/LM/LM358.html
page 13 right most cirquit.
>If you are exceeding the common-mode range of the 358 then all bets >are off anyway (note the restriction to no more positive than 2V below >the op amp V+).
There is a note in the datat sheet that says:
:Note 7: The input common-mode voltage of either input signal voltage
:should not be allowed to go negative by more than 0.3V (at 25C). The
:upper end of the common-mode voltage range is Va b1.5V (at 25C),
:but either or both inputs can go to a 32V without damage, independent
f the magnitude of Va.
Wich sugests this old fart is one of these "top over supply" Opamps! No?
And if yes then i could supply the opamp with +5V and have my +30V
applied to it as common mode range to sense the current!
Is the differential current sensing a better approach then this?
Kkrafs