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powering op amps

m3vuv

New Member
hi all,im after using an op amp as a current sense amp on a psu,30v max output,the op amps i have are jrc4558 ic's,the max supply voltage is 18v,i wanted to use them from the same power rail as the psu,obviously i cant do that,now the question is can i power it from say a lm 7824 reg?,the differential voltage is 30v max,can the voltage at the current sense resistor be higher than the op amp supply voltage?hope that makes sense,cheers m3vuv.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
There ar special amplifiers for high side current sense which the voltage can exceed the op amp supply voltage. See analog Devices.

You can also look at hall effect sensors from alegro which will provide isolation and use a 5V supply.
 

MikeMl

Well-Known Member
Most Helpful Member
I would just buy some modern rail-to-rail in/out opamps. The 4558 is an ancient design which has some serious operating limitations. Here is what is going to make your life difficult:

4558.png
which is for +15V,-15V supplies.
 

rjenkinsgb

Well-Known Member
You can also configure an opamp for "differential" input, eg.
https://upload.wikimedia.org/wikipedia/commons/9/92/Current_feedback_op_amp_differential_line_receiver_single.png

The two R1s are the same value, the two R2s the same value. The ratio between R1 and R2 sets the gain. With suitable resistor values the inputs can be far outside the supply; only the r1:r2 junctions need to stay within the opamp common mode range.


Though, as I think you mentioned 10A or more in another thread, you may be better off with a hall effect current transducer - that avoids the power dissipation of a shunt resistor. They give a voltage (or current) proportional to the current in the conductor through them, with total isolation.
Something like this:
https://uk.farnell.com/lem/lts-25-np/current-transducer-25a-pcb/dp/1617410#

(And yes, the opamps need their own power supply, separate from the variable output).
 

AnalogKid

Well-Known Member
Most Helpful Member
Max 4558 power is +/-18 V, or 36 V total. However, the input common mode voltage range does not include the positive rail. While you can power the opamp directly from the +30 V and GND, you cannot use its inputs to sense the voltage across a shunt in the +30 V line without a circuit trick. Also, the output cannot swing very close to the negative rail, so low values of sensed current will not produce the correct output voltage.

Overall:
Yes, the part can be made to work in your circuit.
No, it won't work very well.

ak
 

audioguru

Well-Known Member
Most Helpful Member
The maximum supply for a 4558 dual opamp and most other old opamps is 36V or plus and minus 18V.
The old 4558 cannot use an input voltage near its positive or negative (0V in your circuit) supply but many modern opamps can.
 

audioguru

Well-Known Member
Most Helpful Member
The datasheet for the OP05 opamp shows that its inputs do not work if they are within 1.5V
from the supply voltages.
 

audioguru

Well-Known Member
Most Helpful Member
You can power an opamp from anything that supplies a voltage or plus and minus voltages that are within the spec's on its datasheet. The input voltage range also must be according to the datasheet spec's. Usually a voltage regulator is not needed to power an opamp.
Your problem is that you are selecting old opamps instead of modern rail-to-rail opamps.
 

MikeMl

Well-Known Member
Most Helpful Member
Even simpler for high-side current sensing are parts like the ZXCT1009
 

shokjok

Member
Over-voltage concerns can be addressed by adding a resistor ladder to activate a fixed voltage regulator that supplies the IC, much like a power transistor can supply extra current beyond the IC's rated limit. If you examine "power supply limiter circuits" online, you'll find several methods of excess voltage controls.
 

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