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Current sense help

Ian Rogers

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Tis I , The non analogue specialist..

I have oodles, and I mean oodles, of LTC1051's and LT1112's dual op amps..

I aim to use one as a current sense.... Now I know some op amps will not work in this config, I normally find an example in a datasheet and use that.. But there isn't one.
 

rjenkinsgb

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As far as I can see, you would have to use a differential config with resistor dividers to pick up voltage from a sense resistor, to avoid the inputs being near either positive or negative supplies.
The 1112 cannot work within around 0.7V of either supply and the 1051 cannot accept anything below its negative supply.

If you are sensing with a resistor in the negative of a battery or power source, so voltages fairly near 0V rather than in a positive or negative supply, you could use either if you added a negative supply somehow; but I suspect a different opamp is simpler.

You could possibly cheat slightly with the near 0V sense resistor, if it's not too critical?
Add two similar diodes close enough together to keep them the same temperature, from each end of the shunt, with bias resistors to a positive supply and the 1051 opamp working from the shifted voltage from the diode anodes.
 

Ian Rogers

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Its a psu... I will be using 1.25 ~ 24v input .. I have 3 LM317T devices paralleled to get me 4.5A I have a micro to determine voltage and current and display it... I'm not going to current limit, just sense... I have a batch of 0.1ohm resistors (5) which I'll also stick in parallel to get the 0.02ohm current detection.. The LTC1051 is rail to rail, but I don't need that... All I remember is some opamps are more comfortable with this sort of application.. As there wasn't ANY sign of an example of this kind I thought I may be barking up the wrong tree...

If an LTC1051 would do the job.......... Hey Boll**cks to it.. I have enough, I'll see if it works.... I'll try a low side first..
 

Nigel Goodwin

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Must admit, I was having problems trying to current sense - in a buck/boost converter - as where ever I put the sense resistor it was too far away electrically to work with my opamp.

In the end I swapped to using an AD8210, which is specifically intended as a current monitor - this worked perfectly.
 

AnalogKid

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Still missing some details. Do you want to sense the current from the DC source into the regulator array, the current into or out of each regulator, the current into the common load, or something else?

Are you leaning toward high side or low side?

Can you post a schematic of the regulator design so we can see what we have to work with? Paraphrasing the Admiral, an engineer don't take a dump, son, without a schematic.


ak
 

AnalogKid

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AND ...

Paralleling three LM317's is not the best way to achieve a higher output current capability. Better is to add a PNP power transistor around a single 317 (circuit is in the datasheet). If you want to stay within the 3-terminal-regulator space, best is to change to an LM350 (3 A) or LM338 (5 A) and get it all done in one device. Both of these methods have a single output voltage adjustment point, no tracking issues, etc.

A possible advantage to the external power transistor method is that is lets you control the relative power dissipation in the 317 and the transistor. This can reduce heatsink size or complexity, and both devices will run cooler than either a 350 or 338 for the same input voltage / output current conditions.

ak
 
Last edited:

Ian Rogers

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AND ...

Paralleling three LM317's is not the best way to achieve a higher output current capability. Better is to add a PNP power transistor around a single 317 (circuit is in the datasheet). If you want to stay within the 3-terminal-regulator space, best is to change to an LM350 (3 A) or LM338 (5 A) and get it all done in one device. Both of these methods have a single output voltage adjustment point, no tracking issues, etc.

A possible advantage to the external power transistor method is that is lets you control the relative power dissipation in the 317 and the transistor. This can reduce heatsink size or complexity, and both devices will run cooler than either a 350 or 338 for the same input voltage / output current conditions.

ak
I'm just using what I have here... The LTC1052 seems to work well BUT!! Christ it doesn't seem to be linear..

I used two 68 ohm for R1's and two 6k8's for R2's I used 0.02m ohm on the sense... I'll just do a calibration sequence.. I did have a couple of 3055's somewhere.... I only really need 2~3A.. would two 317's in parallel be better??

1606910900079.png
 

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