High voltage current sense?

[Sajjadkhan]

Hi every one.

I have been measuring current at low voltage levels using classic current sense circuit as shown in Fig 1. I am using LM358 series but it can handle single supply voltage up to 32V. As input voltage must be 2V less so i can measure current at 30V. My voltage level goes up to 60V DC.

There are lots of OP-amps and comparators like LT series and AD series which i have seen on websites and also hall effect sensors but unfortunately are unavailable in my Area. LM series are common so if you know a comparator which which can handle high voltages, please let me know.

With the existing circuit i worked around a little and managed to get result for high voltage levels. See Fig 2. Using resistors r4,r5,r6,r7(voltage division networks), the voltage across R.sense i.e. 0.1 ohm is reduced to half. U1B is a voltage follower and is used because connecting r3 directly will disturb the voltage division network. Now the problem is that it is working fine on simulation because resistors are set to 0% tolerance and R4, R5, R6 and and R7 are perfectly matched. In real life its not true. When i change value like R4 = 101 k, R56 = 99 k, R6 = 100.5 K and R7 = 100 k, results were not that accurate. For example if SUM of r4 and r5 is even a little bit higher than the sum of r6 and r7 the comparator voltage never goes to zero for 0 amps. Don't know the tolerance effect yet.

So if u guys have any better or robust method the plz guide me, thanks.

 

[dv_sa559]

Could you use Shunt resistor for measuring current ?

 

[JayantD]

**broken link removed**

https://focus.ti.com/lit/an/slla190/slla190.pdf

hope this helps..
JayantD

 

[FvM]

Basically, you can refer to circuits, where a PNP or PMOSFET current source is utilized to span the potential difference. A basic circuit is given in "scheme2" of this link: https://www.edaboard.com/threads/140817/ You'll possibly need to create an auxilary power supply to meet the voltage range of your OP. An OP with a common mode range including the positive supply would considerably reduce the effort.

There are also circuits using only a PNP current mirror and no OP at the high side, but the achievable precision may be insufficient for your purpose.

You also mentioned comparators to be used for your circuit which seems to indicate some confusion. Comparators can be imagined as uncompensated OPs and will never result in stable feedback amplifier circuits.

 

[Sajjadkhan]

Thanks FvM but i m not a pro like you so i might not pick quickly what you say (bear it with me).

Basically, you can refer to circuits, where a PNP or PMOSFET current source is utilized to span the potential difference.

What i understand from this is instead of using voltage division network i should use like bjt? for example see figure 3. If this is what you mean?

A basic circuit is given in "scheme2" of this link: https://www.edaboard.com/threads/140817/ You'll possibly need to create an auxilary power supply to meet the voltage range of your OP. An OP with a common mode range including the positive supply would considerably reduce the effort.

I don't get it. he just inverted it. it still can't measure current for higher voltages as 32V supply is the Op-amp limit. 2nd. this IC is not "over the top voltage" Like " LTC6101 LTC6102 LTC6103 LTC6104 LT6109" so the voltage at high side is exactly the same as supply voltage( which should be kept low than supply voltage), therefore op-amps unlike LT series as i mentioned wont work.



---------- Post added at 18:13 ---------- Previous post was at 18:11 ----------

Could you use Shunt resistor for measuring current ?

I guess you are asking so yes. 0.1 Ohm is being used as shunt.

 

[FvM]

To understand the difference, it's possibly helpful to study the concept of commercially available high-side current sense amplifiers, e.g. **broken link removed**

I already mentioned the key points, but you apparently missed it. One point is to supply the measuring OP with a reduced voltage below the V+ (high side) node. The other is to use a level shifting means, usually a current source.