Current limiting circuit i.e. Ampere to milliamps using Current transformer or MOSFET

[Santhoshksk]

Guys i want to convert an 100Ah of dc current into 30mA using an current transformer or MOSFET but i am not getting it correct.. can anyone help me with the circuit .. thanks in advance..

 

[tpetar]

Guys i want to convert an 100Ah of dc current into 30mA using an current transformer or MOSFET but i am not getting it correct.. can anyone help me with the circuit .. thanks in advance..

Hi Santhos,

First you mix two different things 100Ah (Ampere Hour) and A (Ampere or mA miliampere) are different things. 100Ah is NOT 100A.

Simply you can use resistor to limit current for your needs.
To limit current on 30mA @ 12V you need 400R resistor, can be 1/4W power ratings.

Current transformer is used to measure current in AC circuit. Do you want to measure current in DC circuit ?



Best regards,
Peter

;-)

 

[Santhoshksk]

Hi peter,
Thanks for the reply.. I want to measure DC current.... i want to limit 100A to mA Or can i use wirewound resistor to convert the 100A to voltage.. can u just tell me the value of resistor.... i want to display the value in the LCD via microcontroller

 

[tpetar]

If you want to measure DC current I suggest Hall sensor, but if you insist on shunt resistor ok.

Look to use smaller shunt value to avoid higher voltage losses. For higher currents such as 100A shunt resistor values can be 0,03Ω (30mΩ) to 0,075Ω (75mΩ) with adequate power rating.

When current flow, measure voltage on shunt resistor ends with OpAmp to get 0-5V on output, which is suitable for uC ADC.


Picture example :



http://letsmakerobots.com/node/18517




Best regards,
Peter

;-)

 

[Santhoshksk]

Thanks a lot peter for replying again... i ll work it out and i ll reply u yar....

 

[tpetar]

Higher currents I suggest to measure with Hall sensor, see this example with UGN3503 (UGN3503 is obsolete and new replacement is A1302). You dont need to cut wire or to do anything with wires, just put clamp around wire and read value, this dont have impact on voltage in circuit such as case when you use resistor shunt.

See illustration for Hall sensor usage:

Projects are located here:

Current Clamp Adaptor For Multimeters
**broken link removed**

**broken link removed**



Compact 0-80A Automotive Ammeter
http://archive.siliconchip.com.au/cms/A_30551/article.html




Best regards,
Peter

- - - Updated - - -

Ferrite core take from some PC PSU choke.

 

[tpetar]

Hi peter,
The last reply is ok when i want to measure a current when we want .. but when it comes to any product like things we can go and measure it using a clamp.. i want to make it as a product .. please help me with that... i ll attach an circuit which i am working right now.. In this circuit i get my R2 resistor burned up... i don't know why it happens?.. if u have any solution to this .. please let me know... thanks in advance...

I must admit circuit is very scary. :|

Generaly I give point and direction for current sensor in earlier posts. Better is to use Hall sesnor then power resistor (shunt) for this job. Shunt must have appropriate power to handle that current.


Best regards,
Peter

 

[Santhoshksk]

hi peter,
Thanks for the reply again.. can u do me another favour.. can u help me to convert the 7A dc current to 5v using an opamp r 4-20mA using any simple components..thanks in advance..

 

[tpetar]

hi peter,
Thanks for the reply again.. can u do me another favour.. can u help me to convert the 7A dc current to 5v using an opamp r 4-20mA using any simple components..thanks in advance..

Can I ask for you on this way ?

You want to measure 0-7A current and show result as 0-5V voltage.



Best regards,
Peter

- - - Updated - - -

Current sensing
http://letsmakerobots.com/node/18517

 

[Santhoshksk]

Hi peter,
The question which u asked is correct.. i want to measure 0-7A and have to display as 0-5V.. but the thing is it is to be measured from a 12Vdc, 7Ah battery.. thats my trouble..

 

[tpetar]

There is no circuit difference for 12V, circuit is the same. Its very important to know principe of measuring. You measure voltage drop on shunt resistor ends. That voltage drop is usually measured in mV or even in V if resistor have higher value and we have higher amperage. OpAmp make that small voltage from mV to 0-5V for uC ADC.


Best regards,
Peter

:wink:

 

[Santhoshksk]

Thanks Peter....