How much current??

[tkvenki]

Hi all.

In the diagram i have attached i'm having a few doubts.

i) The ade7754 can have 0.5V as its full scale diferential input.
is there any problem if i give the full 0.5, or keep a little
overhead space...like i'll give 0.4v. Any suggestions for this?

ii) To acheive 0.4v or 0.5v at the ade7754, what is the current,
that i must supply?

Thanking you
Venkatesh T.K

 

[Russlk]

The input impedance is 370K so the current will be microamps.

 

[tkvenki]

HI,

How did you get that??
By the way, do you mean to say,
that if i pass some micro-amps from my current generator,
then i will get some 0.5V at ade7754 end??

I'm not sure of that. I think i must source 1A through my
current source, to get 0.5V at ade7754.

Please clarify....
Thanking you
Venkatesh T.K

 

[tkvenki]

Please confirm....

Hi russlk,

I calculated, wghat happens if i pass 1A current through my current source,
When the input impedance of ade7754 is about 370kohm.

1A current, when, ade7754 i/p imp is 370Kohm.

Then the current that will appear accross the ade7754 will be 0.1uA.
Have i calculated right???

Please guide me.

THanking you
Venkatesh T.K

 

[ljcox]

I would connect 2 diodes (1N4148 or similar) across the input. One pointing up, the other down. These will limit the differential voltage to around 0.5 ~ 0.6 V.

 

[eblc1388]

I don't understand your circuit at all.

The path of the current from the current source will be through SH1A, common point and then return via SH1B. With both shunt resistance being 8 ohm, you will get a hugh voltage, not a small differential voltage as the inputs to the ADE7754.

For example, with just 1A passing through, the voltage to the inputs of the ADE7754 is 16V total, +8V on one pin and -8V on the other.

Did I miss something?

 

[ljcox]

eblc has a good point. There must be something wrong with your circuit.

 

[Russlk]

The ADE7754 input is high, so current is not needed to drive it, but the 16 ohms across the current source input will require 31mA to get 0.5 volts. The diodes mentioned above to limit the voltage is a good idea.

 

[tkvenki]

Dear Russlk,

The circuit is actually for "current measurement".
In real-world application i need to connect a CT accross the terminals.
Here what i did was to build a current source.

I think the minimum secendary current a CT can offer is 1A.
i.e. 50/1, 100/1....etc.

So, if i pass 1A current through my current source, what will happen?
Will the voltage at ade7754 greater than 0.5V??
The circuit is a standard document by analog devices.

I will be attaching the complete circuitry, in which,
i think they intend to pass 1A....through the input.

But....THey have mentioned 1:1800...which means if they pass 1800A
in input of CT then they will get 1A at CT output.
But...Imax is 40A....so CT max output is 40/1800 = 0.022A.
Am i right?

Please find the ckt diagram attached.

Can you please give me a suggestion?

------------edit----------------
Russlk,

You are right.
If i pass 22mA through my current source,
then, i will get 314mV accross my ade7754.
And the 0.5 is the maximum(must not be given..some head-room should be there.)..
and they have specified...355mV.(giving some factor of safty).

So it is all clear now....So if i need to simulate the current, then i must vary the current(from my current source). from 0-35mA.....this will be equivalent to simulating current in the line(whatever the range may be it depends upon the CT).

Kindly confirm my approach.

Thanks a lot for your replies.....
It really helped me a lot.

 

[ljcox]

I will be attaching the complete circuitry, in which,
i think they intend to pass 1A....through the input. Now I understand, a picture is worth a 1000 words.

But....THey have mentioned 1:1800...which means if they pass 1800A
in input of CT then they will get 1A at CT output.
But...Imax is 40A....so CT max output is 40/1800 = 0.022A.
Am i right? Yes

If i pass 22mA through my current source,
then, i will get 314mV accross my ade7754.
And the 0.5 is the maximum(must not be given..some head-room should be there.)..
and they have specified...355mV.(giving some factor of safty).

So it is all clear now....So if i need to simulate the current, then i must vary the current(from my current source). from 0-35mA.....this will be equivalent to simulating current in the line(whatever the range may be it depends upon the CT).

Kindly confirm my approach. Looks good to me

The diodes I suggested could be connected across the secondary of the CT.