is it an integrator amplifier or a differential amplifier?

can someone teach me how to do this?

i need this circuit for an AC to DC power supply...

 

what side do you want to sense current on?

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acutally, it is for a cellphone battery charger with auto shut-off feature...

the charger has the possibility to drain the current of a cellphone regardless of not-charging condition of the charger... i want to put a current sensing device at the battery output, or any part which ables the charger to charge the battery if it detects the current of the battery goes down or reduces( constant current for the battery, even if the charger stops from charging because of auto shut-off feature of the charger)

*auto shut-off feature lets the charger to shut-off when it detects the voltage of the cell.battery to be fully charged.

 

Isn't this usually done with a trickle charger?

 

our designed cellpone charger does not have this trickle charge feature...

so is it possible to have a current sensing device for our project?

 

hi,
A simple current to voltage convertor is a low value resistor in the low side of the charger.. A resistor of about 0.5R with say 100mA current flowing thru it will give a voltage drop of 0.5R * 0.1A = 50mV.
This voltage could be amplified or fed directly into a 200mV digital panel meter.

Is this the sort thing you are looking for?

Eric

 

my friend gave a schematic to me, he put a low resistor at the lower part of the charger and he attached a differential amplifier as a current sensing part, i'll post the schem later (can't understand the operation of the diff.amplifier.

 

Have you Googled it?

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sir, please check some of my schematics at the attachment..

the circuit is intended for a cellphone battery charger with an auto shut-off feature, the current sensing device should monitor the load current of the fully charged battery(it remains a constant load current for the charged battery)

please see the schematics and make some observations, about the values of resistor, i already have that except for the voltage comparator and diff.amplifier...

Attached Files

	20070807151239904.pdf (36.8 KB, 14 views)
	charger_block_diagram.pdf (28.5 KB, 8 views)

 

hi,
Ref Fig#1,
The ground connection should be on the low side of the sense resistor, else you will measure ALL the current flowing in the external circuitry. I am assuming that the either the battery or charger will be powering the circuits?

As I understand you just want to measure the battery charge current??

The circuit layout looks ok, although I havn't checked the component values.
On Fig#1, you have called the OPA a 'diff' are you sure???
Also whats the purpose of the Cap in parallel with the OPA feedback resistor?

Eric

 

my big problem is i can't understand the operation of the differential amplifier on the figure1, but i have my thoughts on my mind as i've analyzed the circuit given by m friend(forumite)

so just for clarification, the diff.amplifiers tends to compare the voltages before and after the sense resistor? actually the circuit i've attached is not finished, because we don't know where to attached the output of the differential amplifier...

yes, fe told me to use any gen.purpose operational amplifiers for the diff.amplifier.. he didn't told me about the capacitors on the diff.amplifier, but i think those will serve as filters for unnecessary AC signals.

 

hi,
I misread the handwritten 'diff' on Fig1 as 'differentiator'

I can see how you expect the voltage sensor/relay will work using the comparator. It would be a good idea to have some positive feedback on the comparator in order to add/increase the hysteresis.
As the charging voltage to the battery is switched off, by the relay when the batteries terminal voltage is reached, the voltage of the battery will appear to have dropped..
This could cause the voltage sensor to detect a 'low' battery voltage which would switch off the relay,,, the relay would 'chatter' on/off.

With hysteresis the battery voltage would have to fall much further before the comparator switched... Do you follow that explanation?

With ref to the current sensor OPA, the output would go to a voltmeter or a ADC [PIC] input. OR do want in some way to use this output to control the battery charge current?
If so, the charge current can be 'limited' by a simpler circuit arrangement.

Eric