SparkFun Forums

[Mee_n_Mac]

Just reconnect the unity gain buffer circuit, the value of the resistor, capacitor is the same as the what Mee_n_Mac drew.
...
when the supply is around 9.2v to the circuit, using the 100 ohms burden, the circuit output is 4.90vdc

So within 0.1v of what it should. Given the tolerances involved, I'd say it's working.

For the circuit output current, with 250ohms resistor connecting the output, both 100ohms and 4.5kohms burden give around 10mA

That's odd as the min current spec for your LM324 is 20 mA. How sure are you about that 10mA and/or that the total load was 250 ohms ? That the output was still 5v with the 250 ohm resistor load ?

[DCward]

Hi Mee_n_Mac! im using LM358N now, the 10mA is measured series with the output resistor. There is only one 270ohms resistor at the output (no more 250ohms resistor). The measured voltage across the 270ohms resistor is 3.2v.

[Mee_n_Mac]

The measured voltage across the 270ohms resistor is 3.2v.

That explains the 10 mA. But you said ...

the circuit output is 4.90vdc

What was measured under what conditions ???

[DCward]

The 4.90v is measured directly from the circuit output when there is no resistor.

[Mee_n_Mac]

OK, I give up. Was there ever a case where you measured the buffered output (voltage and current), fed into some resistance, with the supplies set to a voltage that gives the op-amps some headroom (>+/-7v), with the "gain" = 100k/4k and 5A input and a burden of 100 ohm ... like when you measured 4.9v at the unbuffered output ? You know, the desired output under the expected conditions ?

[DCward]

No, i have not get our dream output =( . But it is ok! I need help to understand the circuit more, what is the use on the capacitor in the circuit? I always thought capacitor is connected to ground. And for the didoes i read that it allow current in 1 direction, so it helps to convert AC to DC voltage?

[DCward]

@Mee_n_Mac - Your buffer circuit worked! Today i decided to re-wire the circuit, the only thing i changed is a new LM358N. Now the output voltage across the 270ohms resistor is around 5v and the current is 20mA.
Thank everyone who helped me!

plotted a graph too, voltage to current, and it is proportional.