# Help function generator...

Discussion in 'The Projects Forum' started by JonnyP, Mar 7, 2013.

1. ### JonnyP Thread Starter New Member

Mar 6, 2013
4
0
I need some function generator help in AC bridge circuit.

First of all sorry for any English mistake, as it is not my native language.

First post here and I expect not to be the last. Just found this forum in google.

Just some background… I work as a researcher in a laboratory at my University. My main focus application in eddy current testing (http://en.wikipedia.org/wiki/Eddy-current_testing).

So let´s go. The main focus of eddy current testing is using coils interacting with conductive materials to measure a variation of characteristics of those materials. I am using an AC bridge circuit to measure difference of two identical coils as can be seen below:

Everything is going well except that when I connect the AC generator to the bridge circuit (the coils have ~50ohm and I used another 50ohm resistor and a trimpot to balance the bridge) I see that the voltage amplitude of the sine wave drops. So I am supposing that the circuit is draining more current than the AC generator can supply, which can damage the equipment (I´ll post all my assumption, to show all my steps, please correct If I am wrong at any part!).

So I made a non-inverting buffer using a LM324, which works well while disconnect from the bridge, but when I use the output of the buffer to feed the bridge the sine wave become all fuzzy. And now I am stuck at this point… I need a circuit that feeds the bridge without damaging the AC generator. Can you help me?

Just some more data:
- Frequency of the sine wave: 5-30 kHz
- Amplitude of the sine wave: 4 volts peak-to-peak.

[]´s
João Rocha – UFRJ - Brazil

2. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
What is the inductance?

3. ### JonnyP Thread Starter New Member

Mar 6, 2013
4
0
It is about 6mH each coil.

4. ### MrChips Moderator

Oct 2, 2009
12,447
3,362
You need a power amplifier, something like an LM675.

5. ### JonnyP Thread Starter New Member

Mar 6, 2013
4
0
Thank you! With this chip the only modification that I must do is not to use as buffer, but decrease the amplitude of the input signal and have a gain of 10 or more in the op amp, right?