peak detector capacitor discharge

[kum111]

Hello,

i have built a peak detector for by photodiode circuit(https://obrazki.elektroda.pl/7910969900_1371546262.jpg) this photodiode will detect a laser intensity , i need to measure the peak intensity at sum points on my set up, but capacitor do not hold the charge,its discharges time is around 10ms. i need sum information on how could it discharge with any discharge path because of transistor and op amp.the capacitor store value has to discharge only when i provide a pulse signal from my labview program. my circuit is in the below links

any help will be great thanks in advance

https://obrazki.elektroda.pl/7910969900_1371546262.jpg


<a title="circuit.jpg" href="http://obrazki.elektroda.pl/7910969900_1371546262.jpg"><img src="http://obrazki.elektroda.pl/7910969900_1371546262_thumb.jpg" alt="circuit.jpg" /></a>

 

[AdvaRes]

Hi,
Why dont you use big capacitances ?

 

[keith1200rs]

That is a poor circuit. You will have errors due to the series diode and I doubt you will find a 22uF capacitor which is suitable for a peak detect.

I would suggest using a transimpedance amplifier for the amplifying the photocurrent then use a standard opamp based peak detect circuit after that. You can then reset the peak detect with a transistor or analog switch. You would also most likely be using capacitors <1nF for the peak detect so you can use a capacitor with a decent dielectric and not subject to dielectric absorption. Use a FET or CMOS input opamp for the peak detect.

Keith.

 

[kum111]

hey,

sorry it is a mistake will drawing the circuit, capacitor is 0.22µf .

thanks

 

[GUMY]

What is +Vcc and -VCC voltage and where does it come from? And I think You will allways get 9V on capacitor. You should detect current, not voltage.

 

[kum111]

Hello gumy , both the bias voltage to the photodiode and ad620 is given from battery. no i dont get always 9V on capacitor, it depends on the laser light intensity. more intensity greater voltage,less intensity lower voltage.

thanks

 

[jiripolivka]

The circuit is wrong. The 9V battery cannot push any current through the diodes and the capacitor, nor there is the second line from diodes to the opamp.

A peak detector is a (photo) diode connected to a capacitor load while a load resistor determines the rate of discharge of the capacitor. Make first such simple circuit, only the photodiode feeding a capacitor, and connect an oscilloscope with 1 MOhm input resistance.Then you will see the response to laser pulses, and using a shunt resistor (potentiometer) across the capacitor, you can adjust the rate of discharge.

 

[AdvaRes]

In your figure, Is the V- battery connector and the ground connected to each other ?