radiation meter November 2011

Equipment, circuits, projects, procedures

Postby vasik » Wed May 01, 2013 12:00 am

Hello,

I am trying to get improved radiation meter kit to work and I have a problem with amplifier. With 8.8v power supply voltage it gives about 6.5v offset at the output. It still works as amplifier but offset to high (outside ADC range).
Any ideas how to adjust the DC offset at the output ? Or what is wrong with my amplifier board ?

Thanks!
vasik
 
Posts: 3
Joined: Thu Jan 02, 2014 10:47 am

Postby thijsbeckers » Thu May 02, 2013 12:00 am

Hi vasik,

That seems a bit high. Did you mount the opamp in a socket? If so, what is the voltage on pin 2 of K2 when you take the opamp out?

It should be zero. If it isn't, check your solderings! If it is, try a different opamp, preferably the same type, but for testing any type will do. Output still ok? Then it's probably a bad opamp.
thijsbeckers
 
Posts: 1134
Joined: Thu Jan 02, 2014 10:37 am

Postby vasik » Thu May 02, 2013 12:00 am

Hi thijsbeckers,

Thank you for the suggestion.
I have a opamp in a socket. There is zero on the output (K2 pin2) when I take opamo out.
I have a suspicion that something wrong with my pin diode. Do you happen to know what voltage should be on T1 gate ? (pin diode anode)
I have about 5v there no mater in what direction I connect pin diode.

Thanks!
vasik
 
Posts: 3
Joined: Thu Jan 02, 2014 10:47 am

Postby vasik » Fri May 03, 2013 12:00 am

Hi,

I am using 10x probe and oscilloscope.
There is about 6v on opamp pin 2.
I will try replace pin diode.

Thanks!
vasik
 
Posts: 3
Joined: Thu Jan 02, 2014 10:47 am

Postby thijsbeckers » Fri May 03, 2013 12:00 am

The voltage on the gate of T1 is a little difficult to measure. Since the resistor values are so high you're probably affecting the voltage the moment you measure it with a DVM.

What's the voltage on pen 3 of the opamp? I measured about 2.5V. On pen 2 of K2 there should be around 2.5V as well, when there's no radioactivity measured, that is.
thijsbeckers
 
Posts: 1134
Joined: Thu Jan 02, 2014 10:37 am

Postby thijsbeckers » Mon May 06, 2013 12:00 am

Ah, ok. So what's the impedance of the probe? Is it near the 20M Ohm of R14+R3?

6V is definitely too much, as you already concluded. Let us know how the replacement diode works out.

Thanks!
thijsbeckers
 
Posts: 1134
Joined: Thu Jan 02, 2014 10:37 am

Postby ifriad » Tue Jun 25, 2013 12:00 am

Hi All,
I bought lately the improved radiation meter and after putting together the sensor board I tried to test the output of the board using an oscilloscope.

What I noticed that I get a square wave at the output when I am enclosing the circuit in a plastic container or even when I am having my fingure over the sensor.

I am using a bench top power supply and I am connecting the ground of the scope with that of the power supply to that of the sensor board.

Could you please advice me on how I could further procced.

Cheers Ihab
ifriad
 
Posts: 2
Joined: Thu Jan 02, 2014 10:47 am

Postby thijsbeckers » Wed Jun 26, 2013 12:00 am

Hi Ihab,

Ah, ok. That's ok, then (the covering).

100Hz looks like the AC grid voltage is rectified, resulting in a 100Hz interference signal. So poor grounding of your PS seems like a totally plausible cause for your problems. Have you tried to power the circuit with an ordinary 9V block battery? Or a wall wart with 9V DC(!, check voltage with your oscilloscope) output?

Regards,
Thijs
thijsbeckers
 
Posts: 1134
Joined: Thu Jan 02, 2014 10:37 am

Postby ifriad » Wed Jun 26, 2013 12:00 am

Dear Thijs,

Many thanks for the reply. I covered the inside of my plasitc box using an aluminum foil but I could still see the sqaure wave the frequancy was about 100Hz.

Is it possible that this might be due to poor grounding in my PSU.
Cheers Ihab
ifriad
 
Posts: 2
Joined: Thu Jan 02, 2014 10:47 am

Postby thijsbeckers » Wed Jun 26, 2013 12:00 am

Hi Ihab,

You shouldn't use a plastic enclosure! Have you read the accompanying article in the November 2011 edition of our magazine? Please do.

Regarding the square wave: is its frequency by any chance a 50 (or 60) Hz? Fat chance you're your body is acting like an antenna and when you approach the sensor, the circuit amplifies the signal 'you are receiving'.

Use a metal/shielding enclosure and make sure to connect it to the ground of the circuit.

Cheers,
Thijs'
thijsbeckers
 
Posts: 1134
Joined: Thu Jan 02, 2014 10:37 am

PreviousNext

Return to Test & Measurement

Who is online

Users browsing this forum: No registered users and 3 guests