Wheatstone Bridge Light Sensor Problem

Thread Starter

Veraborn64

Joined Apr 15, 2019
6
Hello,

You'll have to excuse me if this has already been covered at some stage I'm pretty new to the forum and certainly couldn't find anything similar. I've recently built a Wheatstone bridge circuit not dissimilar to the one shown below and its been working reasonably well.

The key differences are:
- I have a feedback resistor Rf from the output to the point D.
- I have two LEDs which act as signally LEDs; a green one parallel with the LDR and VR1 tells me my board is powered on and a red one at the output of the op-amp to tell me when the board has triggered to compliment the relay.
- The circuit was originally powered by 24V, a voltage regulator dropped the voltage down to 12V to power the rest of the circuit.

All is dandy with all that anyway, my problems occurred when I was told that the design needed to be powered by mains (not my choice, I liked the 24V option with a dedicated supply but heyho).

I achieved this using a transformer (a Myrra 45030) I've attached the datasheet but bottom line of it is it supplies 24V AC, a full bridge rectifier changes it to DC and a smoothing capacitor removes the ripple. Power on the circuit and

A) The red led comes on as well as the green which it isn't supposed to. The red is supposed to stay off until the board triggers.
B) When the light is introduced both LEDs switch off and the relay triggers.

Not sure what the problem is here. Am i not supplying enough current? Is something shorting my circuit somewhere along the line and I've screwed up my soldering?

Any advice would be greatly appreciated, I'd be happy to answer any questions as well provided they're not laced with insults and sarcasm of course.

Cheers!
 

Attachments

AlbertHall

Joined Jun 4, 2014
10,015
That transformer has two secondary windings. Are you using just one of them; both in series; or both in parallel?
Do you still have the 12V regulator connected in circuit?
If so, what voltage regulator is it?
 

ericgibbs

Joined Jan 29, 2010
10,187
hi V64,
Welcome to AAC.
That datasheet states 2*125mA at 24V for the 45030.

Have you measured the DC voltage on the smoothing capacitor.??
E
AA1 15-Apr-19 16.19.gif
 

bertus

Joined Apr 5, 2008
20,567
Hello,

A 24 volts transformer will result in about a DC voltage of 32.5 Volts on the smoothing capacitor.
It is not clear to me where and how you have connected the leds.
Could you draw the leds in the schematic?
Do you use current limiting resistors?
What are the values if they are used?

Bertus
 

Thread Starter

Veraborn64

Joined Apr 15, 2019
6
Hello,

Currently using just one of the secondary windings at present and I still have the regulator attached to the board, the regulator is an LM317T (data sheet also attached) I have a pair of resistors that set the output voltage from it to 12V.

The DC voltage across the smoothing capacitor is a shade over 16V DC on measurement, this should be enough for the regulator to supply the circuit shouldn't it?

My concern was the current drain from the circuit might be more than the 125mA specified during a trigger?
 

Attachments

bertus

Joined Apr 5, 2008
20,567
Hello,

What is the value of the smoothing capacitor?
It sound to me that the capacitor is way to small.

Bertus
 

Thread Starter

Veraborn64

Joined Apr 15, 2019
6
Hello, I've attached the datasheet for the relay I'm using for this project. Its the first one on the list (the 34.51.7.12.0010 to be particularly specific), I chose that one because of its size more than anything.

Nothing attached to the contacts other than shown on the original schematic (i.e. the protective diode).
 

Attachments

AlbertHall

Joined Jun 4, 2014
10,015
The coil resistance of that relay is 840Ω, that is 14mA so the current would be nowhere near what would be required to drag the smoothed supply down to 16V.
Please check the voltage across the smoothing capacitor and the regulated 12V.
 

Thread Starter

Veraborn64

Joined Apr 15, 2019
6
I think i may have found the problem! My voltage reg isn't outputting nearly enough voltage for some reason. Around 4V, may be a bad connection somewhere or my regulator resistors may be wrong somehow, i'm away from the workshop today so i'll have a proper look tomorrow when i'm back but i'm fairly sure this is what is causing my problem.
 

Thread Starter

Veraborn64

Joined Apr 15, 2019
6
Hello folks,

Update on my system is that it was a silly error on my part. I went over each connection with a multi-meter, for some reason my big old 10,000uF capacitor was only supplying 16V to the regulator so i swapped it for a 12,000uF one which seemed to do the job. Remeasured close to 33V DC across this capacitor.

Just to be extra cautious I replaced my regulator for a new one (still an LM317T though) then tested the circuit with only the regulator section and my green LED, this also worked and I had a refreshingly normal 12V output. My green LED also lit up as expected which was all good.

I had removed my op-amp from its chip holder prior to replacing the regulator and I replaced this now and powered on the circuit and voila. The red LED only comes on when the light is applied to the sensor, the relay also triggers at this stage.

The relay trigger signal will signify to my controller to stop increasing intensity on the lights for the rest of the project.

Thanks for everyone's help! Very much appreciated.
 
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