Power Supply missing part

[Zedman]

Hi,

I got a PCB, it's a small remote controlled relay. It has this non isolated power supply part. As far as I figured it out it looks like the attached schematic, except it uses a transient suppressor instead of the two zeners.



But i cannot see the PCB lines under some parts (eg. the relay), so I couldn't figure out what is between point A and B.
From the TL431's datasheet there can be just a resistor which limits the current of the output and the current to sink when there's no load.

I built it with just a 220 ohm resistor between A and B.
At the point Vcc 12 there's 13.4V on the original PCB, and it gives 3.1V at Vcc.
Mine works the same way with small differences in the voltages, BUT when I attach the relay between the Vcc 12 and GND points the output voltage gets down to 1.6V causing the MCU to reboot.

I know due to the 0.47 uF cap it will only give max 30mA but on the original PCB the 3.1V remains 3.1V even if the relay is on!

(I am using the same 12V/400mW relay)

What could be between A and B points causing the stability of the output?

thanks,
Zedman

 

[Awarapunshee]

Did you measure the resistance between these two points without connecting input? or check for the diode etc between both points

 

[Zedman]

I will measure it again when I get home (Iam @work now), and will post the results.

Added after 1 hours 9 minutes:

Another thing came to my mind, when I tried it with my MCU remote system, it won't boot up until I put it before the inductor L1.
I think it makes the Vcc rise time lower and that's not enough for the MCU.
But the inductor is there for noise reduction and it's obviously needed, so how can I solve that?

thanks,
Zedman

Added after 4 hours 19 minutes:

I measured the resistance, both directions it seems I start to load a capacitor because the resistance grows from a couple of kohms to Mohms slowly.

any suggestions?

 

[Awarapunshee]

Measure it for a diode

 

[xulfee]

use any resistance there n take care of rc time constant

 

[Zedman]

I finally managed to carefully remove the relay and I figured out how it works, it switches it through some transistors.

xulfee: thanks I'll try.

thanks