2
\$\begingroup\$

I have a garage door opener for which I need to replace the radio receiver. The opener outputs a DC supply voltage for a receiver at 34 VDC. The receiver has an input range specification of 12-24 V AC/DC.

I'm attaching a photo of the receiver's input stage below. It consists of an MB6S bridge rectifier and a 78L05 (SOT-98) linear regulator with 5 V output.

According to the datasheets I could find for the 78L05 it has an input-voltage range of 7-20 V with 30 V absolute maximum. It has 220 uF/50 V input and output filter caps and apparently a 51 R series input resistor. The receiver has a specified input current of < 10 mA.

What would be the simplest modification or addition to the receiver's input that would make it compatible with the 34 V supply voltage?

receiver input stage

\$\endgroup\$
0

1 Answer 1

4
\$\begingroup\$

I would cut the trace after the MB6S's positive terminal and replace it with a 15V zener, like this:

enter image description here

This way, the input cap will 'see' around 19V, which would be in spec.

\$\endgroup\$
4
  • \$\begingroup\$ Be aware that that zener's gonna be dissipating a lot of power, dependent on current draw; get a big one. Do the math and see if you need a 5 W zener or if a 3 W one will do. \$\endgroup\$
    – Hearth
    Mar 24 at 0:11
  • \$\begingroup\$ OP said it's consuming < 10mA, so even a small DO-35 500mW zener diode will work. \$\endgroup\$
    – anrieff
    Mar 24 at 0:26
  • \$\begingroup\$ This seems to be the easiest solution indeed. However, I'll measure the input current with a 12 V input before I try this. I've grown suspicious of the 10 mA specification in the receiver's data sheet, as the relay coils alone each seem to have a power consumption of 450 mW. \$\endgroup\$
    – tg85
    Mar 24 at 23:46
  • \$\begingroup\$ @tg85, checking it is definitely a good idea. But it seems, if the 78L05 supplied the coil current, it would have been fried at 24V input, as the SOT-89 would have had to dissipate more than 1.7 watts \$\endgroup\$
    – anrieff
    Mar 25 at 8:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.