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


1 Answer 1


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.

  • \$\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
    Commented Mar 24, 2023 at 0:11
  • \$\begingroup\$ OP said it's consuming < 10mA, so even a small DO-35 500mW zener diode will work. \$\endgroup\$
    – anrieff
    Commented Mar 24, 2023 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
    Commented Mar 24, 2023 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
    Commented Mar 25, 2023 at 8:38

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