I have a 433MHz RF antenna connected to a HT12D. The transmitter is powered by a 9v battery with a HT12E. The receiver is powered by 5Vdc. I am trying to power the unit using a 12-0-12 center tap transformer. I have each lead of the transformer going through a diode, then they connect together and go through a capacitor to smooth out the current. That power then goes through various voltage regulators to power different components. I use a 5V (LM7805) regulator to provide power to the receiver and the HT12D.
When I hook the power at the capacitor from a 12Vdc power source that is already smoothed out I can take the transmitter up to 15-20 yards away and receive the signal no problem(I have a 220uF@25V capacitor connected here).
However, when I use the power from the transformer described above I do not receive the transmission at all. I have tried various sized capacitors, 220uF@25V,100uF@35V, 470uF@35V,
1000uF@25V, 2200uF@35V, and 3300uF@35V.
At one time I could get the 220uF@25V to work at about 5 yards but now nothing. Can anyone tell me how to calculate the size capacitor I would need and why this isn't working possibly?

  • \$\begingroup\$ When you say 9V battery, do you mean one of those rectangle things that belong in the smoke detector? There's a good reason you don't ever see them powering things with motors or radios. They simply aren't capable of delivering much current. They're built with 6 AAAA cells. Try with a handful of AA or AAA and see if you don't have better results. \$\endgroup\$
    – Delta_G
    Commented May 13, 2020 at 22:04
  • \$\begingroup\$ If the cap is just there to smooth the power then the exact size doesn't matter much. \$\endgroup\$
    – Delta_G
    Commented May 13, 2020 at 22:04
  • \$\begingroup\$ I am using 2 18650 3.7v batteries in parallel recharged by a solar panel. I get to the 9v by a 9v boost which is only turned on when certain sensor criteria are met. Like I said using the same transmitter I can get reception at 25 yards when using the already rectified 12vdc source. But not when using the 12-0-12 center tap transformer I rectify. \$\endgroup\$ Commented May 14, 2020 at 16:25

1 Answer 1


lets suppose you need 0.1 amps to run the radio.

lets suppose you indeed have implememted a successful full-wave rectifier, which produces 2 * 60 Hertz output ripple of 120 half-sinusoids every seconds. We need to store that energy.

Suppose 1 volt ripple is tolerated by the regulator.

Let us suppose the transformer is good, the core is good, and the transformer produces 12 volts * sqrt(2) or about 17 volts peak.

The regulator may or may not be happy with 17 volts peak,including OVERHEATING, or not surviving the 17 volts.

But lets soldier on.

Using Q = C * V, taking the derivative, and assuming dC/dT is ZERO, we end up with

I = C * dV/dT, and rearrange that to have the very useful

I * dT/dV = C

Sticking in numbers, we have

0.1 amp * 1/120 second / 1volt = C

0.1 * 1/120 = C

0.000833 = C ==== 833 uF should suffice.

But if that has failed, then

  • the wiring is wrong

  • the regulator is broken

  • the transformer/rectifiers are bad

  • \$\begingroup\$ As I said I run each leg of the 12-0-12 transformer through a 1N4007 diode, then join them together to create the positive side. This output produces 17vdc as you predicted. Then I connect a capacitor between the negative and the positive (1000uF). This then goes to the lm7805, which has an output of 5vdc. So the 433Mhz antenna and the HT12D are both receiving 5vdc. The NTE1904 produces 3.3vdc and the LM7812 gets 12vdc. Still when hooked up to the 12vdc source everything works fine but the transformer doesn't get a signal even at close range. Has to be the transformer, will try new one. thx \$\endgroup\$ Commented May 14, 2020 at 16:56
  • \$\begingroup\$ The new transformer did the trick. Up to 25 yards no problem. I am going to try upping the voltage on transmitter to 12v and see how far I can reach thanks. I have another question about putting two transistor switches on a dual coil latching relay but will do that in a separate post. Thanks again. I used a 1000uF@25v capacitor. \$\endgroup\$ Commented May 15, 2020 at 16:22

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