Ham radio power supply circuit with RFI elimination

This is the Ham radio power supply circuit with RFI elimination for Amateur radio. The walkie-talkie requires RF Immune Power supply only.


A typically Radio transmitters use High-frequency transistors as a current driver. So, it can cause very severe electromagnetic.

Sure, The power supply must be able to make the current higher than 10A.

How Ham radio power supply works

And most importantly, our power supply circuit must eliminate RFI (Radio Frequency Interence) as well. Let’s try to design a circuit better.

Pick it up(Designing)

A good power supply circuit for a walkie-talkie should have the following characteristics:

  • Supply current at least 10A
  • Keep constant voltage at 13.8V
  • There is a function of Short Circuit Protection.
  • Excess RF must be eliminated for antenna systems well.

From design principles For the most economical We choose to use the instant IC, LM350T.

Read How to use LM350 and pinout

LM350 pinout
LM350 pinout

And it has a good cooling system with IC regulator regulations on the TO-220 chassis.

Read also: 7805 datasheet

The heatsink has a maximum temperature resistance of 1K / W. This IC is about 30 watts and has a voltage drop of up to 3 volts. This gives it a low drop voltage. It doesn’t work very hard.

Paralleling LM350 Regulator to increase current

See in the Block diagram below.

Diagram of Paralleling LM350 Regulator to increase current

Normally, this IC can supply about 2.5A current, so we connect 4 of them in parallel, so the total current is 10A and or max 12A.

Set voltage IC to the same

But one of a problem in the many IC connections is the tolerance of the power supply system. Which is usually not too much.

However, a different voltage level may cause the output voltage to not be exactly as specified. Therefore, in parallel, it is necessary to have a voltage of 13.8V on all four units.

Ham radio power supply circuit with RFI elimination
Ham radio power supply circuit with RFI elimination

So we split this circuit into 2 sections. In order for the first IC to act as a reference IC. While the other 3 ICs have to adjust to that reference IC. By controlling VR1, VR2, and VR3 respectively.

In this design, we designed the IC1 to be able to supply a voltage of 13.8V constant (theoretical).

This circuit defines voltage with the network resistors circuit consisting of R1 to R5 to adjust voltage as needed.

In its design it was found that instead of connecting the ADJ of the first IC directly to the resistor as usual.

We try to design it to maintain constant stability. There is R4 to reduce the total tolerance for this system.

If the value of R4 reduces, the output voltage also reduces. If the value of R4 increases, the output voltage also increases.

When IC1 has set the voltage level to 13.8V, we adjust VR1, VR2, VR3 so that the voltage is equal to the first IC (13.8V). And to compensate for mistakes We put the resistor R2, R3 to get a resistance equal to 2.2 ohms.

Meet Rs protection resistors

According to the output principle of each IC regulator. Should have a protective resistor Commonly known as Rs. But because Each output Must be connected to the real wire with a small resistance. Served as Rs.

RS protection and paralleling
RS protection and paralleling

We use the power cable about 30 CM long, 0.7MM diameter. Go into the positive power outlet point and all 4 ground are connected together.

We assume that the resistance is now formed. On the output side of the regulator that is Rs equal to 30 milliohms.

If a short circuit occurs, it will stabilize itself well.

Asked if used with a high power booster How will it affect?

When a current of 3A is drawn, the voltage drop across the power cable is approximately 100mV. In practice, the output only changes 0.75% from the maximum voltage in the case of full load.

This is not a problem for radio transmitters.

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How to build

Although this circuit has a high current. But we can make it easy Because the essential equipment is IC and capacitor. It makes the circuit is smaller use transistors.

First prepare the components below.

Parts you will need

Resistors 0.5W
R1: 47 ohms
R2, R3: 1 ohms
R4: 10K
R5: 560 ohms
VR1, VR2, VR3: 50 ohms, POT potentiometer

C1,C3,C5,C7: 4700uF 35V: Electrolytic Capacitors
C2,C4,C6,C8: 100uF 25V: Electrolytic Capacitors
C9-C12: 0.1uF 50V, Polyester Capacitors

BD1: 10A Bridge Diode
IC1-IC4: LM350T, 3A Adjustable Regulators

Thanks: Photo LM350T

T1: Toroidal transformer 18V 6A
and others

And build the PCB as the layout below.

PCB layout Ham radio power supply
PCB layout

Then assemble the parts on PCB as component layout below.

Components layout


We change R4 value for control the output voltage at 13.8V. Then adjust VR1, VR2, VR3 to control the output2, output3 and output4 at 13.8V all.

Then, parallel all output in one. Next, test dummy load at 5A. the voltage will be a stable output.

High Current 13.8V ham radio power supply (up to 30A)

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