Powering Your Radios: Choosing the Right Power Supply

Selecting the right power supply unit (PSU) can be challenging. With thousands of options available, how can you know which one to trust?

Many radio operators are unsure because the source of these units is often unclear, and quality varies widely.

Types of Power Supplies: What to Consider

When it comes to powering radios or other equipment, there are various types of PSUs to choose from.

Many inexpensive models can be found online, but there are several factors to consider:

• Switched or Linear Design: Switched PSUs are cheaper but produce noise due to their electronic circuitry. Linear power supplies, on the other hand, are essentially a combination of a transformer and a rectifier, offering cleaner output at a higher cost.
• Quality Control: Where are these units manufactured, and are they safe?
• Reliability: Will they last, or will they fail quickly under load?
• What should I spend my money on?

Voltage

Lets remember that our equipment is designed to run on 12 roughly 14 Volts. Higher voltage, means more power is available, this usually comes into play when the supply is loaded.

When loaded the voltage drops, we call this V-Droop. It rises back when the load is removed. Some PSU’s will make up for this by increasing their duty cycle, others don’t monitor it at all.

V-Droop happens due to Ohms law V=IR’s effect on the internal components of the PSU. In short, the more power the supply has to provide; the more resistance it has to overcome within its circuits, leading to a small drop in voltage under load.

An example with some ballpark figures: a 12V supply dropping to 11.8-11.6V under load. A 13.8V supply dropping under load to around 13.6-13.4V This provides more power to the radio keeping your output power as high as possible.

You WANT a 13.8V supply

Calculating the correct amperage for your radio

In general, most modern 2M/70CM radios are about 50% efficient to their output power.

That means for a 50W radio, the supply needs to give 8.33A @ 12V (100W) to give us ample power.

• Adding 10-20% headroom on the amperage can help keep a higher voltage when the radio is transmitting.

Watts are the product of voltage and amperage: Watts (P) = Volts (V) × Amps (I).

If you have a 12V radio and it says it outputs 50 watts, you can bet it will take 100 watts from the supply.

That means 100W = 12V at 8.33A. A 10A supply should be sufficient, but if you can find 12A, that gives better headroom, it will also mean the supply has to work less hard.

PSU design

Any power supply can power your equipement. But we use a lot of power while TXing.

How will the supply handle that load? Is it stressed or being overworked? Is your SMPS of good quality?

Switched Mode PSU design (SMPS):

Switched-mode power supplies (SMPS) produce DC by first rectifying the AC from the mains. A high-frequency MOSFET switch then chops this DC into high-frequency pulses, which are fed into a step-down transformer. This transformer operates at the high frequency, allowing for a smaller size than a linear transformer. Finally, the pulsed output is rectified and smoothed using inductors and capacitors to create a stable DC output.

While more complex than linear power supplies (due to more components and high-frequency operation), SMPSs are generally reliable and highly efficient, with typical efficiencies ranging from 80% to 95%.

The noise!

This is a problem with SMPS’s.

The pulsating circuitry can be picked up at their respective frequencies.

• UHF: You don’t hear much of anything at 70cm (gmrs falls into this catagory) and above in my experience and are a good match.
• VHF: Can be affected by the SMPS’s harmonics and spurs. A good SMPS is generally not something that is a problem in 2 meter and should be okay.
• HF: Things change considerably. The band is particularly susceptible to SMPS noise. Your SMPS is producing EMI Electro Magnetic Interference.

It is radiating HF Signals, in line with FCC Part 15 Subpart B for unintentional radiators.

You will want to use a linear supply for this. I’m sure there are some SMPS’s that will work, but is it worth the trouble?

• Use ferrite beads on your cables
• Ground the PSU and shielding cables

Linear PSU design:

Linear PSU’s are simple and effective. A simple Linear PSU’s internals look like this:

A transformer drops the voltage to a certain set ratio. Of course this is 1:12 for 12V. We then rectify this AC into DC and probably add some extra smoothing capacitors.

That’s it! It produces NO noise anywhere. It is less diverse because the voltage cannot be set easily. You would have to replace the transformer with an approiate ratio for another voltage.

These are my favorite design. They are original, simple and effective. They are also very pricey.

An Astron 13.8V/35A linear like the LS-35A runs nearly $400!

I have seen them as cheap as $150 online used. This is a great deal if you need one.

Look for used, used is good for the wallet and should be one of the best things you can purchase used because it is so simple there is less to go wrong.

What should I spend my money on? Things to consider:

• Required voltage and amperage with headroom.
• Choice between linear and SMPS based on radio frequency usage.
• Brand reputation and quality.
• EMI considerations for HF use.

Final recommendations

For those on a budget and or non HF users an SMPS is typically fine, their typically inexpensive compared to a linear.

Currently I am using an EVGA Computer PSU and tapping the 12V rails to some T-Type connectors. This works well for me. I was able to put out 48W from a 50W rated Motorola no problem.

• Cheaper radios may not tolerate the V-Droop as well as a Motorola however and you may end up with less RF output power.

Brands I recommend for SMPS’s:

• MeanWell
• EVGA
• Corsair

If you plan on doing HF Get a linear. You will not have to deal with the noise of an SMPS that ruins the band.

Brands I recommend for Linear supplies:

• Astron
• Ameritron