DC Power Supply Ratings

Except for handheld transceivers (HTs), most modern ham radio gear uses 12V DC power.  You may find vintage radios and more recent designs that require AC main power, but the VHF/UHF mobile radios and the latest HF transceivers run off of 12-13.5VDC.   Nearly all of the modern all-band transceivers use 12VDC power as well so that they can be operated mobile or portable.

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If you want to install a mobile radio in your home instead of your vehicle (an excellent idea, by the way, if you don’t have an all-band rig) you will need a DC power supply for it.  Same goes for the latest HF transceivers; they require 12VDC power but don’t often include the power supply.  Additionally, many radio accessories require 12V nominal power.

Astron RS-35M 35 Amp DC Power Supply with Meters 120V AC in / 13.8 Volts DC Out     MFJ 4230MVP 30 AMP Switching Power Supply With Meter, 4-16 Volts & Power Poles    NEW Pyramid PS8KX AC to DC 8 Amp 12V Fully Regulated Low Ripple Power Supply     NEW QJE PS50SWIII 50 Amp Switching Power Supply with Volt/Amp Meter     JETSTREAM JTPS30LCD - 9 ~ 12 Volt 30 AMP Max Switching Power Supply w/ LCD Meter

Examples of ham gear DC power supplies.

The 50W mobile radios and 100W transceivers need a lot of current at 12V to operate at full output power.  Sizing a DC power supply for these plus accessories isn’t quite as straightforward as it would seem.

For one, you cannot simply take radio output power and divide by 12V to get amperes.  These radios are not all that efficient; quite a bit of power is used in the control circuitry in addition to amplifier heat loss.  For example, a 50W mobile transceiver specifies a maximum power supply draw of 9A at 12VDC in transmit mode.  9A*12V=108W for a 50W transceiver.  A 100W HF rig specifies a power consumption of 22A in transmit.  22A*12V=264W for a 100W transceiver.   In both cases, the input power is more than double the output.  So look at the radio specifications, not just transmit power.

Another complication is in how these DC power supplies are rated.  There may be values for peak, surge, continuous, and constant output current.  All mean something different and even then may not match actual performance.  Carefully read the specifications for any power supply before buying and always get more power than you think you need.

For example, I got burned on a power supply I bought for a mobile transceiver when used indoors.  From the radio specs of 8.5A max draw I thought that a 10A constant current-rated power supply would be fine, especially when it indicated 12A surge capacity.  Turns out that the specifications were very misleading: Specs claim it will supply 10A “constant” but when you read the fine print, it says only 7A (70%) continuous. Boy, I’m still fuzzy on the difference between “constant” and “continuous”.  When using it to power my transceiver on a net training session, it shut down twice due to over-current (thrermal trip). I was talking for 5 minutes and it shut down. According to specs it should have lasted at least 15 minutes at 10A (100%), so it performed worse than even the misleading de-rated specification, and that was at reduced transmit power.

Note that radios are specified for 12VDC nominal, which corresponds to a fully-charged vehicle battery.  With the engine running the alternator will elevate the battery level to just under 14VDC, which is what charges your car battery.  So most of these ham gear DC power supplies provide a regulated output in the 13.5-13.8VDC range (battery level under charge.)

Always use a regulated DC power supply.  Unregulated power supplies can have large output voltage swings depending on the load and input fluctuation.  The good news is that unregulated 12VDC supplies are uncommon and you are unlikely to come across one.  Make sure anyway that it’s regulated before you buy or use one.

Another consideration when choosing a DC power supply is linear vs switching regulator technology.  Switch mode power supplies (SMPS) are smaller and lighter and often cheaper but they are electrically noisy and may present electromagnetic interference (EMI) on certain frequencies (harmonics of the switching frequency).  They are also more complicated and less reliable with high-failure components (power semiconductors, numerous small electrolytic capacitors).  Linear power supplies are robust, extremely reliable, and do not emit EMI but are very heavy, large and often expensive.

Further consider features of the power supply themselves according to your needs and wants.  These would be things like output terminal type, multiple terminals, “cigarette lighter”-type socket, voltage and current meters, frequency adjustment (to move EMI on a certain frequency).  Of course you’ll pay more for these extra features but sometimes they are worth it; it’s your call.  It’s generally better to get more than you need if budget permits.

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