Ham Radio Power Supplies: The 12-volt Question

See also the Ask Dave 17 video, which discusses power for amateur radio stations.

MFJ-4225MV power supply

MFJ-4225MV power supply. This one's mine. See text for its good points and bad.

HF radio power supplies. Why so many options? There are lots of ways to power your HF transceiver. But first, why can’t we just plug the transceiver into the wall?
 
Well, a combination of tradition and an opportunity to tinker. Back when rigs had vacuum tubes, a radio was often sold sans power supply. Lots of hams made their own, and the manufacturers all had high-cost options available. Back then, the power supply had to supply several voltages: the filament voltage, either 6.3 or 12.6 vdc, the plate voltage, usually a few hundred volts, and various bias voltages, sometimes even negative! Tube rigs are great for nostalgia, but powering them was always an issue.

13.8 vdc Emerges

This changed when separate transmitters and receivers started to merge into a single transceiver—these often had a built-in power supply. But an opportunity came when rigs became all solid-state. No high voltages were needed. The most frequent supply voltage was almost the same as the old filament voltage: 12.6 vdc. Except…well, any good rig will work within a range of supply voltages. So the standard input voltage became 13.8 vdc. Why 13.8? Because that’s an average voltage that an automobile electrical system runs at. So if you purchase a stand-alone power supply, such as the MFJ-4225MV shown in the image, you’re looking at a 13.8 vdc supply, nicely matched to today’s HF radios and VHF mobiles.

Powering your station with a battery instead of a power supply

Battery box protects "marine" 12v deep-cycle battery

A battery box from Walmart protects "marine" 12v deep-cycle battery

Okay, now here’s where all the options joust for front and center. You can actually power your rig from an automobile battery, and there are lots of hams who do this. But if you’re going to get a battery just for your ham station, get at least a “marine” deep cycle battery instead of a standard automotive battery. Why? Because auto batteries are designed for lots of current for short periods of time, whereas a deep cycle battery is designed for smaller amounts of current over longer periods of time. You can purchase a suitable battery at Walmart for not very much money—I found a suitable “marine” battery for $58 + $10 core charge + tax. You can hook your radio directly to the battery. My experience is that these batteries have a useful life of about three years and then must be replaced. I just replaced mine.

You do, however, need a way to charge that battery. You may have an old charger laying around, but what you really want is a smart one because, after all, this battery is going to be in your house and you don’t want lots of acid haze everywhere. Walmart’s website is really hard to use—can’t find anything—but it looks like you can get a simple battery maintainer for about $20 at the low end. Remember, most of the time your rig draws hardly any current, so keeping it charged is not hard. Now, you need one more thing: a suitable box to put that battery in. You need it for several reasons, not the least of which is safety. While 12vdc won’t cause harm if you happen to touch both terminals at the same time, if you happen to put a piece of metal across the two contacts at the same time, that battery can release several hundred amps all at once, which means over a kilowatt of power, which will pretty much vaporize the metal, not to mention the fire hazard. Plus, even though it might be a maintenance-free battery, it still emits some acid haze, which you don’t want getting on anything. Okay, looking at cost here, we’re up to about $120 or more, including the battery, charger, and box. Don’t forget to disconnect the charger while you’re using the battery, because chargers often create RF noise that you’ll hear in your receiver. Oh…don’t forget to reconnect the charger when you’re done. Don’t throw your back out getting that very heavy battery into place. Hmmm…lots of work.

Using a Standard Power Supply

Now here’s the rub. For that same $100, you can purchase a standard power supply built especially for radio use, meaning you won’t hear them in your receiver. The picture at the top of this post shows one I have. It’s an MFJ-4225MV Switching Power Supply that plugs into the wall (120 vac) and has an output that’s adjustable from 9 vdc to 15 vdc. It has a detent in the voltage knob for 13.8 vdc. I can’t discern any RF hash—very clean power supply. I use it to power my APRS station, consisting of an old AEA (now Timewave) DSP-232 and an ICOM IC-2100 2-meter mobile rig. My only issue with the MFJ-4225MV is the fan noise. The QST review said the noise was moderate. Ha! It sounds more like an airplane taking off. So I neutered the fan so it’s very quiet. You’ll note in the picture I’m careful to keep the sides open to the air because that’s where the air vents are. I really, really wish it had a temperature-sensitive fan. If I had to put this into service for my main HF station, I’d turn it off when not in use and would use headphones when operating.

Powerwerx supply rightsized for HF, with temperature sensitive fan

Powerwerx supply rightsized for HF, with temperature sensitive fan

Here’s something worth looking at, a very inexpensive power supply from Powerwerx that will give you ample power for your HF rig and has a temperature-sensitive fan. Now I’ve not used one of these, so if you get one, please comment on this post and tell everyone about your experience. (Update 24 Sept 2011: I purchased one of these. Nice little unit. It has a temperature-sensitive fan and makes less noise than the MFJ unit.)

I’ll mention here that power supplies are something you can pay as much or as little for as you want. You definitely want a power supply designed for HF radios! A supply you might find at a retail chain may create lots of RF hash which you’ll hear in your receiver. That noise interferes with your ability to hear the other station’s signal. One place to go for them is Ham Radio Outlet—they have quite a few to choose from. Essentially, you want a 13.8 vdc power supply of at least 20 amps, because that’s what your 100-watt HF rig will draw on transmit. You can buy them with fancy meters (like the MFJ-4225MV) or a simple black box. Most power supplies these days are of the switching type (the modern way), but the old linear (energy-inefficient) supplies are still available and work fine, though they’re rather heavy (not nearly as heavy as that battery, though!).

You can tell what I recommend: do what works for you. Overall, I’d say go the commercial power supply route because it’s about the same price, will last for a long time (rigs come and go, but power supplies have a habit of sticking around), it’s safer, and it’s a lot less trouble.

One very last comment. Do not attach your 13.8 vdc power supply to a battery which is then attached to your rig. Yes, this will give you a power supply in the event of utility power failure. BUT…the float voltage for a flooded lead-acid battery is 13.3 volts. Keeping it on 13.8 vdc will eventually cause damage to the battery and greatly shorten its life. So don’t use your power supply as a battery charger!

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51 Responses to Ham Radio Power Supplies: The 12-volt Question

  1. Dave says:

    Steve, there are gobs of 2m base station antennas. MFJ has several with the MFJ-1521 only one example. Or you could build a simple J-pole. I think you’ll find that any output power above about 20 watts won’t gain you anything, but you never know. Don’t put antennas where they can be touched. 73, Dave

  2. Steve Cox says:

    I am just getting started in ham radio with a 2 meter rig consisting of a Yaesu FT2900R mobile transceiver and a Yaesu FP1023 power supply. The radio is capable of 75 watts transmit power, and I appears my selected power supply should have no trouble delivering the necessary amps. My question is regarding antenna choice – I picked up a Mig welder cart at Harbor Freight to put this outfit on so I would be able to move it from inside the house to out on the deck when weather permits. I would like to have as simple of an antenna to connect this with that will give me sufficient capability without being a choke-point for my transmit/receive functionality, and I don’t want to break the bank getting this on the air. What can you recommend for me?

    73s to all –

    SFC
    Former Army Security Agency Morse Listening station operator
    (back when there was an Army Security Agency)

  3. johnymac says:

    Just went a couple of days without power. It was no problem operating my radios as I grabbed a fully charged marine battery.

    In the future if the juice goes out again I would like a place to plug in my radios rather than lug a heavy battery around. My plan is to run a line from my solar battery bank to my shack to plug into when the electric goes down. The run is ~55 feet and I was wondering what gauge wire di you recommend.

    Thanks,
    JohnyMac

  4. Jack says:

    Excellent point Dave

  5. Dave says:

    I don’t recommend letting a battery ride the 13.8vdc line. That’s about a half volt too high for a trickle charge. On the other hand, the battery will never be fully charged.

  6. Jack says:

    An earlier recommendation was to buy a good 20A power supply because later you would want to be able to adequately power a 100W rig. Consider a couple of extra bucks and go for a 25A regulated power supply. That way you could also use it to parallel supply your antenna tuner, which you will want, or perhaps even a 12V emergency desk lamp. The whole rig could be riding on a good 12V deep cycle battery making your rig a good emergency communications system.

  7. Dave says:

    Steve, it looks like you’re on the right track. 15 amps is what it needs. I advise a 20a supply to prepare for when you’re a general and want to power a 100 watt HF set. 73

  8. Steve C. says:

    I’m no electrical engineer, I can barely plug wires into the correct connectors and I need some advice on this subject of power supplies regarding my new Yeasu TF2900R. The specs say it requires 13.8 vdc at 15 amps. I am not wanting to risk damage to this radio – it is my first ham rig. The transmit wattage maximum is 75 watts, and I need to know how many input amps to look for to be able to run at maximum for extended periods. I will probably only run at half power most of the time but I don’t want to be limited to that or operate at reduced performance potential because my power supply is inadequate.

    Can you give me a range of amperage that will allow me to do what I want to without risking overpowering the radio? I am really new with all this stuff, returning to radio since my Army experience with the old AN/PRC 25 back in the early 1970’s. So much is different with these new small transceivers that I feel a bit lost in the weeds with the technical information but I also want to better understand what I am using so I don’t put some frankenradio shack that is a maintenance nightmare – or a fire hazard.

  9. Dave says:

    Hello to New Zealand! Yes, you can use one battery to power multiple devices. That’s the way I’m set up. You’ll probably need more than a solar trickle charge for the battery, though, especially if you’re pretty active on the air. Good luck with your experimenting! 73, Dave, KEØOG

  10. john Mac says:

    Hi from Nelson NZ. Can more than one radio work safley on a single battery supply? and would a solar trickle down keep it charged

  11. Don says:

    Hi Dave,

    On that follow up (I needed to edit it, but didn’t know how) Those wires should be reversed from what I posted. This is how it should have read: The Red & White wires (are positive) and the Black & Grey wires are the Negative according to my Ohm meter readings.

    Sorry for the confusion, but more important, thanks.

    PS: Us old men make mistakes like that at times….

    Don

  12. Don says:

    Hi Dave,

    I appreciate your answer on the dc power 4 wires, in your stating the two red would be positive and the two black would be negative.

    However, on this one, the 4 wires are: Red, Black, Grey and a White one. However, after thinking about it, I took an ohm meter and checked from chassis ground to each of the four wires. The result was: The Black & White wire had zero ohms and the Red and Grey wires showed Open. So I take it, the Black and White are negative and the Red & Grey would be positive.

    PS: I haven’t hooked it up yet but felt I’d drop this to you for your opinion. I’ve had this radio since 2005 and have never hooked it up. I’ve been running my Kenwood TS-940 S AT.

    Thanks again
    Don

  13. Dave says:

    Hi Don, you should go color-to-color. So the two black wires coming from the plug go to the negative (actually ground) terminal on the power supply, and the two red wires go to the positive terminal, which is very likely to be red also. Hope that helps! 73, Dave

  14. Don says:

    Hi Dave,

    On a Kenwood Transceiver TS-850, there are 4 wires coming out of the 6 pin connector for the power supply. I’m using the Kenwood PS-52 power supply.

    My question is: Which of the 4 wires connect to the Positive and which of the 4 wires connect to the Negative on the power supply?

    This is the sad part. There is nothing in the Kenwood owners manual concerning why the 4 wires or which go to the power supply. My best guess is that possibly two wires go on the positive and two wires go on the negative Power Supply terminals, in order to make sure the terminals on the supply don’t overheat from them not being as strong as they need to be to handle the current under a load. But I’m not sure I’m right. If so, I’d still need to know which of the two wires are positive and which are negative, without having to remove the covers to find out what is what.

    Thanks for any information you can give me on this. I’ve never powered it up since I’ve owned it, for that very reason of not wanting to hook it up wrong.

    Sincerely,
    Don

  15. Dave says:

    Mike, it may well work. Something to consider, though. Check the voltage output. It may well be 12v instead of the 13.8 that’s erroneously called “12v” in amateur radio. Some transceivers (such as my Yaesu FTDX3000) don’t work well with a 12v input. Other radios, usually those designed for mobile work, will work okay with a true 12v. Another thing to consider is that the computer supply is likely designed for a fairly constant load, with occasional and benign changes in load. A transceiver, on the other hand, provides a highly-variable load to the power supply. You need to be sure that your computer supply will handle it. BTW, the usual amp rating for a 100 watt rig is about 20 amps, so you’d be in the ballpark. The only way to know is to try it, although I do recommend that you get a power supply that’s actually built for ham radio. Good luck! 73, Dave, KEØOG

  16. Mike Solomon says:

    I wrote the previous reply recalling from memory while I was on vacation this week. I just got back home and checked the power supply in question. It is actually listed as 18A on the label. So… That would cover it, yes? That’s over double the (theoretical(on paper)) wattage needed.

    I would then obviously need to listen for noise issues.

  17. Mike Solomon says:

    Hey Dave,
    I bought a 12 V power supply at a ham fest a few weeks ago. It was 8 or 9 amps and designed for a Dell server as I understand it. I bought it under the premiss of my re-sharpened Ohms Law skills (freshly brushed up for, for my test).
    I figured that if I bought say… a 100 W transceiver and dialed it back, it would not need the full amperage. Being that this PS has at least 96W and obviously the radio is going to need a little juice just to keep it running, would it work? It would seem it should.
    Or is it similar to audio amplifiers, you need some umph in reserve for the loud bits.
    Also… Reading your other responses, it sounds like it has the potential to introduce some noise. Aside from being somewhat annoying, is it dangerous to the radio at all?
    Thanks for all you are doing.

  18. Jack says:

    Thanks to all of you who offered questions and especially those with answers.
    A new General here trying to move up from temp. use of a garden tractor battery
    and “smart” charger to something more permanent. The exchange here has
    convinced me to such up my guts and spend the loot to get a decent power supply
    and stop playing Russian roulette with my new Yaezu 857.
    73
    KM4QJY

  19. Dave says:

    Mark, if the lights dim at 55 watts, it’s likely the power supply can’t provide adequate current. You need to measure the supply’s output voltage and current across a variety of loads. Your mobile rig probably has a low power setting of around 5 watts, a medium setting of around 25 watts, and full power around 50 to 55 watts. If you get satisfactory results with 25 watts, you may be able to use your rig very successfully with that. With FM, you either have a connection or you don’t—there’s not much in between. With a good outdoor antenna, you can work most anything with 10 watts. Good luck! 73, Dave, KEØOG

  20. Mark Symms says:

    Dave, Got any suggestions on troubleshooting an old power supply? Not quite sure it is output the correct amperage. Everything dims when I try to tx on 50W.

    tia
    -mark
    WB5SRQ

  21. Dave says:

    John, in general, wall warts are designed to charge the batteries internal to the device they come with. Often the output voltage is poorly regulated and has an AC component. You’ll need to check the owner’s manual for each rig to determine power requirements. In general 5W rigs will need at least twice that to transmit, closer to 1 amp. But if I were you, I’d look for a power supply intended to drive radios. Hope that helps. 73, Dave

  22. Dave says:

    To Mike Baird, potentially it could. Check it with a couple different meters. Most radios will accept 13.8 vdc +/- 10%, or a high of 13.8 + 1.38 = 15.18 vdc max. You might want to check the voltage when the rig is receiving vs transmitting. If there is a way to adjust the voltage downward to 13.8, that’s good, in the meantime the 15vdc may be okay for your rig—check the specs.

  23. mike baird says:

    my 12 volt/ 10 amp power supply output reads 15 volts out…..is that a problem?

  24. John Colvin says:

    Don’t know if you are still giving advice-if not no biggie. I’ve got a bunch of AC-DC wall plug voltage adaptors from 3 VDC to 36 VDC output. I probably have 10 – 15 that put out 12.5 VDC from 200 ma to 1.5 amps. I also have an HTX 202; HTX 404; an Icom 4AT; and an HW 8.
    My question is this: can I use one of these (12VDC @ 500ma) to power one of the HTs or do I have to another “wall wart”.
    BTW Callsign is NoYHX.
    Thanks much. 73s

  25. Dave says:

    John, possibly. If it’s an HF radio, it’s worth noting that HF bands are noisy anyway. The best way to tell is to try other power supplies or hook the radio to a 12v battery. I recommend getting a ham radio power supply from a ham radio source, such as MFJ or PowerWerx. If you call Ham Radio Outlet or Amateur Electronic Supply, they have lots of models that work. BTW, if you’re powering a 100-watt HF radio with this power supply, you really should be looking for a 25-amp supply. Good luck!

  26. John says:

    I just bought a radio shack 19 amp power supply and its causing noises in my radio, should i take it back and buy a power supply from a local radio store that specifically made for radios ?
    i cant hear anything receiving.

  27. Dave says:

    Hi John, I also like to see “dancing meters” while I’m transmitting. I have an MFJ-4225MV, which is a variable voltage supply, and I’ve always used it set on 13.8 VDC. The dancing ammeter is nice, but the unit is so noisy that it gets relegated to a spot under the bench anyway. Normally I use my photovoltaic system to power the rig, and I do have an analog voltmeter and ammeter for that. So, bottom line, power supplies just work. I have no scenario for anything other than that. One thing that concerns me about the variable voltage supplies is how easy it is to accidentally kick up the voltage beyond what is safe for the rig. Of course what you choose will be based on your personal preferences, so if you like the dancing ammeter, go for it! 73, Dave, KEØOG

  28. John says:

    Hi Dave,
    I’m shopping power supplies for a Kenwood TS850. The Powerwerx supply in your post would get the job done, but I’m a sucker for meters, knobs, and switches (needed or not.) Was hoping you have 1 or 2 “real-world” scenarios you can share where a variable output supply with a meter was the best choice for the application.
    73, John

  29. Dave says:

    Hi Karen, yes, the 757 should operate directly from a 12VDC marine battery. Be sure it’s fully charged. Best practice is to not let the voltage drop below 12 volts. Beware of charging it at the same time you’re operating—it could introduce hum into both your transmitted and received audio. Good luck!

  30. Karen says:

    I have a Yaesu 757GX Hf mobile rig. I have never gone PP but want to soon. A simple marine battery directly to rig would be my best bet?

  31. Dave says:

    Hi Dave, that certainly sounds like an approach that could be tried. But if you discharge the lead-acid battery down to 12 VDC, you’ll want to charge it up to 14.4 (for flooded batteries, 14.1 for sealed batteries) and then float it at 13.3. Otherwise it won’t fully recharge. There are 3-stage battery chargers out there, although they’re a bit pricey. Common 25 or 30 amp power supplies can be found for less than $100. The battery does offer the advantage of powering the rig if the utility service is disrupted. Hope that helps. 73, Dave

  32. Dave Sunde says:

    Since the float voltage of most batteries is 13.3 VDC and if I’m going to operate my radios directly off the battery the voltage will eventually drop to 12VDC or so. Why shouldn’t I just adjust the output of the power supply to 13.3 leave it hooked up as a charger and be done with it?

  33. Dave says:

    Yes, you can power multiple radios with a single power supply.

  34. Christine says:

    I recently bought an Astron SS-30M power supply. I want to use it for two of my radios – a Yaesu FT-2800 and a Yaesu FTdx100. The power supply has two places in the back for the end of a wire – one for the red and one for the black. Can I put two wires in each place? I don’t want to do it this way, but I don’t know how else to connect both radios to it.

    Thanks,
    Christine

  35. Dave says:

    I suggest getting one for 13.8v. 14.1 may be within your radio’s specs, but they’re really built for 13.8 v. There are many suitable power supplies out there from MFJ, PowerWerx, and others.

  36. Mel says:

    Hi I am a newbie I just purchased a Yaesu ft857d trying to figure out the power supply so I can have a home station I found a power supply 14.1 V and 25 amp continuous is this okay the radio says 13.8 V not sure if 14.1 V will damage the radio

  37. Dan Long says:

    This is good news for me. I have the MFJ-4225MV on backorder and I hope to receive it soon. I am a Ham newbie. I have had my technician license (KC9VZU) for a couple of years, and I just got a radio, an ICOM IC-7100. I have started going through your videos for General Class. You mentioned that you would like a thermal sensitive fan for your power supply. I think it would be easy to build one with an Arduino or Arduino like microcontroller. I think you could get most of the parts (the microcontroller and temperature sensor) from Adafruit.com, add a 5v fan and a little code and it would be up and running (I am not affiliated with Adafruit, I am just a fan)

    73

  38. Dave says:

    Vaughn, your issue with the Samlex SCM 1235 puzzles me. I can only recommend that you discuss the matter with the store where you purchased the SCM 1235. Another option is to consult with a local ham radio club—perhaps members there can help you. You can find a club by clicking here. 73, Dave

  39. Vaughn says:

    I have an SCM 1235 and it is making a ringing sound when the radio is connected to it. Is there anything I can do to fix this? It really drives me crazy. My radio is an Icom 2200. Thanks

  40. Dave says:

    You’re probably fine connecting it to the central power supply.

  41. Tom Kolodz says:

    Dave:

    I think I found the answer to my question. The antenna tuner does not need its own power supply to run. The power supply is for the meters on the tuner. The info. in the manual calls for a 12 vdc transformer at 200 ma. That’s way smaller than the PS for the transceiver.

    Tom
    KC9MDN

  42. Tom Kolodz says:

    Dave:

    Hope you are still answering questions. I’m a newbie. I have a Yaesu 897 powered by a PS from AES. I’ve just installed a 43 foot vertical from DX Engineering which will need a tuner and the tuner a PS. Would I be able to connect the tuner to the PS I am using for the 897 or does each piece of equipment need its own power supply?

    Thank you for any help you can give me.

    Regards,

    Tom Kolodz
    KC9MDN

  43. Dave says:

    Mike, the short answer is “whatever you want.” Often power supplies have binding posts you can use, which is the simple way to do it. I prefer Anderson PowerPole connectors, which are quite common, but then you do have to add the connectors to the power cord supplied by the radio manufacturer.

  44. Mike says:

    What connectors are used to connect a power supply to a mobile radio being used as a fixed station? thanks.

  45. Mike says:

    I want to hook up a power supply to a mobile radio as a fixed station. What connectors are used to connect a power supply to a radio? thanks.

  46. Dave says:

    Rodney, no problem, the power supply merely needs to be big enough to handle the load. A power supply will give a radio only the current it needs. So, for a relatively modern radio, a 25 amp supply is a good choice.

  47. Rodney K7ROD says:

    A question about the amps out of a standard PS. I am looking at an HF radio that requires 20.5 A @ 13.8 VDC. If the PS puts out 25 A continuous is this bad for the HF in the long run? Can you tell I am still new to Amateur radio. By the way Dave I like the General Class teaching videos. Keep up the good work.

  48. shakey white boy kfg8296 says:

    thanks helped alot!!!!

  49. Tony says:

    Hi,

    I am glad I found this post! I have been trying to find some information to help me on selecting a power supply for an HF rig I am hoping to buy. The specifications on the radio says you can use any power source that is 13.8v, 22 amps. I could find many 13.8v supplies, but very few with 22 amps. I was not sure how critical it was to match the 22 amps exactly. SO, at least now I have a better understanding of what to look for!

    Thanks,
    Tony

  50. Robert Smith says:

    Thanks for the info, KG4UGT

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