What size power supply

[TMGRN901]

Hello all,
I am a new ham, and don't understand power supplies all that well. I have a 50 watt mobile radio attached to a 15 amp / 13.8 volt power supply. Every time I key up the radio just shuts off. I'm thinking it's the power supply but, as I said earlier, I don't understand it very much. But from what I do understand, I=P/E. So that would mean I could get by with a 3.6amp power supply, so the 15 amp would be just fine correct?

 

[n5ims]

Hello all,
I am a new ham, and don't understand power supplies all that well. I have a 50 watt mobile radio attached to a 15 amp / 13.8 volt power supply. Every time I key up the radio just shuts off. I'm thinking it's the power supply but, as I said earlier, I don't understand it very much. But from what I do understand, I=P/E. So that would mean I could get by with a 3.6amp power supply, so the 15 amp would be just fine correct?

Your calculation assumes that your radio is 100% efficient (not even close) and that 100% of the power consumed goes to developing the transmitter power output (nothing for the computer controller, lamps, etc.).

Next, a power supply rating is often the peak amount it can supply (for very brief periods). Assume it will actually supply only 75% or so of that power for normal use. Check the supply's documentation to see what it is rated for during continuous use.

Check your radio's documentation (sorry to keep bringing up that dirty word, but they create documentation for a reason) to see what it needs to properly power it. Failing that, check the fuse rating on the power cord. Make sure your power supply will provide at least that much power during continuous use.

Many of the mobile radios require 20 amps or so. To properly provide that much power to your radio continuously, you'll probably need a 30 or 35 amp power supply.

 

[jeatock]

Power supply ratings are sometimes optimistic and for short-term draw. Long winded TX will cause issues in a bargain power supply. Stick with ITC, Astron or Dura-Comm. The 'good' ones will deliver nameplate amps at continuous use, and more short term.

A typical 50 watt land mobile pulls 0.5 amps in receive and 12.5 amps in transmit, and normally has a 15 amp fuse. Higher TX power ham radios need more (check the specs in the book). I suspect either your power supply isn't delivering enough amps at the required voltage, or the wire size is causing a voltage drop. Check the volts at the radio when in transmit; then at the power supply. A difference between the two points means bad wiring or undersized wire. A difference of more than a few tenths of a volt is bad.

Also, try transmitting at low power and recheck the volts. If there is still a difference (voltage drop) between volts at the radio and power supply your wiring is at fault. Low power TX will be in the 3~6 amp range.

An ammeter would also help diagnose the issue if you have (or can borrow) one.

Bad power supplies (even new) are not unheard of.

Try direct to a car battery to check your radio. If the 15 amp fuse pops you have radio issues.

Anything less than 10 or 11 volts will cause the radio to shut down.

 

[cmdrwill]

Is your transmitter antenna close to your power supply? A very bad practice, RF can get into the power supply causing problems.

 

[robertmac]

I'm a relatively long term ham. This was one of the things covered in the course I took. As there have been many similar posts, do they not teach the proper power supply needed? Bigger the better I was taught. As in may need one that is more than 40 amps. As a minimum.

 

[wyShack]

A 'rule of thumb' I use is 2-1/2 times the output rating so a 50 W rig would be about 12-13 amps. That assumes nothing else connected to the supply and also the rating of the supply is continuous (not peak). For the most part you may have several things running off the same supply although only one unit is likely transmitting at any given time the other equipment would still draw some.

Hope this helps

 

[zz0468]

...Bigger the better I was taught. As in may need one that is more than 40 amps. As a minimum.

That's entirely too much power supply for a 50 watt radio. Bigger can be better, if one plans ahead and wants to use a single supply for several radios and/or higher power. But many people have economic and space considerations as well.

To the OP, just look at the operator manual for the radio. There should be a specification page that states what the current requirements are. The 15 amp supply is probably fine.

Every ham, without exception, should have a dummy load capable of handling the power level of the transmitters on hand. Make that the first piece of test gear you buy. Run your transmitter into a dummy load and see if you still have power supply problems when you transmit. You'll probably find you don't. That would mean that your antenna is either too close, or maybe you have a problem causing the coax to radiate in the shack.

 

[N4KVE]

Like others have stated, always get a supply that supplies at least twice what you need. Some of the no name supplies are optimistic in their ratings. What brand is your supply? My Astron which is rated at 35 amps, is continuously rated at 25 amps, so I just ignore the 35 rating. One of the most important pieces of equipment you will own is a QUALITY power supply. Remember, having a power supply that is too big does no harm at all, but too small is bad.

 

[k6cpo]

Hello all,
I am a new ham, and don't understand power supplies all that well. I have a 50 watt mobile radio attached to a 15 amp / 13.8 volt power supply. Every time I key up the radio just shuts off. I'm thinking it's the power supply but, as I said earlier, I don't understand it very much. But from what I do understand, I=P/E. So that would mean I could get by with a 3.6amp power supply, so the 15 amp would be just fine correct?

My 50 watt mobile radio draws 8 amps at full power transmit. Without knowing what kind of radio you have, my though is the 15 amp power supply should be sufficient for your radio. If it is, then there is something else wrong in your setup.

 

[davidzimmerman]

i have 2 Motorola CDM-1250 mobile's set up as a base radio both are wired to a Astron 10AMP power supply but i have my radio's on low power like 20Watts work's good.

 

[N4KVE]

My Kenwood TK880, a 25 watt radio draws 8 amps, while my Motorola XPR4550, a 45 watt radio draws 13.5 amps, so I don't see how a 50 watt radio can draw 8 amps. The OP needs a bigger power supply. Plain & simple.

 

[AK9R]

The OP needs a bigger power supply. Plain & simple.

Since the OP has yet to tell us the make and model of his radio or the rated amp draw for his radio at high power, the answer is not plain and simple. It certainly sounds like his "15 amp power supply" is not up to the task, but if there is excessive voltage drop in the power cable due to insufficient wire gauge or faulty connectors, solving his problem may take more than a "bigger power supply".

 

[n4yek]

My Kenwood TM 241A manual says at 50 watts, current draw is less than 11 Amps.
My Yaesu FT 5100 draws 11.5 Amps at 50 Watts.
Both run on 15 amp supplies and have for years.

 

[Voyager]

A 'rule of thumb' I use is 2-1/2 times the output rating so a 50 W rig would be about 12-13 amps. That assumes nothing else connected to the supply and also the rating of the supply is continuous (not peak). For the most part you may have several things running off the same supply although only one unit is likely transmitting at any given time the other equipment would still draw some.

Hope this helps

Sounds about right. A typical 100W repeater draws about 22 amps, so I don't see a 50W mobile needing more than about 12-13.

I too am suspect that it's a switcher and is getting RF into the PS.

 

[bharvey2]

A 15A power supply might be hard pressed to power a 50W radio especially if it's unregulated or if it's a poorly made power supply. One scenario that was mentioned was having the antenna too close to the power supply. That can definitely make it go haywire. Also, if the wiring is insufficiently sized or too long, that could cause problems as well. Also, don't discount the importance of quality wire terminations and connections. A poor connection will cause a voltage drop enough to shut down the radio. I've experienced it first hand trying to cobble things together too fast.

 

[junkdr]

Does it shut down when connected to a dummy load? If not you have an RFI problem.

 

[N8IAA]

Hello all,
I am a new ham, and don't understand power supplies all that well. I have a 50 watt mobile radio attached to a 15 amp / 13.8 volt power supply. Every time I key up the radio just shuts off. I'm thinking it's the power supply but, as I said earlier, I don't understand it very much. But from what I do understand, I=P/E. So that would mean I could get by with a 3.6amp power supply, so the 15 amp would be just fine correct?

First, I'm going to agree with W9BU's comments that the OP hasn't listed the radio and power supply he is using.
Most 15 amp supplies have a continuous use at about 12 amps or less.
Just because the math is saying 3.6 amps, doesn't mean your radio is using that at 50 watts.
It is more likely RFI getting into the radio through the antenna as another poster wrote. Most radios will shut down when the match is well over 3:1.
Just my thoughts,
Larry

 

[k6cpo]

My Kenwood TK880, a 25 watt radio draws 8 amps, while my Motorola XPR4550, a 45 watt radio draws 13.5 amps, so I don't see how a 50 watt radio can draw 8 amps. The OP needs a bigger power supply. Plain & simple.

The amp meter on my power supply, an Alinco DM-330MV indicates an 8A draw when I key up my Yaesu FT-7900 on full power. Different radios have different current requirements.

 

[prc117f]

The Samlec SEC 1235M is a nice unit it is from what I heard the OEM power supply to some ham equipment builders.

I have one and it has 2 meters so you know your Amp usage etc..

I use one for a 100watt radio and it handles 100% duty cycle transmits no problem.