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Author Topic: Power Supply size?  (Read 4482 times)
K5UNX
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« on: February 16, 2013, 05:41:02 PM »

I just bought a FT-857D. I am going to mount it in a portable case of some sort and need a power supply. I am looking at the MFJ-4125. It's 25 amps. According to the manual for the FT-857, it can consume 22 amps. I assume that's at a full 100 watts transmit level. I don't think I would run the radio at full power all the time . . . but I am new to HF.

I am also planning on possibly charging or powering my FT-60R. According to it's manual, that would take 1.6 amps. The only other things I can see using that power might be a small LED light. Not sure what else.

I am planning the following in the case. FT-857D, Power Supply, Antenna Tuner. Maybe the LDG FT-Meter.  I was thinking about a rig runner power dist panel but now I am thinking I might not need that, just a Power Pole spitter maybe.

Am I cutting the power too close with the 4125 at 25 amps??

Wayne

« Last Edit: February 16, 2013, 05:43:18 PM by K5UNX » Logged

K8AXW
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« Reply #1 on: February 16, 2013, 05:46:49 PM »

Wayne:  IMHO, yes.  I'd go for a 35A supply.  Even though you'll be using SSB and CW, I'd still go with a supply with a bit more headroom.  The 25A supply will work but....as I said, my opinion.
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AA4PB
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« Reply #2 on: February 16, 2013, 05:59:21 PM »

The MFJ-4125 is rated 25A intermittent and 22A continuous. My opinion is that you a cutting it pretty close.
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W8JX
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« Reply #3 on: February 16, 2013, 06:53:28 PM »

Wayne:  IMHO, yes.  I'd go for a 35A supply.  Even though you'll be using SSB and CW, I'd still go with a supply with a bit more headroom.  The 25A supply will work but....as I said, my opinion.

Bit of overkill here. I have a 20 Astron that I used as a main supply for many years and now is a backup. I replaced it several years ago with a light weight Astron SS 30.
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AF5C
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« Reply #4 on: February 16, 2013, 07:59:19 PM »

Jetstream makes a 28 amp supply I have that isn't much bigger than the MFJ one. Still small and lightweight.  I would check the QST review on the FT857D.  It will tell exactly how much current it consumes at full power. I am guessing it is less than 22 amps. Probably more like 17 to 18 or so.

John AF5CC
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G4IJE
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« Reply #5 on: February 17, 2013, 02:42:00 AM »

IMHO a 25 amp supply would be fine. After all, that's what they are made for - powering a typical 100 watt transceiver. I've owned several 100 watt HF rigs over the years and don't recall any of them taking more than about 18 amps "key down".
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K2DC
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« Reply #6 on: February 17, 2013, 03:51:32 AM »

I've never had a problem using a supply rated at 25A peak.  However, I didn't have much luck with the MFJ-4125.  Mine lasted about 6 months before the power switch failed, so I bypassed it (not worth replacing).  Then after another 6 months it died altogether.  You mileage may vary.

73,

Don, K2DC
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K1CJS
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« Reply #7 on: February 17, 2013, 05:57:43 AM »

A good rule of thumb to follow is to use a power supply that will deliver twenty percent more than the required constant draw on it.  If your system will use 23.6 amps (the total of the two rigs) you should use a power supply that will deliver not less than 28.3 amps--or a thirty amp power supply.

If this is done you will never run your supply at full output, will have a comfortable margin for overdraw if it ever happens, and the power supply should last you a lifetime.  Scrimping on the power supply may be the worst thing you can do, especially with some of the rigs today that just may be adversely affected by any undervoltage that may occur.

If you do use a twenty five amp supply to supply that 23.6 amps, there is a possibility of overdrawing it--possibly causing premature failure, especially with economy supplies that just may be too closely rated, not to mention possible damage to your equipment.  The question is this:  Do you want to spend a little more now--or spend a lot more later?
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AA4HA
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« Reply #8 on: February 17, 2013, 06:02:16 AM »

I think that the 25A supply is cutting it a bit close as well. You also stated that you are going to be using a portable charger and a tuner as well. I am assuming that your portable case is something you can use for field days or EMCOMM. You will end up finding something else you want power for, like a small LED light for night operations or a TNC or maybe to float charge a backup battery for when you do not have AC hookup.

An unspoken thing about electronic specifications is "duty cycle". Any manufacturer (not just MFJ) will give you an intermittent rating but it almost always has been tainted by sales and marketing types who would happily say "25 amps intermittent duty", even if that intermittent duty cycle was 5-10%. Consider what a radio amateur who does data modes would have for a duty cycle (as high as 30-50% of the time they are transmitting). Also that power supplies are rated in a nice environment at 20C (68F) with plenty of air flow to keep things from overheating. You on the other hand are going to pack this up in a box, probably use it where the outdoor temperature may be 30-35C (80-95F) and possibly with an incoming electrical service that is not rated right at utility specifications. You should mentally derate any product specification by another 20%. (wow K1CJS, we were thinking the same thing)

I deal with electronics specifications all the time when pulling together designs for communications systems in less than desirable environments. (too hot, too cold, locked in a cabinet on a utility pole somewhere, maybe a fan but not allot of airflow) It is embarrassing to install a 12 volt, 5 amp supply in 30-50 sites where you know the load is only 12 volts, 3 amps and then to find out that in the months of June, July and August that they had to replace 2/3 of those supplies because they burned up. Naturally the client wants you to credit them for the replacement supply and their tech labor to go out there and swap those things out. Also they justifiably left with a sense that this half a million dollar priced system is less than reliable because they cannot go a week without something failing. Now I just spec in a 10 amp supply from the same manufacturer and seldom have a problem unless it is caused by lightning. BTW, these are not cheap supplies either; they are industrial rated and cost a few hundred $ each.
« Last Edit: February 17, 2013, 06:09:02 AM by AA4HA » Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
KE3WD
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« Reply #9 on: February 17, 2013, 06:44:44 AM »

Switching Supplies are good, lightweight and can solve the portability problem well. 

The one thing that I encounter on the testbench all the time when switching supplies fail is the fact that people don't realize the effects of powerline transients on the Switchmode supplies. 

Very common that the supply as purchased has little to no transient protection built in, also very common to see various supplies used in the field, such as the switchmode supplies used in pro audio gear, to take a transient hit that typically will take out the rectifier(s) that are usually rectifying the AC line current before chopping it up and presenting it to the smaller transformers or autoformer coils involved. 

Therefore, if you are contemplating a Go Kit for Emcomm and contemplating use of a Switchmode supply, I recommend adding Powerline Transient Supression to the kit for the AC input line. 

This problem will be greatly exacerbated by the sometimes necessary use of generator power, auxiliary power sources, etc. that EmComm operations necessarily may throw your way. 

Even just a good quality powerstrip that includes good quality MOV transient protection can go a long way towards keeping your switchmode supply running. 

I also have my suspicions that many of the reports of premature switcher failures, even when plugged in at home in the shack, become victims of powerline transients and thus get the bad name.  Transient Surge Protector strips are cheap insurance, not only for the switchmode supply as a separate, but for many of the appliances and accessories found in the modern ham shack as well.  Like the ubiquitous computers, etc.


73
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W8JX
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« Reply #10 on: February 17, 2013, 07:03:20 AM »

A good rule of thumb to follow is to use a power supply that will deliver twenty percent more than the required constant draw on it.  If your system will use 23.6 amps (the total of the two rigs) you should use a power supply that will deliver not less than 28.3 amps--or a thirty amp power supply.

If this is done you will never run your supply at full output, will have a comfortable margin for overdraw if it ever happens, and the power supply should last you a lifetime.  Scrimping on the power supply may be the worst thing you can do, especially with some of the rigs today that just may be adversely affected by any undervoltage that may occur.

If you do use a twenty five amp supply to supply that 23.6 amps, there is a possibility of overdrawing it--possibly causing premature failure, especially with economy supplies that just may be too closely rated, not to mention possible damage to your equipment.  The question is this:  Do you want to spend a little more now--or spend a lot more later?


The big problem with this logic is there is NEVER a high constant load. The only constant load is a few amps or so with rig in receive mode. The is not commercial 100% key down with high current draw operation. When using SSB your average load is around 8 to 10 amps max when transmitting. When digital you may have I higher load at times but it is NOT constant and is intermittent in nature with a duty cycle of 50% or less typically. Also I have never tested a rig that actually drew the amount of power that it was rated at. It is typically 10% or more less and the nominal figures in spec's are more of a "never will exceed figure" than a actual consumption. 
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N4NYY
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« Reply #11 on: February 17, 2013, 07:18:30 AM »

25A with 30-35 amp surge will be more than enough. I know people that have used Astron 20A with no problem
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K5UNX
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« Reply #12 on: February 17, 2013, 08:08:07 AM »

To answer or confirm a couple thought. Yes I am planning on this being a portable rig for emcomm, field day, comm volunteering at local events and such.  This being my only rig, I want it portable. I can see running a small LED light, and possibly something else that I don't know about now. A guy in my club recommended a MightyLIte Supply. I just now realized what he was talking about  . . http://www.mfjenterprises.com/Product.php?productid=MFJ-4235MV   This one is 35A.

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AA4PB
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« Reply #13 on: February 17, 2013, 10:03:52 AM »

25A with 30-35 amp surge will be more than enough. I know people that have used Astron 20A with no problem

An Astron RS20A is rated for 20A intermittent and only 16A continuous. Thats not good for a radio that is specified at 22A key down plus the addition of some other accessories.
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AC5UP
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« Reply #14 on: February 17, 2013, 03:16:41 PM »

Lest we forget, each mode has a duty cycle that affects the current drawn from the radio. There's also the RF power adjustment to consider.

If you're running RTTY bulletins at full power you will be very close to the maximum rating for the radio.  You should also realize that dropping power from 100 to 80 watts for a mode like RTTY could avoid a premature equipment failure with little affect at the other end of the Q.  SSB at 100 watts PEP is like FM at maybe 60-ish watts unless you're James Earl Jones and have the speech compression cranked to the max.  The duty cycle of CW depends on the speed and to some extent on the fist sending it.

Point being that for normal SSB @ 100 watts PEP a 20 amp Astron is more than adequate.  Especially for a temporary / portable setup when size, cost and weight are considerations.  If you think you have to run full rated power to make Q's you're either new at this or have a G5RV in your attic.  With a halfway decent antenna and normal band conditions 50 watts of RF is more than enough to do business.

OTOH, there are Hams who wouldn't dream of calling CQ at less than 400 watts because they have a linear and it makes no sense not to use it for every Q........  Part 97 be damned.   Tongue
« Last Edit: February 17, 2013, 03:21:29 PM by AC5UP » Logged

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