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Author Topic: Power supply question  (Read 2165 times)
KG6WLA
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« on: February 08, 2005, 12:15:26 PM »

I am looking at getting a getting a power supply. Its a 22amp (20 constent 22peak???) MFJ-4322. My in the future i plan on getting a HF rig that puts out 100 watts, most of the specs i have seen say that max power at transmit is around 20 amps (i think a FT-857 is 22), will a power supply that provides the max be fine, or would it be recomended to get something with a little more power....

the radios im looking at are:
FT-857D
IC-706MkIIG
IC-718
FT-840

I know the amp will work fine for my VHF/UHF radio but i dont want to limmit my self to much for the future and I dont intend on running any amps.

THanks
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AA4PB
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« Reply #1 on: February 08, 2005, 01:28:31 PM »

I'd suggest you go a bit larger. For one you may want to power some other accessories from it as well.

You might also want to look at the Astron supplies.
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WB2WIK
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« Reply #2 on: February 08, 2005, 03:16:45 PM »

I'd suggest you go a bit larger, also.  30-35A is a good compromise.

Lots of reasons for this, but one obvious one is if you ever want to transmit on HF and VHF at the same time, with your two rigs, even just once.  With the 20-22A power supply pretty well maxed out by your HF rig, you go to squeeze the mike on VHF and -- lights out!  The power supply won't do it.

SSB usually only taxes power supplies fairly lightly, e.g., 22A peak but only 7-8A average, so SSB is easy on the supply.  However, you might want to work digital modes one day, or may even find that you like working 10 meter FM, which is a continuous-duty mode and will draw the 22A all the time you're transmitting.

Unless you have some real need to keep the power supply very small and light weight (like you pack it in your suitcase and travel with it), I'd go bigger.

WB2WIK/6
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KG6WLA
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« Reply #3 on: February 08, 2005, 05:18:56 PM »

I would only max it out at 100 watts (digital/FM) right, what if i reduced power to lets say 50 watts out.... would it only need half as many amps?
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WB2WIK
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« Reply #4 on: February 09, 2005, 08:29:31 AM »

Half as many amps at half power?  No.

The rig's final amplifier stage (PA) draws a lot of the power consumed, but all the driver stages, PLL circuitry, display and other parts of the rig always draw the same amount of current regardless of output power, so there's no direct relationship between output power and current consumed.  You'd have to measure that, as it varies from rig to rig.

Also, it's unreliable to count on a power output control to limit current drain.

I'd just get a 35A power supply.  If a new one is too expensive, I'd get a used one, there are thousands out there.  Unless weight is an issue for portability, the Astron RS35M is a really good choice: Heavy, but inexpensive and extremely reliable -- they've been on the market for more than 20 years and most of the original ones are still in service.

WB2WIK/6
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N0IU
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« Reply #5 on: February 09, 2005, 10:38:59 AM »

I also vote for the Astron. If you want to save about $20, get the "plain Jane" RS35A for $150 (from AES) as opposed to the RS35M that has a volt meter and ammeter on the front and sells for about $170. I still have my 35A and after 12 years the only problem is that the LED that lights up the power switch when the unit is on has burnt out.

You also might want to consider some sort of power distribution device such as one of the Rigrunners from West Mountain Radio (http://www.westmountainradio.com/RIGrunner.htm). With these, you only make one connection to your power supply and then tap off of the Rigrunner unit to the other devices in your shack. These also use Anderson Power Pole connectors (as opposed to one of the MFJ units which will also do a fine job, but use banana plugs to make the connections). A lot of ARES/RACES groups spec this type of power connector so that if you ever chose to use your equipment in that capacity, you won't have to worry about whether or not you have the proper power connection. These are also keyed so that (assuming you have wired the connector properly in the first place), you don't have to worry about reversing the polarity and risk damaging your equipment.

After all, electrical devices need one thing in order to operate properly; electricity!

73,
de Scott NØIU
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N0IU
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« Reply #6 on: February 09, 2005, 10:44:13 AM »

I stand corrected.

MFJ also makes Anderson Power Pole multi-outlet power strips.

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AA4PB
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« Reply #7 on: February 09, 2005, 06:37:10 PM »

Half power doesn't mean only half the current draw but it will draw considerably less current at half power than it does at full power.

If you are running PSK31 you'll want to reduce the power to at least half anyway. On the other hand if you are running RTTY with a rig rated for continuous duty you may want to run 100W output. You may also want to run a full 100W output on 10M FM. You may also forget to reduce the power before keying up some time and overload the supply. I just wouldn't cut myself short on the power supply current capacity.

If size and weight are an issue then consider a switching supply such as the Astron SS-30M. Switching supplys do generate some RF noise so you'll want to be sure that your antenna is not located close by (like directly over an upstares shack) if you use a switching supply.
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KG6WLA
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« Reply #8 on: February 10, 2005, 08:08:13 PM »

Thanks for pointing me in the right direction everyone!
I have relized that saving a few $'s now may limit my self in the hobby, which i really dont want to do. Are there any precautions i should take while using such a device. I have heard that they can fry a radio... Is this true? Thanks again!
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N0IU
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« Reply #9 on: February 10, 2005, 08:29:51 PM »

Precaution #1: The red wire is the positive.

Precaution #2: The black wire is the negative.

Precaution #3: Put a fuse on BOTH the positive and negative wires coming from the power supply to your equipment or outlet strip.

Precaution #4: Follow #1, #2 and #3 and you shouldn't have to worry about frying your equipment!

Good luck and Via Con Dios!

NØIU
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AA4PB
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« Reply #10 on: February 11, 2005, 03:18:29 PM »

Of utmost importance is to make sure that whatever supply you use as an overvoltage "crowbar" circuit in it. If a pass transistor shorts and you don't have an overvoltage protection ciruit then the radio gets hit with something in the neighborhood of 25 Volts which usually means it gets "fried".

I believe all of the Astron supplies have the overvoltage protection.
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N9DG
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« Reply #11 on: February 11, 2005, 07:19:21 PM »

I too would only go for something bigger than the absolute minimum supply. A common mistake I see many hams make is to buy a power supply that is "just big enough", inevitably their station layout plans change with added gear and then they quickly run into a shortage of power with multiple radios running. Power supplies run cooler overall if the combined RX only current of all the radios is a smaller percentage of the power supply’s maximum capability. The only noticeable heating will then be during heavy TX usage.

To cover the typical 100W HF radio + 1 or 2 VHF/UHF radio case I wouldn't buy anything smaller than 30-35A, for example an Astron RS-35 as previously mentioned (have been using one here for 21 years and still counting). If weight and size is critical then the Astron SS-30 would be a good bet too. Have one of them as well, no problem with RF noise here. It does however have a fan and in my case it was a wash between the hum of the linear supply vs. the cycling cooling fan of the switcher as far as audible noise is concerned.
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KG6WLA
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« Reply #12 on: February 14, 2005, 05:47:34 PM »

I was looking at the SS-30 at gigaparts and relized that it was 25 continuous and 30 at peak, so should i assume its enough for a 20 amp radio? What do they mean by 30 peak and 25 continuous? Why is the item number (SS-30) not SS-25 if that is what its continuous value is?
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WB2WIK
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« Reply #13 on: February 15, 2005, 10:21:21 AM »

Model numbers can be whatever the manufacturer picks them to be, and don't necessarily mean anything.

The SS-30 should suit your needs just fine.

The difference between "continuous" and "intermittant" can be defined by the manufacturer to be whatever he says it is; however, *usually* "continuous" means the power supply can deliver the rated current without interruption, and "intermittant" means it can deliver that current for five minutes, followed by a five minute cool-down period, and then deliver the current for five minutes again, etc -- this yields a 50% duty cycle with 5 mins maximum operating cycle.  That's a pretty standard definition.

However, the manufacturer can re-define the "max" time to be something else, like 1 min, 2 mins, 3 mins, etc.  Whatever the "max" time is, there's normally a cool-down period equal to that time during which the load current must be reduced to nearly zero before the maximum load current may be drawn again.

Since the definition can be a moving target, it's best not to even look at "maximum" or "peak" current ratings, and just focus on the continuous rating.

WB2WIK/6

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AD5X
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« Reply #14 on: February 24, 2005, 05:33:22 AM »

I've been using an MFJ-4125 with my IC-706MKIIG for about a year.  This supply works great with this radio - no measurable voltage drop at full output, and no noise that I've been able to hear.  It may also be the smallest and lightest switcher out there - so it is great for portable operation.  The supply is very well filtered on both the AC input and the two DC outputs.  I did replace the low current rear connectors with PowerPole connectors so I could draw full current from the rear of the power supply.

There are two versions of the MFJ-4125, so be carefull if you see a used one for sale.  The earlier one was about 10 inches long and did have problems according to some.  The new version is only about 5 inches long and seems to be well-built (my experience).

Phil - AD5X
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