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Author Topic: How much power does a transeiver need?  (Read 3074 times)
LB5KE
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« on: March 24, 2012, 10:16:42 AM »

Many 100 watt transceiver specs are 20 amp (12 volt) input. I am currently using a power supply that are rated to give 15amp continuous. It doesn't have a max rating like many power supply has. I am not sure of what the consequences of having a littel current deficit. But i guess that 20A input is peak? Will i loose a little power or will my tone/voice sound distorted? I have a new 125watt VX-1700, and the spec are 24 amp. So i suppose i need a new power.


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AA4PB
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« Reply #1 on: March 24, 2012, 10:34:44 AM »

You can't assume that a 15A maximum rating equates to 20A peak. It all depends on the power supply design. What happens if you draw more than the maximum rating is also dependent on the particular design. Some power supplies will "trip" off with over-current. Some of those will reset automatically and some will require a manual reset. Some will drop the output voltage in an effort to limit the current and that will generally cause distortion in your transmit signal. Some may not protect themselves and over-current may result in component failure. Failure of a pass transistor (if the supply doesn't include a crowbar circuit) will usually apply too much voltage (perhaps 25VDC) to the radio which will usually cause components in the radio to fail.

The bottom line is that you really want a supply that is never overloaded by the radio.
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K8AG
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« Reply #2 on: March 24, 2012, 10:36:57 AM »

But in reality if you can turn your rig down a bit you may find the 50W is all you need to work the world.  Until you know your supply can handle 20A peak, consider a bit less output on transmit.

72, 73, JP, K8AG
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AA4PB
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« Reply #3 on: March 24, 2012, 10:45:32 AM »

Yes, you can certainly reduce the RF power output in order to reduce the current drawn by the radio. You've got to be really careful with SSB however because simply talking louder into the microphone can increase the peak current. If you use any metering like a current meter or wattmeter you've got to be sure that it is capable of reading "peak" values. Many meters read "average" values which are much lower than "peak" on SSB voice.
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ONAIR
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« Reply #4 on: March 24, 2012, 03:14:22 PM »

With that PS, you can probably run around 65 Watts on SSB with no issues.  When conditions are right, you can work the world with less than 25 Watts.
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W8JX
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« Reply #5 on: March 24, 2012, 04:01:30 PM »

But in reality if you can turn your rig down a bit you may find the 50W is all you need to work the world.  Until you know your supply can handle 20A peak, consider a bit less output on transmit.

72, 73, JP, K8AG



My Kenwood draws less than 18 amps at 100 watts out. Also all power supply's have a continuous and intermittent rating (usually 50% duty cycle for this) that is usually 25 to 35% more than continuous rating. SSB is low duty cycle on a power supply so I would really not worry about reducing power if you do not want to.
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KD8DEY
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« Reply #6 on: March 24, 2012, 04:04:32 PM »

1 read the manual for the radio
2 read the manual/Specs for the power supply
3 compare

If Radio needs more peak power than Power supply peak rating
A reduce TX power
or
B Buy Bigger Supply

Hope this helps.
(Just yanking your chain) Smiley

More seriously, I have read that for good performance on SSB that supply voltage shouldn't drop by more than 5%

So for a "12v" supply (13.8V) if voltage drops below 13.11v I would cut back on the TX power setting.....

BTW You never mentioned what supply you are referring to

PS
I just looked up the radio pdf.
They recommend a 25 amp continuous (minimum 22 amp) supply. If you have a Astron or similar 723 based supply maybe you can do the floating battery mod and draw the needed current from the battery??
« Last Edit: March 24, 2012, 04:31:50 PM by KD8DEY » Logged
HB9PJT
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« Reply #7 on: March 24, 2012, 04:55:39 PM »

For SSB voice mode a 15 Amps power supply is adequate for 125 watt output power. But for FM or CW you need more. You can check that for examply by reading the data sheet of the commercial radio Micom 2BF which makes 125 watt HF and is rated with 13 Amps for voice mode.

http://www.royal-communications.com/downloads/MicomBF.pdf

73, Peter - HB9PJT
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K8AXW
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« Reply #8 on: March 24, 2012, 05:15:09 PM »

If you own an analog meter, connect it to the terminals/connectors near the transceiver, crank it up and talk.  Watch for needle "flicker."  The voltage on a well regulated power supply should maintain it's rated voltage at full load.  If you overload it, the voltage will sag or drop and you will see this "flicker" in the analog meter.  A digital meter works too slow to catch this.

If you find that you don't have much of a voltage drop when you operate at full power, check the pass transistors for excess heat.  All you have to do is touch them.

Another test is to load the operate the transmitter in the CW mode and see how much of a voltage drop you have with the key down.  If the voltage drops significantly, then I'd avoid using this power supply for that rig.  If it doesn't drop much, then keep an eye (finger) on the pass transistor temperature.

Some Kenwood transceivers will let you lower the power in the CW mode but not the SSB mode. 
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N6AJR
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« Reply #9 on: March 25, 2012, 10:42:10 AM »

ost100 watt radios need 22 or 23 amps to hit max power.  You can operate at reduced ower out  but I reccommend  getting a 35 amp suppllllllly minimum, so you can alsso hook up a 2 m fm radio, or lights for  the meters and such.  more is ok, less is questionable.  use that 15 amp PS to run the mobile style rigs in the shack.
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WX7G
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« Reply #10 on: March 26, 2012, 07:02:14 AM »

Key the transceiver in CW mode raise the RF power while measuring the power supply DC voltage. See if the transceiver reaches full power and the power supply voltage is above 12 volts. If it is above 12 volts there is no voltage sag that will cause distortion on SSB.

While transmitting 100 watts CW the power supply will supply about 11 amps average or 15 amps RMS. It is the RMS value that we want to know to calculate power supply transformer heating. Your power supply is rated for 15 amps and should be good to go on CW or SSB.
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W8JX
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« Reply #11 on: March 26, 2012, 08:39:50 AM »

ost100 watt radios need 22 or 23 amps to hit max power.  You can operate at reduced ower out  but I reccommend  getting a 35 amp suppllllllly minimum, so you can alsso hook up a 2 m fm radio, or lights for  the meters and such.  more is ok, less is questionable.  use that 15 amp PS to run the mobile style rigs in the shack.

The three 100 watts rigs I have here use LESS than than 20a at 100 watts out. (highest was 18.4a)  Spec's are for worst cases scenarios and is higher that what is usually needed.  I used a 20a supply (16a continuous,  20a@50%) for nearly 20 years with 3 different rigs and never had any problems. Still use it as a back supply. More is not always better and usually is not really needed.  My main supply is now a 30a switching supply (25a continuous, 30a @50%) which is very compact, light weight and cheaper too. I should add that I even ran 100 watt SSB portable with a 12 amp supply and a 100,000 ufd capacitor in parallel with output. Worked fine and supply did not overload.
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LB5KE
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Posts: 141




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« Reply #12 on: March 26, 2012, 01:48:23 PM »

I should add that I even ran 100 watt SSB portable with a 12 amp supply and a 100,000 ufd capacitor in parallel with output. Worked fine and supply did not overload.

I am glad you brought up the use of capacitors, i know how effective they can be in providing peak current to hi-power car stereo. A scenario that might resamble transceivers in SSB mode.

OK, I have measured the voltage. When the ts-690 is just receiving the voltage is 12.6 volts, on key down CW it drops to only 12.3 volt. I have bought a shunt on ebay so in a few weeks i can messure current also.
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LB5KE
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Posts: 141




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« Reply #13 on: March 26, 2012, 03:04:37 PM »

If i look at the spec on the kenwood ts-430 it says : power supply provides 13.8 VDC at 20 amps intermittent (25% duty cycle), 15 amps standard (50% duty cycle) or 10 amps maximum at continuous load.

http://www.universal-radio.com/catalog/hamhf/ps430a.html

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