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# Thread: Watt / Amp questions

1. Ham Member
Join Date
Mar 2012
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## Watt / Amp questions

Just bought my first radio (Yaesu FT-880r) and the manual states that I needs a power supply that has a constant 15 amps @ 13.8 volts. If I use the P, I, E formula that was taught in class that would equal 202.5 watts yet the radio is only rated to transmit 50watts max.
Am not using the formula correctly?

Thanks
Nathan

2. Originally Posted by KF7UUC
Just bought my first radio (Yaesu FT-880r) and the manual states that I needs a power supply that has a constant 15 amps @ 13.8 volts. If I use the P, I, E formula that was taught in class that would equal 202.5 watts yet the radio is only rated to transmit 50watts max.
Am not using the formula correctly?

Thanks
Nathan
15A @ 13.8V = 207W. How'd you get 202.5W?

But the rig's transmitter output power isn't well related to its DC power consumption. For one thing, the transmitter final power amplifier stage is probably 50% efficient, so to run 50W output power that stage alone would need to consume 100W. Then, while it's doing that, DC power also goes to the driver stages, frequency synthesizer, display and display illumination, any other pilot lamps or LEDs in the rig and microphone, possibly a cooling fan, and everything else that consumes power.

3. Ham Member
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Mar 2012
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Originally Posted by WB2WIK
15A @ 13.8V = 207W. How'd you get 202.5W?
I'll just go ahead and blame that on fat finger typing on the calulator

Thanks for the comment - It makes sense that all the other circuits and such in there are using up power as well.

4. Ham Member
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Sep 2001
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14,976
We cover that exact subject in our club's Tech and General Class training. Also, the question as to why a modern 5 Watt H-T gets hot during operation. 5 Watts out, means approximately 10 Wtts in, and what happens to the other 5 Watts? Anyone want to wrap their hand around a 5 Watt nightlight bulb for very long?

5. Ham Member
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May 2003
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The manual for the FT-8800R says it needs 8.5A on transmit (worst case). A power supply that can continuously produce 10A or more will work. So much the better if you buy a power supply that'll handle more load -- you can simultaneously run more ancillary equipment and the power supply will run a tad cooler.

6. If this is your first power supply do yourself a favor and buy at least a 35 amp power supply then you will have one that can even run a 100w rig and your 8800 on receive plus a few other small acces. It will save you money in the long run and not that much more a quality supply might last you a lifetime think of them as cost per year. An Astron RS-35M (35amps) supply is a good choice.

7. Ham Member
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Sep 2001
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Originally Posted by N0AZZ
If this is your first power supply do yourself a favor and buy at least a 35 amp power supply then you will have one that can even run a 100w rig and your 8800 on receive plus a few other small acces. It will save you money in the long run and not that much more a quality supply might last you a lifetime think of them as cost per year. An Astron RS-35M (35amps) supply is a good choice.
Just be aware, that Astron supplies are numbered according to PEAK Current capability. The RS-35 is rated for only 25 Amperes continuous operation; which is more than sufficient for most HF radios and a few accessories. (The Astron RS-20, for example is only rated for 16 Amperes continuous operation, so it's marginal for an HF radio; on low duty cycle modes [CW, SSB] you can get away with the RS-20 supply, but it will be severely overloaded if you use an HF radio in FM or FSK modes.)

8. Ham Member
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His current rig needs only 8.5A and he made no mention (or even hinted at) anything that would require a bigger power supply. Given what we know for certain, an RS-20 will work just fine (even an RS-12 would just about handle the load). And an RS-35 is ridiculously overkill. These days, because of greater efficiency, PWM supplies are taking over, especially for heavy loads and battery-operated gear. They've figured out how to tame the RFI problem.

9. Ham Member
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Originally Posted by KF7UUC
Just bought my first radio (Yaesu FT-880r) and the manual states that I needs a power supply that has a constant 15 amps @ 13.8 volts. If I use the P, I, E formula that was taught in class that would equal 202.5 watts yet the radio is only rated to transmit 50watts max.
Am not using the formula correctly?
This is not out of line at all. I do a lot of stuff with audio amplifiers, and I can count on an input wattage of around twice the output wattage. Due to the way an amplifier circuit works, you're always going to be throwing away energy, and the more powerful the amp - the more energy you throw away.

In the case of your Yaesu, you're actually pulling around 9 amps, so you actually draw around 100 watts while transmitting. Notice how that fits my output=2x input rule of thumb?

The reason you need a bigger power supply is that you want the power supply to have some reserve power. Depending on the type of power supply and the output filtering, you could find that the voltage drops on transmit or that you get some AC ripple when you put a load on the PS output. Having a power supply that can provide more power than you're drawing ensures that your output voltage is a nice, stable 13.8vdc.

For a 50 watt mobile, you can do just fine with a 15 amp power supply. I ran a Yaesu FT-7800 for a long time on a 13-amp unit that I bought at a swap meet, and I never had a single problem with that setup.

However, you have been given some good advice here: the power supply is a critical component in your shack, and it's probably the only component that you will buy today
and hand down to your kids in your will... so get a nice, big, heavy brick of a supply. I'd get something like a 35 or 45 amp linear power supply. There's not much that can go wrong with those, and you can run the thing for the next 20 years without a single problem. More to the point, when you start adding components to your shack, you'll be able to drop them in without having to worry that you'll run out of amps.

10. There is a conflict in the Yaesu documentation (surprise surprise). The "specification" states 8.5A at 13.8Vdc as the current consumption, which is only 117.3W. However in big bold print at the top of Page 10 in the owner's manual it does state to not use a power supply rated less than 15Adc continuously.

I suspect the rig actually draws more than 8.5A. If the PA is 50% efficient, which is typical for those modules, that alone would draw 7.25A, leaving only 1.25A for all remaining circuits, the cooling fan, backlighting for the panel display and front panel controls, lighting for the microphone key pads, etc. I'm not sure I can believe that.

"Most" 50W output mobile rigs draw more like 10A when running full power; at least all of mine do, and some draw a bit more than that.

Still, a 12A power supply should work, if it's any good; and I might understand Yaesu's emphatic stipulation of using only a 15A minimum rated supply, since many consumer-grade (amateur/CB/stereo/hobbyist) power supplies are overrated and cannot produce their claimed current continuously -- that includes switch mode designs as well as linear ones. Even the "600W" ATX12V type SMPS's commonly found in computers cannot produce 600W continuously without overheating and shutting down to protect themselves (or burning up!).

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