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Watt / Amp questions

Discussion in 'General Technical Questions and Answers' started by KF7UUC, May 4, 2012.

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  1. KF7UUC

    KF7UUC Ham Member

    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. WB2WIK

    WB2WIK Premium Subscriber

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

    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. KF7UUC

    KF7UUC Ham Member

    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. WA9SVD

    WA9SVD Ham Member

    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?:p
     
  5. WA7PRC

    WA7PRC Ham Member

    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. N0AZZ

    N0AZZ Ham Member

    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. WA9SVD

    WA9SVD Ham Member

    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. WA7PRC

    WA7PRC Ham Member

    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. K6ABZ

    K6ABZ Ham Member

    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. WB2WIK

    WB2WIK Premium Subscriber

    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!).
     
  11. WA9SVD

    WA9SVD Ham Member

    I wasn't necessarily stating a NEED for something like a RS-35A/M supply. But if the OP foresees expansion or addition of a HF rig, or additional equipment, it WOULD be a wise investment, rather than having to purchase an additional supply in the future.

    My main point was to emphasize that an RS-35 is only rated for 25 Amperes in continuous operation, and a RS-20 is only rated for 16 Amperes continuous operation, which is just barely able to support the radio in question, since FM is a mode that requires continuous current drain in transmit.
    Sorry you misunderstood the main thrust of my post.

    Certainly, switch mode supplies are an option, but not all of them are "RF silent." Reputable manufacturers certainly should be able to produce switch mode supplies that don't cause interference, but there are some lower quality manufacturers and products that do cause horrendous interference. IMHO, one of the greatest liabilities of the switch-mode supplies is the proprietary nature of the circuits and components. With most linear supplies (e.g. Astron,) the schematics and components are "off the shelf" should repair ever be required.
     
  12. WB2WIK

    WB2WIK Premium Subscriber

    With SMPS's it's a bit of a dice roll unless you buy a laboratory grade product.

    MANY small SMPS's violate FCC Part 15 requirements even though they carry the label; but for hams it's worse than that because even good SMPS's that meet Part 15 requirements very well can generate enough noise for our sensitive receivers to easily hear. We're listening way down below the Part 15 Class (B) certification threshold.

    I'm on a SONY VAIO laptop right now that has its matching SONY-brand "brick" power supply/charger which I can hear from 100 yards away on HF. I'll bet it meets Part 15 okay, it's just too noisy to run near sensitive receivers. And it only outputs about 50W. Larger power supplies can often generate more noise.

    In my large server in the garage there are two 620W SMPS's running 24/7 and I can't tell they're even running...however those are expensive, well-shielded and well-filtered supplies. The pair cost almost $500. Nobody would pay that much for a home PC supply.

    For the shack, I stick with old fashioned Astron linear units which are not only quiet but also cheap and easy to repair should they ever fail. Other than the transformer, there's nothing in there that costs more than $3.
     
  13. WA7PRC

    WA7PRC Ham Member

    I'm sorry you mistakenly think that.
     
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