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Author Topic: can I do this?  (Read 2427 times)
K5BDJ
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Posts: 18




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« on: September 05, 2012, 02:18:33 PM »

Can I run small (5 watts) solid state 2m ham radios on a power supply of 7 amps?    Seems like it would be OK.   
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K2OWK
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Posts: 1037




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« Reply #1 on: September 05, 2012, 04:17:03 PM »

Should work OK if the voltage of the supply is greater then 3/4 volt.

73s

K2OWK
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WB6BYU
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Posts: 13026




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« Reply #2 on: September 05, 2012, 06:52:02 PM »

What does the manual say the current draw is at 5 watts?

Likely you can, since 12V x 7A = 84 watts, which is well beyond what a well-designed
radio would require for 5 watts output.  (1 to 2 amps would be more typical.)  But
a rig designed for higher power being run at a low power setting may require more
than that.

It also depends to some extent what radio you are using:  some of the older HTs
did NOT work well at 12V or higher, as they were designed for 9V battery packs.
In that case you may need an additional voltage regulator to operate the radio.
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KE4DRN
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Posts: 3714




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« Reply #3 on: September 05, 2012, 08:36:43 PM »

hi,

what make and model radio are you going to use?

my older kenwood th-78a can run 12vdc off the
power jack on the radio or using battery packs.

best to check the manual before you try it.

73 james
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KF6QEX
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Posts: 590




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« Reply #4 on: September 05, 2012, 08:58:20 PM »

Should work OK if the voltage of the supply is greater then 3/4 volt.

73s

K2OWK
....and "radios"  < 2    Smiley
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KA4POL
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Posts: 1913




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« Reply #5 on: September 05, 2012, 10:13:38 PM »

This is not just a question of amperage. You definitely have to make sure the voltage is right for your equipment.
If all else fails, read the manual. There you'll find the specification which tells you what you need.
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K1CJS
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Posts: 5875




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« Reply #6 on: September 06, 2012, 05:20:02 AM »

Can I run small (5 watts) solid state 2m ham radios on a power supply of 7 amps?    Seems like it would be OK.   

First, check the voltage that the radio can use.  The best way to do that is to look at the specs in the owners manual.  Second, is the power supply going to supply voltage in the range that the radio can stand? 

If that power supply is a twelve volt supply, and the radio has a cord that will allow it to be plugged into a car power socket (a cigarette lighter plug), yes, you can use the power supply to run the radio--using that cord only.

Many handheld transceivers made today can operate on a wide range of voltages, but if the manual says not to use 12 volts to supply the radio, don't try to.  You'll end up with a nice looking paperweight that resembles a radio.  BTW, don't worry about the amperage.  The radio will only use what it needs, and the power supply will supply only what is required.   
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KA4POL
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Posts: 1913




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« Reply #7 on: September 06, 2012, 05:40:04 AM »

I think the question of voltage has been clearly addressed by now. A rough calculation results in 5W out requiring 10W in. So taking the 13.8V as the voltage you would be below 1A. Or the other way around, with your 7A you could go down as far as about 1.5V. If all else fails read the manual.
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KC9NVP
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Posts: 65




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« Reply #8 on: September 06, 2012, 06:00:21 AM »

K1CIS, I did agree about not looking at the amperage requirements.  The power supply may start out at 12 volts but if rated for less then the radio needs for transmission, you are going to have problems.  The power supply will attempt to supply the current, but the voltage may drop to near zero.  You can run into a similar saturation if the incorrect wire size is used between the power supply/battery and radio.  If 22 gage wire is used, you will see 12 volts at no load (i.e., no current draw), but as soon as you start to draw current, the voltage is going to drop.  If you use 16 or 14 gage wire, the IR (current x resistive) voltage drop would be less, then for the 22 gage wire.  The length of the wire between the power supply/battery and radio also plays into this discussion, the longer the wire the greater the voltage drop for a given current draw.

Still need to look at the radio spec's and see what it requires along with the power supply capabilities to see if the two will work together, along with selecting the proper wire size.

73,
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K5BDJ
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Posts: 18




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« Reply #9 on: September 06, 2012, 07:29:03 AM »

Thanks for all the info.   
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K1CJS
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Posts: 5875




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« Reply #10 on: September 07, 2012, 02:12:01 PM »

....The power supply may start out at 12 volts but if rated for less then the radio needs for transmission, you are going to have problems.  The power supply will attempt to supply the current, but the voltage may drop to near zero....

A five watt rig isn't going to draw 7 amps.  I believe that you need to look more closely at the thread before you respond with an answer that takes things to an opposite extreme.  Even though he isn't saying so, it's all but apparent that he's speaking of a handheld rig or an old 'lunchbox' type rig.  I don't believe the old Gonset communicators drew seven amps either.  73.
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KC9NVP
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Posts: 65




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« Reply #11 on: September 07, 2012, 06:43:58 PM »

someone miss the point of my comment, yes the HT is 5 watts and will not draw 7 amps, but if powered from a wall wart or similar that puts out very little current, good luck on getting very far. 

I recall some kids years ago who tried to use several clip leads made from 24 gage wire to jump start a car and wonder why they kept burning up the wire when they turned the key, they measured 12 volts on the car they were attempting to start, and figured voltage was ok, so everything must be alright.

73
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