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Author Topic: Is 12 VDC good enough?  (Read 1026 times)
KC2MMI
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« on: January 30, 2004, 01:42:59 AM »

I'm looking at buying a mobile radio (Yaesu FT-857 or ICOM 706MkIIG) for mobile and home use. In the car, no problem, I've got power wiring for it.

But I'm wondering, will they work OK with a nominal 12VDC power supply (just pulling more amperage) or will they really perform best with a matching 13.8VDC power supply?

Are these radios internally regulated to the nominal 12VDC? Or should I plan on a real 13.8VDC supply for them?

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KC8AXJ
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« Reply #1 on: January 30, 2004, 02:33:20 AM »

Per their spec:

Power supply requirement: 13.8 V ± 15%

That would be 11.73 V at the lowest, so as long as your O.K. with the amps required(20)and hold volatage, I would say it should work.
But I'm no expert.

73
Steve



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KC8AXJ
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« Reply #2 on: January 30, 2004, 02:38:28 AM »

Above is Icom 706

Yaesu:

same voltage but you'll need 22 amps at full power.
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N7NBB
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« Reply #3 on: January 30, 2004, 10:43:00 AM »

The power supply MUST supply the correct AMPERAGE, in your case up to 20 Amps, and also be able to do so CONTINOUSLY if you are into MODES of operation that require LONG transmit cycles (RTTY, SSTV,some DIGITAL MODES, etc). Some supplies ADVERTISE they are 20 amp supplies, but in reality they can only provide 20 amps for a very short period of time, and have LONG duty cycles ( short high current draw (transmit) followed by a LONG low amp draw (receive)

The supply does not have to say "ICOM" or "YAESU" or whatever (a matching supply)... it can be built or sold by another manufacturer.  Most (good) supplies are designed to provide 13.8 at the rated current draw..

Hope that helps some

CAM

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KC2MMI
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« Reply #4 on: January 30, 2004, 12:38:45 PM »

Thanks, Guys. What I didn't say <G> is that there's of couse a bigger picture. I have a kilowatt power supply I built 30 year ago with the "radical" new LM-317 type regulator and eight bootstrap power transistors, which will supply 25A at up to 16VDC all day without any fuss. Except, the only transformer I could find at the time wasn't up to the load and it will fold back from a peak voltage of 17V (no load) pretty soon after 15 amps. Seems like high power transformers are hard to find, and expensive to ship.

Rather than looking for a new transformer for that beast (and it is a rugged well-filtered beast in every other way) I was thinking about using a couple of  computer surplused switching supplies, which is the problem in that they only put out 12V unless you really start kludging them. Still, 20-30A at 12V from a couple of those new supplies will be smaller, cheaper, lighter, more efficient than going with a new analog supply. Which brings me back to wondering, how the new radios like running on 12V instead of 13.8.

I'd expect something like the FT-817 to be internally regulated (since it can run off an even lower voltage internal battery) but if the larger portables are really making use of 13.8V...that's the long story for why I'm looking to see how the 12/13.8 question affects them.

Jared
KC2MMi
WCO3103
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WB2WIK
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« Reply #5 on: January 30, 2004, 01:10:33 PM »

First, I wouldn't try paralleling two PC power supplies; they're not designed for this, won't load share (no remote sensing), and if you use the highly recommended diode OR gate to combine outputs, you'll have far less than 12Vdc, since you'll be adding the voltage drop of the ORing diodes.  (You can use power FETs in lieu of diodes for lower drop, but still have the technical issue of load sharing.)

Also, PC power supplies are specifically designed to be as inexpensive as possible and thus omit any circuitry designed to withstand pulse modulation of the load, which replicates CW or SSB operation.  For continuous carrier modes, like FM, they work far better.  The transient response of every PC power supply I've ever tested stinks.

You might refer to "Technical Correspondence" in QST magazine, the October 2003 issue for a bit more information on this.

WB2WIK/6

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AA4PB
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« Reply #6 on: January 30, 2004, 03:16:36 PM »

I think its pretty unusual to find a "regulated" power supply that puts out 12VDC. Most of them put out 13.8VDC. However, if you have a "regulated" supply with enough current capability that is truly 12VDC it should work. You'll have to be careful about the voltage drop in your connecting cables however because when you start out with only 12V you can't drop much without going too low at the radio during peaks. As an example, a 10-foot run (20-foot of cable down and back) will drop about 1/2 volt when drawing 20 Amps thru it. Use heavy guage wire and keep it as short as possible.

If you are going to purchase a supply, I'd opt for the 13.8V supply.
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KC2MMI
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« Reply #7 on: January 30, 2004, 05:46:08 PM »

WB2WIK - I'd have the same question as you about running computer supplies in parallel but there's an artilce out there saying it works:
"15 VOLTS up to  40 AMPS for  $25By August Hoecker, W8MIA w8mia@antennex.com"

I haven't contacted him to doublecheck that.

AA4PB - Thanks, I'm aware of those issues. The voltage drops with cable size can be critical on boats, where masthead gear may need 150' (75 round trip) of wire to reach the battery, which typically runs from 14.4 with the alternator on down to 11VDC on a bad day with the batteries overdue for a charge. The computer supplies are indeed rated at 12VDC, along with 5VDC supplies. And they're tightly regulated to that voltage.

All of which is a moot point right now, the FT-857 seems to be out of stock all over except for one dealer who's asking a sttep enough price that the rebate isn't so atractive.<G>
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KC2MMI
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« Reply #8 on: February 01, 2004, 02:37:54 PM »

I did a little more legwork, read that article over carefully, and spoke to Yaesu support. The article is primarily suggesting using the +5VDC sections of three computer supplies (sometimes rated 20A on 5V) in series to get 20A 15VDC out of them, as opposed to diode isolating the three 12V sections and using them in parallel, which would result in maybe 11.6VDC after the diode losses.

20A at 15VDC isn't a bad idea...Yaesu points out most of their radios are rated 13.8VDC +-10% or 15% and I'm assuming that just like marine VHF's, the ham radio will produce higher output power when it gets higher input voltage. So 13.8VDC +10% would be 15.18VDC, which means a 15VDC supply would be VERY good to work with. Since computer power supplies can often literally be obtained free from the scrap pile...That's not a bad price for a 20A+ tightly regulated clean 15VDC power source!
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AA4PB
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« Reply #9 on: February 01, 2004, 07:24:04 PM »

Another thing to consider is that most computer power supplys generate a lot of RFI. Many of them also require a minimum load in order to operate. Another thing to consider is how the supply will operate if you get RF into it from the transmitter. Will it go into an over voltage situation. Does it have a crowbar to protect the radio from over voltage if the supply fails.

I guess I have to question the advisibility of spending $700 plus bucks on a new radio and then hooking it to a rig of 3 old computer power supplies in an attempt to save a few bucks. Certainly you can do it if you are careful but there is some degree of risk to the new radio. If you wipe out the radio then you haven't saved any money.
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WB2WIK
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« Reply #10 on: February 02, 2004, 12:52:02 PM »

PC (and most other) power supplies usually have a voltage adjustment, especially on the lower-voltage regulators, e.g., the +5V bus on a PS/2 or ATX type power supply.  If running three +5V supplies in series (which I think is a terrible idea, but what is ham radio without experiments?), I'd crank the +5V down as low as it can go, probably about 4.75V or so, on each power supply first.

Reason: Yes, +15V will probably work, and most FM transmitters will actually develop greater power output at this higher potential, but it's likely they'll wear out faster, too.  Most VHF FM rigs use hybrid PA modules not having great cooling systems (usually just a small copper plate that bolts to the rig's heatsink, which is always undersized for the job), and these can just barely handle their design power, which is based on a +13.8Vdc source.  At +15V, if they run 10% more output power, they'll also be dissipating 10% greater power and getting hotter, especially in the warm weather months when ambients are already likely too hot.

If you "series" PC power supplies, be really careful.  Remember their cases are at ground potential, but the 5V return lines are not, so if you're very careful to do the job correctly, it should work.  Hopefully!

WB2WIK/6
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KC2MMI
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« Reply #11 on: February 02, 2004, 03:56:47 PM »

Minimum load: No problem, covered in the article. That's what pilot lights are for.<G>

RFI: Apparently he's been able to deal with it, he doesn't mention any problems happening. The supplies are fully caged and that leaves the leads, which probably can be dealt with in the usual ways.

Crowbar protection: Yes, a good idea. Simple & cheap enough to add. But with many years of computer experience, I don't recall any systems being damaged by an overvolt failure from the power supplies. I *think* they are already protected and they usually fail by simply failing.

Voltage? Well...Consider the 13.8V rigs are supposed to be used in cars. Most cars run 14.3-14.4 volts, not 13.8, so this really shouldn't be a stretch. And the extra voltage shouldn't matter unless I'm running at full tx gain, should it? As long as I've got the tx gain set down, I'm still going to be using "just enough" power across the finals, whatever the input power voltage is, yes? No?

Considering that I've got several "surplused" PC power supplies on the back shelf, and the first one I grabbed was rated 5v 25A...a free digital power supply would be nice. I'll also be using my analog supply, which will probably saturate out and crump in the 12A/14v region with the existing transformer. But I already know that one is clean & safe, crowbar regulated, and only has the one weakness of "not enough transformer".<G>
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WB4QNG
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« Reply #12 on: February 06, 2004, 10:37:06 PM »

I am like the other guy if you are going to spend $700 on the rig. I would spend another few dollars and buy a power supply. Then you wouldn't have to worry about it. Just my 2 cents worth.
Terry
WB4QNG
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TOASTY
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« Reply #13 on: February 09, 2004, 04:06:53 PM »

I just finished building a powersupply out of 3 old AT psu's.  it works find and it puts out 15.4v.  no problems yet
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