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Author Topic: Supply voltage lower than 13.8 V for FT 817ND  (Read 3626 times)
RADIOPATEL
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Posts: 16




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« on: June 01, 2012, 04:06:12 AM »

Dear Elmers

I could not find answer in other forums hence putting my query here.

I have a FT 817ND radio. I wish to operate this radio with diode in series for reverse voltage protection. During field opration with 12V batt the voltage available to radio shall be about 11.2 to 11.4.
My Query

Shall I operate this Radio with  such voltage lower than 13.8.

Thanks in anticipation.

Regards

Dinesh Patel - VU2DCI
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K2DC
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« Reply #1 on: June 01, 2012, 04:55:54 AM »

Dinesh,

   Well, it may or may not work well.  The manual for the 817ND says the supply voltage should be 13.8 +/- 15%, which means it should work just fine down to 11.7V.  Some rigs really don't like running with low supply voltages.  I've heard a number of mobile stations attempt to turn off the engine and run on the 12V battery, with several of them tending to FM quite a bit.  But I doubt you could do any damage, so go ahead and give it a try.

73,

Don, K2DC
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KA4POL
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Posts: 2125




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« Reply #2 on: June 01, 2012, 05:17:16 AM »

The manual says Supply voltage 'Operating' 8.0 - 16.0 V whatever they mean by operating. There is also an NiMH battery pack that only is specified as 9,6V. So obviously no problems with even lower voltages down to 8V.
At 2A on transmit you could use a Schottky diode which only brings a voltage drop of about 0.3V.
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WX7G
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« Reply #3 on: June 01, 2012, 05:36:19 AM »

When operating on the internal battery pack (low voltage) the FT-817 drop the RF power to one-half. When powered externally with low voltage you can set the RF power to one-half to make sure the PA is always linear. Or simply turn down the mic gain a bit.
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G3RZP
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« Reply #4 on: June 01, 2012, 06:26:26 AM »

Or use a suitable fuse - say 25 or 30 amp - and a 50 amp diode across the supply leads. Reverse biased in normal use, of course.
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K5LXP
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« Reply #5 on: June 01, 2012, 07:08:49 AM »

Use something other than a diode for your reverse voltage protection.  In many applications, a single PFET is all you need.  Or key your power connection so that a reverse connection isn't accidentally possible.

Mark K5LXP
Albuquerque, NM


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W8JX
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« Reply #6 on: June 01, 2012, 07:48:52 AM »

Use something other than a diode for your reverse voltage protection.  In many applications, a single PFET is all you need.  Or key your power connection so that a reverse connection isn't accidentally possible.

Mark K5LXP
Albuquerque, NM


External diode protection is a wise idea and fail safe with that rig as it is easily damaged.  While radio has reverse polarity protection with internal power it does not for external power. A properly keyed connector does not guard against someone hooking up leads to power source wrong.

As far as voltage drop, I would not give it a second thought. You might loose a little output power but it will not be seen down range.
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AA4PB
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« Reply #7 on: June 01, 2012, 08:14:30 AM »

The problem with the diode drop is that it limits how low you can take the external battery. For example, if the minimum voltage spec is 11.7V and you have a silicon diode with 0.7V drop then you can't discharge the battery below 12.4V.

With larger radios, power dissipation and heat becomes an issue. A diode with 0.7V of drop and 20A flowing through it is dissipating 14 watts of power.

I like the idea of a fuse in series with and a diode across the power line. There is no voltage drop - the downside is that you'd better carry some spare fuses. I've often used a zener diode to provide both over-voltage and reverse polarity protection. For QRP level radios I've sometimes used a polyfuse which automatically resets itself when the fault is corrected.
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KB3HG
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Posts: 404




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« Reply #8 on: June 01, 2012, 10:28:45 AM »

Another way.
Series is one way,  what about parallel ? Cathode of a diode to the red wire or Positive line Anode tide to the Black wire or negative. The Red lead should have a fuse  between the diode and the source, The diode should have a rating larger than the fuse, you want to blow the fuse not the diode. For that matter if you have room in the radio put the diode in the radio. No voltage drop to speak of.

Tom Kb3hg



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VA7CPC
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« Reply #9 on: June 01, 2012, 08:11:11 PM »

Many FT-817 users run off batteries at less than 12 volts.

While the rig _automatically_ goes to 1/2 power when it detects low supply voltage, it's possible to run it at full power (5 watts) off the internal battery by adjusting the PWR menu item.   That gives about 9 volts during transmit, if I remember right.  Nobody has complained about my audio.

It's a very forgiving rig.

       Charles

PS -- there's an FT-817 group on Yahoo Groups that will have more expertise than I do, and the archives are very useful.
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VE7RWN
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Posts: 73




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« Reply #10 on: June 02, 2012, 12:02:26 AM »

I use a conditioner kit on mine from Phil Silas, AD5X,
See http://www.ad5x.com/images/Articles/Condx817RevA.pdf
It addresses a number of concerns including reverse polarity.
I have run my 817 from a variety of sources down to about 8.5 volts without issue.

rob.
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STAYVERTICAL
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Posts: 875




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« Reply #11 on: June 02, 2012, 12:26:47 AM »

Hi,

I use my FT817ND with 10V or less all the time on an external PSU - it works fine.
The FT817ND is very forgiving of external PSU voltage.

The forward voltage drop will not be much of a problem at all, but be careful, under transmitting operation with a couple of Amps
going through the diode, it can get quite hot. For example, 3Amps x 0.7V  = 2.1Watts dissipated in a small package.
Also, you will need to use a 3 Amp rated diode to be safe, but voltage is no problem.

By all means use a series diode, but another system is to put in a crowbar type of protection.
If you put a power diode across the + and - leads, such that it is reverse biased when the polarity is correct,
and add a fast blow 3 Amp fuse between the diode and the PSU, it will act as a crowbar protective circuit.
The diode will conduct if the wires are reversed and draw a large current which should blow the fuse.
The advantage of this system is that it will cause no voltage drop or dissipate any appreciable power, unless the polarity is reversed.
One danger is however, that if the fuse does not blow, and the diode burns out you will still be at risk.

Another system is to get a bridge power rectifier package, and put the + and - outputs to the rig, and the A.C. input pins  to the PSU.
With this system, no matter which way you put the leads on the PSU, the correct polarity will still be passed to the rig.

The series diode is probably the easiest, and from memory I think the FT897D has this protection, since I once mistakenly reversed
voltage and it did nothing. Thinking the worst, I found it was fine, and checking the schematic think I saw a protective diode.

Good, luck and check twice - 73 Rob.

« Last Edit: June 02, 2012, 12:29:02 AM by STAYVERTICAL » Logged
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