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Author Topic: Astron RS12A enough for a 65W mobile?  (Read 1658 times)
NEVBEN
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« on: November 23, 2017, 01:11:30 AM »

I have a Astron RS12A and a Kenwood TM281A.  The manual for the 281 indicates it can use up to 14A.  That seems high.  Do I really need an RS20?  Ammeter is showing 11.47A transmitting, but I don't know if that's the limit of the Kenwood or the Astron.  I do want full power when I need it.  I don't need it continuously.
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K9YLI
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« Reply #1 on: November 23, 2017, 07:23:02 AM »

If the book  says  14 amps  then thats required on  high power.
An analog meter may be slow to respond to  current changes and show low.

also is the astron rated  ICAS  or continuous draw.   ICAS  may be pushing it with  FM.


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AA4PB
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« Reply #2 on: November 23, 2017, 07:45:53 AM »

The RS12 is rated for 12A peak and 9A continuous. It is not large enough for reliable operation of your transceiver at full output FM.
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Bob  AA4PB
Garrisonville, VA
K0UA
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« Reply #3 on: November 23, 2017, 07:56:21 AM »

Obviously not enough power supply.  Also in the future and in general terms, never run up against maximum ratings. Always, always run well under maximum ratings. For example if your 100 watt class radio say it draws 22 amps, you would be much better off having  a 30 or even 35 amp rated power supply even though you might "get by" with a 25.  Good engineering practice requires always having a "fade margin" or reserve capacity.  People are always trying to "get by".  Getting by is never a good idea in any facet of life. It will bite you in the arse sooner or later. 
A 12 amp power supply is a good match for a 25 watt class radio.
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W1VT
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« Reply #4 on: November 23, 2017, 08:04:57 AM »

I would use the Kenwood at the low power 25 watt setting until a larger power supply could be obtained for high power operation.
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G8YMW
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« Reply #5 on: November 23, 2017, 10:07:43 AM »

I have a Astron RS12A and a Kenwood TM281A.  The manual for the 281 indicates it can use up to 14A.  That seems high.  Do I really need an RS20?  Ammeter is showing 11.47A transmitting, but I don't know if that's the limit of the Kenwood or the Astron.  I do want full power when I need it.  I don't need it continuously.

That is voltage "sag" due to the load.
As others have said, throttle back on the rig's output until you get a bigger PSU
BTW Ignore the peak rating, ALWAYS go on the continuous rating, in other words this is a 9 amp psu but I would allow a bit more.
Your minimum requirement is 15 amps continuous but I would go for a minimum of 20 amps but then if you can afford a bigger psu, buy it.

Carry on running your setup like that, you will put the psu into Apache Mode, you will start sending smoke signals
« Last Edit: November 23, 2017, 10:13:27 AM by G8YMW » Logged

73 de Tony
Windows 10:  Making me profane since March 2017
K5LXP
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« Reply #6 on: November 23, 2017, 10:53:37 AM »


Ammeter is showing 11.47A transmitting

I'll be the one to dissent here.  What is the voltage under load?  If it's in regulation then I wouldn't sweat "when I need it" at all.  Even if it's starting to sag a bit under that load I'd still say good enough for "when I need it" as long as you're not dipping below the minimum input voltage for the radio .  This isn't a continuous duty application and this won't hurt a thing "once in a while".  As others have said it's plenty for mid or low power.  Maybe look around for a 20A + supply and upgrade when it's convenient but I'd run it for now, knowing that you're maxed out and might have a bit extra ripple "when I need it". 

Mark K5LXP
Albuquerque, NM
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AA4PB
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« Reply #7 on: November 23, 2017, 05:45:33 PM »

Mark, the supply is rated for 9A continuous. If he overheats the pass transistors then one of them will probably fail SHORTED. The power supply will then attempt to output 20V or more. Yes, the SCR is supposed to protect the radio by blowing a fuse in the power supply BUT I've personally had the SCR fail too and blow a bunch of transistors in the radio. Personally I would not recommend running an Astron power supply beyond the factory load ratings. Not even on a temporary basis.

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Bob  AA4PB
Garrisonville, VA
NEVBEN
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Posts: 98




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« Reply #8 on: November 23, 2017, 07:34:31 PM »

The Astron is supplying 13.99 or 14.0 V in receive.  Transmitting at full power, it is sagging to 13.85V.  I did not adjust it.

Because of the measurements, I believe the RS12A is adequate for this configuration, but only just adequate and only for the "50%" duty cycle of it's 12A rating.  So I think Mark's assessment is accurate, but I appreciate the sentiment of the other comments.  This isn't really where I want to be, and I feel the need to replace it with an RS20A.  Because of the light duty of this transceiver and the option to run reduced power, I don't think it's a risk for imminent failure, but I'll get a bigger PSU to address this issue soon.  I would rather be where the PSU is well within it's continuous rating and I've got enough headroom that I don't have to worry if conditions change unexpectedly.
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K5LXP
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« Reply #9 on: November 23, 2017, 09:46:41 PM »

If he overheats the pass transistors then one of them will probably fail SHORTED.

I don't think *over* heating is possible in "typical" FM operation, say 60 or 90 second transmissions with a 50% or less duty cycle.  ICAS is 5 minutes at a 50% duty cycle, and this current level isn't that far out of spec for this supply.  Stressed perhaps, but not to failure.  It's not like a fault current that would cause a spike in junction temperature and shorting failure.  Ambient conditions will play a role as well as actual duty cycle but "once in a while" I believe this supply can put up with it reliably.

As a data point, I have a 2M rig I use for simplex set at mid power running on an astron 7 that drops down to around 12V key down.  It's been running this way about 8 years now.

Mark K5LXP
Albuqueruque, NM
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M0GVZ
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Posts: 343




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« Reply #10 on: November 24, 2017, 05:08:54 PM »

Just run it at 25W there really is absolutely no benefit in running it at full power over 25W other than seeing the needle go up higher on a SWR/Power meter. The other end won't be able to tell the difference between mid and full power.
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W8VVE
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« Reply #11 on: December 06, 2017, 02:23:10 PM »

If you are interested in a RS20A send me an email...which you can find in
QRZ.  Price is $40 plus shipping.  73 Sam W8VVE
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