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PSU question; PSU capacity
Topic Started: Wednesday, 13. December 2017, 00:48 (544 Views)
Dipolestaffs
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Ok I must be being stupid, if so feel free to point this out!

Take, for example, something like an at5555 ssb transceiver capable of transmitting 20w

Now loads of psu vendors are suggesting you need at least 8 amp psu's to run this...

Now 8 amps @ 13.8v is in the region of 105w

Now the manufacturers specs on the transceiver suggest it doesn't draw anything like that even transmitting, and most of the time it's recieving so it's a fraction of that.


I haven't measured the consumption but some basic maths tells me it won't be drawing more than about 40w peak unless the design is pants.

So why do the vendors suggest 8/10 amp psu's rather than Say 5 amp?

Are the psu's so 'optamistically' rated as to question their whole build ( bought a 5 amp one recently that chucked out so much RF it killed reception on a radio Inthe same room running off a battery!)


Obviously a bigger PSU gives headroom etc and allows running more kit latter etc...but is it actually necessary?


Opinions please! ( And yes use a big battery is starting to appeal as psu's are starting to look a bit marginal if you can't trust the ratings)
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vendee85
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26 Charlie Tango 2201

Hi. Patrick.

I use a Battery in the car an exT5 75ah. I only charge it once a week & I'm usually on 3/4 nets + DX a week. I
use a KL203p amp. I have too as my car is all Electric i.e. Doors/G/box/Steering a Pug 1007. At my Qth. I use a
30A linear power supply for all my Radio's HF/PMR/CB. Don't get a switch mode if you only use 11m. they cause
loads of problems with RF. like IT/Drones/Models/Household gadgets. You could use a battery @ your Qth with
a Battery charger if it's not switch mode & will do 10A +. Good Luck Hope this help's. 73s Mike 2201 [Yeovil].
:pirate3:


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Smokey Jay
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Dipolestaffs
Wednesday, 13. December 2017, 00:48
Ok I must be being stupid, if so feel free to point this out!

Take, for example, something like an at5555 ssb transceiver capable of transmitting 20w

Now loads of psu vendors are suggesting you need at least 8 amp psu's to run this...

Now 8 amps @ 13.8v is in the region of 105w

Now the manufacturers specs on the transceiver suggest it doesn't draw anything like that even transmitting, and most of the time it's recieving so it's a fraction of that.


I haven't measured the consumption but some basic maths tells me it won't be drawing more than about 40w peak unless the design is pants.

So why do the vendors suggest 8/10 amp psu's rather than Say 5 amp?

Are the psu's so 'optamistically' rated as to question their whole build ( bought a 5 amp one recently that chucked out so much RF it killed reception on a radio Inthe same room running off a battery!)


Obviously a bigger PSU gives headroom etc and allows running more kit latter etc...but is it actually necessary?


Opinions please! ( And yes use a big battery is starting to appeal as psu's are starting to look a bit marginal if you can't trust the ratings)
Your maths formula is correct but you are overlooking that fact that no amplifier (external, or within the radio is 100% efficient)
So if you radio is drawing 8 amp @ 13.8 v, then perhaps only 50% of it is resulting in RF out.
The rest goes to warming you shack up with dissipated heat etc!

I don't know what the power output of the 5555, but as a measure most HF radio's require 18 -20 amps to put out 100 watts

J


Update: Just looked up the spec for your radio> Input voltage 13.8 21w / 6amp

So If you don't want to run your PSU at full load all the the time (which you don't) then an 8 amp would be better than a 6 amp.
Edited by Smokey Jay, Wednesday, 13. December 2017, 16:19.
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time machine
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163 Charlie Tango 169

I Have measured the current drain on full power with a very similar radio using my fluke multimeter in series and the current drain was around 7 amps. Always best to use a psu with quite a lot more current output capability at least 25% more in my opinion.
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Ste
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Also bear in mind if purchasing a power supply to allow for future expansion. Ive ended up with the HF rig, Belcom, and a 2M set to be run from the same supply, and if going for a power supply a linear supply is always going to be quieter than switched mode.

second hand units do occasionally appear on Ebay, and the odd bargain is out there,
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Dipolestaffs
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Thanks guys, similar wasn't going nuts! I would always oversize a supply (background in electronics) and am currently running from a battery ...though I believe that may well! Give a reduction in transmission power due to the voltage being about a volt lower.

Thinking a 110ah battery and a 100w solar panel might be a fun and green way to go.....for the amount of time I get to operate....
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Ste
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Dipolestaffs
Tuesday, 2. January 2018, 22:58
Thanks guys, similar wasn't going nuts! I would always oversize a supply (background in electronics) and am currently running from a battery ...though I believe that may well! Give a reduction in transmission power due to the voltage being about a volt lower.

Thinking a 110ah battery and a 100w solar panel might be a fun and green way to go.....for the amount of time I get to operate....
hows about this

https://www.ebay.co.uk/itm/B-N-O-S-13-8-Vdc-25-Amp-Linear-CB-Ham-Amateur-radio-power-supply/232617336530?hash=item362912ded2:g:0roAAOSwUlxaMF-6
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Dipolestaffs
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Thanks guys, need to put some thought into it as considering doing my foundation license this year so more capacity would be a good idea, plus at the moment getting truely awfully reception home based so think I need to sort antenna issues first, or decide if have to go mobile ...in which case a power supply will become a bit redundant....
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Dipolestaffs
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Update : stumbled upon a 40amp toroidal PSU on Facebook locally(isn't) for 40.....seems to do the job :thumbs: thanks for the advice guys
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_amw_
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26 Charlie Tango 2742

In my experience of these things PSU manufacturers are rather optimistic in their claims, so it's best to view some of their ratings with a degree of skepticism, espeically when it comes to peak vs. continuous current ratings.

Also, transmitting using FM or AM will cause a continuous, full power load on your power supply even if you are not speaking, where as SSB the load will fluctuate along with the peaks of the modulating signal (your voice).

Either way, if you are concerned about the rating of your supply then connect a voltmeter across the supply and then start transmitting into a dummy load. You will see the supply voltage drope a bit - if that drop is more than a 100 millivolts (0.1 volt) or so then that is a good sign an power supply upgrade might be a good idea. NOTE: if you are doing this test using SSB you will also need to blow into the mic or whistle to get the true reading.
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AlphaPapa
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26 Charlie Tango 2966

Generally, I would say if radio manufacturer says 5 amp draw on transmit, you could probably add 50%. If a power supply manufacturer says 5-7 amps you could probably estimate it at a stable 3-4 amps. I would recommend if a radio says 5 amp draw on TX, I wouldn't go lower than 10-12 amp on a PSU.

In fact the difference in cost between a 10 amp psu and a 20 amp PSU is not so great. Go for the best power supply you can afford then at least you will have plenty of headroom, especially if you move on to bigger and better things. In many ways your power supply is the most important piece of kit you can buy.
Edited by AlphaPapa, Monday, 5. March 2018, 14:08.
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