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Author Topic: Power supply ?  (Read 3357 times)
KD2BDG
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Posts: 16




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« on: January 17, 2012, 04:47:47 PM »

I am a bit confused. i am looking at my radio's specs to try and figure out what power supply to buy to use it as a base station. One line says operating voltage:11.7 V DC -15.8 V DC. (negative ground) and then a few lines below that it says Battery supply voltage: 13.8 V DC. What is the difference and how do  i figure this out?

Thanks
73
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N4NYY
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« Reply #1 on: January 17, 2012, 04:55:43 PM »

Standard 12V DC power supplies are regulated at 13.8V DC. You won't find one at 12V. There may be variable power supplies that can go 11-15V. But the standard 13.8V DC is all you will need.

You will likely need a 20-22 amp minimum for a 100W rig.

Now you will find a couple different types. Switchers are very light and portable. Isolated (linear) are heavy and bulk because they have a BA (Big Ass) transformer. However, in my 25 year service experience, the switchers fail much more often, than isolated. Also, I think isolated supplies are fairly easy to repair.

Either way, 20-22 amp is what you need. Mine is an Astron.
« Last Edit: January 17, 2012, 05:00:14 PM by N4NYY » Logged
N4CR
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« Reply #2 on: January 17, 2012, 05:39:22 PM »

I am a bit confused. i am looking at my radio's specs to try and figure out what power supply to buy to use it as a base station. One line says operating voltage:11.7 V DC -15.8 V DC. (negative ground)

This is the range of voltages where the radio can operate safely/normally.

Quote
and then a few lines below that it says Battery supply voltage: 13.8 V DC.

This is the optimum design voltage where all the rest of the specifications were calculated. Such as current draw for receiving and maximum power output.

And 13.8 happens to be the voltage of a battery that is being charged that has reached a full charge without causing excessive gassing. So this voltage that equipment is designed around is based on the maintenance charge voltage of a typical lead acid battery. And this voltage is based on the chemistry between lead and sulphuric acid.

So when you buy a bench supply that is fixed voltage and not variable it will likely be set to 13.8 volts to mimic the classic quiescent charge voltage of a lead acid battery.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
N6AJR
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« Reply #3 on: January 20, 2012, 10:33:41 AM »

You will find that a shack continues to grow after you start it up.  So I reccommend at least a 35 watt Power Supply.

This will give you 22 -25 amps for your 100 watt hf rig, and a spare 10 amps of power available to run a couple of 2m/440 radios and the lights in your meters, and perhaps even charge your cell phone..

The price for a slightly larger PS is a good investment.  In my shack I have a 70 amp PS for the 400 watt 2m and 6 m amps, and a 50 amp  ps for most of the radios, and a couple of 30 amp supplies, one for the 1.2 ghz stuff, and 1 for the 900 mhz stuff. You always need a power supply.  Either kind is fine, switching or linear.

I have used mostly Astron, with a few pyrimids, and samlex supplies, but you can't go wrong with astrons.  You will find they are all rated a bit higher than the useful rating, like a 20 amp supply is rated 20 amps peak, 18 amps sustained draw. So always buy a bit bigger than you need , it is money well spent.
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W6EM
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« Reply #4 on: January 20, 2012, 06:52:44 PM »

You will find that a shack continues to grow after you start it up.  So I reccommend at least a 35 watt Power Supply.

Ahhh, 35 Watts at 13.8V is a tad less than 3 Amperes.  Might work for powering an HT via an adaptor, but that's about all.  You probably meant 35 Amps.

Quote
This will give you 22 -25 amps for your 100 watt hf rig, and a spare 10 amps of power available to run a couple of 2m/440 radios and the lights in your meters, and perhaps even charge your cell phone..

He'd only need to worry about heavy demands from both at the same time if simultaneously transmitting on both.  Receiver demand usually isn't more than an amp or two.  (Buddy in the shack.....for a contest or some such, sure, he'd need it then)

Quote
The price for a slightly larger PS is a good investment.  In my shack I have a 70 amp PS for the 400 watt 2m and 6 m amps, and a 50 amp  ps for most of the radios, and a couple of 30 amp supplies, one for the 1.2 ghz stuff, and 1 for the 900 mhz stuff. You always need a power supply.  Either kind is fine, switching or linear.

I have used mostly Astron, with a few pyrimids, and samlex supplies, but you can't go wrong with astrons.  You will find they are all rated a bit higher than the useful rating, like a 20 amp supply is rated 20 amps peak, 18 amps sustained draw. So always buy a bit bigger than you need , it is money well spent.

Good advice.  However, switchers can, often times, generate HF harmonic noise from their switching circuitry.  Some of the better names designed for ham use don't.  Or, have means to tune the 'birdies' away from your chosen frequency, if operating.  Switchers are more tolerant of AC line over and undervoltage transients than most linears and won't be damaged.  Astrons are well protected from transient over voltages, but Pyramids aren't.  I've repaired and modded enough Pyramids to have seen the results.

For instance, a Pyramid 50A rack mounted supply that I bought new had one AWG 12 insulated wire running from the 8 paralleled regulator transistors to the + output lug.  Great design.  If I ran more than 15A through it, it would be smokin'!!!!!
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NO6L
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« Reply #5 on: January 21, 2012, 12:23:55 AM »

It's actually very easy to estimate the size power supply you need. Divide the peak power out of the transmitter by 6 and obtain the supply that is one size up. Example: 100W / 6 = 17 amps. Get at least 20A. 30+ if you are going to use a full duty mode like RTTY etc.

The reason I use "6" for voltage is that the output stage is around 50% efficient and the voltage is 13.8V. Instead of multiplying the power out by 2, dividing result by 13.8 and adding about 10%, I just remove two steps. This works great for estimating the fuse you may need, too.

And always, always, always make sure the supply has over-voltage protection!

Hope this save folks some time.
73
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AC5UP
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« Reply #6 on: January 21, 2012, 07:23:34 AM »

For instance, a Pyramid 50A rack mounted supply that I bought new had one AWG 12 insulated wire running from the 8 paralleled regulator transistors to the + output lug.  Great design.  If I ran more than 15A through it, it would be smokin'!!!!!

Remember the amount of power a wire can carry is determined by both the diameter and the length of the wire... A thicker wire can carry more current but a shorter wire drops less voltage when both are seeing the same DC load. While a 12 gauge line might look light inside a 50 amp supply, given the short length any voltage drop across that line should be negligible, even at full load. Power = Amps x Volts and if volts across the wire is small, the power dissipated within the wire should be small as well. No Power = No Heat = No Smoke.

Example: 100W / 6 = 17 amps. Get at least 20A. 30+ if you are going to use a full duty mode like RTTY etc.

And sometimes the real trick is reading (or finding) the small print that tells you the " 20 Amp " power supply is rated 12 amps continuously, or 20 amps SSB at 20%/80% RX/TX and 50% modulation with long pauses between words..............   Wink
« Last Edit: January 21, 2012, 03:56:20 PM by AC5UP » Logged

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N4CR
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« Reply #7 on: January 21, 2012, 08:58:35 AM »

Remember that the current capacity of a wire is determined by both the gauge and the length

No it's not. Current capacity of a wire is determined by it's cross sectional area and the material the wire is made of. Heat dissipation may be limited by insulation and conduit so that derates the current capacity.

Length has nothing to do with it. Length has effect on Power delivery and voltage drop, not current capacity.
« Last Edit: January 21, 2012, 09:07:16 AM by N4CR » Logged

73 de N4CR, Phil

Never believe an atom. They make up everything.
KA5IPF
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« Reply #8 on: January 21, 2012, 09:24:53 AM »

Any 13.8v DC power supply will work. Cheap is not good as they are usually not well regulated or filtered. I prefer Astron Supplies. Look at your pocketbook and buy the biggest you can afford, min of 35A. Always buy a metered supply, you may not ever look at them but when working on a radio the first thing I look at is the current draw. And if someone has a transmit problem that's the first thing I ask, "How much current is it drawing?" Nice to see if you're getting to the limits of the supply too, as you add on all the new gear (and you will).

Clif
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AC5UP
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« Reply #9 on: January 21, 2012, 10:10:05 AM »

Length has nothing to do with it. Length has effect on Power delivery and voltage drop, not current capacity.

And that was my point... If a short piece of 12 gauge drops almost no voltage, and it won't, what's the power dissipated by the wire?

When W6EM claimed:
Quote
...If I ran more than 15A through it, it would be smokin'!!!!!
that would be true if there was enough of a voltage drop across the wire to build some wattage at that current. A short piece of 12 gauge is not going to have any significant DC voltage drop at 15 amps otherwise the regulation would go soft near full output.
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W9KDX
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« Reply #10 on: January 21, 2012, 12:26:24 PM »

I have never seen a transceiver that specified whether the required "23 amps" for example, indicated the need for 23 amps continuous or 23 amps ICS rated.  Assuming SSB transmission, and a transceiver that calls for 23 amps, as many of them do, which rating do they expect you to use?  I purchased a 30 amp Astron so I have amps in reserve but it would be helpful to know how much.

Thanks
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Sam
W9KDX
N4CR
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« Reply #11 on: January 21, 2012, 01:59:43 PM »

Length has nothing to do with it. Length has effect on Power delivery and voltage drop, not current capacity.

And that was my point... If a short piece of 12 gauge drops almost no voltage, and it won't, what's the power dissipated by the wire?

You said the current capacity of a wire is determined by it's length. (and also by it's gauge)

Current capacity of a wire has nothing at all to do with it's length.
« Last Edit: January 21, 2012, 02:06:53 PM by N4CR » Logged

73 de N4CR, Phil

Never believe an atom. They make up everything.
AA4PB
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« Reply #12 on: January 21, 2012, 03:35:15 PM »

A longer wire has more resistance. Since P = I*I * R, for a given current the longer wire will indeed dissipate more power (as heat). The thing is that the longer wire also has more surface area over which to disipate that heat so its current carrying capacity is NOT dependent on its length. A 1-foot length of #12 wire with 20A flowing through it will dissipate the same amount of heat as 1-foot of a 100-foot length of #12 wire with 20A flowing through it so the wire will not overheat as it gets longer - unless you wind it into a coil that has a limited air flow. Restricted air flow is why electrical codes require you to derate the current capacity when the wire is inside conduit filled with other current carying conductors.

The length (due to its resistance) may limit the voltage (and therefore the power) that can be delivered to the load for any given supply voltage. That may require the use of larger wire for longer runs but it is not a "current carrying capacity" issue - its a voltage delivered to the load issue and it can vary depending on the requirements of the load. That's why electrical codes can require you to increase the wire size if you have a very long run.


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AC5UP
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« Reply #13 on: January 21, 2012, 04:00:52 PM »

I've edited the post N4CR took issue with... Should have used the word 'power' instead of current.

AA4PB: Thanks for adding to the discussion with your response. It helped me understand the nit that was being picked.
N4CR: Avoid any work that involves politics, sales or public relations.
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W6EM
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« Reply #14 on: January 22, 2012, 09:41:52 PM »

For instance, a Pyramid 50A rack mounted supply that I bought new had one AWG 12 insulated wire running from the 8 paralleled regulator transistors to the + output lug.  Great design.  If I ran more than 15A through it, it would be smokin'!!!!!

Remember the amount of power a wire can carry is determined by both the diameter and the length of the wire... A thicker wire can carry more current but a shorter wire drops less voltage when both are seeing the same DC load. While a 12 gauge line might look light inside a 50 amp supply, given the short length any voltage drop across that line should be negligible, even at full load. Power = Amps x Volts and if volts across the wire is small, the power dissipated within the wire should be small as well. No Power = No Heat = No Smoke.


Actually, the V in your equation is voltage drop across the piece of wire in question and no, it is not negligible for heat dissipation purposes.  Much simpler to understand using I squared R.  Assuming that the regulator senses output voltage at the output terminals and 50A continuous output current.

Oh, I'm remembering a tad.  Resistance of AWG 12 at about 90F is about 1.7 Ohms per 1000 feet.  So, 4 inches worth at 50A is about 1.5W assuming a continuous 50A if my calculator is correct.  And, you'll at least smell the PVC.  Or, more accurately, the anhydrous hydrochloric acid gas that's released will start to deposit green goo on any exposed copper in the neighborhood, like board traces.  Tinned or not, it makes little difference.  Connections don't like green goo......and it's very corrosive to boot.
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