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Author Topic: Linear Power Supply  (Read 7021 times)
KB1PXU
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Posts: 33




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« on: June 30, 2012, 04:34:58 PM »

I need to buy a linear power supply for the shack. I have decided on an Astron.  At the same price my preference is for the Astron VS-70m which is a 70 amp variable power supply with meters.  Alternatively if I don't buy this I would purchase the Astron RS-70m fixed power supply with meters.  They are both within a few dollars of each other.  I can't think of any reasons not to buy the variable power supply; however, am I overlooking something?  Are there reasons why I should not buy a variable power supply?

73
Will
KB1PXU 
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WB2WIK
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« Reply #1 on: June 30, 2012, 06:10:05 PM »

Not really, other than to save a few dollars.

The RS-70M has an internal adjustment for voltage (I have two of them).  The VS-70M brings that adjustment up to the front panel.

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AC5UP
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« Reply #2 on: June 30, 2012, 06:10:56 PM »

Are there reasons why I should not buy a variable power supply?

Probably because you don't need the variable output feature..... Is there any reason why you think you'd need to set the supply for 9.7 volts @ 12.6 amperes?

Unless you have a specific need, a power supply is generally a set-it-and-forget-it sort of device. The fixed voltage model is also adjustable (internally) should you decide 14.0 volts is more fun that 13.8 VDC, but in the overall scheme of things having a "12 volt" power supply that's always set where it ought to be isn't a bad thing......
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KB1PXU
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« Reply #3 on: June 30, 2012, 07:08:33 PM »

thanks for the comments
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WB6DGN
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« Reply #4 on: June 30, 2012, 08:09:27 PM »

Basically second AC5UP's comment.  A high current supply rarely needs the voltage changed unless you're setting it to the float voltage value for a different make or type of standby battery and the internal adjustment will take care of that if needed.  That said, the external current limit control can be worth its weight in gold when working on a troublesome piece of equipment that may be in danger of "self-destructing" if unlimited current is available at its power connector.  With that control, you can advance the available current slowly until you are certain that the unit is functioning properly; often saving a trace or two on the unit's PC board.  So, while variable voltage is mostly unnecessary, the ability to limit the current available does, at least in my opinion, favor the variable supply.  Just get in the habit of checking those meters BEFORE connecting equipment to be sure that the voltage hasn't changed to an unsafe value.
Tom
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AC5UP
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« Reply #5 on: June 30, 2012, 09:53:28 PM »

...the external current limit control can be worth its weight in gold when working on a troublesome piece of equipment that may be in danger of "self-destructing" if unlimited current is available at its power connector.

True, but even with a non-adjustable supply an automotive brake lamp or headlamp can be wired in series as a current limiter. Kinda' like the old boatanker technique of running the AC through a 100 watt incandescent lamp in series with the line cord as a way of testing for a shorted transformer / rectifier / filter condenser when you notice your latest swapmeet acquisition has a vaguely smoky smell to it, and I'm not talking about tobacco..........     Grin
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TANAKASAN
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« Reply #6 on: July 01, 2012, 04:39:40 AM »

Don't hams build things like power supplies any more?

Tanakasan
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W6EM
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« Reply #7 on: July 01, 2012, 07:36:10 PM »

In constructing projects, a variable supply is useful.  Especially so if you want to test the limits of satisfactory operation.

On the operating side, though, an incidental twist to the voltage control while dusting the shack with the supply off might go unnoticed.  If you then turn things on and don't notice, maybe you'll be shipping your gear off to have a shorted voltage suppressor diode replaced.  Or, something more expensive.  Ouch.
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WB6DGN
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« Reply #8 on: July 01, 2012, 08:35:33 PM »

Quote
Don't hams build things like power supplies any more?

You'd have a hard time buying the parts for what a good used Astron costs.  All mine are "basket cases" that I refurbished or just repaired; usually lightning hits so this is as close to "building" as I've gotten.  Bought a trashed RM-60M for $20.00 recently,  all the expensive parts are good and get the metal work "free".  Hard to beat.
Tom
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W0BTU
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« Reply #9 on: July 01, 2012, 09:42:58 PM »

Quote
Don't hams build things like power supplies any more?

You'd have a hard time buying the parts for what a good used Astron costs. 

That may be true, but you could sure build a much better one than an Astron. Take a good look at the issues with Astron supplies at:

http://www.repeater-builder.com/astron/astron-index.html
http://www.repeater-builder.com/astron/astron-intro-stuff.html
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K7MH
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« Reply #10 on: July 01, 2012, 11:45:52 PM »

Do you really need a 70 amp power supply?
You won't be moving it around a lot!

I have been using an Astron RS-20M for over 15 years now with no problems.
The light in the switch even still works.
It is under the desk. I turn it on and off with my foot.
The meters are not all that worthwhile. I have hardly ever looked at them.
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K8AXW
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« Reply #11 on: July 02, 2012, 08:00:31 AM »

Quote
Don't hams build things like power supplies any more?

Yes, I have but no more.  Reason for building initially was money.  Didn't have any!  But, I did have a nice junk box and learned how to wind transformer secondaries on an old TV transformer.  It was a variable voltage supply and I still use it on the bench.

The second was also a variable voltage supply for a breadboard and I also still use it.

The third was a heavy duty supply, fixed voltage set by a pot that would supply 13.8V @ 50+ amps and the meter wouldn't even flicker!

This supply used a voltage regulator PCB that I Chinese copied from the Astron.  It had overvoltage protection accurate and repeatable to within 1 volt.  Circuit for this came from a QST article and used a chip from Motorola.  Again, the reason for building this beast was a large junkbox and no money. (10 of the overvoltage chips were provided free from Motorola)

Otherwise, as another said here, you can't buy the parts as cheap as you can buy a new Astron.  Plus the Astron will look nice.   Grin
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W6EM
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« Reply #12 on: July 02, 2012, 05:04:08 PM »

Quote
Don't hams build things like power supplies any more?

You'd have a hard time buying the parts for what a good used Astron costs. 

That may be true, but you could sure build a much better one than an Astron. Take a good look at the issues with Astron supplies at:

http://www.repeater-builder.com/astron/astron-index.html
http://www.repeater-builder.com/astron/astron-intro-stuff.html


Repeater-builder has a collection of 'improvements' to the Astron design.  Most notably are the suggested changeouts of the 2N3055s and 2N3771s.  Astron tends to make people think their ratings are quasi-continous, but they aren't.  By replacing the series pass transistors with 2N5686 devices, it really stiffens the supplies.

But, be prepared to pay a high price per 2N5686 transistor to do so.

As has been said, the cost and appearance of home-brewed supplies can leave a lot to be desired.  A sick Astron or Pyramid can usually be repaired and rebuilt to better than original condition.

Lee
W6EM
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N4CR
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« Reply #13 on: July 02, 2012, 05:20:44 PM »

I have repaired a lot of Astron power supplies and the worst thing I find on them is that the thermal compound is dried out. If you want one to last long, you need to clean off and reapply the thermal compound every few years. If you don't they really aren't anywhere near their stated rating.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
KG4NEL
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Posts: 443




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« Reply #14 on: July 03, 2012, 06:27:47 AM »

Do you really need a 70 amp power supply?
You won't be moving it around a lot!

Yeah, but deadlifting one is a good way to save on a gym membership. Smiley
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