[GreenKeys] Loop power supplies

[Charles]

Remember that resistor power ratings are specified at 25C case temp, which never happens in the real world!
If there is 100 volts being dropped across 2K, that is 50 watts while continuously marking, too.
Bigger is generally better when it comes to power resistors...

Charles
WB3JOK/0

From: drlegendre . 
Sent: Saturday, March 03, 2018 9:38 AM
To: Paul Birkel 
Cc: Green Keys 
Subject: Re: [GreenKeys] Loop power supplies

I made my own 120V / 60mA supply from a cheap, extremely common 28V / 1A control transformer. The key here is to wire the transformer with caps & diodes as a voltage +tripler+ (google it). For me, this yields just a bit over 120V DC which I limit to 60mA with a 2K / 25W resistor. 

Technically, a 10W resistor is OK, but they get awfully hot. I use a 25W Dale in a finned aluminum case which is mounted to the chassis of the supply and even that makes a fair amount of heat. You can also use a 24V transformer. The voltage will be around 105V DC, which is still way, WAY more than enough for the kind of work we do. 

On Sat, Mar 3, 2018 at 3:12 AM, Paul Birkel <pbirkel at gmail.com> wrote:

  A previous thread on the topic suggested a pair of 48VDC supplies in series as being more-than-adequate for use-cases involving local-loops.



  Some inexpensive examples (Hengfu):



  http://www.mpja.com/48V-057A-25W-Power-Supply-Hengfu/productinfo/17325%20PS  ($15)

  http://www.mpja.com/48-Volt-Power-Supply-08A-40W-Switching/productinfo/16004%20PS  ($16)

  http://www.mpja.com/48-Volt-Power-Supply-13A-60W-Switching-Hengfu/productinfo/16009%20PS  ($20)

  http://www.mpja.com/48-Volt-Power-Supply-22A-100W-Switching-Hengfu/productinfo/16016%20PS  ($26.50)



  I have no personal experience to offer.



  From: greenkeys-bounces at mailman.qth.net [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Jordan Spencer Cunningham
  Sent: Saturday, March 03, 2018 3:52 AM
  To: Green Keys
  Subject: [GreenKeys] Loop power supplies



  For those of you who have aftermarket ~120-150v DC power supplies for your loops, can you share where you got them? If something is commercially available at a reasonable price, I will elect that route, but I'm still interested in custom designs. I've read of a very simple one I may try if nothing else.



  I have an original 20ma loop power supply built into my Model 19 table, but I'd like to get more compact versions for mobility's sake. Plus I'd like to be able to operate more than one loop at a time.

  I've been doing research on regulated supplies from parts stores like DigiKey, Jameco, and Mouser, but there's unfortunately not much that meets the necessary criteria that I've been able to find. Acopian makes more obscurely voltaged supplies like this, but not for less than $200.



  Thanks much.



  --Jordan


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