[GreenKeys] Loop power supplies

Sat Mar 3 11:00:01 EST 2018

@Charles

You're using the wrong figures also.. not sure where the 100V is coming
from in your equation. The two factors we know are the supply voltage
(120V) and the desired current (60mA). To calculate power use I^2 * R. So
0.60(a) ^2 * 120(v) = 7.2(w).

So 60mA (I) across a 2K (R) resistor will dissipate 7.2W (P) power. This
rounds to a 10W resistor, which as I mentioned, still runs too hot for my
tastes. I use a 25W part.

On Sat, Mar 3, 2018 at 9:53 AM, Charles <charlesmorris800 at centurytel.net>
wrote:

> OOPS, I screwed up... 100^2 / 2k is only five watts. Sorry, haven’t had my
> morning coffee [image: Smile]
> Charles
>
> *From:* Charles <charlesmorris800 at centurytel.net>
> *Sent:* Saturday, March 03, 2018 9:46 AM
> *To:* drlegendre . <drlegendre at gmail.com> ; Paul Birkel
> <pbirkel at gmail.com>
> *Cc:* Green Keys <greenkeys at mailman.qth.net>
> *Subject:* Re: [GreenKeys] Loop power supplies
>
> 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 . <drlegendre at gmail.com>
> *Sent:* Saturday, March 03, 2018 9:38 AM
> *To:* Paul Birkel <pbirkel at gmail.com>
> *Cc:* Green Keys <greenkeys at mailman.qth.net>
> *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/product
>> info/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 mail
>> man.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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>
>
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