[GreenKeys] Powering a Model 15 currentl loop from a 5VDC 250mA power supply - theory

[ldesoto at comcast.net]

As long as you stick to 0805 parts and larger, it isn't hard to build SM projects by hand.  Even 0603 parts aren't that bad.  I've built plenty of stuff by hand using a microscope and a fine tipped iron.    The smallest parts I've ever done by hand are 16-pin resistor packs with 0.5 mm pitch.  The whole chip is only 4 mm long by 1.5 mm wide and they were a real pain to do. 



One really nice way to make things is to have a solder stencil made and use solder paste.  The only real problem is the solder paste is expensive and has a finite shelf life.  The stencils can be made from your Gerber's and is about $100 or so.  You don't really need a computer controlled oven if you're careful about things and you have a way to monitor the temperature.  Some of the stencil manufacturers will give you a telltale that you put on the board where you can see it.  When it melts, your board is done. 



I like the stencils but they're bit on the pricey side for one-offs.  If you're making a small 5 to 10 unit prototype run, they're well worth the price. 



Larry 






----- Original Message ----- 
From: "John Nagle" <nagle at animats.com> 
To: greenkeys at mailman.qth.net 
Sent: Thursday, May 7, 2009 8:36:39 AM GMT -08:00 US/Canada Pacific 
Subject: Re: [GreenKeys] Powering a Model 15 currentl loop from a 5VDC 250mA power supply - theory 

W2HX wrote: 
> How many devices in series do you think your model could handle? 
> Just curious 
> 
> 73 Eugene W2HX 

    One.  It has to be matched to the load.  It's possible to 
build ones for other loads, of course.  You'd need to increase 
C2 and change out the LT3484-1 for a LT-3484-0, which has 
a higher current limit.   You'd have to adjust R4 for a higher 
holding voltage (probably double it), and increase V1 to maybe 7V or so. 
If you want automatic load matching, that costs extra. 

    My goal was to run a Teletype from a USB laptop without much 
extra gear.  Multiple machines would go on multiple USB ports. 

    Does anybody on here build surface mount, with the little 
computer-controlled toaster oven? 

                                John Nagle 
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