[GreenKeys] Fusing Requirements for Loop Model 26

[Jordan Spencer Cunningham]

How forgiving are selector magnets? Has anyone stress tested them? Maybe even Teletype themselves? 

I had an unfused loop burn out an optoisolator in a prototype board and get up to something like 150 - 200mA. My 15's magnets started smoking. But since it all happened so quickly and I shut off power when I realized what was happening, I may not have measured the current properly, but I thought I glimpsed something in that range. Also, the melted component in question was rated for 150mA. The other component on the board that did NOT burn out was rated for 200mA, I believe. So the current had to be somewhere in that range, and I know the magnets were getting dangerously hot.

A local electronics shop provided some 63mA fuses that I used in the circuit after that. The shop said they were rated for DC, and the filament apppears to be the spring-type that's supposed to pull the ends apart to avoid DC arcing should it blow. And the fuses have been working well for over a year.

I also sourced some 63ma picofuses from ebay, but I have not at all tested them. I don't even remember their voltage ratings now.

-
Jordan Spencer Cunningham
teletype.net


On Wed, Sep 18, 2019, at 8:47 PM, Jim Cooper wrote:
> On 18 Sep 2019 at 21:44, Zach Zarzycki wrote:
> 
> > I'm going to be setting up my model
> > 26 with a laptop power supply, eBay
> > boost converter (boost to 97v)
> > resistor for my loop supply (60mA) and
> > the usb/tty board (Thanks
> > Jordan!) I'm trying to find a good
> > supply of the 63mA dc fuses to run
> > this, 
> 
> I think if you ever, ever found a 63mA DC fuse
> it would blow on the first mark transition, due 
> to surge current ... I'm sure an 1/8 A fuse would 
> do just fine, maybe even a 1/4 A ... the selector 
> magnets are a lot more forgiving than transistors !! 
> 
> w2jc
> 
> 
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