[GreenKeys] Fusing Requirements for Loop (Model 26)?

Thu Sep 19 01:14:49 EDT 2019

Eric-- regarding your 32VDC fuses:

While an AC fuse can handle high DC voltages just fine, it's when the fuse blows that the DC rating comes into play. It's easy to keep a AC current from arcing due to the frequency at which its polarity changes, so a blown fuse element can still have its severed halves somewhat close together without a problem. However, since DC current is just plowing one direction, it's prone to arcing (I'm sure someone else here can give a much more scientific explanation than I). So a 32VDC fuse handling 120VDC that blows will almost certainly arc, continuing to provide power to the circuit it was supposed to arrest, just more haphazardly.

I've had a hard time finding a fuse online that indicates any DC voltage ratings at all, which is why I went to the local electronics shop (which burned down a few months later-- talk about blue smoke!). What I understand is that cartridge fuses rated for any appreciable DC voltage will have some kind of spring element so that both ends will pull outward if blown so as to keep an arc from forming. I'm no expert, though.

Now that I've hunted around on Mouser a bit more, I see a few promising cartridges with DC ratings starting at 140VDC on up, depending on what sweet spot you want to hit (the search below includes 80, 100, and 125mA):

https://www.mouser.com/Circuit-Protection/Fuses/Cartridge-Fuses/_/N-ba8h0?P=1z0x7lfZ1z0x7l0Z1z0x65qZ1yo195qZ1yo19ldZ1yzs9nvZ1yx4arc

DigiKey did not have the same selection, and I had lost all hope. I should shop Mouser more often-- I keep finding things I need there that DigiKey doesn't have.

-
Jordan Spencer Cunningham
teletype.net

On Wed, Sep 18, 2019, at 10:32 PM, E. wrote:
> 
> I’ve used these fuses:
> 
> https://www.mouser.com/ProductDetail/littelfuse/0312062mxp/?qs=RpX4E3T4PRatEZ%252baqtg6Cg%3D%3D&countrycode=US&currencycode=USD
> 
> …with this cheap bench supply:
> 
> https://www.circuitspecialists.com/benchtop-power-supply-csi12001x.html
> 
> …using these fuse holders:
> 
> https://www.autozone.com/electrical-and-lighting/fuse-and-accessories/bussmann-agc-fuse-holder/32421_0_0
> 
> …without blowing a fuse. Just noticed that the maximum rated voltage value for those fuse holders is 32vdc… but up to 250vac. Too tired to think about it…..
> 
> 
> > On Sep 18, 2019, at 10:44 pm, hwhall at compuserve.com wrote:
> > 
> > Fie upon yon picofuses. Verily, thou must desolderest and resolderest when anon they givest up ye magik smoke. Leavest thou said Pico to yon de Gallo.
> > 
> > The local surplus store (still hasn't gone down but is taking on water, fast) has literally hundreds of pounds of 3AG & similar in wide range of values. Like the song said, "I can get it for you wholesale."
> > 
> > Wayne
> > WB4OGM
> > 
> > -----Original Message-----
> > From: Ralph Mowery <rmowery28146 at earthlink.net>
> > To: 'Zach Zarzycki' <shardsofaperture at gmail.com>; greenkeys <greenkeys at mailman.qth.net>
> > Sent: Wed, Sep 18, 2019 9:25 pm
> > Subject: Re: [GreenKeys] Fusing Requirements for Loop (Model 26)?
> > 
> > They are easy to find if you know what to look for.
> > 
> > Look for PICO fuses. There are plenty on ebay. Here is some 125 ma which is probably what you may want to start at. If you try using some 60 some ma fuses they will probably blow like popcorn.
> > 
> > https://www.ebay.com/itm/QTY-20-125A-1-8A-125V-AXIAL-PICOFUSE-VERY-FAST-ACTING-275-125M-LITTELFUSE/121665969044?hash=item1c53db5b94:g:cXMAAOSwpdpVa0jM:sc:USPSFirstClass!28146!US!-1
> > 
> > Here are you r American brand name 63 ma fuses if you insist in trying.
> > 
> > https://www.ebay.com/itm/FUSE-BUSS-NEW-GMA-63-R-5-X-20-mm-FAST-ACTING-63-ma-125-volts-box-pack-of-5/163011534976?epid=9018371927&hash=item25f43ea080:g:lX8AAOSwFE9a2~Md
> > 
> > Here are some rated for 250 volts
> > 
> > 
> > https://www.ebay.com/itm/LOT-OF-5-NEW-Lf-S-T63mAL250V-LITTELFUSE-FUSES-250V-5mm-x-20mm-T-63mA-SLOW-BLOW/400849456673?hash=item5d547d4e21:g:LLAAAOSwm8VUw1Xu
> > 
> > Ralph ku4pt
> > 
> > 
> > From: greenkeys-bounces at mailman.qth.net [mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Zach Zarzycki
> > Sent: Wednesday, September 18, 2019 9:43 PM
> > To: greenkeys at mailman.qth.net
> > Subject: [GreenKeys] Fusing Requirements for Loop (Model 26)?
> > 
> > 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'm not trying to burn up my magnet, and don't really know if say 100mA fuse would work or if the magnets are so delicate that I should just keep looking for that "right" fuse. 
> > 
> > DC fuses are as in previous conversations, a pain to find and not nearly as plentiful as the blade fuses or glass fuses.
> > 
> > 
> > Thanks,
> > 
> > Zach - W8ZZW
> > 
> > Sent from my Android BlackBerry KEY2
> > 
> > Virus-free. www.avast.com
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