Which fuse to use?

Interactives

·hello, I currently am working on a project that controlls several strips of LEDs via BS2. The stamp is sending signals to a ULN2803A, which is firing several relays at different times, which are conecting the LED strips to a seperate power source. I got the power source with the LEDs, and it outputs24VDC, 20W max, but I've had to connect the 120 line to it myself, via a creepy and dangerous looking plug, cut off of some old something or another. I was thinking that an in line fuse would help reduce the danger of having 120 current at exposed terminals in my enclosure. I figure if something goes wrong, then the fuse blows, and if I'm shocked, it's only for a second- much less lethal, but still... my question is about choosing the right fuse. How will I know what current is being used, and how much more than that should the fuse be rated at? Plus, I'm using a plastic enclosure, so there's no place for a chassis ground connection. What should I do with the ground wire? Would a isolation transformer help reduce the danger here. But most importantly : which fuse. Please help- I dont like fires. Thanks!

HavoKane

use a GFI (Ground Fault Interrupt)

it will save your life if something goes wrong

they sell them at home-depot,

only downside is theyre build into power-outlets,
so youd have to wire the power-outlet in-line (after) your PSU, before your circuitry.

hope that helps

Orion

First thing I would do is replace the "creepy and dangerous looking plug". Find a cord cap @ Home depot or where ever, there less than $10.

Second a fuse will not blow when the power is going through your body. Don't think that because you have a fuse it will be any safer in that respect. Plug your device in a GFI outlet if you want protection against that.

As for a fuse a .2 A fast acting should do the job. Measure with a meter and see what you device is using, and then pick a fuse a hair bigger. Maybe 1.2 times your actual draw.

Interactives

GFI- sounds good, I just looked them up on McMaster-carr (the have everything) and there was one surface mount GFI that is listed at $58! I guess its really worth it when you think of all that could happen- then I noticed that they have some GFI adapters, that you plug into any three pronged outlet. They were much cheaper ($18). Do you think that would offer the same protection? I'm plugging this monstrosity into a fused power strip, so it mat be overkill, but I'd rather·err on the safe side.·Also, what's a PSU?

Interactives

Thanks Orion! since the stamp is sending out several signals, and the relays turn on and progressively draw more power, I was kind of worried that the power surges might trip a fast fuse. I guess I'll know once I try. What's a cord cap? Any advice on the dangling ground wire?

Orion

Cord cap is the part that plugs into the outlet, attaches the wires to the three prongs. As far as the GFI keep in mind if you place it on the PCB, it will not protect anything before it. Your best bet may be to find one that is part of and extension cord. Something like this - http://www.aquaticeco.com/index.cfm/fuseaction/product.detail/iid/11571/cid/3010 or http://www.nh-di.org/special_dizzyderby022105.htm. Cut the female end off and feed your project with that. This way you have a good plug and GFI protection. Your talking of a "creepy and dangerous looking plug"·is bringing nightmares of old appliance plugs that were ceramic with frayed cloth covered wires. The ground, if your case is plastic there is not much to tie it too. I assume you have a metal transformer or power supply, you should attach it to that.

Post Edited (Orion) : 4/29/2005 1:06:12 AM GMT