Jocklandjohn Posted May 29, 2020 Share Posted May 29, 2020 (edited) Just fitted a small 4-way fuse box in the back of the 110. Took a line off it to a small USB power socket I'm fitting - the kind that has a built-in volt meter. Despite having no fuse in the 4-way I'm getting 11.5V at the USB socket. Put the fuse in the holder on the 4-way and I get 12.7V. Metering any of the four outlet tabs on the 4-way shows voltage (11.5 or 12.8) fuse out/fuse in respectively, and the same with the meter at the USB socket. I'm not any kind of expert but I assumed that a circuit minus a fuse should be dead - 0V? Do I have a problem? Edited May 29, 2020 by Jocklandjohn Quote Link to comment Share on other sites More sharing options...
hurbie Posted May 29, 2020 Share Posted May 29, 2020 where did you get the feed from ? direct from the battery , and how about the earth ? Quote Link to comment Share on other sites More sharing options...
Jocklandjohn Posted May 29, 2020 Author Share Posted May 29, 2020 (edited) Feed comes from a main distribution board which has switched & unswitched feeds (so not directly connected to the battery). Its on the switched side so only active when ignition is on. Its a one-wire 4-way box (live connection only) and I;ve securely earthed it to the chassis at the rear. I've never seen a fuse box that passes current witout a fuse - what it does have though and which may be significant is some led's that illuminate should a fuse blow - would that circuit account for this? Edited May 29, 2020 by Jocklandjohn Quote Link to comment Share on other sites More sharing options...
Peaklander Posted May 29, 2020 Share Posted May 29, 2020 (edited) It’s because without the fuse you’d normally have an open circuit, but in this case even without the fuse, the led is there allowing current to flow and you therefore have voltage on the output side of the fuse connection. The led is in the feed but it’s normally short circuited by the fuse and so no current flows through it. With the fuse out or blown, the led can pass current and therefore illuminate. You then see the lower voltage as the led drops a little when it conducts. Edit: should add that the usb socket is providing enough load to provide current through the led but I assume not enough to illuminate it. Edited May 29, 2020 by Peaklander 1 Quote Link to comment Share on other sites More sharing options...
Jocklandjohn Posted May 29, 2020 Author Share Posted May 29, 2020 Aha - so not abnormal then just a 'design feature'! Never come across that before - but I did wonder how the led was fed. Is this any less 'safe' than a non-led open circuit? Quote Link to comment Share on other sites More sharing options...
Peaklander Posted May 30, 2020 Share Posted May 30, 2020 There must be a resistor in each led circuit and that will limit the current that flows through the led and so through the load (your usb / voltmeter) when the fuse has blown. Quote Link to comment Share on other sites More sharing options...
Jocklandjohn Posted May 30, 2020 Author Share Posted May 30, 2020 Ok thanks for that, was concerned it was defective in some way. I live and learn! Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.