WFCO "mystery" - branch LED4 stays on, even though fuse replaced.

[rickst29]

I blew the fuse with a momentary short (while "optimizing" my multi-way fridge with an external DC On/Off/On switch. Oops. Was careful with my hands, but not ALL of the loose contacts.) The title says nearly everything I know, except for one additional "mystery" - the hot side of that branch circuit shows a tiny bit of voltage - roughly 0.26 VDC. I don't know what to make of the non-zero value. The new fuse looks perfect, and I'll test it for continuity tomorrow AM.

I've also pulled AC power, and the Battery cable to let everything "quiet down" before plugging AC back in. But LED4 turns back on, and the same tiny voltage reading. re-appears. (All other DC branch circuits have full voltage.) I thought that the WFCO LED tested for continuity across the fuse - and with a good fuse, it should go out. Incidentally, I have some slightly sloppy wiring which treats a couple of ground wires as "common" DC grounds. I'll SWAG that doesn't perform any "GFCI-like" testing for leakage on it's individual secondary DC circuits. Am I wrong?

So, I suspect that I've blown something in that branch circuit of the WFCO fuse board, and the 0.26 Volts is some sort of downstream "leakage" from the LED or blown WFCO components.

I could try a couple of tests:

(1) Pull the Fuse Board (everything cold, this time!) and re-wire to one of the empty, unlit LED fuse slot near the bottom. See if it works with current 20A fuse.

(1b) Rewire as in #1, but (if it comes up), subsequently replace the 20A fuse with the smallest fuse I've got in that form factor (probably 5A), and see if it blows in a no-load configuration of accessories. If 5A doesn't blow, then I might be able to test for amps in-line - but not sure if I should bother,because a "short circuit" which can only accept something less than 5A would not be taking the voltage differential on "open" wires all the way down to 0.26 VDC.

anything else?

[ZekenSpider]

Just a guess without being there.

I think the new fuse is also "blown" or maybe it "blew" as soon as you plugged the AC in because there is still a short on circuit 4. It is common for the "hot" side of a blown fuse wire (or any "open" line) to show a small amount of voltage. This is especially noticeable when using a digital voltmeter that has high input impedance because the meter doesn't put a significant load on the circuit. It is caused by the hot AC circuits near the "open" wire inducing a small voltage into the open wire.

If you are familiar with using your multimeter, try to check the resistance to ground of your circuit 4 with the fuse removed. If it is shorted, it will be less than .5 ohms. If it is open (ie with nothing turned on like the refrig, fan or radio) it should be near infinity (greater than 500,000 ohms).

[QUOTE]Incidentally, I have some slightly sloppy wiring which treats a couple of ground wires as "common" DC grounds. I'll SWAG that doesn't perform any "GFCI-like" testing for leakage on it's individual secondary DC circuits. Am I wrong?[QUOTE]

There is no "GFCI-like" testing performed on the DC circuits by the WFCO.

Just a guess.

Jerry

[rickst29]

Originally Posted by ZekenSpider

Just a guess without being there.

I think the new fuse is also "blown" or maybe it "blew" as soon as you plugged the AC in because there is still a short on circuit 4.

Resistance between the legs of the "new" fuse is zero (it's good).

If I jump between fuse holder legs 4 and 5 (closest to the LEDs), with both fuses removed, then LED4 goes out. If this is the "secondary" side of the fuses, I would have expected any short circuit (along circuit 4) to still have it's full effect effect (i.e. short circuit downstream) on circuit 5, when they are jumper'd together (still no voltage for the LED, because it all gets leaked to ground at short-circuit in Secondary wiring).

If this is the "primary" side of fuses, then WFCO circuit 4 isn't getting +12V unless I "give it some" from an alternate circuit. That would imply WFCO circuit for LED is failed, inside WFCO.

Measuring resistance hot->ground on some "behind the fridge" wires next ...

[rickst29]

I've got "infinite" ohms on all hot wires (PID controller power and relay +control pins, and power wires into the DC power Relay and Selector switch.

The only non-infinite resistance lead is from the selector switch into the actual fridge heater, and that's right where it is supposed to be - a bit less than 2 ohms.

Next Step, I think: pull all the power, and then rewire the fuse distribution board (move "circuit 4" into an empty, unused circuit slot). First, however, I'll need to buy a couple of 20A fuses - right now, I'm one short.

[ZekenSpider]

Sounds like you are on the right track. Maybe check for a bad solder joint or burned trace on the circuit board at Fuse 4 (If you can get access to it)?

Jerry

P.S. I would think the blown fuse LEDs all need 24 VDC on the + buss side and a path to ground (24-) on the load side to get the energy to illuminate. LED 4 must have a path to ground that disappears when you short 4 to 5. What am I missing?

[rickst29]

I'm going to ignore it, unless my Fridge fails to cool down while running on PID-driven DC power.

And that seems extremely unlikely: When I activate the PID-controlled power circuit (using my external ON/OFF/ON switch, the WFCO fan turns on. And when I switch to one of the two other positions (both equivalent to "OFF" when DC/Battery has not been selected at Norcold dial), the fan goes off. I'm not going to bother with re-wire of the Secondary Circuit into a different (empty) fuse link, it's a bit involved. I think that the original fault blew some component on the "bad fuse!" LED circuit, and nothing more than that.

BTW, you were correct about "nothing GFI-like on the DC side". All of the current-carrying neutral wires are connected to a common screw-down bus bar.