Opus 120, 12V line from battery, what size fuse?

**01black_ac** · 03-27-2006, 08:20 PM

What should the Amperage be on the fuse that sits on the 12V line coming directly off the battery and is connected to the Opus 12V yellow line?

Also, is 10G wire the way to go from the battery to the trunk, or will 12G suffice?

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**Specimen** · 03-27-2006, 09:33 PM

I thought the yellow line was from the ignition switch. Are you not using the ignition on feature, or are you running a 12v line through a manual switch to the opus ps to manually power on and off the computer?

I have a 10 amp fuse going from my bat to the red +12V wire (12 Gauge). A fuse is not really needed for the Ignition on/off yellow wire since the current draw from the ignition on/off input is very small. If you do put a fuse on that, I doubt it will draw more than 1 amp. I have 18 gauge going into my Opus ignition on/off Yellow wire (18 gauge).

**01black_ac** · 03-27-2006, 10:08 PM

On the Opus 120, the yellow wire gets connected to the 12V Battery+ terminal. The red wire is the ACC ON Ignition wire.

I plan to put a fuse (but what Amperage?) on the 12V line somewhere very close to the battery itself. I also plan to use a manual switch on the 12V yellow line in those instances when I plan to drive a short distance, like just pulling the car out of the garage to wash it. With a manual switch on the 12V yellow line, I'll be able to temporarily override the ignition startup functionality.

But perhaps you're confusing the Opus with another PSU (Your sig says you're using the M1-ATX)? The schematic for the Opus 120 clearly shows the Battery Positive terminal gets connected to the Opus yellow line. The red is ignition.

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**hugeness** · 03-28-2006, 04:24 AM

I can't remember where I read it, but I think you want a 20 amp fuse... That's what I ended up installing.

**Miles** · 03-28-2006, 02:57 PM

Since Power (in Watts) = Voltage (V) x Current (A)

Not familiar with the Opus 120, but assuming it's 120W, then Current = Power / Volt = 120W / 12V ~= 10A

For my M2-ATX, it has an on-board 15A fuse, and I have also installed an inline 15A fuse a few inches from the battery.

**01black_ac** · 03-28-2006, 04:51 PM

Yeah, that's what I don't quite understand... The Opus 120 is 120W, which would be 10 Amps running from the battery, as you stated. When I spoke with the tech support guy at mp3car.com earlier today, he recommended a 10-12Amp fuse on the 12V line close to the battery. However, the onboard fuse on the Opus is 15Amp, so why would the fuse on the 12V line *to* the Opus be 10-12 Amps? .

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**Miles** · 03-28-2006, 06:01 PM

Yeah, the 15A opus fuse really wouldn't do anything then with a 10-12A near the battery. The opus can probably safely handle 'up to' 15A (ie. it's over-engineered to that spec) but anything beyond, who knows..

You can just use a 10A fuse, see if it blows regularly, then up it.. better lower than higher when it comes to fuse

**Specimen** · 03-28-2006, 06:37 PM

My sig says M1-ATX cause I switched from the Opus 150. 2 years ago I was using the Opus 150 with a 10 amp fuse and it worked fine so I’m familiar with the opus. Just looked at the Opus 120 and it seems that they've changed the wiring color and some specs. Sorry for the confusion but the older models have it the way I described.

Seems that they still use the 15 amp blade though in the opus so a 10-12 amp is fine. You’d rather have the fuse blow at the battery terminal than have to replace the blade in the opus. A 20 amp blade stated above would be useless. As for the wire gauges, 12 for the power and gnd and 18 for the ignition on. They state that in that manual so you should be fine with the 12 gauge.

**Specimen** · 03-28-2006, 07:53 PM

This is why the 12 amp fuse is fine to use. The PS can supply 120 watts to the computer this is computed with the rails and there max amps

3.3V @ 8A max = 26.4 Watts max
5V @ 8A max = 40 Watts max
12V @ 3.5A max = 42 Watts max
-12V @ .5A max = 6 watts max
5V @ 1.5A max = 7.5 watts max

Total 121.9 Watts MAX (They just say 120 to be safe)

Now the spec says that the PS is 91% efficient. Worst case scenario the PS is going to need an extra 9% more power than 121.9 Watts (10.971 Watts) to run itself. This means that the PS is going to use 132.871 Watts MAX to provide ~120 Watts to the computer. Divide 132.871 by the 12V line in from the battery, you are going to get 11.07258A draw from the battery. So for the worst case and MAX wattage, the most you will draw is ~11 Amps on a constant 12V coming in. It would be better to use a 10 Amp that way if it blows you know your getting close to the MAX Wattage. The reason why they use a 15A fuse is to compensate for fluctuating Voltage and to protect the electronics. The electronics might blow if they receive > 15A.

So why don’t they use a 12A Blade? Since the operating battery voltage is not always at 12V, the 15A fuse allows for voltages to go lower than 12V if you are using all your 120 Watts. (132.871/15 = ~8.8 Volts)

I hope this clears up your questions. If by all means I completely jacked up the math or I’m completely wrong please post, I don’t want to give the info.

Cheers,

**01black_ac** · 03-28-2006, 08:56 PM

That was a very helpful post. Thanks for that.

You think I'd be okay running a 10G wire for the Opus 12V, rather than a 12G? I have about 18ft of 10G wire and was thinking of using that instead of 12G.

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