Using a 10a Mini add a circuit for power - possible issues?

[Martinjmpr]

So I'm thinking of trying to install my old 2m mobile into my 4runner before DEIV which starts on Monday the 31st (I'm leaving here Saturday.)

Since the hardest part for me is connecting the power for the radio, I thought I'd try something I saw on another forum which was to use a mini add-a-circuit to the fusebox.

The mini add-a-circuit packaging says its good up to 10A.

Problem is, my radio (an old Kenwood TM261A) is rated at 11A (DOH!)

So here's my question: Would it be risky to run a 15A fuse in this? I know the purpose of the fuse is to protect the other components, but is the difference between 10A and 15A that big a deal? I'd probably use the fuse for the cigarette lighter, which is a 10A rated circuit, my thought being that if it blows, I haven't lost anything important. Is that a good idea or should I find a circuit that's already got a 15A fuse in it and use that?

Has anyone else done this? If the radio is "rated" at 11A does that mean it's always drawing 11A or just when transmitting at full power?


I'm somewhat inexperienced when it comes to electronics so I'd appreciate any input that comes from "real world" experience.

 

[tanglefoot]

The max current rating is while transmitting at full power. If the radio has a "mid" power setting, you could just use the "low" and "mid" and stick with the 10A fuse.

Using a bigger fuse than the original has always been considered a no-no. Sure, it's been done without issue, but it's a "do at your own risk".

I no longer have radios installed in vehicles but for mobile and HF rigs, it's hard to beat the power wires run directly to the battery.

You might borrow or acquire an ammeter so you can see what the actual current draw of the radio is at high power. The ratings are usually somewhat conservative (most radios don't put out quite as much as the brochure says they do, when measured with a power meter). It's possible that the radio doesn't exceed 10A at full-power transmit. Powering a mobile radio from a cig lighter circuit is not the best way to go. Even if you don't blow the fuse, you might run into voltage draw-down issues.

 

[DaveInDenver]

It's not a good idea to just increase fuse values randomly. If the circuit was rated to 15A Toyota would have put a 15A fuse in. Fuses protect wiring and connections so there's probably something in the path that would not like >10A eventually. Often it's the high density plugs in the harness and not the wires, but without going through the whole circuit it's hard to say for sure. Manufacturers tend to do everything they can to use just enough and not have anything more, which costs weight, space and money.

Like tangle mentions, that 11A is full power TX so if you use low and med you could run from a lower than 11A circuit and probably be just fine. And if not, the worst case is you pop a 10A fuse and replace it. There would be a duty cycle aspect to this, transmitting for just a few seconds here and there might not be a problem, but an extended transmission might blow the fuse. Circuit protection devices don't instantly open at exactly their rated value like a light switch. They might never blow with a current just 10% over rating or might take a very long time (that being like 120 seconds or something). If you want to understand this Google 'fuse time-current curve'. Keep in mind that the circuit you're tapping is running other things, so you'd have to be aware of what's on or off. Like it might only blow when the dome light or blinkers are on or you're talking and adjusting the mirrors.

Using a cigarette lighter would work but again, tangle nails it, the voltage drop is likely to be quite high and the radio will run inefficiently. This will actually compound any issues because as voltage goes down to a given load the current will increase. Not necessarily proportionally, but the characteristics change in any case and I'll go on a limb and say you would /definitely/ blow 10A fuses, maybe even at mid power. I would only do this in an emergency. It'll also probably be very noisy with ignition interference and the like.

 

[java]

Id stick with the 10A fuse. Give it a try and see if it works. Im betting it will if the radio is rated at 11.