How to safely connect a 12v radio to a 12v leisure battery?

Discussion in 'Electrics UK' started by Jul17pri, 1 Sep 2021.

  1. Jul17pri

    Jul17pri

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    Hi

    How can I connect a 12v radio to a 12v leisure battery without damaging it?

    I have:
    - A 12v radio (Roberts Stream 94i)
    - A connector that fits into the radio power socket
    - A length of power cable (2 wires)
    - A soldering iron
    - A 12v leisure battery
    - Terminals to connect the wire to the battery

    The radio's 12v mains power supply is rated at 2.2 amps. A little diagram on the radio case by the socket shows the power supply connector is +ve on the inside and -ve on the outside.

    So I soldered the wire onto the connector, held my breath, and plugged it in. It worked first time!

    However, the second time I connected it up, it didn't work. The radio never worked again, so I replaced it.

    I guess I got the polarity wrong the 2nd time - the wires were both black and not labeled -ve and +ve.

    I though the radio would just draw the power it needed, but maybe the battery amps exceeded the capacity?

    Or maybe a combination incorrect polarity and excessive current. i.e. Maybe the radio was protected against incorrect polarity but only up to the recommended 2.2 amps.

    A) What do you think went wrong?
    b) How can I connect it up while avoiding bricking my radio? Or do I just have to be careful?

    Thanks,
    Jules
     
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  3. bernardgreen

    bernardgreen

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    It does sound like reversed polarity could have been the reason

    One option is to connect a polarised socket to the battery in a way that the polarity of the connection to the battery cannot be reversed/
    Then put a matching plug on the lead to the radio observing the polarity.

    Similar to this set up which I use for temporary lighting. Makes it nigh on impossible for the users to reverse the polarity to the lamp. I did not fit a fuse in the battery lead but to do so would be sensible to protect the battery.
    battery to flood.jpg
     
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  4. ericmark

    ericmark

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    So in the main one would use a 3 amp fuse, however that seems a lot of power, my transceiver does not use that much power. The other point is a 12 volt lead acid battery is normally over 12 volt, on charge up to 14.8 volt, it does depend on the radio, my IC290 would drop off on power output under 13.8 volt, but my FT290 worked the same 10.8 volt to 14.8 volt.

    It says 2.2 amp DC at 12 volt, but it also says use only the adaptor supplied, it does not say it can be powered from a lead acid battery.
     
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  5. AndyPRK

    AndyPRK

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    Wrong polarity sadly
     
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  6. 333rocky333

    333rocky333

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    i would have thought a Fuse was essential for safety, an accidental short across a Battery can be nasty
     
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  7. Jul17pri

    Jul17pri

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    Good suggestion, I'll add a 3 amp fuse, which as I understand it would protect the battery in the event that my circuit or a connected device short-circuits.

    So my battery might deliver over 12v and damage the device? What can I do about that? Can I add a component to limit the voltage to a maximum of 12v? Either way I could use a volt meter to check the battery output, right?

    If the battery outputs less than 12v, it's unlikely to damage the device, but it might not work, right? OK, but is there a component that can input a voltage of 12v +-3 and output exactly 12v?

    If the device says it requires 12v and then doesn't work when you input 12v, that would be strange, but I could live with that, I just don't want to damage the device.
     
  8. Jul17pri

    Jul17pri

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    I'm using a 12v DC plug that fits the radio, and then a similar 12v DC plug & socket to connect/disconnect from the battery without messing with the terminals. I'll also add a switch and a fuse between the socket and the battery. So if I can get it right first time, and never switch the battery terminals by mistake, all will be well.


    However, I've still got to switch the circuit live at some point and if there's a mistake I risk blowing the device.

    e.g. In the top right of your lighting rig photo, where the terminals attach to the battery box, if you connected the red wire to -ve and the black to +ve then the polarity would be reversed.

    I'm currently using a cable with brown and blue wires. Maybe if I use cable with red and black wires then mistaked are less likely to happen.

    But is there a component that blocks the current if the polarity is reversed? If you connect + to + and - to - then current flows as if the component wasn't present, but if you connect + to - and - to + then it breaks the circuit.
     
  9. winston1

    winston1

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    Yes. It is called a diode.
     
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  11. mattylad

    mattylad

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    Put an inline fuse holder in the +v line.

    That's how to safely connect it up, without on it's not safe.
     
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  12. ericmark

    ericmark

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    Turning 16 volt to 12 volt reasonable easy, a 7812 chip can do that, but 11.5 to 14.8 volt to 12 volt not so easy. A normal diode will have around 0.6 volt drop across it, a red LED around 1.2 volt, a white one around 3 volt, but the problem is 2.2 amp is over what you can drop with cheap devices, so you need some thing like this [​IMG]basic idea it turns DC to AC transforms it and then changes back to DC and it alters the mark/space ratio to maintain the voltage.
     
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  13. Colin Brenton

    Colin Brenton

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    Bung a diode in series to prevent any current flowing if you get the polarity wrong?
     
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  14. oldbutnotdead

    oldbutnotdead

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    As @winston1 , put a diode in the line, one of the 1N400x series will work (they cost about a penny each). And will reduce the voltage a bit, winner winner.
    Back in the glory days of CB radio, a lot of the rigs had a diode across the DC input that created a short circuit and blew an internal fuse if supply polarity was reversed. Wonder if your Roberts radio had the same feature ..
     
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  15. mattylad

    mattylad

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  16. Jul17pri

    Jul17pri

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    Awesome! I'll build & test my reverse-polarity-protection with a bulb or volt meter before connecting the radio. As they cost pennies, I'll get some of each type, try a 1N4001 first, and if that blows then try a 1N5401.

    Does the diode go between the fuse and the -ve battery terinal? So if that wire's connected to the +ve by mistake it's the first component in the firing line. Or it can go anywhere in the circuit?

    BTW I found this useful
    https://www.mouser.com/datasheet/lrg/345/1n5400-1n5408-27324.jpg
     
  17. Jul17pri

    Jul17pri

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    Possibly. Hopefully with help from you lot, that'll never happen again, but if it does I'll check for a blown diode before throwing the device in the bin.

    The original radio was bricked, didn't work on it's mains power supply or any other, so I replaced it with a new one.

    The Roberts Stream 94i is the best radio I ever owned BTW, great sound, packed with features, looks ok, reasonable price. The only thing it lacks is pause live radio.
     
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