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Thread: Current limit circuit needed

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    Default Current limit circuit needed

    I'm after some help with a circuit for a current limiter ..
    What I'm trying to do is charge a battery (slowly) in a trailer by tapping into the car parker light wire .. As the battery is not used a lot but is a car battery and the voltage has the potential of going flat I would like to limit the current to 2 amps max, no matter the output voltage .. I think the Parker lights should handle an extra 2 amps as the trailer has all leds .. I'm not smart enough to work out all the part numbers although there are a few circuits on the web .. Can someone draw me a circuit with all parts & values that I can pick cheap from JayCar ..



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    Why not do the job properly and use a typical dual battery set up.
    You will need an extra cable to the trailer, but it will be worth it.
    See this months Silicon Chip if you want build your own dual battery controller.

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    6 and 7 pin trailer sockets should have an auxiliary fused 12V supply available but whether it's been wired in is another matter. A typical setup involves a voltage detecting relay that connects the trailer battery to the vehicle wiring only when the voltage indicates the motor is running (to prevent the trailer battery discharging back into into the vehicle). Off-road shops should be able to advise on these.

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    The 2 replies so far are not suitable for my use as it's not a camper trailer .. It's a tipper trailer with 12 volt hydraulic pump and it's own battery & don't get tipped up very often .. As my car has neither the permanent power wired to the plug nor do I have extra power wired on separate plugs (the same as some other cars that will tow this trailer) I guessed the limited power off the parkers while driving would keep the power up as required .. Just by telling others to drive with lights on while towing the trailer .. To me its less circuitry .. Idiot proof (no fuse to blow ) and wired on & into the trailer so no need to modify other cars .. Still open for suggestions ..

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    I don't think wiring it to the parker circuit will work out well for a couple of reasons
    IE won't that be energising those lamps regardless of dash switch setting?
    maybe underestimating the rating of that wiring considering the worst case battery demand too (but the proposed 2A limit addresses that)

    How many pins are on the trailer plug and does it have any unused? (should be parker, brake, turn and maybe reversing light plus ground at least)

    Edit: the standard arrangement puts auxilliary supply on pin 6 of a 6 or 7 pin socket, reversing light is pin 7, 6 or 7 pin sockets are round, common 5 pin is flat and no spare pins.
    Last edited by Skepticist; 11-07-19 at 09:53 PM.

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    Maybe this is what you need.


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    You could try a Little DC PWM Controller from Fleabay.
    They can be set to pretty much whatever current you want up to the max they will pass which is more than you need. I'd start with an amp meter and the car running as that will give the the highest amps. You wouldn't need a charge controller because the battery will only be able to get to the max voltage of the car it's attached to which will be regulated by it's alternator. As long as the thing isn't towed by a 24V truck it will be fine.

    You should be able to run a lot more than 2A. have a look at the fuses on the car and Pull out the parker fuse and measure the current with an amp meter with the trailer attached. Whatever that is up to the rating of the fuse less an amp or 2 for good measure is what you can give the battery.

    You can also adapt an Lm317 to go from a voltage to a current regulator but that may be a bit iffy in this application or use a Mosfet.
    If you want a packaged application, You should be able to get a DC PWM for about $6 and set that up on the trailer to do what you want. Set the thing, glue the control in place, house it in a waterproof box with some silicone and you should be sweet.

    You should note that trying to charge a car battery at 2A will take a long time, like 24 Hours long. Car batteries left to go flat also have a short life. This would be fine for maintaining the battery when the trailer is in use but you really need the battery to be kept charged in between. Letting a battery go flat and sit for any period then charging it up and letting it sit again will kill even a new battery VERY quick.
    If you can't put a regular AC charger on it, I'd be getting a regular House Solar panel and a DC converter and plugging the trailer into that. The panel could sit ontop of or inside the trailer and even if shaded, would provide enough grunt to keep the thing well and happy.

    A solar controller probably WON"T do what you want.
    The cheapy PWM ones ( and they are ALL PWM under $50 no matter what the description or the label on the unit says!) won't handle a 24v+ Input and a 12 V battery.
    Depending on the panel and the number of cells, You may be able to open the junction box at the back and tap off 2 of the Diodes instead of using all 3 or even one may be enough depending on the panel voltage and put that to a cheap controller. You'll get lower amps but 1-2 amps will be all you need. Again check with a Multimeter. If you are hooking to a 12V battery then any voltage into the controller up to 18V is OK. Don't go over that though.

    There are plenty of Voltage controllers on fleabay with wide enough inputs for what you want to go direct.

    I'm not a fan of those solar battery maintainers. They are all over priced in my tightarse opinion but more importantly, a great many do not have the output to maintain a battery at all. They will allow it to just discharge a little more slowly. Anything under an amp rating I wouldn't bother with because solar and marketing being what it is, you'd be lucky to get half or less of that into the battery and that's probably going to be marginal in a lot of cases.

    I have an old panel on my Dads shed roof Hooked to some cheap controllers and that keeps 2 Mowers, a chipper, a splitter, a tractor and a small hatchback for his wood wagon all topped up nicely. They all have 2 Pin connectors wired to the battery which can be disconnected easily without damage ( when he forgets and drives off) and are isolated from each other.
    MUCH cheaper than those plug in battery maintainers he was using before that which were about $90 each and crapped out after a few years.

    Only thing he dosen't like but has got used to is the setup dosen't have red and green lights so he can see it's working.
    I told him, if the thing starts and you haven't used it in 3 months, it's working. What more you need to know? :0)

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    It's a tipper trailer with 12 volt hydraulic pump and it's own battery & don't get tipped up very often
    Given that scenario, would it not be reasonable to expect that a full charged battery would be able to tip the trailer several times before re-charging became necessary. Would it not be most simple just to store the battery on a trickle charge at home, an not worry about chraging on the road?
    I'm out of my mind, but feel free to leave a message...

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    To charge a battery, you need a higher voltage (13V-14.2V) than the battery rated volts. You would be lucky to get much more than 12.5V at the trailer light circuit with anything more than a few hundred milliamps drain. This will never charge your battery, no matter what current you have available.

    Skepticist is on the right track, 7-pin domestic trailer plugs use pin 5 for electric 'service' brakes. A dedicated wire would need to be run, if the vehicles service brake circuit cannot be isolated, or is missing. This would be the best way to get power from a voltage-sensitive relay near the start battery, that only turns on when the start battery is being charged. Then a current/voltage adjustable buck-boost module in the hydraulics box would turn whatever voltage it had available, to a decent voltage (14V) that may actually charge the battery.

    A cheap and cheerful system uses a standard 12V relay that uses the ignition power to connect the auxiliary circuit. That way, the motor is probably running while the aux circuit is connected.

    A small solar charger would probably work, but you may lose valuable current/time in the regulation process. We're only talking about charging a couple of amps while the vehicle is running, not much for a 40-60AH car battery, less for a bigger battery.


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    I'll try and reply to each answer although I think people are overthinking the whole thing .. A simple 2 transistor circuit with all values would be fine.
    Skepticist : unless the transistors allow power to pass from the trailer battery backwards through the current limiter then no it shouldn't turn the lights on unexpectedly .. part 2 you answered yourself as the 2 amp limit covers the wiring limit .. part 3 I have already explained why I don't want to use the extra pins and put a full time active at the plug ..
    LoopyLoo : yes I have 1 of those thigs on there at the moment but it would take a lot of them to make 2 amps
    George65 : I have not seen a PWM that can variable current limit .. I'm using 12 volt (car) to 12 volt (trailer) The battery is charged and If I notice it getting low I can charge it with a normal charger but I'm not the only one that uses the trailer .. As for putting a large panel in the trailer it would not look good after driving a bobcat over it or covering it with several ton of dirt .. I have room for a panel on top of the control box but I haven't yet found a panel that will fit the lid ..
    lsemmens : the trailer is used 2 or 3 times a week so a constant method of charge for many different cars is what I'm after .. Just turning the parkers on seemed so easy and it would just keep thigs topped up as required ..
    Thanks All for trying anyway

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    Onefella : is this what you had in mind .. it looks like it could work but I don't know if it will lift the voltage aswell as current limit ..

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    Quote Originally Posted by Sambo2 View Post
    Onefella : is this what you had in mind .. it looks like it could work but I don't know if it will lift the voltage aswell as current limit ..

    That one only steps the voltage down. The link in my post was for an adjustable buck-boost module. It can supply a higher (or lower) voltage than it receives, and has adjustable current limiting.

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    Quote Originally Posted by Sambo2 View Post
    George65 : I have not seen a PWM that can variable current limit .
    Then what do the ones you have seen Vary? All the ones I have used deliver full voltage at varying times ( Hz) so if on a motor, torque is maintained but speed is reduced.
    I was using one on a panel today for just such a charging job.

    As for putting a large panel in the trailer it would not look good after driving a bobcat over it or covering it with several ton of dirt .. I have room for a panel on top of the control box but I haven't yet found a panel that will fit the lid ..
    Ahhh, I meant to keep the battery topped up while it was sitting round and then removed before use....

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    Just use one of these they state 5 amps but on my test bench they only pass around 2.5 amps peak. Should be fine for what you need.

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    Quote Originally Posted by carjackma View Post
    Just use one of these they state 5 amps but on my test bench they only pass around 2.5 amps peak.
    Interesting but Typical. I was looking at those the other week for something but went for a different higher capacity version. Lucky I did.

    Should have known that anything Chinese would be over rated. They just slap a number on things that has no semblance to reality and call it good.
    Thanks for the heads up.

    You could spray a steaming turd gold and sell it as bullion and fleabay wouldn't give a damn or do anything for the person that got ripped off long as they got their seller fees.

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    Quote Originally Posted by carjackma View Post
    Just use one of these they state 5 amps but on my test bench they only pass around 2.5 amps peak. Should be fine for what you need.
    To get the correct performance from these, you need to add a low esr cap in parallel with the existing cap.
    The existing cannot handle the ripple and lets the output sag.

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    Quote Originally Posted by Reschs View Post
    To get the correct performance from these, you need to add a low esr cap in parallel with the existing cap.
    The existing cannot handle the ripple and lets the output sag.
    Which again would be completely typical of the Chinese Mentality.
    Anything to save half a cent even if it means the product does not work properly or will fail quickly. According to normal practice, in this case if they made the thing handle the 5 amps through using the proper cap, they could advertise it as being good for 10A!

    Can they not get it through their heads that people wouldn't give a shit if they had to pay a dollar more for something if it meant the thing worked as it should and was dependable.

    Saving 2C here or a Dollar there is NOT what drives the purchasing decision to start with.

    I just bought 20x 555 Timer chips for $1.20. I'll wait to see if they actually work because I cannot believe they can do them that cheap. I was paying more than a buck for the things when I was a kid.

    Of course if I bought them from the US, They would cost $1.50 but $96.48 Shipping.

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