Need to isolate a charging circuit-need help

I have to hook up a hydraulic brake actuator to the 12 volt positive input from my tow vehicle to my travel trailer. But this lead also connects to the trailers 2 12 volt batteries so when the truck is hooked up power from the batteries will flow back towards the truck when it is off and power the actuator all the time. The brake actuator is on board the trailer to activate the disk brakes but it must be connected to switched power from the truck. When the actuator loses 12 volt power it automatically applies the trailer brakes. For this automatic "breakaway" protection to work properly I must isolate where the actuator connects from where the 12 volt line comes into the trailer to also charge the batteries.

So I will connect the brake actuator to the incoming 12 volt positive line coming from the truck, then I need to add diodes after the brake actuator connection so that the batteries will still get their charging power but will not back power the actuator. This positive lead on the truck is 12 volts and protected by a 40 amp fuse. What type and specification diodes do I need? If I have to I can use several wired in Parallel to handle the load.

Thanks for the help.

Use a relay on the + wire between the truck and the trailer. Power the normaly open relay coil from a switched (as it only powered when trucks key is on) wire.

When the trucks key is off the relay will open. Whey the key is on the relay will close and release the trailer brakes.

My advice is to checkout the RS Components, Farnell and Digikey catalogues.

52CPQ030
MBRF2545CT - only raded to 30A but the package contains two diode which can be paralleled.

Well I found a 85 Amp blocking diode on Ebay that is used to preven batteries from back powering solar panels or wind generators. It comes with a heat sink and is rated at double my load so this should work right? It seems the most simple which is what I want, am I on the right track here?

Thanks

I don't see why it won't work but don't forget you need two diodes.

Why two diodes? I can't put one only on the positive feed to the battery after my pump actuator tie in? I am good at the mechanical end but circuit design is not my area of knowledge so a bit more explanation is appreciated.

Thanks

Forgive me if my drawing is not to standard but it should get across my plan. Is this what you mean Hero999 by two diodes and where and how they should be wired in?

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Anyone? I am running into a time crunch.

It should work OK with just the one diode in the positive lead. You will lose about 0.7V across the diode so the trailer batteries won't charge all the way up to 14.2V.

Losing 0.7V is ok, I just need the batteries to be close to topped to run the DC slide motors on the trailer when we are not hooked up to AC power.

I was just wondering why Hero999 said 2 diodes?

Anyone know what Hero999 meant by two diodes and where they would go? I just want to clarify single vs. dual.

? for Hero999, isolating circuit

In a previous thread you said to remember that 2 diodes are needed. Can you expand on that answer and apply it to this schematic. Is my drawing correct? If not where would the second diode go?

Some background, I need the hydraulic pump to draw power only from the truck connection and not be back powered by the trailer battery. This is due to the pump powers the disk brakes on the trailer and when it loses 12v power it engages the brakes as a breakaway feature. Thus I need it to only see the truck connection so that if the trailer breaks away the power lead will disconnect and the brakes will engage. The trailer connector is routed such that I can only tap power near the trailer battery on the trailer and cannot run a dedicated wire. Plus this wire is 6 guage.

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They're two diodes in your circuit.

If you search for the datasheets of the diodes I' posted then you'll see that they actually contain two diodes in the package. You can easily parallel them which will help minimise the voltage drop and lower the power dissipation.

I guess I am still not following. As an experiment I wired in today one diode with heat sink on the positive side and hooked up my battery. It works as I want it to. The pump only works when the truck is hooked up.

Looks okay (soley from the info in this post since I know nothing else about it).

Personally, I do not see why you even need two diodes. Seems to me it will do the trick as long as one of the two is there.

It is working with only one on the positive lead side. I will continue to monitor and see how it works in the long run.

I was merely making the point that there are two diodes in the circuit, not one so you need two diodes to build this.

Whether you use both of the diodes in the package is immaterial, providing the combined current rating is sufficient for your hydraulic pump. However, it is good design practise to use both of them connected in parallel as it will improve reliability as each diode will only carry half the current.