Diode for 30A, 15VDC with low voltage drop

I looked at some Schottky diodes, and learned that they do not offer voltage drop that is that much lower than regular diodes. 0.46 vs. o.70, not that a big of a deal.

I have these diodes - Ixys 2X31D40J

Can anyone confirm that their current rating is 60A? I am a little confused, since they are comprised of two parallel diodes, so I am not sure if their rating in datasheet needs to be doubled or not.

I think the easiest and most bullet-proof solution might be the following.

I assume you are using some kind of connector between the vehicle and the trailer wiring. You can use a third wire if there is one to power the LED. Or, you can simply add an additional wire and connector at the trailer end that connects only to the LED. Sounds confusing, but the only way the LED will light is if the trailer box is connected to the vehicle. No power-wasting diode in the circuit. LED cannot light via the battery in the trailer.

Good luck.

Chuck

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That's true only if the two wires -- one going to led and one going to the rest of the trailer -- are powered by different cables AND different plug pins AND are on different relay contacts. Which unfortunately is not the case for a 4 way trailer plug.

I feel highly apprehensive about putting a diode into that electrical box, because I do not want something to fail while towing, say, a heavy load on a hot day and braking on a steep descent (worst case scenario for brake power use).

Even worst case numbers are not that bad. If, say, these brakes draw

I have a ultrafast Ixys diode Ixys-APT-2X31D40J, see

If I mount it to the back of the electrical box (bolt it to the outer wall, from inside) and bolt a small heatsink outside, and use heatsink paste, that ought to be more than enough to safely dissipate those 30 watts.

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Your best bet is a Shottky diode. They have a lower forward drop than their conventional diode counterparts. See, for example,

The ones I looked at, had voltage drop of 0.46v or so, which is still not that much lower than regular diodes. Am I missing anything?

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So ty-wrap a #18 insulated wire to your existing cable and add one small connector at the trailer end.

Alternatively, I don't know how the 4-wire connector is usually wired, but surely there are other connectors with more than 4 conductors that you can use.

Chuck

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Big power mosfet

Why don't you take a look at the SED page of my website, at the several schemes for implementing an ideal diode (or bilateral control) using power MOSFET's.

...Jim Thompson

The Schottky's forward volt drop rises with current, rtaher than being relatively constant like a normal silicon diode (but youknew that). IMOE the only way to get lower forward drop is to use a higher rated diode than the actual load current requires. The greater the over-rating, the lower the drop.

You could use a relay. 20 or 30 amp rated automotive relays draw about 150 mA, so that's not much burden on your truck battery, and you wire it so that the truck ignition must be on to energize the relay.

• from_truck ---[Relay_in_trailer]---ground

|> | ----+ +-----[Battery_in_Trailer]---

The battery in the trailer is isolated by the relay contact, unless the truck is on.

Ed

I am going to try this Schottky diode:

I would consider that to be a significant difference. 21 watts vs. ~14 watts for the lower Vf diode.

As a practical matter, I would just use an off-the-shelf battery isolator. You'll get two diodes and a nice chunky heatsink in one unit. That is essentially what you're doing anyway - charging two batteries with one alternator, right?

Chris

Have you checked the reverse leakage at 12 volts? It will keep an LED lit.

Make it simple by using one of the seven pin (or more) round trailer connectors made for commercial trailers so you don't need any extra circuitry. Parallel two contacts on the vehicle side, and run separate leads to the two loads. It has worked well on tractor - trailer rigs for decades.