Specifications for Roadmaster Diodes # RM-690 and # RM-790 for Use on a Semi-Trailer

Updated 05/14/2025 | Published 12/04/2017

Products Featured in This Question

Roadmaster Single Park Light Diode

(120)

$22.21

Roadmaster Hy-Power Diode - (Qty 1)

(336)

$18.91

Question:

I am interested in the RM-790 and RM-690 Diodes that you sell. Do you have prints and specifications for these diodes? My application is for battery isolation for a 25A fused circuit on a semi-trailer. I need to assure the diodes will withstand 35 amps continuous and how much heat they will generate, what the physical size is, what is the environmental capability need -40F to 150F .

asked by: Jim

Helpful Expert Reply:

I do have some technical information for you about the Roadmaster Diodes # RM-690 and # RM-790, but I do not have any prints available.

Both part numbers are 1/2" tall, 1-9/16" wide, and are 1-5/16" long. They can withstand up to 85 of continuous amps, plus they have been tested to work when they are frozen and tested in an oven up to 180 C without failing.

My contact at Roadmaster said that these can definitely take a beating and will work great for your application.

expert reply by: Jon G

Scott M.

5/13/2025

Hi There. I want to use my trailer battery to illuminate its reverse and running lights while it is disconnected from the tow vehicle. I also want to make sure that the trailer battery does not backfeed to the tow vehicle when it is hooked up. I have a 7-way hookup. My thinking is that I should place several Roadmaster 690s on the trailer as follows: One diode between the 7-way plug and the trailer battery One diode between the 7way plug and the running lights (brown) One diode between the 7-way and the reverse lights (purple) One diode between the 7way plug and the ground wire (white) Does this sound correct and are there other diodes I should consider? I contact Roadmaster technical support and they told me they have no idea how to do this. Thanks!

Jon G.

5/14/2025

@ScottM The Roadmaster Single Park Light Diode # RM-690 will prevent backfeed, yes. But I'm honestly not sure that you'll actually need it because you'll need to have disconnect switches like part # SE73FR between the battery and each of the light functions otherwise your running and reverse lights will always be on until your trailer battery is drained. When you disconnect them for travel they won't be causing any sort of loop effect to take place, if that makes sense. It would make more sense to put the disconnect switch for the light functions closer to the battery and then run a wire closer up to your 7-Way and splice in your battery power via the Roadmaster Hy-Power Diode # RM-790 which has 2 inputs; one for the 7-Way from your vehicle, and one from the battery on your trailer. Hopefully all of this makes sense. Please let me know if I misunderstood, or if I can explain something better!

Scott M.

5/14/2025

@JonG Thanks. I actually am going to install a battery disconnect switch, but figured I'd better install diodes for safety, in case somebody forgets to disconnect power when hooking it up to tow. So, just to be clear, I will still need an RM-790 for each of the circuits above, correct?

Jon G.

5/14/2025

@ScottM You'll want one of the # RM-690 for the 12V power (I forgot about the part where you didn't want backfeed to the vehicle - sorry!) and then one of the # RM-790 for each of the lighting circuits. You won't need one for your ground.