I had to do some creative wiring on my car's A/C to get it working without a $1000 repair bill. Now the A/C works perfectly, but I need to isolate the current on one of the wires to so that it only flows one direction.

A manual switch is wired between the negative battery terminal and the A/C relay, with a second wire between the A/C relay and the Fan 2 relay. This is because the condenser coil would overheat and blow the high pressure valve unless I keep constant air-flow across the condenser coil while the A/C is running.

The downside is, when the A/C is off and the engine is hot enough for the fans to run, the short wire between the Fan 2 relay and A/C relay carries the Fan 2 current to the A/C relay and the A/C compressor kicks on too. If I add a diode inline in the short wire between the relays, then it will keep current from flowing backwards when only the fans should be running.

The wire carries 10 Amps at 12 Volts. Since there is a 10 Amp fuse on the circuit, is it okay to install a diode that is rated higher on Amps and Volts? For instance, a 15 or 20 Amp, 45 Volt diode? Or do I have to match the diode's specs exactly to the circuit?

What are the consequences of using a 15 or 20 Amp, 45 Volt diode on a 10 Amp, 12 Volt circuit?

Edited - 2020.09.11 (added diagram and outcome)

The Amps pulled across the jumper wire between the A/C relay and the Fan 2 relay is only 0.4 Amps. So the power dissipation is insignificant. The DIY wiring is like this:

Diagram of New Wire                           ╔═════════╗
┌───────────────────────┬─────────────────────║ switch  ║──────────[neg battery terminal]
|                       |                     ╚═════════╝
|       A/C Relay       ┴      Fan 2 Relay
|     ╔════════════╗    ▲     ╔════════════╗
└─────║→ 1─load─2  ║    |     ║  1─load─2  ║
      ║  3──/ ──5  ║    └─────║→ 3──/ ──5  ║
      ╚════════════╝          ╚════════════╝
                  Inline Diode
                  on wire with
                  0.4 Amp draw

A 20SQ045 diode, sinked to the car body with plumber's tape, worked perfectly. The temperature increased from 90°F when the A/C was off, to 97.5°F after running the A/C constantly for 5 minutes.

When the A/C is on (via the new manual switch), the radiator fans also always run. When the A/C is off, and the engine temp is high enough to kick the fans on, it no longer kicks the A/C compressor on.

  • Thanks for letting me know that there's a mechanics site, I didn't know that. However, the question itself and the answer can be applied in any situation (home, auto, etc.). I just appreciate that there are people like @dandavis who know more about electronic than me and can provide a meaningful answer. – James L. Aug 26 '20 at 19:41
  • @dandavis - Quick question. The MBR2045CT looks like a rectifier, with 3 leads. I was thinking more like a 20SQ045 to install inline in one wire to only allow current to flow one direction through that one wire. That should still work, right? – James L. Aug 26 '20 at 19:49
  • The MBR2045CT is a 3 lead rectifier. I'm sure I could get that to work because I am trying to control the flow of ground current on one wire. But wouldn't it be more simple to use a 20SQ045 and simply wire it in-line on the one wire where I want to force ground current to flow in one direction? Is MBR2045CT a better long-term solution than 20SQ045? – James L. Aug 26 '20 at 20:02
  • (It is disappointing that the original comments were lost when this question was migrated. There was some very good information in those comments, which is now lost. And the original question on 'diy.stackexchange.com' auto-forwards to this migrated question. So valuable and useful information was lost.) – James L. Aug 26 '20 at 20:17
  • Comments normally do migrate (as often seen by surviving comments from graypeople who haven't signed up for an account at the destination). I think they were either self-deletion or mod cleanup. Answers belong in the answer field. I'm sorry that StackExchange doesn't work the way you want it to; there's a process to propose changes. – Harper - Reinstate Monica Aug 26 '20 at 20:26

Yes, you CAN use a 15 or 20A rated capacity diode for a 10A circuit. The current rating of a diode is the MAXIMUM that it's rated to carry without damage. As long as your diode is rated >= 10A you are ok.

Keep in mind that at 10A, you will have significant power dissipation in the diode. For example a typical forward drop for a diode is 0.6V. So 10A x 0.6V = 6 Watts. That's enough that you probably need to use a heat sink on it but check the spec sheet to see what is needed.

  • Another user on DIY (before the question was migrated) suggested using a MBR2045CT rectifier (with 3 leads). Does that rectifier have the same power dissipation as a normal inline diode, such as a 20SQ045? – James L. Aug 26 '20 at 21:49
  • Here is the data sheet for that part: onsemi.com/pub/Collateral/MBR2045CT-D.PDF Yes, that should do it, it's two 10A diodes in one package. The concern is the junction to ambient thermal rating of 60 C/W. At 6W that's going to be 360C. WAY too hot. You'll need to sink it and remember that the tab is connected to the diodes. – jwh20 Aug 26 '20 at 21:59
  • Use a mica or silicone washer (plus some heat sink compound) to electrically isolate the diode from the heatsink. Washers to mount standard semiconductor package designs on heatsinks (e.g. TO-220 for this component) only cost a few cents. – alephzero Aug 26 '20 at 23:19
  • The car is a 90 minute drive away, so I can't quickly/easily measure the amps on the new wire (from battery neg terminal to manual switch to A/C relay to Fan 2 relay). I think it's going to pull less than 10 amps -- just enough to close the relay(s). Which means the power dissipation and heat will be MUCH less than I previously thought. Thank you for your answers and comments. – James L. Aug 26 '20 at 23:56
  • Even if it's only 5A that's going to be 3W and at 60 degrees C/Watt will get you to 180 C which is also well above the temperature that will destroy this diode. You're going to need to attach it to a heat sink. – jwh20 Aug 27 '20 at 10:10

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