Application and circuit:

Powered from a 6 circuit fuse block that has both constant and switched power to the output terminals. Item powered is a dual USB 2.0 port, max forward current if both ports are used simultaneously with a device drawing max current is 1 amp @ 12VDC (supply voltage does very from 11.8 to 14.8VDC)

The USB port has two circuits feeding it so it can be powered and shut off with normal key on bike running (circuit 1) and powered with a switched second circuit when the bike is off.(Circuit 2)

The issue I have is, the fuse block is being energized by by circuit 2 The switch used for circuit 2 is limited to a SPST miniature push button switch that fits a specific mount.

I need to prevent current flow back to the fuse block when circuit 2 is used. I have been using two 1N4004 diode in parallel successfully. I understand some of why pairing diodes like this is not the correct solution.. So the question, what is the correct diode for this application? As said, an alternate switch is not an option, nor is altering the fuse block. So the solution needs to be something between the fuse bloc output terminal and the usb PORT

Thank you advance for your time enter image description here


2 Answers 2


You just need a single diode that is rated for something greater than 1A forward current and something greater than 15V reverse voltage. There are many such diodes. Go to your favorite distributor's web site and do a parametric search.

If you want to reduce the voltage drop across the diode, look for a Schottky diode instead of a PN silicon diode.

  • \$\begingroup\$ Thank you for that. Ive been looking at Mouser and Digikey all morning, which is what led me to ask> Using your suggestions the search returned 23 options. But all of them have a forward voltage in the 1-3 volts mouser.com/datasheet/2/90/4d02120a-838539.pdf If that is the max allowable forward voltage, that's not going to work i think...as I will have 12VDC going forward Or I don't understand the spec of "Vf - Forward Voltage " \$\endgroup\$ Commented Sep 8, 2018 at 20:09
  • \$\begingroup\$ You are looking for a Schottky diode, NOT Silicon Carbide. Consider something like this: mouser.com/datasheet/2/115/SBR10U40-464741.pdf This type of diode will have a forward voltage in the 0.25-0.7 Vf range depending on it's maximum current rating. \$\endgroup\$ Commented Sep 8, 2018 at 20:42
  • \$\begingroup\$ I actually had "Schottky diode" Selected when I did that other search.. But anyway>> The Diode you linked me to, thanks for that.. But here is the part I still don't get.. All three of those linked to..SBR10U40CT, SBR10U40CTFP Show a very low forward voltage Vf - Forward Voltage: 0.25 V 0.25 V 0.47 V Forgive my inaccurate terms but... Is that saying those are the voltages losses from the forward voltage?? If so, great, they will do just fine.. Or the max allowable forward voltage of that diode? If so, no way will that diode live when i will have 11.8 to 14.8 VDC going "forward" ? \$\endgroup\$ Commented Sep 8, 2018 at 21:24
  • \$\begingroup\$ The forward voltage is the voltage across the diode (i.e. the voltage "loss") when the diode is conducting. It should be less than 1V for any diode you would consider. The "maximum reverse voltage" is the maximum voltage across the diode when it is not conducting; that's the parameter that must be more than 15V. \$\endgroup\$ Commented Sep 8, 2018 at 23:03
  • \$\begingroup\$ Elliot, Perfect thanks, so I was misunderstanding the term "forward voltage" Ok got it now......That helps tremendously as I was discounting diodes based on the forward voltage... \$\endgroup\$ Commented Sep 8, 2018 at 23:15

Pairing only matters when they are not closely matched and if yours are evenly warm, this is OK. If for some reason you are in doubt of burning your finger or not close, use three.

Using 2 1N400x's works for this use if as you say is OK if convenient.

For mass production, others would be better.

How did you verify Imax ? power supply spec or measured?

  • \$\begingroup\$ I claim ignorance... I Max? Seems I have read the term (other than at movie theaters) but I don't remember how used use or definition. 2 1N4004's were not warm to the touch at all... I used three today because I had them, and had not yet figured out the rest of this \$\endgroup\$ Commented Sep 8, 2018 at 23:16
  • \$\begingroup\$ sounds good with 2 if under worst case load. Imax means max DC current \$\endgroup\$ Commented Sep 8, 2018 at 23:23
  • \$\begingroup\$ How about one if these. 15SQ045 Schottky Ratings look right. The 10 amp would cover all the applications I have installed them in so far, and is overkill in many (that only need 1 or 2 amp capacity ) Other then the voltage drop a larger diode might have....are there any other downside to just using the largest diode I need on all the like applications? \$\endgroup\$ Commented Sep 9, 2018 at 20:48
  • \$\begingroup\$ Oh and " Imax means max DC current" Tony, thanks for that. I have verified each time with a test meter, as well as, USB has established specs that are widely adhered to so when its the USB port circuit being 2 circuit powered Im working with the 500Ma per port is a pretty safe bet. \$\endgroup\$ Commented Sep 9, 2018 at 22:00

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