What is the voltage output at the end of a USB connector. The input plug is at the cig lighter receptacle.
I assumed it would be 12v, but, heard it might be 5 v ???

USB power is always 5v

USB 2.0 - 5V - Max output 2.5W
USB 3.0 / USB3.1 - 5V - Max output 4.5W

Thanks, thought it

Looks like USB can go up to 48V for power delivery but is 5V otherwise:
https://en.wikipedia.org/wiki/USB

I did it myself for a USB outlet. Turns out that I had to make resistor connections to keep the data pins at the right voltages to suit Apple.

What size resistor would drop 12v to 5v. ?

Depends on the current load you are expecting to pull. At a minimum you need two resistors in series to tap the middle point, or resistor/zener diode combination.
Rather than build it yourself though, go buy a couple cigarette lighter adapters and open them up to see what they use for circuitry. Build that into a USB port if you want.

Goggle “divider network.”

Why not use something like this?

First one that came up with initial search, I am sure many other can be found.

Click to view attachment

https://www.amazon.com/Cllena-TOYOTA-Dashbo...vD_BwE&th=1

@jim_hoyland a better question might be: what device are you trying to get power to?

- Tony

A USB charger that plugs into a cigarette lighter plug already does the voltage step down process for you.

Unless a device plugged into the USB port specifically requests a higher voltage (like a tablet, or a laptop), USB will ALWAYS default to outputting 5v.

I’m going to power a HUD directly to the fuse panel. The HUD say max voltage is 5v.
So, I want to step the v down under the dash.
This is A MPH HUD, not the fancy ones that plug into an OB receptacle

https://en.wikipedia.org/wiki/Voltage_divider

Interesting; if I go to the electronic shop, what should I be buying ?

What does the power connector for the HUD look like?
How does it normally get power?

No affiliation, but I don't think you can build something for this price...

https://www.temu.com/ul/kuiper/un9.html?sub...YBoCb5oQAvD_BwE

Buy a 5 dollar cigarette lighter converter and break it open.
There will be the circuit and resistor

That's sort of cool if it really can deliver 5a at 5v.

Why not just use the power supply that came with it?

The power cable has USB connectors at each end; I want to power it directly off the fuse panel.

Ok, then get a USB adapter that normally plugs into the cigarette lighter and simply connect 12V from the fuse panel to the center contact and ground to the side contacts on that adapter.
That way, you can simply use the usb cable that came with your HUD and everything fits with no other modifications needed.

Like dis ...

Found this on Amazon:

That should work.
And if you're running this off the fuse panel, make sure you're piggy-backing off a switched terminal so the power to your HUD is turned off when the key is off.

Otherwise, that thing will drain your battery in a hurry ...

or one of these:
https://www.amazon.com/gp/product/B01FJ8OXX...=UTF8&psc=1

red to + and black to ground. plug in the adapter andy shows in post 23 and bob's your uncle.

put two female spade connectors on it, get two piggybacks, pull the ashtray, reach in and use the connections on the back of the cigarette lighter. one most forward in the car is + and the one on the side is ground.