The 9 V a USB port does not have
Both kits have a USB-C socket, and plugging it into a computer does nothing at all. The print head needs 9 V, and a USB port hands out 5 V unless something asks it for more.
The first evening
A thermal printer draws a line by heating a row of tiny elements until the paper under them turns black. Heat costs power, and the head in both of these kits is built for 9 V. Nothing about that is negotiable: at 5 V the elements warm slightly and the paper stays white.
The socket you plug into is USB-C, and every USB-C port on earth starts at 5 V. It only goes higher if the thing plugged into it asks, and the asking is a conversation on two extra pins in the connector. A laptop, a desktop or a phone port has no part in that conversation — it has 5 V and that is the end of it.
The base board is a PD trigger: its whole job is to have that conversation on your behalf, request 9 V, and pass it through to the printer. Give it a charger that can say yes and it works. Give it a computer port and it asks, gets nothing, and sits at 5 V forever.
What to plug in
Any USB-C charger that lists 9 V on its label. Phone chargers, laptop
chargers and the small square bricks sold as "PD" or "fast charge" almost all
do — the label usually reads something like 5V⎓3A 9V⎓2.22A 12V⎓1.67A, and the
middle number is the one that matters.
What will not work, however good the cable:
- A computer's USB-C port, on any machine.
- An old USB-A charger with an A-to-C cable.
- A 5 V-only brick, including most of the ones bundled with small electronics.
The base board says the same thing on its own sticker, which is worth reading before you conclude the printer is dead.
Why it fails silently
There is no light that means "not enough volts" and no error the printer can send you, because at 5 V the printer's own controller may well be awake and listening. It will take your bytes, acknowledge them, and produce blank paper — which reads exactly like a broken head or a wrong baud rate, and sends people looking in the wrong place for an hour.
The quickest way to tell the difference costs nothing: hold the button on the printer down. If it feeds paper, it has power. If it does not, it does not, and no amount of code will change that.
When it does not work
That is the expected result, not a fault. A computer's USB-C port supplies 5 V and has no 9 V to offer. Use a USB-C charger that advertises 9 V — most phone and laptop chargers made in the last few years do, and it will say so in small print on the plug.
Check the cable. A charge-only cable with no data lines cannot carry the negotiation, and some very thin cables drop enough voltage under load to make the printer stall mid-line. Try the cable that came with a phone charger before you suspect the board.
The base board's printer power output is a 9 V JST socket, so a bench supply set to 9 V can drive the printer directly. What you lose is the 5 V or 3.3 V the base board was also making for your microcontroller, which then needs its own supply and a wire joining the two grounds.
The printers in these two kits are 9 V units; both say so on their own stickers. Leave the board asking for 9 V. The 12 V marking exists because the same base board is used with printer modules that want 12 V, and setting it wrongly puts a third more voltage across a head that was not built for it.
What the base board makes out of that one cable, and which of its two microcontroller headers your board belongs on.
One board, two rails →Edit this page — content/books/thermal-printer/the-9v-usb-does-not-have.mdx
Questions about this product
See what other owners have asked, and read their solutions.
This page covers several products. Choose yours to see the right discussions.
Discuss this article
Ask about this page. The answer stays here, on the page it belongs to, for whoever hits the same wall next.