Can I connect this USB PD device to an external device?
Can I connect this USB PD device to an external device?
I'm searching for a way to run a 20W external USB device directly from a laptop's USB 3.2 port.
USB 3.2 ports can supply up to 15W, even with PD, while USB PD supports up to 100W for charging the laptop.
Double of one, cables 2 to one are quite straightforward. Plus and minus are linked in parallel using diodes in the current direction, which prevents them from interfering with each other.
Are you attempting to connect USB-A or USB-C ports on your laptop to energize the external "20W" device? Wikipedia outlines the usual power outputs for various USB connections:
https://en.wikipedia.org/wiki/USB#Power
If you plan to use USB-A ports:
- A high-power SuperSpeed (USB 3.0) device can deliver up to 900mA at 5V = 4.5W
- Multi-lane SuperSpeed (USB 3.2 Gen x2) can provide up to 1.5A at 5V = 7.5W
Your laptop might not meet these limits, but even if you connect two USB-A ports in parallel, you’d only achieve around 15W, falling short of the 20W requirement.
If your gadget needs 20W to function (beyond just charging its own battery), then two USB-A ports won’t suffice.
For charging a battery inside the "20W" device, it would take longer when drawing power from one or two USB-A ports.
USB-C (multilane) can supply either 1.5A (7.5W) or 3.0A (15W), which may not be enough if the device demands 20W.
PD (Power Delivery) can output up to 5A at 20V = 100W or up to 240W at 48V with the latest PD 3.1 standard. This would be ideal if your laptop supports PD, though it’s unlikely.
Extracting 15 to 20W from a laptop without a charger will drain its battery quickly, unless you connect a mains charger simultaneously.
A standard laptop battery stores about 55,000mWh, or roughly 55Wh.
If your "20W" device includes a battery, what is its rating in milliWatt-hours?
Does your "20W" device operate exclusively at 5V, or can it function at PD rates from 5V, 9V, 15V, 20V, or even higher voltages up to 48V DC?
Thank you for your message. I can design my device to be PD compatible, but according to your note, I’m unable to locate a laptop with a PD source (5 to 25V / 20W). My current setup runs on 15Vdc, which is suitable for medical devices. Regarding the battery inside my device—there isn’t one yet, so it seems an option to add one would be necessary. I’ll need to charge it after each use, right?
If you're creating the unit yourself, consider using a dedicated DC input from a power supply of your selection. This approach is superior to depleting your laptop's battery and is likely safer as well.
This application could be portable. Providing 15V DC for the medical device adds a bit more complexity, though it remains manageable.
You might employ an LM2577-15 boost converter IC to transform 5V DC from a portable USB power bank into 15V suitable for your medical device. Once the device is no longer in use, simply recharge the battery bank using a standard 5V 2A USB charger.
It's probable that ready-made hobbyist modules converting 5V to 15V DC are available, often incorporating this IC or similar components. Ensure the module delivers at least 1.5A at 15V (22.5W) for safe operation. Keeping the converter separate from the 5V USB power bank can simplify the setup.
Any further queries are welcome.
Hi,
I would like to switch off the external P/S and connect directly via USB to the laptop (preferring the commercial option). The laptop's battery capacity isn't a concern; it will always be connected to the mains. The device draws 1.2A at 15Vdc. Increasing the voltage to 15V still requires at least 1.3A, including converter losses... do you think USB3.2 can manage this?
The different USB specifications set the upper limit for how much current a device can consume under various circumstances. Manufacturers adapt these rules according to their preferences.
USB-C supports multiple voltage levels and current capacities; PD-enabled models can deliver significantly higher power, usually used for charging laptop batteries and comparable gear.
When connecting a USB port on a laptop, the outcome depends largely on the laptop’s design and the product specifications provided by its maker. Unless there’s a demand for laptops that can deliver substantial power to external devices, designers stick to the established norms. Higher power components are more expensive, so efficiency matters.
The required power threshold is over 3.0A at 5V for USB 3.2 Gen x2 using a Type A connector. For 20W applications, consider USB-C and its related Power Delivery protocols. If you link your device to a laptop, it’s likely to experience limitations and may enter a foldback current control mode.
You can easily locate a USB-C power adapter suitable for medical equipment, though finding a standard laptop with a 20W output is unlikely. Specialized industrial laptops with unique features exist but are costly and uncommon.
For confirmation, reach out directly to laptop manufacturers within your budget range and inquire about the capability of their USB ports to deliver 20W. It’s probably more practical to use a conventional mains power supply rather than relying on a laptop for high-power tasks.
A standard 15V 3A mains unit is a straightforward alternative, offering better value at just $10.99 while being simpler for laptop integration.