OTG Cable to Micro USB: Turning a Phone Into a Host
Quick answer / TL;DR: Short answer: an OTG cable to Micro USB has the ID pin grounded, which tells a phone or tablet to act as a USB host. It supports keyboards, mice, flash drives and card readers, and supplies about 100 to 500 mA at 5 V, so it cannot drive a mechanical hard drive without external power. A standard charge cable will not work.
Five shells, one family: the plug shape does not set the speed.A plain Micro USB cable connects a phone to a computer, with the computer as host. An OTG cable reverses that relationship, letting the phone act as host to a keyboard, flash drive or card reader. The two cables look identical and differ by one connection inside the plug.
That single connection is the ID pin. In a standard Micro-B plug it floats, so the phone behaves as a device. In an OTG plug it is tied to ground, and the phone's controller switches to host mode.
The ID Pin Is the Whole Difference
The Micro-B connector has five pins: VBUS, D minus, D plus, ID and ground. In the USB 2.0 specification, a host port grounds ID and a device port leaves it floating. A phone's Micro-B port is normally a device port, but its controller can switch roles when it detects a grounded ID.
An OTG cable contains a short between ID and ground inside the Micro-B overmould. Nothing else changes. A standard cable leaves ID unconnected, so the phone stays in device mode and ignores whatever is plugged into the other end.
| Cable | ID pin | Phone role | Works with a keyboard |
|---|
| Standard Micro-B | floating | device | no |
| OTG Micro-B | grounded | host | yes |
| OTG Y-cable | grounded plus power | host with external power | yes, with charger |
| Micro-B to USB-C | depends on design | usually device | no |
What Actually Works
- USB keyboard and mouse, including through an unpowered hub
- Flash drives formatted FAT32 or exFAT
- SD and microSD card readers
- USB MIDI keyboards and controllers
- Game controllers with standard HID drivers
- USB Ethernet adapters, on devices with driver support
Storage is the case that fails most often. A flash drive draws under 100 mA and works. A 2.5 inch mechanical drive draws 500 mA to 1 A at spin-up and will either fail to mount or cause the phone to drop the connection. The solution is a powered hub or an OTG Y-cable that accepts a charger.
Power and Charging Conflicts
In host mode the phone supplies VBUS, so it discharges faster. Most phones will not charge while acting as host over the same port, because the port cannot be both source and sink. An OTG Y-cable solves this by injecting power from a charger into VBUS while the data lines go to the peripheral.
Not every phone supports charging and OTG simultaneously even with a Y-cable. The behaviour depends on the charging controller, and the only reliable test is to try it with the specific handset.
Real-World Applications
Copying photos off a camera card in the field
A card reader plus OTG cable lets a phone import raw files to an SD card or cloud service without a laptop. This is the single most useful application and the reason the accessory survives.
Typing on a phone or tablet with a real keyboard
A USB keyboard through OTG works with standard HID drivers, no app required. Layout follows the phone's keyboard settings, so a UK keyboard on a US-configured phone will swap the at and quote keys.
Backing up a phone to a flash drive
File managers on Android can write to an OTG-mounted drive. The transfer runs at USB 2.0 speed, about 30 MB/s, so a full backup takes a while but needs no computer.
Diagnostics on an embedded Android device
Set-top boxes and kiosks often have only a Micro-B service port. An OTG cable plus a keyboard is the standard way in when the touch panel fails.
Related reading: USB OTG adapter types — blogs/news/usb-otg-adapter
Related reading: Micro-B to USB-C cables — blogs/news/usb-micro-b-to-usb-c
Related reading: USB OTG cable overview — blogs/news/usb-otg-cable
Standards and Verification
USB On-The-Go is defined in the USB 2.0 On-The-Go Supplement, which introduced the Mini-AB and Micro-AB receptacles and the ID pin convention. A Micro-AB receptacle accepts both Micro-A and Micro-B plugs; a Micro-B receptacle accepts only Micro-B, which is why some devices cannot host at all regardless of the cable.
The supplement also defines the host's power obligations: 8 mA minimum in suspend, 100 mA for a low-power device, and up to 500 mA for a high-power device if the host can supply it. Many phones limit the port to 100 mA, which is why unpowered hubs and mechanical drives fail.
Verification: plug in a known-good flash drive and check whether the file manager shows a new volume. If nothing appears, the cable's ID pin is not grounded, the phone lacks OTG support, or the drive needs more current than the port provides. Testing with a keyboard separates the first two from the third.
Frequently Asked Questions
How do I know if my cable is an OTG cable?
Test it with a USB keyboard. If the phone responds to typing, the cable grounds the ID pin. A charge cable produces no response.
Why does my hard drive not mount?
A 2.5 inch drive needs 500 mA to 1 A and the phone port cannot supply it. Use a powered hub or an OTG Y-cable with a charger attached.
Can I charge while using OTG?
Only with a Y-cable, and only on phones whose controller supports simultaneous charging and host mode. Test before relying on it.
Does OTG work on iPhone?
Lightning and USB-C iPhones support accessories with the Camera Adapter, but the behaviour and power limits differ from Android. The Micro-B OTG cable does not apply.
Will it work through a hub?
Yes for low-power devices, but the hub and the device share the phone's current budget, so keep the total under about 100 mA.
Sourcing cables for a project, a store, or a private-label line? Contact our B2B sales team for specifications, certification evidence, and volume terms – FOB and CIF options available. Request a quote or sample.
