The XL-10AL is a small 12.8x22.7mm breakout board with a highly power-efficient bistable switch that works both with 3.3V and 5V microcontrollers at just 120-350nA quiescent current.
The 10AL in XL-10AL stands for 10A and low, and there is also a XL-10AH. The H stands for high and refers to the voltage range: it can handle voltages from 4.5-26V (i.e. if you wanted to switch 12V car electric)
Overview
The board comes with four solder pads: Vin and GND are connected to the power supply (i.e. a battery), and Vout and GND are connected to the device you want to control. The voltage you feed in is the voltage you get out. A push button can be used to manually toggle the switch.
When the built-in push button is pressed, it essentially pulls a pin in the circuitry to low. That’s why you can toggle the switch via a GPIO or an external push button just as easily:
- Programmatically: pull down either one of the two marked solder pads for a short period of time with a microcontroller GPIO, simulating a button press.
- External Button: connect an external momentary push button (normally open) to GND and one of the two solder pads marked GPIO low.
Caveat
The vendor has published instructions on how to connect external buttons and added this misleading image for illustration:
It appears as if you could toggle the bistable switch either by pulling up one pin, or by pulling down another pin. That’s however not the case.
The labels VIN and GND in the image refer to the power connectors and do not relate to the push button. Both left push button contacts are interconnected, and both right push button contacts are interconnected as well.
So both external push buttons in this drawing do exactly the same and short-circuit the built-in push button.
Since the left side of the push button is connected to GND, toggling is performed by pulling the right side of the push button to ground - either by pressing the built-in push button, or by short-cutting the built-in push button with an external button, or by pulling either one of the right contacts to low.
Voltage and Current
The vendor markets this bistable switch as power switch for high currents of up to 10A, and the published specs apply to high current use cases only:
| Item | Value |
|---|---|
| Current | max. 10A |
| Voltage | 3.5-5.4V |
| Voltage Drop | max. 130mV |
| Quiescent Current | 300nA |
For currents >5A, the voltage should be >4V, and an additional heat sink may be required.
Microcontroller Use-Case
With low currents (around 200mA, as is common in microcontroller projects), the specs look quite differently:
| Item | Value |
|---|---|
| Current | max. 200mA |
| Voltage | 2.8-5.4V |
| Voltage Drop | none |
| Quiescent Current | 120-300nA |
3.3V Microcontroller
Operating the XL-10AL with 3.3V is officially out of spec but entirely possible: as long as you switch small currents, the MosFET works just fine.
With currents around 200mA (as is common in microcontroller projects), voltages below 3V perform well, and the quiescent current in off state is much smaller than 300nA (around 120nA). Another benefit is the absence of any noticeable voltage drop.
To verify, the graph below shows a measurement series at 2.98V input voltage (simulating a nearly empty LiIon battery), and drawing a current of 200mA:
The graph in the middle shows that 200mA could be delivered ok in on state. In off state, the quiescent current is merely 120nA in the average.
High Currents
This board can switch surprisingly high currents of up to 10A, however when current exceeds a few Ampere, the voltage must be higher than 4.5V.
At lower voltages, the MosFET can only handle small currents. If the current exceeds the transport capabilities of the MosFET, its resistance raises, and the MosFET heats up, causes a voltage drop, and can be destroyed.
There are enough electrons available in the MosFET to conduct small currents, even at very low Vgs: no heat is generated, no voltage drop occurs, and all is good. As current increases, more electrons need to be “pulled” into the channel to maintain conductivity. A higher Vgs enhances the electric field, which strengthens the channel and allows more current to flow. So if the voltage is too low to pull in enough electrons to carry the current, this increases resistance, which in turn produces heat, causes a voltage drop, and can eventually destroy the MosFET. That’s why you get away with very small voltages in low-current microcontroller scenarios, and need a higher voltage with currents exceeding a few Ampere.
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(content created Oct 14, 2024)