You may not know what quiescent current is, but it is a very important and often overlooked specification in almost all electronic devices. What is a quiescent current, and what is the effect of quiescent current on electronic mobile devices? Continue reading this.
Oops, running out of power!
You must have had this experience: You placed brand-new flashlights or other battery electronic devices (cell phone, power bank watch, anti-theft monitor, earbuds, and more) in the warehouse or on the shelf. Several months later, you are ready to sell them, but they are found can’t work, or the output is much dimmer than the original one.
Or you go out for a business trip or holiday and put your electric control car in the garage for ten days without driving. You are back from holiday and try to drive the car again, but you find the vehicle can’t start again. How frustrating they are!
Some people may think that there must be a quality problem with these lights or devices. They may indeed be quality defects, but more often than not. Let’s suppose it was brand new and functional before being placed. Then it is very likely that a quiescent current causes the problem.
If the quiescent current causes it, is it considered a quality problem? It depends on how high is the quiescent current.
- If the static current is less than dozens uA, it’s a normal phenomenon.
- If it’s more than hundreds uA, then consider as a quality issue. The quiescent current is too high and affects the battery efficiency.
I am sure you need more clarification. To explain this, it is important first to understand what quiescent current is. And why does it affect the efficiency of battery usage?
What’s quiescent current?
Quiescent current is the current consumed by a circuit when it isn’t driving any load and its inputs aren’t cycling. Quiescent current is also called parasitic current and dark current. It is the current consumed by the device when it is not operating. For example, the flashlight gets less battery power automatically is a quiescent current discharge process.
Unit conversion and formula of the quiescent current.
Quiescent current is a low current. The units of quiescent current are generally uA and nA.
Their conversion with Ampere is as below:
1A=1000 mA=1000,000 uA=1000,000,000 nA
How to calculate the quiescent current?
Iq=Iin-Iout
While the quiescent current theoretically can be calculated, it is more often measured with a Multimeter.
Why is there a quiescent current in electronic devices?
The right amount of quiescent current is designed by electronics engineers, but too much quiescent current results from design flaws or distortions in electronic components.
As mentioned above, the quiescent current consumes battery power, so why do we design it in our devices? Can we cancel it? The answer is no. There are some reasons:
- Most of the devices use electronic switches. When the device is off, it also needs a standby current. Because the current keeps the IC active, so can restart to operating mode quickly.
- A quiescent current support an electronic device with a memory function. For example, the flashlight turns on again in the same mode it was the last time off. To achieve this memory function, the circuit must keep working. If there is no quiescent current, the circuit is disconnected, and the memory goes.
In short, almost all electronic devices are designed with quiescent current. This current is necessary to exist, but the lower, the better. Too high a quiescent current is a quality problem. It will shorten the standby time of the battery.
What is the biggest challenge of designing for the quiescent current?
As our flashlights are now smaller and more compact than before but still expect more functionality and higher performance, the efficient use of battery capacity must become higher and higher. Who wants a flashlight that runs out of power whenever they are ready to use it, and who wants to recharge their device frequently? Therefore, it is unacceptable for the static current to consume much battery power.
If the quiescent current can be extremely low, it is equivalent to double the standby time and more power saving. Then the number of charging reduce to a minimum.
But now, flashlights are getting more and more functions. And need more circuits to achieve these functions. More circuits mean a higher quiescent current.
So, the biggest challenge is that electronic devices should have a quiescent current but require it to be as low as possible.
Some experience in quiescent current.
- The mechanical switches-controlled devices do not have quiescent currents because it’s an open circuit when the switch is off. But now, the trend is that electronic switches are gradually replacing mechanical switches. Because now the power of the flashlight is getting higher and higher.
- High-power (over 30W) electronic devices (over 30W) cannot be controlled by mechanically switched because the switch must be big enough to get the output current through the button. But this switch will be too big for a flashlight.
- The quiescent current for the regular device needs to be under 30 uA.
- For a device with a USB function, its quiescent current is usually close to 150 uA.
- The more functions, the higher the quiescent current. For example, the quiescent current of a typical car is not more than 20mA, but the more advanced the car is, the more electrical devices are used, and the more IC chips are used, so the quiescent current also increases. With the increase in electrical equipment and battery capacity, the quiescent current of the car will become larger and larger. If you are more interested in car parasitic current draw, read this on Youtube.
How to classify if the quiescent current is in a value of expectation or too high?
Below is a rough way to judge if you cannot access a multimeter to test the quiescent current.
If a fully charged device runs low on power after three months without use, there must be a quality defect.
If an electronically controlled car fails to start, usually after a week of running, it is considered the cause of the failure.
Now, most devices have a quiescent current. But we generally do not pay attention to their quiescent current for direct power supply devices, such as TV, because it is relatively small. But for battery-powered devices, such as flashlights and power banks, because it has limited battery capacity, we must improve the effective use of the battery as much as possible.
How to store your flashlight?
- Take the battery out of the flashlight.
- If you can’t take out the battery, charge your light every three months to prevent the quiescent current from consuming the battery and causing the light can not work.
