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I want to take a reading from 5 different mq sensor at a time. do I need an external power source to power these sensors or it will be ok to power sensors from 5v available from the board? I am using arduino uno board .

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  • How much current does each sensor draw? – VE7JRO Mar 19 '18 at 8:07
  • @VE7JRO maximum current drawn by each sensor is 240 ma . I have measured using multimeter altough its power rating is given as 800 mw . – r0000 Mar 27 '18 at 10:18
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I only had experience with the MQ2 sensor (more info here), so I'm going to use it as an example. If you have different sensors double check their datasheets.

Now, the main current consumption for these comes from the heater. Can you turn the heater off to save some power? Apparently not, since they say that

Please note that the best preheat time for the sensor is above 24 hours.

So, for the moment, let's consider all five sensors turned on at the same time.

The heater specs say that its resistance is around 33 ohm +/- 5% in the worst case this means 31.35 ohms. At 5V, this means 160mA for each sensor. Five of them are 800mA, and then let's add something like 50mA for the rest of the circuit (just a random number, and highly depends on the rest of the circuit).

Now, how is the UNO powered? If it is powered from the USB, the maximum current you can draw is 500mA (so 850mA is not allowed). So.. No powering from USB. If, on the other hand, the board is powered by the barrel jack, the onboard regulator can dissipate roughly 1-1.5W (taken from this thread). The power dissipation comes from the input voltage. With a 9V input, about 3V (slightly more) will need to be handled by the regulator. At 850mA, this means 2.5W (Not ok). In both cases you will need an external 5V power supply. With a 7V input (the very minimum you can apply) you will have to dissipate roughly 1.2V, at 850mA this means 1.02W. Please beware that this means something like 80-90°C of temperature increase in that area, so the part will become very hot. And moreover its lifespan will be reduced. So use this path only if really necessary.

If you reduce the performances and start powering just one sensor at a time (this means longer time before having it fully operational, and adding some transistor to switch them on and off) this data can change. I mean, if put a transistor to stop powering it, and then turn on one sensor at a time, you will be able to reduce the maximum current consumption to 210mA.

This means that you will be able to get power from the USB port; moreover you will be able to use the external power connector for voltages up to 10.5V (0.8V on the diode, so 4.7V on the regulator - around 1W). This comes at a price, of course: the heater will work for 1/5 of the time, so it will be harder to reach the temperature required for the measurement

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  • 160 ma is the maximum requirement. For safer side, if I take 120 ma per sensor that is total 720 ma. Now can I use an external power source whose output is 5v and power rating is 3500 mw or around this? . One more doubt is that although I will provide the external power source to sensors for input but, will there be any problem for Arduino due to input from all six sensors at the same time which may exceed its input capacity? – r0000 Mar 27 '18 at 10:19
  • @r0000 You can also assume 100mA per sensor. Even 10mA. But it will be wrong. Usually a designer makes designs in the worst case, which in this case is 160mA. If you use the nominal value, it is 152mA. But this will be an underestimation, and may result in bad working. As I told you, if you want to dimension it, assume 850mA. At 5V, this means 4.25W. Your 3.5W will be undersized. As for the inputs, the UNO has six inputs so six sensors are feasible – frarugi87 Mar 27 '18 at 20:46

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