user44882: 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 .

user10214: 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

44882: <p>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 .</p>
 10214: <p>I only had experience with the MQ2 sensor (more info <a href="http://wiki.seeed.cc/Grove-Gas_Sensor-MQ2/" rel="nofollow noreferrer">here</a>), so I'm going to use it as an example. If you have different sensors double check their datasheets.</p>

<p>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</p>

<blockquote>
  <p>Please note that the best preheat time for the sensor is above 24
  hours.</p>
</blockquote>

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

<p>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).</p>

<p>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 <a href="http://forum.arduino.cc/index.php?topic=168964.0" rel="nofollow noreferrer">this</a> 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.</p>

<p>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.</p>

<p>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</p>
