Power consumption calculation

Thread Starter

loganjon

Joined Sep 1, 2017
17
Hi,
I have a circuit of 7 daisy chained shift register "CD 4021b" and 56 Hall effect sensors connect to them. All Hall effect sensors have a pull up resistor of 10K per each. How can I calculate the power consumption of this circuit. I am new to electronic design. It will be very nice of, if you could help me. I am very interested in the calculation steps. I read the data sheet of both components, but I am not able to do this myself.
 

Alec_t

Joined Sep 17, 2013
13,153
If you post a link to the Hall sensor's datasheet it might give us a clue :rolleyes:. You can probably ignore the CMOS chips' power consumption, unless you are clocking them extremely rapidly.
 

LesJones

Joined Jan 8, 2017
3,813
If you assume all the Hall sensors are pulling the bottom of all the 10 K resistors to ground then it would be like putting 10000/56 (= 179) ohm resistor between ground and the positive power rail. We can't do the calculation for you as you have not told us the supply voltage. I agree with Alec about ignoring the CMOS logic power consumption.

Les.
 

LesJones

Joined Jan 8, 2017
3,813
The information I gave you in post #3 is wrong. I was assuming the Hall sensor was a 2 wire device for which the only current would be in the pull up resistor. The device can take a maximum of 8 mA on it's supply pin so 56 of them could take up to 56 x 8 mA = 488 mA plus the current through the pull up resistors. this would be (5V - 0.25V)/179 = 26.5mA so the total could be as high as 514.5 mA. (The 0.25 volts subtracted from the supply voltage is the typical output saturation value.) The current taken by the CMOS deviced depends on the clock frequency. I have never had any reason to do this calculation but I think the data sheets will give a graph of current agains clock speed.

Les.
 

Alec_t

Joined Sep 17, 2013
13,153
These are the relevant datasheet figures:
HallConsumption.PNG
I take SHigh to mean the internal Schmitt trigger has a high output, which will turn on the output transistor.
The supply current clearly depends on whether the output transistor is on or off.
If I were designing a power supply for the Hall devices I would use the maximum figures above, not the typical ones. I note Les has used 8mA as the supply current per device. This is the value I would use too.
To know the CMOS current you would have to know the switching speed and frequency. Average current could typically be measured in uA per device, so negligible in the present context.
 

Thread Starter

loganjon

Joined Sep 1, 2017
17
Now I am a little bit confused.
Why does my circuit work ? I use in same circuit a Teensy 3.2 as microcontroller which I power it by USB. The Teensy shares this 5v of USB, so I use it as power supply for the hall sensor an CMOS. As I know the USB current is limited to maximum of 500 mA. And it's less if the computer USB port is used. I think it is even about 100mA. And I use a computer USB port.
 

Alec_t

Joined Sep 17, 2013
13,153
Why does my circuit work ?
The calculations of maximum current above are based on all Hall sensors having their output transistors on constantly and simultaneously. If only some are on at any time then clearly the current draw from the supply will be less.
 

Alec_t

Joined Sep 17, 2013
13,153
Assume one sensor draws 8mA, plus ~ 0.5mA through the 10k resistor. So number of sensors allowed ON simultaneously if the total current available is 100mA will be 100/8.5 = 11 (rounded down).
 

Thread Starter

loganjon

Joined Sep 1, 2017
17
Thank you very much for that wonderful explanation.
Just one question more.
Is there any help to learn to understand the data sheet of electronic components.
 

Alec_t

Joined Sep 17, 2013
13,153
Apart from such things as Absolute Maximum values and Recommended Settings, many items in a datasheet won't make much sense unless you have a good understanding of the basic operation/function of the components. So you need to do a lot of background research about the components themselves.
 

Thread Starter

loganjon

Joined Sep 1, 2017
17
I am still thinking about the calculation.
What, if the switches are off and there is no activity that mean if the transistors are open. Which consumption will have the circuit?
 
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