Since my PCB design could be improved, it might be good to know what the maximum or average current draw is for my design. Here is my recent post about my PCB design for reference:
There are portions of this circuit that aren’t overly difficult to analyze, however, the integrated circuit does add a level of complexity because we don’t know the exact details of what makes up the chip. The application note gives us a generic diagram, but again, there aren’t part numbers:
This is very daunting for students and honestly, it kind of scares me too! There are some parts that I don’t even know how to interpret (I’ve never seen a transistor like Q19 for example). There aren’t any specifications to go on here either, so that means you have to make a lot of assumptions. We can use an oscilloscope probe on something called a shunt resistance to get an inferred measurement!
O-Scope Limitations and Finding Values
I’m using B&K Precision 2190E. This is limited to 2mV/division, it’s going to be hard to distinguish anything that small, so we need a mV value that we can “see better.” Let’s say that value is 10mV. The only load that I know is going to be relatively constant is the two LED’s and the maximum currents across the timing resistors. If you only add up the LED current’s together, that will be approximately 46.12mA. We can round this up to 50mA. Take the 10mV value and divide by this value to get a minimum shunt resistance. This value is 0.2Ω’s. Since I don’t have a through hole resistor this small, I found four 1 Ω resistors to get me close enough. See the explanation below:
When you hook up the o-scope across the shunt resistor, you can calculate the current, even if it is a complex wave form! If the display is 10mV, that value is approximately the same as 40mA because of V=IR. 20mV is 80mA and so on.
Sample Data From Prototype
Since the signal across this resistor is very noisy, you have to decide what values matter the most. In this instance, I only care about the maximum value and average current. I measured the amplitude from the zero reference to figure out the max value ten times and then calculated the average current drawn.
On average, my circuit draws approximately 140.388mA and very briefly reaches 167.116mA (these are transient spikes from the quick pulses). The shunt resistors in parallel aren’t being overwhelmed by this as that power based on the current calculates to about 0.7W. Some of this current is not actually passing through the 555 timer, some of this is being directed to the decoupler capacitor in the form of voltages being charged and discharged to ground (there will also be losses due to some leakage). There might be some error introduced in this setup because I only considered the LEDs when I formulated the notes. For example, this will drop the Vcc down more than I initially calculated because of the total current required. On average, it would drop by .0281V (140mA times .2 ohms).
Full Excel Data
Here is the Excel sheet for reference:
shunt_mvSamples_data.xlsx (9.9 MB)