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Measurement of voltage and current at the GPIO

November 26, 2017 by Walid Salame

Raspberry Pi Measurement of voltage and current at the GPIO output with high level under load

GPIO output with high level under load

If you connect a high-level GPIO output to a Raspberry Pi, then you expect a voltage of 3.3 volts. Unfortunately, a GPIO output changes its voltage and current behavior under load.

How exactly can be determined with a series of measurements.
Measurement series

Mtensionelectricityresistanceresistanceelectricitytensionelectricity
Volt (V)Amps (A)ohmohmAmps (A)Volt (V)Amps (A)
givengivencalculatedgiven (E12)calculatedmeasuredmeasured
03.3Comparative measurement without resistance3,309deleted
13.310,0000.00033,2980.00033
23.382000.00043,2950.00041
33.368000.00053,2920.00049
43.356000.0013,2890.00058
53.347000.0013,2840.00071
63.339000.0013,2700.00086
73.30.001330033000.0013,2640.00101
83.327000.0013,2540.00122
93.322000,0023,2420.00149
103.318000,0023,2280.00182
3.30,0021650
113.315000,0023,2100.00215
123.312000,0033,1850.00268
3.30,0031100
133.310000,0033,1670.00317
143.30,0048258200,0043,1280.00393
153.36800.0053,0910.00472
3.30.005660
163.30,0065505600,0063,0530.00556
173.30,0074714700,0073,0010.00662
3.30,008413
183.33900,0082,9450.00779
3.30.009367
193.30,0103303300,0102,8830.00910
3.30.011300
203.30,0122752700,0122,7930.01100
3.30,013254
3.30,014236
213.30,0152202200,0152,7090.01241
3.30.016206
223.31800,0182,5820.01527
233.31500,0222,4720.01633
243.31200.0282,3130.01929
253.31000.0332,1660.02289

The table consists of a total of 3 parts. The first part consists of the voltage of 3.3 volts (column 2) applied to the GPIO output (high) and a current from 0.001 A to 0.016 A (column 3). In order for the given current to be set below the specified voltage, a corresponding resistor must be used for this purpose, which was calculated for this purpose (column 4).

The second part of the table is that the calculated resistance values are not real resistances, but real resistances with other values have to be used. In this case from the E12 series (column 5). In some cases, there are overlaps with the calculated resistance values (between columns 4 and 5). Since a given current is set at a specified voltage at a given voltage, it was calculated (column 6). This current is what would be expected at the specified voltage of 3.3 volts.

The third part of the table deals with the actual measurement of voltage and current (columns 7 and 8). Here, first the voltage at the GPIO output (high level) was measured (column 7) to see if there are really 3.3 volts at the resistor. The current measurement was then performed (column 8) to see if the calculated current actually flows through the resistor.
The measurement 0 was made to check whether there really is a high level at the GPIO output and how high the high level is to be expected. The current measurement is omitted here, because there is no closed circuit here. Subsequently, the measurement was carried out 1 to 25. Once each for the GPIO output voltage and the GPIO output current.
Observation and evaluation of the measurements

  • As the resistance decreases, the output voltage at the GPIO decreases.
  • As the resistance decreases, the output current at the GPIO increases.
  • As the output current increases, the output voltage at the GPIO drops.

While a stable voltage and current ratio is set up until measurement 22, from measurement 23 the measured value can only be read off the meter with difficulty. From 0.016 A or 16 mA, a GPIO output starts to float. The values for voltage and current fluctuate.

Findings and conclusion
The measurements show that a GPIO output under load has a lower output voltage than expected. When a current flows, the voltage drops. However, the actual output current remains behind the calculated current.

A GPIO output is a digital output that should not be loaded because the output voltage drops due to the output current. That is, the subsequent wiring must also be able to detect a voltage below 3.3 volts as a high level. That is to be tested. Or you have to make sure that the GPIO output is not loaded.

Basically, the output current must be limited. It would be better, however, to understand the GPIO output as a digital output, which you cannot remove power, but may only pick up the voltage level. A subsequent switch stage should be voltage and not current controlled. So a FET instead of a bipolar transistor.

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Filed Under: Raspberry Pi Tagged With: Raspberry Pi GPIO

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