Raspberry Pi 3.3V pin Measurement

 Measurement of Raspberry Pi voltage and current at the 3.3V pin under load

If you connect a 3.3V pin to Raspberry Pi, then you expect a voltage of 3.3 volts. Unfortunately, the voltage and current behavior changes under load. How exactly can be determined with a series of measurements.
Measurement series

M	tension	electricity	resistance	resistance	electricity	tension	electricity
	Volt (V)	Amps (A)	ohm	ohm	Amps (A)	Volt (V)	Amps (A)
	given	given	calculated	given (E12)	calculated	measured	measured
0	3.3	Comparative measurement without resistance				3,309	deleted
1	3.3			10,000	0.0003	3,309	0.00033
2	3.3			8200	0.0004	3,308	0.00041
3	3.3			6800	0.0005	3,308	0.00049
4	3.3			5600	0.001	3.307	0.00058
5	3.3			4700	0.001	3.307	0.00071
6	3.3			3900	0.001	3,306	0.00086
7	3.3	0.001	3300	3300	0.001	3,306	0.00102
8	3.3			2700	0.001	3,305	0.00123
9	3.3			2200	0,002	3,305	0.00152
10	3.3			1800	0,002	3,304	0.00185
	3.3	0,002	1650
11	3.3			1500	0,002	3,303	0.00219
12	3.3			1200	0,003	3,301	0.00273
	3.3	0,003	1100
13	3.3			1000	0,003	3,300	0.00325
14	3.3	0,004	825	820	0,004	3,298	0.00409
15	3.3			680	0.005	3,295	0.00495
	3.3	0.005	660
16	3.3	0,006	550	560	0,006	3,293	0.00589
17	3.3	0,007	471	470	0,007	3,290	0.00703
	3.3	0,008	413
18	3.3			390	0,008	3,286	0.00843
	3.3	0.009	367
19	3.3	0,010	330	330	0,010	3,282	0.01005
	3.3	0.011	300
20	3.3	0,012	275	270	0,012	3,275	0.01241
	3.3	0,013	254
	3.3	0,014	236
21	3.3	0,015	220	220	0,015	3,270	0.01420
22	3.3	0.016	206
23	3.3			180	0,018	3,260	0.01825
24	3.3			150	0,022	3,255	0.02062
25	3.3			120	0.028	3,245	0.02424
	3.3			100	0.033	3,225	0.02989

The table consists of a total of 3 parts. The first part consists of the 3.3V (column 2) voltage applied to the 3.3V pin and a current from 0.001A to 0.016A (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, the voltage at the 3.3V pin was first measured (column 7) to see if there were really 3.3 volts across the resistor. The current measurement was then performed (column 8) to see if the calculated current actually flows through the resistor.
Measurement 0 was made to see if there were really 3.3 volts. The current measurement is omitted here, because there is no closed circuit here. Subsequently, the measurement was carried out 1 to 25. In each case once for the voltage and the current.
Observation and evaluation of the measurements

• As the resistance decreases, the voltage decreases.
• As the resistance decreases, the current increases.
• As the current increases, the voltage drops.

Findings and conclusionThe measurements show that the 3.3V pin has a lower voltage under load than expected. When a current flows, the voltage drops. The actual flowing current remains only slightly behind the calculated current. However, the difference increases from about 0.02 A or 20 mA more and more. The 3.3V pin can be charged only conditionally, because of the current drops the voltage. That is, the 3.3V pin is only partially suitable for the power supply of the external wiring. Subsequent circuit components must therefore be able to manage with less than 3.3 volts or require its own power supply.
The extent to which the findings are disadvantageous for a particular circuit depends on the exact requirements and the dimensioning of the circuit.