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The connections are as follows:

  • [+5 V from arduino pin] => 680 Ω resistor => C1815 transistor's base pin (3)
  • [+14 V from variable power supply] => C1815 transistor's collector pin (2)
  • [C1815 transistor's emitter pin (1)] => Load => [Negative from variable power supply].

Oscilloscope probe connected at C1815 transistor's emitter pin (1).

Oscilloscope probe's ground connected at negative from variable power supply.

The arduino PIN in outputing 5v for 1 second, and then 0v for another second in a neverending loop, producing a square-wave pattern on the oscilloscope.

The problem is as follows:
When the variable power supply is turned off, the oscilloscope displays alternating voltage between 0 and 1.5 V.

However, when it's turned on, it displays alternating voltage between 0 and 5 V.

My understanding it that it should alternate between 0 and 14 V, since the collector voltage is 14 V.

Where am I wrong?

Circuit image : enter image description here

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  • 5
    \$\begingroup\$ Draw a schematic. \$\endgroup\$
    – Hearth
    Feb 7, 2023 at 2:39
  • \$\begingroup\$ @Hearth There you go \$\endgroup\$ Feb 8, 2023 at 14:52

2 Answers 2

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The 600 Ohm resistor connects to +14 V, and the coil and diode to the collector, and the emitter to GND.

The negative pole of your 14 V source must also be connected to ground which is negative of the 5 V source. Without this connection of nothing works really. Maybe you forgot it in the sketch. But even then your circuit can not give more voltage on the emitter a (base voltage - base drop).

Something like this

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  • \$\begingroup\$ So if i understand correctly, per your schematic, ( -14 V & -5 V & emitter) should all share a common ground ? \$\endgroup\$ Feb 8, 2023 at 22:33
  • \$\begingroup\$ Connecting both ground worked. Thanks ! \$\endgroup\$ Feb 9, 2023 at 1:35
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With the load between the transistor emitter and Ground/negative, you have an emitter follower. The emitter voltage will be about 0.7 volts below the base voltage.

I assume the Arduino is applying a pulsing voltage to the transistor base.

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  • \$\begingroup\$ mmm Interessting , never tought of that. Considering the schematic above, any advice on what i should change to break the emmitter follower pattern ? Its a new concept to me, and im not sure why it's hapenning \$\endgroup\$ Feb 8, 2023 at 15:25
  • \$\begingroup\$ @Charles-UgoBrouillard Use the transistor as a low-side switch, with the load between +14 V and the collector. \$\endgroup\$
    – Hearth
    Feb 8, 2023 at 15:46
  • \$\begingroup\$ @Hearth Perfect, and just to be sure , also connect the emitter of the transistor to the negative side of the 14 V power supply ? \$\endgroup\$ Feb 8, 2023 at 20:43
  • \$\begingroup\$ Read on the low-side switch, much needed insight in this situation. Thanks @Hearth \$\endgroup\$ Feb 9, 2023 at 1:36

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