I'm using an NPN to pull a 12v (5mA) signal low from an MCU (logic level). My problem is that when I pull the 12v line low, 7.4v are still present at the 12v side of the line using the circuit below:

Circuit 1

When I remove R2 and connect the signal directly to the collector pin on the NPN, about 110mV are still left on the line. See below: Circuit 2

Why isn't the NPN fully pulling the line low, and what can I do to fix it?

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    \$\begingroup\$ In your 2nd schematic, your output signal will always be 12V, and your input !signal will only control how hot your poor transistor gets. Are you sure this is what you intended to draw? \$\endgroup\$ – brhans Apr 25 '19 at 18:42
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    \$\begingroup\$ Are you sure the resistor is actually 4.7K? \$\endgroup\$ – Spehro Pefhany Apr 25 '19 at 18:46
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    \$\begingroup\$ reduce R1 and increase R2. or use a logic-level N-chan FET \$\endgroup\$ – dandavis Apr 25 '19 at 18:49
  • \$\begingroup\$ @brhans - I'm a hobbyist, so bear with me -- Yes, this is how I've wired it on my breadboard and tested with my multimeter. Perhaps I'm measuring incorrectly. \$\endgroup\$ – t3ddftw Apr 25 '19 at 18:50
  • \$\begingroup\$ @SpehroPefhany - Actually, you're right -- it's a 5.6k resistor. I don't have a 4.7k to test with, but I tried 270ohms and still found about 1.2v at the signal. \$\endgroup\$ – t3ddftw Apr 25 '19 at 18:51

Most of the commenters so far seem to think that the line you labeled "12V" is a power supply. However, I take it that you mean that it is a control signal that is pulled up to 12V — presumably through a pullup resistor of 2400 Ω, since you also specify a current of 5 mA.

Your second circuit is correct, and 110 mV is perfectly reasonable for a saturated NPN transistor @ 5 mA. If your equipment really requires a lower voltage drop than that, then you'll have to try either a logic-level MOSFET or a mechanical relay. Are you saying that the radio is failing to mute using the transistor?

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  • \$\begingroup\$ Dave -- Bingo! I should've made it more clear that the 12v line is a digital logic line only passing 5mA (so your presumption is likely correct). I assumed that the equipment required 0v to "respond" to the mute signal, but I will do some testing to see if the 110mV is low enough to trigger the mute. Given the 5mA being passed, I'm safe to use schematic #2 without risking the BJT, right? \$\endgroup\$ – t3ddftw Apr 25 '19 at 20:14
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    \$\begingroup\$ Right. The 2N3904 would safely handle several hundred mA. \$\endgroup\$ – Dave Tweed Apr 25 '19 at 20:15
  • \$\begingroup\$ Perfect! I will test the circuit with speakers connected to ensure the output is muted. Thanks for your help! \$\endgroup\$ – t3ddftw Apr 25 '19 at 20:26

The NPN should pull the signal line down when enabled to the 10mV's range. In the second schematic you show an NPN connected directly to the 12V line. This would dissipate approximately 1 watt which is exceeding the absolute maximum rating of the part and probably causing it to fail.

Check the 2n3094 with a multimeter, if it's bad replace it (or just replace it with a new one). IF it looks good then check the resistor or the wiring. Do a conductivity test on the circuit with the meter to make sure it matches the schematic.

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  • \$\begingroup\$ Actually, the NPN tested out good. I tested at signal and I'm indeed getting sub-100mV voltage using schematic #1. However, why am I still seeing ~7v on the 12v side of R2? Essentially, I need to pull that 12v signal to GND, so I'm just a little confused :) \$\endgroup\$ – t3ddftw Apr 25 '19 at 19:52
  • \$\begingroup\$ How much current can the 12V signal source? \$\endgroup\$ – Voltage Spike Apr 25 '19 at 20:13
  • \$\begingroup\$ 5mA is what it tested at. \$\endgroup\$ – t3ddftw Apr 25 '19 at 20:14

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