# Analysis of a NOT gate circuit

BS"D

Hi all.

I'm having trouble analysing the voltages/currents in various logic circuits.

For example, I can't understand WHY a NOT circuit actually works.

The circuits he presents there is attached here:

Why is it that when you press the switch, no current flows through the LED?

As far as I'm concerned, pressing the switch equals to creating a short between points A and B. So I could replace the transistor with a wire that connects to A and B.

I don't see how these prevents current from flowing through the LED.

Using the equations for parallel loads, some current still flows through the LED.

He also says that when you press the switch, it zeros the potential difference between points A and B. I can't see how this happens using the equations I have.

Thanks! :)

• BS"D Thanks for all the replies. I now see how it works :] – Arye Segal Jan 23 '18 at 23:38

The description is simplified.

When you close the switch, if the resistor values are chosen appropriately, the NPN BJT will be saturated, so Vce will be reduced to some tens of mV typically.

With tens (or hundreds) of mV across the LED the current through the LED will be very, very small (but not quite zero) and thus no (perceived) visible light will be emitted.

• Maybe change 'emitted' to 'perceived.' The LED will emit light at a visible wavelength, but the intensity will be imperceptibly low. – vofa Jan 23 '18 at 17:59
• @vofa Okay. Apparently < 10 pA is enough to get a bit of visible light. – Spehro Pefhany Jan 23 '18 at 18:07
• @SpehroPefhany at 10mv, I calculate a current in the attoamps (using the schokley equation, which may not be accurate at low V) – BeB00 Jan 23 '18 at 19:08
• @BeB00 I'm not necessarily convinced there is any light there at all, even at 100mV, but not interested enough to research it at the moment, hence the weasel wording. LEDs do appear to have a kind of defect-related threshold current that (in very old LEDs) was easily measurable. With modern LEDs it's now very small, but maybe still present. – Spehro Pefhany Jan 23 '18 at 19:19

Why is it that when you press the switch, no current flows through the LED?

When you flush the toilet it takes normally maybe 30 seconds to refill. However, if you have all the cold water taps on in the house (thus emptying water into their respective sinks and tubs), when you flush the toilet, it will now take several minutes to refill. It may never refill at all if the flow out of the taps exceeds the flow into the house.

So, you operate the switch and you turn the transistor on and it bypasses the current flow from the LED and returns it to ground/0 volts. The little bit of pressure (I mean voltage) left at the transistor collector won't be enough to pass current into the LED and it doesn't illuminate.

• And also what you ate last night... – Trevor_G Jan 23 '18 at 18:51
• @Trevor_G That adds a whole new dimension to the answer. Maybe it just muddies the water too much hehe. – Andy aka Jan 23 '18 at 19:06
• They should add "severe analogy alert" as a subtitle to the whole forum. That and "May contain nuts!" :D – Trevor_G Jan 23 '18 at 19:14

All transistors when used as a switch such as this are inverting voltage with respect to base input and collector output voltage. However normally when a switch is closed it is turning something on. Here it is the opposite.

Normally a series switch is used, but here it is a shunt switch. So Vce only has to be less the the forward voltage, Vf of the LED diode to turn it off. To make that happen Rb/Rc just has to be less the the current gain hFE. e.g. <100 but normally we operate a switch in saturation and hFE drops to 10~20 in this state.