Transistors in Series Problem

Here is of Problem that I ran in to when working on a
AND logic gate

Hope this helps!
for my multimeter video:


for PCBways website:

thanks for watching!
-Daniel

[TheKingOfAces]

I was going NUTS trying to figure out what the bleeding issue is that I was having when making my own logic gates, and you solve it in under 4 minutes. Massive help, good video. Thank you!

[brickmasterg]

Thank you for the explanation. I couldn't find out anywhere why I was shorting it. It makes total sense. Cheers!

[lachlantheobald]

I like what you're doing here, especially the format of: problem then solution.

From a circuit standpoint, an alternative to consider is to put the load on the high side of both switches. If you have a high side NPN BJT, the control current must flow through the load, hence the switching charicteristics of the BJT are load dependent. If the load needs to sit in between two BJT's, a PNP on the high side, and NPN on the low side could achieve the same thing.

[JochemNL]

Funny how YouTube decided to recommend this. Subscribed just to support you.

[kiefac]

I think traditionally this would be solved with a pull-down resistor on the drain of the lower transistor. With snap circuits it's not much of an issue, but on real PCBs/silicon, having the load in the middle would be impractical to route for more complex circuits

[imaginitivity]

I enjoyed your video and explanation. I've been teaching my 9 y.o. son some of these principles, but I'm far from an expert. When we hit this same issue i opted to significantly increase the resistor value. The turn on current for the transistors can be vanishingly small, this then much reduces the flow through current when only one transistor is turned on. I believe there are actually situations where having three states, on/off/floating can be advantageous. IE one can achieve more outputs with the same number of inputs

[SqueakyNeb]

I don't know why YouTube is recommending me this but that was pretty good. Keep it up 👍

[zeiram]

Nice video, and good on you for taking the time to make it. I don’t think snap circuits included and MOSFETs, but in practice you’ll find logic gates are more commonly built from complementary pairs of MOSFETS. FETs are much less fiddly, and don’t generally suffer from the biasing situations you have to orchestrate with BJTs.

[lucapalermo]

Pretty good explanation! what I recommend to make it easier to understand is to show the series problem on the snap circuit first for context, THEN show the diagram to assist your explanation of how it works.

You might also want to try isolating the area from outside sound to make outside noise less audible and distracting, a quick and dirty way to achieve this is to drape thick blankets around your recording area and on the walls, this reduces the sound bouncing around the room, making an echo, and outside noise coming in.

[josephreynolds]

Cool video its nice seeing people entering the electronics scene. I'm subbing

[itsfarseen]

Great video. Please make more. Subscribed :)

[Leadvest]

Here's a lead on challenge for you, make an AND gate with 1 transistor and 0 transistors.
I also have some suggestions for simple but useful tools you may want to look into for future videos. Falstad Circuit Simulator, and OBS.
Keep up the great work!

[Tunamayo]

Try putting both the transistors switching the negative of the LED. So it goes +ve... resistor...

[oskimac]

But the led should be connected to the upper side of the circuit. The emitter of the bottom transistor to ground. And the led at the emitter of the top transistor.

[bamboozledsoftware]

I think you are new to being a youtuber/electrical engineer but at least you are having a go, good work and good luck. You could/will get rich doing this and your confidence will surely grow, don't give up unless it isn't what you want to do. (It''s not or me, atm. I'm too camera shy) Reminds me of early Electroboom without the pain. :)

[ZeldaVerde]

But then what purpose serves the top transistor? You'd be better off just connecting load to vcc and it's collector.

[iliazark]

In digital circuits, you almost always want to use NPN BJTs as your pull down network and PNPs as your pull up network. Otherwise you need more complicated circuitry to drive those transistors. So in your case putting the load between the positive and the upper transistor will work even better.

The base current flowing through the load is only one of the problems... Another important thing to notice is that your upper transistor(s) is not fully on... I bet you that if you measure the voltage drop of the two transistors on your last example, the upper one will be signifficantly higher... This may not be a problem when you drive an led, but if you try and drive a bigger load you will have a lot of unecessary power loss on that transistor.

[JuhaBaxton]

At least you're ahead of 99 million that plan to put up a video, but never did - so find your way and then beat the rest of youtube 😁 Greets from Germany

[steamcastle]

just put the load on the of the transistores, that also lets you use separate power supplies.