Please explain 'no' current to PNP transistor - Page 1
Forum section: electronics / beginners

author: windmill john
author_rank: Contributor
timestamp: February 02, 2023, 05:54:40 pm
content: Hi All,Please have a read of the attached picture.I can see why the NPN transistor turns on the top half of the LED. Not totally sure why the PNP doesn't turn on the bottom half at the same time.To clarify my lack of understanding, if that's not a contradiction in term! I guess this is an OHM's law question.There is a positive feed to the base of the PNP via the 2.2K resistor. So is the reason it does not light the bottom half of the LED purely because the resistance 2.2K ohm is a lot higher than the 1K resistance to the base of the NPN?The comment about the diode confuses me because surely current flow is left to right through it in the diagram?Thanks.Beginner section, so be gentle

author: langwadt
author_rank: Super Contributor
timestamp: February 02, 2023, 06:20:28 pm
content: flip the picture upside down then it is much easier to understand

author: shapirus
author_rank: Super Contributor
timestamp: February 02, 2023, 06:28:19 pm
content: When the switch is open, current flows (remember that current direction is positive->negative, and the arrows on the transistor symbols follow this convention) like this:Code:battery_pos -> 1K resistor -> NPN base -> NPN emitter -> LED anodes -> LED cathode (COM) -> battery_neg`-> NPN collector -^It can't flow via the PNP base, because its possible path is blocked with the diode.When the switch is closed, NPN transistor can't turn on, because current is diverted back through the diode to battery negative, being the path of lowest resistance, instead of going into its base. But now current can flow via:Code:battery_pos -> PNP emitter -> PNP base -> 4.7K -> switch -> battery_neg`-> PNP collector -> LED anodes -> LED cathode -> battery_negThe 2.2K resistor is optional, as I see it. Possibly added to pull the PNP base high to prevent reverse current flow through the germanium diode (I don't remember off the top of my head what their typical values were).It's easy to understand cicruits like this when you consider the possible paths and directions for current to flow.Using animated simulators likehttps://everycircuit.com/apphelps a lot in understanding the basics.

author: bdunham7
author_rank: Super Contributor
timestamp: February 02, 2023, 06:32:37 pm
content: Quote from: windmill john on February 02, 2023, 05:54:40 pmAnd? What is needed to turn on a PNP transistor according to the last sentence of the first paragraph?The operation of the diode and the need for it to be germanium is a bit more complex, but yes current will flow through it from left to right when the switch is closed.

author: shapirus
author_rank: Super Contributor
timestamp: February 02, 2023, 06:40:27 pm
content: Quote from: bdunham7 on February 02, 2023, 06:32:37 pmThis is a good point btw. Considering that germanium diodes are now mostly phased out, this circuit isn't quite an ideal educational example, and even if they were still used, it would need an explanation. A good homework task will be to redesign this circuit with a requirement that the germanium diode must be replaced by a silicon one (no cheating! P-N diodes!), and still have it working properlyup: in fact, I forgot about those LEDs and their junction drop. This circuit, unless I'm wrong, will work as expected with a silicon diode too, provided that the NPN transistor is also a silicon one.

author: floobydust
author_rank: Super Contributor
timestamp: February 02, 2023, 06:44:25 pm
content: The NPN is normally on all the time, due to base current flowing in 46, supplied by the 1k resistor.The PNP is normally off all the time, there is no path for base current to flow out of 40 to ground. The diode blocks current from flowing that way.When the key is pressed, the NPN is turned off because the key shunts the base current from the 1k through the diode to ground, so it does not reach the transistor enough to light the LED's.When the key is pressed, the PNP is turned on because the key provides a path for its base current, to ground. Current flows in the opposite direction in a PNP compared to NPN.Not sure if the transistors are germanium like in the old 100-in-one kits from the 1970's. They could be silicon here but I remember Radio Shack kits calling them the "2SA" and "2SC".The difference is in the emitter-base voltage needed to turn them on, i.e. 0.3V verses 0.7V

author: Zero999
author_rank: Super Contributor
timestamp: February 02, 2023, 08:00:39 pm
content: It's a good educational example of how not to draw a circuit.Upside down.No 0V/earth/ground point.No +V supply voltage reference.I've redrawn it. Hopefully this will make it easier to understand.

author: MrAl
author_rank: Super Contributor
timestamp: February 02, 2023, 08:19:10 pm
content: [ Specified attachment is not available ] [ Specified attachment is not available ]Quote from: windmill john on February 02, 2023, 05:54:40 pmHi,I was off the computer for a while and didnt see that Zero999 had the same idea i had.I already drew it up though so here is a better rendition of the circuit maybe this will help.I also removed the random node numbering for clarity.You have to click on the thumbnail to view the diagram it would not let me post it full size.I have included five different color LED approximate voltages that would appear at the emitter as shown with the switch off. The base is just about 0.7v higher than the LED voltage if a regular silicon transistor or a bit less if germanium so you can take it from there.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 02, 2023, 09:16:57 pm
content: I'd say trow away that book and get another one.If you have a book trying to explain the basic workings of tranistors, and they start by turning the thing upside down and obfuscaiting the power supply, then that book has already gathered so much negativity that it would be a one way ticket to the garbage bin for me.Also, transistors are not "turned on by voltage", but they amplify current. That is another reason to throw that book in the garbage.About the simplest (and incomplete) way to describe a transistor is that te BE junction is a diode, and the CE current is equal to the BE current, but amplified by a constant factor (called Beta, or Hfe).And of course, if the "programmed CE current can not flow, then it is limited by external circuitry, this is called "saturation", and in that case the voltage over CE can be quite low (can be as low as 100mV).

author: windmill john
author_rank: Contributor
timestamp: February 02, 2023, 09:19:47 pm
content: Thanks all. Being very new I still find it a little baffling, especially as I thought and you say the current can flow left to right through the diode..I think I’ll wait until tomorrow, print out a copy of the circuits, so I can read all your posts again at the same time as studying the original and the modded ones; bedtime now!

author: shapirus
author_rank: Super Contributor
timestamp: February 02, 2023, 09:35:25 pm
content: Quote from: Doctorandus_P on February 02, 2023, 09:16:57 pmIn addition, it is either very old (germanium diode, what, really?) or uses circuits taken from old sources.Not that it is necessarily a bad thing, but for education it is generally better to use more recent literature which illustrates circuits as they are normally drawn today, as well as uses components commonly used and available today.

author: james_s
author_rank: Super Contributor
timestamp: February 02, 2023, 09:47:12 pm
content: Quote from: shapirus on February 02, 2023, 09:35:25 pmIt's obviously the book from one of those 150 (or whatever) in one electronics kits with the little spring terminals, hence the numbers at each connection. If you throw away the book you won't be able to build the circuits by following the instructions. These old kits are mostly collectibles at this point, usually they did have at least one germanium diode which was also used as the detector in the radio receiver circuits.

author: shapirus
author_rank: Super Contributor
timestamp: February 02, 2023, 10:01:00 pm
content: Quote from: james_s on February 02, 2023, 09:47:12 pmwhich, by the way, could be used to listen to AM radio without batteries.

author: ledtester
author_rank: Super Contributor
timestamp: February 03, 2023, 12:19:44 am
content: This Falstad simulation might be helpful:(Falstad simulation)When you press the switch note that the current through the 1K resistor flows through the diode instead of the NPN base -- this is the reason why the NPN turns off and the top LED goes dark.Why does no current flow through the PNP when the switch is open?In order for the PNP to conduct current would have to flow out of the base (from 42 to 40). But from node 40 the only path to ground is through the diode and the diode blocks current going from 125 to 126. So no current flows out of the PNP base and thus the PNP does not conduct current from 42 to 41.When the switch is closed, current out of the PNP base can get back to ground via the switch.

author: floobydust
author_rank: Super Contributor
timestamp: February 03, 2023, 12:51:04 am
content: The ground point is wrong, it's at the mid point of the 9V battery and I don't see how you select that. edit: Inputs and Sources -> Add Ground... nuthin' happening

author: MrAl
author_rank: Super Contributor
timestamp: February 03, 2023, 06:41:47 am
content: Quote from: ledtester on February 03, 2023, 12:19:44 amHi,That's cute but can you get the node voltages to show up too. That is also important to understand why the currents flow as they do.

author: shapirus
author_rank: Super Contributor
timestamp: February 03, 2023, 11:29:29 am
content: Quote from: floobydust on February 03, 2023, 12:51:04 am"Ground" is always a virtual, relative thing (unless we're talking real earth ground) and it can be wherever you decide to assign it. There can be no "ground" at all (however many simulators will refuse to run until a ground point is defined). Where is ground in a circuit consisting only of a LED, a dropper resistor and a battery?

author: windmill john
author_rank: Contributor
timestamp: February 03, 2023, 01:06:08 pm
content: Quote from: ledtester on February 03, 2023, 12:19:44 amThanks again all, just hanging in there......ledtester, just need clarification as to why the current doesn't flow through the NPN base as well as the diode. Cannot see the reason for that, even in zero999's picture. Sorry for my (thickness) misunderstanding.

author: shapirus
author_rank: Super Contributor
timestamp: February 03, 2023, 01:24:04 pm
content: Quote from: windmill john on February 03, 2023, 01:06:08 pmbecause in order to do that, it has to overcome the Vbe drop of the transistor (~0.3V for Ge, ~0.6V for Si) plus the junction drop of the LED (~1.8V? not sure exactly) whereas to flow through the diode, it has to only overcome the ~0.3V drop of the Ge diode junction, and that is the way it "chooses": the least resistance path. There is no proportional division of current taking place here (like you see in resistors), since we're talking about potential barriers (correct me if I'm wrong), and voltage at point 46 doesn't have a chance to rise above the point where it would be able to cause current flowing into the NPN base.Imagine a dam on a river divided into two halves: one is a couple meters higher than the other. All the water, unless there is too much of it, flows through the lower section, never having a reason to flow through the upper one. Hydraulic analogies often work very well to help understand electricity, so they do in this case: water height = voltage, volume = charge, flow rate (volume/time) = current, pipe cross section = 1/resistance, p-n junction voltage drop = dam height.

author: Zero999
author_rank: Super Contributor
timestamp: February 03, 2023, 01:26:18 pm
content: Quote from: Doctorandus_P on February 02, 2023, 09:16:57 pmAgreed.QuoteThat's not strictly true.QuoteThat's only valid when considering a transistor as a switch. In a linear application, it's easier to analyses a BJT as a voltage controlled current source, with the base current being necessary to bias the transistor.Quote from: shapirus on February 03, 2023, 11:29:29 amYou're right. It is arbitrary, but in this case, the circuit is easiest to understand with 0V being on the negative of the two 4.5V batteries.A 0V reference is required to perform any calculations. Some simulators will just put it on a node which they think is convenient such as the negative side of a voltage source, in this case. It might be putting the 0V between the two batteries, rather than one end, because that configuration is commonly used to get a bipolar supply for an op-amp circuit, or it could be the negative of first voltage source it sees.Quote from: windmill john on February 03, 2023, 01:06:08 pmThe diode is germanium, which has a forward voltage of 0.3V, whist the transistor is silicon and has a base-emitter forward voltage of 0.6V. The transistor's emitter is also connected in series with an LED, which has a forward voltage of around 2V.

author: windmill john
author_rank: Contributor
timestamp: February 03, 2023, 05:36:22 pm
content: Path of least resistance…..Thanks both again. It does look like Ohm’s law rules the roost. I do appreciate everyone’s posts, the dam analogy works at my level

author: ledtester
author_rank: Super Contributor
timestamp: February 04, 2023, 12:06:33 am
content: Quote from: MrAl on February 03, 2023, 06:41:47 amSimulation with voltage probe across the NPN B-E junction added:(Falstad simulation)You can add more probes with Draw -> Outputs and Labels -> Add voltmeter/Scope Probe.I also added a ground terminal -- kinda surprised that Falstad doesn't require it. It probably just picks an arbitrary node if you don't have a ground symbol in your circuit.

author: floobydust
author_rank: Super Contributor
timestamp: February 04, 2023, 02:34:27 am
content: Thank you for the Falstad simulations you post. I think it makes it much easier for a beginner to see current flow animated and mouse-over for voltages as well. With the oddball ground reference point the sim had +/-4.5V rails which is not straightforward when learning how this works.Funny thing is shorting the diode does nothing, I had to laugh at these circuits they are sometimes what was in the designer's head.The old 100-in-one from the 1970's didn't waste a single part or wire.

author: james_s
author_rank: Super Contributor
timestamp: February 04, 2023, 03:33:45 am
content: The original circuit doesn't have a split supply. the kit just has two banks of AA cells which are connected in series for this particular circuit. I'm actually pretty sure I had this same kit when I was a kid, I vaguely remember this specific circuit, although I suspect the same circuits were used across many of the kits. It's actually pretty impressive they managed to make so many different circuits with such a limited collection of parts.

author: windmill john
author_rank: Contributor
timestamp: February 04, 2023, 02:13:17 pm
content: How dare you all criticise my 130 in 1 Maxitronix kit!I’ll order some breadboard and bits. The kit isn’t visual as components are so spread out. If I stick them on a breadboard and lay then out in a… better manner, it may help me understand better…

author: james_s
author_rank: Super Contributor
timestamp: February 04, 2023, 07:14:44 pm
content: I think in a lot of cases they were scraping the bottom of the barrel in order to come up with as many circuits as possible that could be built using the limited selection of components. It's hardly surprising that some of them are not as well designed as others. In this case it's the way the schematic is drawn and the lack of description of how it works that is the biggest issue.

author: IanB
author_rank: Super Contributor
timestamp: February 04, 2023, 08:01:12 pm
content: Quote from: windmill john on February 04, 2023, 02:13:17 pmThe attached picture may help to explain the behaviour of NPN and PNP transistors when used in a switching configuration. Using a breadboard you should be able to lay things out so that the arrangement follows this diagram.(If I somehow have a mistake here, I'm sure someone will correct me.)

author: EPAIII
author_rank: Super Contributor
timestamp: February 05, 2023, 10:33:09 am
content: Yes, the jackass who drew that schematic did not know how to draw schematics. A schematic should be drawn in a manner that makes the operation of the circuit as plain and understandable as possible. That one fails miserably.And the jackass who wrote the text is equally bad. It states that a "Voltage" applied to the base of what are apparently BJTs is what controls them and causes them to conduct. BJTs are current operated devices, not Voltage. I don't care if there is 1,000,000 Volts on the base of a transistor, if there is no current flowing, it is not going to conduct. Except by turning to carbon.I would toss that book in the fireplace and look for a better source of electronic knowledge.Quote from: langwadt on February 02, 2023, 06:20:28 pm

author: IanB
author_rank: Super Contributor
timestamp: February 05, 2023, 10:59:30 am
content: Quote from: EPAIII on February 05, 2023, 10:33:09 amThe equations that describe the operation of a BJT have the base-emitter voltage vBEas the primary variable, so saying this voltage is what controls the transistor is entirely reasonable:https://leachlegacy.ece.gatech.edu/ece3050/notes/bjt/thebjt.pdf

author: magic
author_rank: Super Contributor
timestamp: February 05, 2023, 12:30:43 pm
content: I have seen several old schematics with negative supply and ground at the top; it seems to be the way things were done in germanium era.Quote from: EPAIII on February 05, 2023, 10:33:09 amIf there 1,000,000 V on the base and no current flowing then there is 1,000,000 V on the emitter too - of course not going to conduct.And if it isn't conducting, it isn't turning into carbon either

author: m k
author_rank: Super Contributor
timestamp: February 05, 2023, 12:56:58 pm
content: Maybe the maker has thought the straight through schematics is too obvious.Was there somewhere that transistor is not Ge?Though "NPN is turned off because negative voltage is applied" and "through the diode" is a bit off, for today.Much better explanation, for today, since turning on and off 7-segment parts was the thing, would have been explaining things towards Common(25) level.Then one threshold voltage needed over the diode vs. two, and then some, needed over transistor and LEDs is also eminent.No current to PNP is also eminent then, since Common is floating, since LEDs are in opposite direction.

author: ledtester
author_rank: Super Contributor
timestamp: February 05, 2023, 02:02:40 pm
content: Quote from: magic on February 05, 2023, 12:30:43 pmRelated thread:https://www.eevblog.com/forum/projects/old-schematics-positive-rail-at-the-bottom/

author: magic
author_rank: Super Contributor
timestamp: February 05, 2023, 03:51:16 pm
content: Did I really write this?Yes, that's what I meant, negative rail at the top. Contrary to modern convention.

author: Zero999
author_rank: Super Contributor
timestamp: February 05, 2023, 04:33:10 pm
content: Quote from: EPAIII on February 05, 2023, 10:33:09 amTo be fair the verbal description of the circuit is fairly good. It's possible there was a reason for the odd schematic. The original poster has stated it came with a kit. Perhaps it matches the layout of the components attached to the unmodifiable breadboard, thus making it easier to build.Quote from: ledtester on February 05, 2023, 02:02:40 pmExcept, the circuit discussed in that thread made sense, with a negative rail. The only thing they got wrong was not denoting the 0V reference, which should be the +V of the battery in that case.

author: floobydust
author_rank: Super Contributor
timestamp: February 05, 2023, 04:59:14 pm
content: This is how you explain electronics... it actually saved me as a kid from disliking the whole thing. I was too young for the kit but got into it due to the cartoons.

author: james_s
author_rank: Super Contributor
timestamp: February 05, 2023, 06:57:40 pm
content: Quote from: EPAIII on February 05, 2023, 10:33:09 amOnce again, that book is part of a collectible vintage circuit lab kit which is useless without the book so it is not helpful to suggest throwing the book away. These "xx in one" things were toys for young children, not serious educational sets.

author: james_s
author_rank: Super Contributor
timestamp: February 05, 2023, 07:13:29 pm
content: This is the one I had when I was a kid, it came from a garage sale, I must have been around 6 years old.https://medium.com/@rxseger/electronic-project-kits-hands-on-with-a-vintage-160-in-1-eea39e6193f4It was much more nicely made than the newer ones, having a wooden case. I had several others over the years, eventually I got the 200 in 1:https://www.petervis.com/electronics-lab/200-in-one-electronic-project-lab/200-in-one-electronic-project-lab.htmlI don't know that they were all that educational really, but it was fun to build the circuits and see them do something.

author: james_s
author_rank: Super Contributor
timestamp: February 05, 2023, 07:19:21 pm
content: Quote from: magic on February 05, 2023, 12:30:43 pmThe earliest transistors were all PNP, circuits that would normally be built today with NPN used PNP instead and were drawn upside down by modern conventions. This was also the era of transitioning from vacuum tubes to solid state so most engineers were used to working with tubes. You saw a lot of vestiges of the tube era, such as output transformers used in solid state amplifiers. The electronic kits in question were popular from the early 70s into the late 80s.

author: Zero999
author_rank: Super Contributor
timestamp: February 05, 2023, 08:44:55 pm
content: Quote from: james_s on February 05, 2023, 06:57:40 pmI agree. I was too hasty to support the idea of throwing it out.It would be better to try to understand it, by redrawing the schematics in a more legible manner and ask more questions here.

author: floobydust
author_rank: Super Contributor
timestamp: February 06, 2023, 01:04:16 am
content: Quote from: james_s on February 05, 2023, 07:13:29 pm100-in-1 $21.95 New for 1972 ($150 in today's dollars) ran until 1976. I still have that one but can't pinpoint the year I got it. Guessing '73. Really expensive with a family income median $11,500/year back then.150-in-1 $29.95 New for 1977 ($135 in today's dollars)source:Radio Shack CatalogsI think RS and Science Fair Kits shaped a nation of kids learning electronics. We'll never see a two transistor VCO or RF Wireless Burglar Alarm again. I don't know who designed these kits.

author: IanB
author_rank: Super Contributor
timestamp: February 06, 2023, 02:24:33 am
content: I remember really wanting one of those kits in the mid-70's, but the price was out of reach

author: floobydust
author_rank: Super Contributor
timestamp: February 06, 2023, 03:36:34 am
content: The entire family pitched in, I think it was my uncle that actually wanted it for himself. Weren't rich, it covered a birthday present as well.60, 75, 100, 150, 160, 200-in-1 there was a craze about having more circuits, every year as part of the competition.I remember the rich kids got the 150-in-1's and rubbed it in my face, and then they didn't build more than a few circuits and shelved it. To get the circuit count up there, it became a game of just adding many useless builds, circuits that were boring and or useless. Too bad it was a number race.Sadly I see Elenco discontinued all their project lab kits: the 500-in-1, the 300-in-1, 200-in-1, 130-in-1 etc. I wonder if these are still made or kids have any interest.