hybridfree/eevblog-posts huggingface dataset:
Why can a capacitor accumulate charge? - Page 1
Forum section: electronics / beginners

author: mpp2508
author_rank: Contributor
timestamp: September 25, 2025, 09:35:18 am
content: Attached is a diagram and oscilloscope trace; it should be clear. The capacitor shouldn't charge when the laser diode shunt is turned on, but it does. I can't figure out how this is possible.

author: mpp2508
author_rank: Contributor
timestamp: September 25, 2025, 10:11:17 am
content: As soon as I asked the question, a guess immediately occurred to me. I checked it, and it turned out to be correct.The analog switch needed to be powered from the same voltage as the operational amplifier.
Why can't we adjust Bandwidth limit in scopes to the value we want - Page 1
Forum section: electronics / beginners

author: JustAnotherGuy999
author_rank: Contributor
timestamp: February 29, 2016, 11:37:02 pm
content: Most scopes give some predefined bandwidth limits values (like 20MHz), and don't give the right to adjust it to an arbitrary value say (17MHz). why?

author: tggzzz
author_rank: Super Contributor
timestamp: February 29, 2016, 11:47:15 pm
content: Have a look atthis.It will help overcome the observation that you ask a lot of questions, i.e. ~14 very different threads started in 4 days

author: Fred27
author_rank: Unknown
timestamp: March 01, 2016, 12:06:25 pm
content: Is the correct answer " because you want to get your post count up to enter the 'scope giveaway"? ;-)
Why can't I test the transistor part of an optocoupler using my multi-meter? - Page 1
Forum section: electronics / beginners

author: ddewaele
author_rank: Newbie
timestamp: August 31, 2020, 07:52:30 am
content: I'm attempting to test if my PC111 opto-coupler is working properly (pulled from a broken PSU).pin 1 = anodepin 2 = cathodepin 3 = NCpin 4 = emitterpin 5 = collectorpin 6 = baseI wanted to use my multimeter first (so the device is not connected to anything), setting the multimeter in the diode test mode.Testing pin 1 and 2 (the LED) was fine. I saw the voltage on 1 end and OL on the other.I assumed that the output transistor could also be testing this way as the base pin is also exposed,I thought base (pin 6) -> emitter (pin 4) and base (pin 6) -> collector (pin 5) would show a voltage drop (like with a regular NPN transistor).However this was not the case and always shows OL (I tried it with another opto-coupler also).I then tried both opto-couplers in a simple circuit with an LED (hooking up 5V through 2K resistor to the optocoupler) to turn on the LED (with separate power source) and that worked fine.Someone also told me that I should check for continuity between emitter and collector when the optocoupler is activated but also that doesn't work.So why can't I test the transistor using my multimeter that way (diode test and continuity) ?I tried it with a second opto-coupler (this time pulled from a working PSU). A CNY17GF-2. Exact same thing.Multimeter diode test on transistor pins fails, simple circuit to turn on LED via opto-coupler does work.I then bought a brand new CNY17 (same pin layout as the previous 2 opto-couplers), and there the multimeter diode test on transistor pinsdoes work.Any idea why the multimeter diode test on transistor pins fails for the first 2 opto-couplers but turning on an LED using a basic opto-coupler circuit does work ?

author: magic
author_rank: Super Contributor
timestamp: August 31, 2020, 08:56:23 am
content: May have something to do with this:https://www.eevblog.com/forum/projects/different-die-pictures/msg3173898/#msg3173898The secondary of the dismantled optocoupler is clearly not a single transistor but an IC.editOkay, H11L1 is specified to have a buffered output so duhStill, there is a chance of some shenanigans in the supposedly "single transistor" optocouplers too.

author: Andy Watson
author_rank: Super Contributor
timestamp: August 31, 2020, 10:17:32 am
content: The manufacturers are messin' with youhttps://www.datasheetq.com/ndown.jsp?url=/datasheet-download/Sharp/556034.pdfDatasheet for PC110, PC111, PC112, and PC113 - says "PC111 and PC113 have no base terminals."

author: ddewaele
author_rank: Newbie
timestamp: August 31, 2020, 10:55:28 am
content: Quote from: Andy Watson on August 31, 2020, 10:17:32 amNo idea how I missed thatAnd same thing with the CNY17GF apparently. Only the newer CNY17 have their base pin exposed.Thx !
Why Cheap LED Lights Keep Glowing - Page 1
Forum section: electronics / beginners

author: Lucky-Luka
author_rank: Unknown
timestamp: July 06, 2019, 09:17:26 pm
content: Hi allI've watched this funny video about a strange (at least for me) phenomena involving even some led lights in my house: they glow even when I turn off their swithes!At the end of the day I haven't understood the part at 7:00The output voltage can rise the input voltage? has he said that? I couldn't understand his pronunciation.So why do those LED keep glowing?Can somebody help me to understand better?Cheers

author: ataradov
author_rank: Super Contributor
timestamp: July 06, 2019, 09:52:46 pm
content: "The output voltage can rise with the input voltage".Even open switch and wires attached to it have some capacitance. It is not big, but it dos not matter, capacitor in the bulb will charge and there is nothing to discharge it, except for the LEDs.

author: radiolistener
author_rank: Super Contributor
timestamp: July 07, 2019, 05:39:57 am
content: Quote from: Lucky-Luka on July 06, 2019, 09:17:26 pmbecause when you touch it, your body will turns into one plate of the capacitor. And the second plate of this capacitor is mains wire in your room (or may be ground). So, the LED is connected through ballast capacitor. This capacitor has too small capacitance, but it is enough for glowing. And this is why it glowing.You can replace your body with a long wire, so there will be capacitance between this wire and ground (or other mains wire).
Why connecting ESD mats in series is a no-go? - Page 1
Forum section: electronics / beginners

author: drogus
author_rank: Contributor
timestamp: December 16, 2015, 05:42:59 pm
content: In the episodeDave mentioned that what you want is to install ESD equipment in a star configuration, ie. all of the mats and wrist straps should be connected to one earth bonding point. Why is it important to connect it this way?I have also another related question. I bought an earth bonding point with 1M? resistors and I just noticed that my connecting cables also have 1M? resistance. Is it OK?

author: Gyro
author_rank: Super Contributor
timestamp: December 16, 2015, 06:25:54 pm
content: It's pretty common (almost universal?) to have wrist strap studs on the front edge of the bench mat, it's not really practical to do anything else due to coiled lead length. Multiple 1M resistors are fine - the important thing is to have a minimum of 1M between any two points to avoid shock risk and prevent any possibility of a fault current passing though the mats. It's common sense, practically, to star ground floor and bench mats to the common grounding point (via resistive leads).edit: typo

author: dom0
author_rank: Super Contributor
timestamp: December 16, 2015, 06:52:57 pm
content: When you have multiple (most mats have two) of these connectors that look like poppers on clothing ... grab a DMM, measure the resistance between them. It'll be low, somewhere around 100 k? or so. It's pretty normal to connect the mat to the PE point on the rear popper and use the popper in the front to connect your wrist strap.

author: Gyro
author_rank: Super Contributor
timestamp: December 16, 2015, 07:31:44 pm
content: Yes, rear popper for ground connection, front one for the wrist strap. Ideally you only want one popper within touching distance as the resistance between two poppers on the same mat can be quite low (due to the more conductive bottom layer of most mats).
Why could increasing biasing currents increase the bandwidth? - Page 1
Forum section: electronics / beginners

author: Evy
author_rank: Contributor
timestamp: August 15, 2017, 04:30:52 pm
content: How can an increase of biasing currents increase the bandwidth of a negative feedback amplifier?Please explain in easy terms since I am pretty new to electronics.Thanks.

author: w2aew
author_rank: Super Contributor
timestamp: August 15, 2017, 05:01:12 pm
content: In general - increased bias currents mean that lower resistances are generally used. For example, a load resistor might be 200 ohms instead of 20kohms. Why does this affect BW? In a word - parasitics. Every component has some (unwanted) parasitics like lead inductance, shunt capacitance. These parasitics are generally related to the physical construction of the device, not the value. So, a 200 ohm resistor will have the same parasitic inductance and capacitance as a 20kohm resistor. At a given frequency, the parasitic elements will alter the value of a 20k resistor much more than a 200 ohm resistor - thus making the 200 ohm resistor usable at higher frequencies.

author: rfeecs
author_rank: Frequent Contributor
timestamp: August 15, 2017, 06:32:11 pm
content: Quote from: w2aew on August 15, 2017, 05:01:12 pmIt could help with parasitic capacitances, but not inductances. Higher current could charge/discharge capacitances faster. As you say, for smaller resistances, capacitance will have a smaller effect but inductance will have a larger effect.For bipolars, higher current would give higher transconductance, which might help with gain BW productThe question is too vague.

author: danadak
author_rank: Super Contributor
timestamp: August 15, 2017, 11:21:00 pm
content: Confirming its a direct impact on Ft.https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-720j-integrated-microelectronic-devices-spring-2007/lecture-notes/lecture37.pdfRegards, Dana.
Why crystal oscillators called XTAL in electronics? - Page 1
Forum section: electronics / beginners

author: hurricanehenry
author_rank: Regular Contributor
timestamp: October 14, 2015, 09:23:54 am
content: Curious: Why are crystal oscillators referred to as "XTAL"?

author: rjeberhardt
author_rank: Regular Contributor
timestamp: October 14, 2015, 09:32:55 am
content: Good question. I wonder when the term was first used.Russell.

author: Balaur
author_rank: Unknown
timestamp: October 14, 2015, 09:39:24 am
content: X is the Greek Chi, the first letter of the word Christos - Christ.In English, X can be used as an abbreviation for Christ (see XMas)The pronunciation in Greece and in some other Orthodox Est European countries of the "Chris" part is close to the English Crys.Therefore Crystal => Xtal

author: rs20
author_rank: Super Contributor
timestamp: October 14, 2015, 09:42:06 am
content: C was already used for Capacitor/Condenser.

author: singapol
author_rank: Frequent Contributor
timestamp: October 14, 2015, 10:36:16 am
content: None of the above...It began in scientific chemistry.How the physical elements are organised and it'sbonding structure...the secrets of nature. As to the X in xtal ,the atoms are connected by lattices so the basic structure is a cross like snowflakes.The following may be hard to digest for some:http://www.chem1.com/acad/sci/aboutwater.htmlhttp://snowcrystals.com/http://www.gcsescience.com/a13-structure-ionic-compounds.htmhttp://www.xtal.iqfr.csic.es/Cristalografia/parte_01-en.htmlhttp://www.chem.ucl.ac.uk/ice/docs/Michaelides_Scheffler_Chapter_Revised_10_Aug_07.pdfhttp://www.scienceinschool.org/2012/issue24/crystals

author: German_EE
author_rank: Super Contributor
timestamp: October 14, 2015, 08:39:18 pm
content: I was always told that the Xmas thing (and presumably xtal) is because Jesus died on a cross and a letter X is also a cross. I don't know if this is true though.

author: singapol
author_rank: Frequent Contributor
timestamp: October 15, 2015, 01:20:28 am
content: Quote from: German_EE on October 14, 2015, 08:39:18 pmIt is the nature of human beings to associate things theybelieve inin order to make sense of the realityand their lives, more myth than facts. Take for instance the invention of the transistor which some attribute to the "generousity" of alienstruth or myth it's much more then the eye can see. How about US president meeting with aliens to strike a deal of maintaining the power status quo and the famous Roswell incident, are we really unique in this universe.Look out for men in black.
Why cutouts for these inductors? - Page 1
Forum section: electronics / beginners

author: lpc32
author_rank: Frequent Contributor
timestamp: January 18, 2015, 12:13:19 am
content: Any idea why L2 and L4 sit in cutouts on this cordless phone board? There are no space constraints.

author: free_electron
author_rank: Super Contributor
timestamp: January 18, 2015, 12:16:08 am
content: because they are reflow soldered. or becaus ethey can;t have pins poking out the back

author: TSL
author_rank: Regular Contributor
timestamp: January 18, 2015, 01:59:24 am
content: Hi,These are tiny wire wound inductors and if they had no cutout, the copper plane would affect their value. Its not unusual to etch clear space under SMD inductors to ensure the copper plane does not affect their value too.Also, given their thickness they may have violated a height constraint inside the case so setting them in the plane of the board lowers their profile. Also a common technique.regardsTim

author: mazurov
author_rank: Frequent Contributor
timestamp: January 18, 2015, 04:33:57 am
content: Or because it's faster to solder this way :-).

author: IconicPCB
author_rank: Super Contributor
timestamp: January 18, 2015, 04:49:26 am
content: Thes are higher current axial chokes. I have seen them blow and take out chunks of PCB directly beneath them. I suspect this way a blown choke will do minimal damage to the surrounding component ( the PCB ).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 18, 2015, 07:52:33 am
content: Interesting, seem to be 1.5uH if I'm reading that right? On each charge pin? Sounds like EMI/RFI, either to isolate and filter the base, or to keep the handset's antenna from getting too screwed up by the connection.They are most definitely hand soldered -- only the most offensive globby crusty job will do!Tim

author: lpc32
author_rank: Frequent Contributor
timestamp: January 23, 2015, 02:04:59 am
content: There are a few thruholes with pins poking out the back, so I don't think that's the issue. But even without going thru, why would this require a cutout? Or if you mean whatmazurovsaid, that it's easier to solder, I assume because the leads are straight, then maybe. It's an interesting idea, but wouldn't it be even quicker if they were just plain thruhole?The board's not single-sided, and anyway, can't you just drill thru a pad?You can just etch the copper where they are, which it appears they did anyway. There are no height constraints in these locations.Assuming they can explode, why would the manufacturer care what sort of damage they do?One thing they did on this board, which I hate, is solder wires just flatly on a pad. Why would anyone want to do that? Wouldn't it just make it more time consuming for them to assemble?

author: albert22
author_rank: Regular Contributor
timestamp: January 23, 2015, 06:47:02 pm
content: My guess is that they designed the board for another type/brand of inductors that required the cutout. And then used the ones that were available.
Why did my mp3 player die after battery replacement? - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: January 22, 2021, 10:08:25 pm
content: I have a mp3 player where the charge port broke off. Also the internal battery was getting weaker, since i used it every day for the past 5 years.I opened it and found the battery was a 3.7v LiPo, 700mah. Power consumption around 70mA.Since I had some 18650 holders and a couple 3000mah 8A limit batteries. So i removed the LiPo and wired the 18650 holder to the battery connections on the PCB. (please note, I did check the battery voltage periodically, and never let it go below 3.9v)It worked great for a few days until i noticed it was slowly getting quieter and quieter, and occasionally had a sound of static coming thru the headphones - ESD damage potentially?Today i went to use it and noticed the volume, when put all the way to maximum - which is usually quite loud, enough to damage some earphones - barely audible.I went to check solder joints and swapped the op-amp, but no improvement.Did i kill it with ESD?

author: rs20
author_rank: Super Contributor
timestamp: January 24, 2021, 10:18:32 am
content: Far too little information to come to such a definitive conclusion. If you're advanced enough to be swapping out op-amps, I'd consider debugging the problem from scratch (i.e., checking the signal strength at the op amp, DAC outputs, etc) and figuring out where the problem lies. Then you'll know what IC is dead (or, what other sort of PEBKAC might be going on). After that, we could start to explore how likely the theory of ESD damage is.PS/ I do congratulate you for considering the theory of ESD damage being the culprit -- the number of people who confidently say "I've never killed anything with ESD" astounds me; you can only really say that if you've spent your entire life in an ESD-safe lab, or every device/IC failure/malfunction you've encountered has been 100% definitively proven to be a non-ESD cause.

author: Dan_Ohm
author_rank: Newbie
timestamp: January 24, 2021, 04:27:31 pm
content: did you make a silly little mistake like put the old battery back?Mixing up the old and new batteries would not be the first time.Try changing the batteries again and check the voltage on the batteries with a volt meter.
why did the 600mA fuse break.. but the 200mA didn't? - Page 1
Forum section: electronics / beginners

author: JPortici
author_rank: Super Contributor
timestamp: November 04, 2016, 11:33:22 am
content: posting this to the beginners section because i think i'm missing something basic..so, we were messing around with the electronics of a car engine. the turbo pressure valve actuator got stuck in a position and the ecu refused to move it after barely tryingmoving it manually was out of the question, we tried applying power to it(SPOILER ALERT: at the end it got unstuck and all has gone back working as if nothing happened)one of the motor input at gnd and because we didn't want to blow something up, the other to the battery, thought a multimeter in amp mode.BRYMEN 867s, 1kV 600 mA fuse for the mA range. didn't work. later i checked and the fuse blew open. I was actually expecting it but...BRYMEN 867s, 1kV 11A fuse for the 10A range. didn't work either. the fuse is ok by the way and verified that the multimeter works in the Amp rangeUNI-T UT33D, 200mA fuse, mA range. it worked. the motor moved. fuse didn't blow up. tried several times.but why?

author: P90
author_rank: Frequent Contributor
timestamp: November 04, 2016, 11:45:10 am
content: Leads plugged in the wrong jacks???

author: JPortici
author_rank: Super Contributor
timestamp: November 04, 2016, 11:48:15 am
content: of course not!

author: rsjsouza
author_rank: Super Contributor
timestamp: November 04, 2016, 11:59:05 am
content: Can you double-check and see if your Uni-T actually has the fuse in series with the mA jack and not with the V inputs?IIRC joeqsmith saw that before in an el-cheapo DMM...

author: JPortici
author_rank: Super Contributor
timestamp: November 04, 2016, 12:12:37 pm
content: the uni-t has two inputs.10A, unfusedv/ohm/ma, fused with 200mA (allegedely)aaaaaand we have no fuse. a very small wire soldered there instead. somebody here may have done something that stupid, i asked, let's see. (yes, somebody did that.)but then, why the brymen wouldn't read anything in the A mode? could it be that the 11A fuse + meter resistance was so high that the voltage at the motor collapsed?

author: P90
author_rank: Frequent Contributor
timestamp: November 04, 2016, 03:14:59 pm
content: That wire you're referring to is the 10amp current shunt.No.

author: JPortici
author_rank: Super Contributor
timestamp: November 04, 2016, 04:07:57 pm
content: If you say soSo, why did the brymen not work with the motor even in 10 amp mode? Combined fuse/shunt resistance respect the modor impedance?

author: rsjsouza
author_rank: Super Contributor
timestamp: November 04, 2016, 06:02:44 pm
content: Quote from: JPortici on November 04, 2016, 04:07:57 pmI wonder if the A jack of the BM867S is defective on the circuit/PCB and not the fuse. Can you measure the conductivity between the A and the GND inputs using your Uni-T? Make sure to connect the cables to the BM867S first, as the input sense may influence things.
Why do ATX power supplies use a transformer after the chopper? - Page 1
Forum section: electronics / beginners

author: Whales
author_rank: Super Contributor
timestamp: March 29, 2015, 02:19:40 am
content: As I understand wound transformers are useful in DC-DC power supplies as they isolate the outputs, but any safety advantage here is completely irrelevant as all ATX power supplies are designed to bridge output GND and mains earth together.Surely it would be cheaper/simpler to use a wound inductor (single wire) rather than a wound transformer (at least two wires)?

author: c4757p
author_rank: Super Contributor
timestamp: March 29, 2015, 02:38:13 am
content: Quote from: Whales on March 29, 2015, 02:19:40 amMains earth and mains neutral aren't the same thing...

author: wraper
author_rank: Unknown
timestamp: March 29, 2015, 02:56:05 am
content: LOL

author: wraper
author_rank: Unknown
timestamp: March 29, 2015, 03:20:25 am
content: I'd like to see what will happen when pass transistor in such buck converter decides to become short.

author: Whales
author_rank: Super Contributor
timestamp: March 29, 2015, 04:24:36 am
content: Quote from: wraper on March 29, 2015, 03:20:25 amAh, I've only been thinking of external failure modes. For example: connecting the output to active or neutral. In this situation whether or not the power supply is isolated internally makes no difference as the shared ground will still form a large current path and destroy the device/start a fire. A 'fully' isolated power supply would not exhibit this.Quote from: c4757p on March 29, 2015, 02:38:13 am"Output GND" as in the secondary side GND, not mains neutral.Quote from: evb149 on March 29, 2015, 03:00:48 amThanks evb. I'm curious as to where a 50% duty cycle avoids losses -- is it to do with differences in harmonics? Transistor switching loss either way would be equal as far as I can see.

author: rs20
author_rank: Super Contributor
timestamp: March 29, 2015, 04:38:29 am
content: Quote from: Whales on March 29, 2015, 04:24:36 amYou're proposing a single-wire indicator, so what on earth do you mean by "the secondary side"? Please draw a diagram to explain what you're proposing, you'll end up with at most a diode between mains earth and mains neutral = not good.

author: Whales
author_rank: Super Contributor
timestamp: March 29, 2015, 04:50:12 am
content: Quote from: rs20 on March 29, 2015, 04:38:29 amI'm aware of that danger now. I thought c4757b may have misunderstood me.Before anyone else gets worried: if I had known the answer I would not have asked the question. No I'm not working on any mains-side circuity.There's nothing wrong in not knowing enough about safety. There is a lot wrong however in pretending that you do if you don't. I'm being open about what I didn't understand: that's why I asked the question here in the beginners forum. The worst thing anyone can do is give people a sense that they should not ask safety questions, especially if they seem dumb to others.
Why do decade boxes go to 11? - Page 1
Forum section: electronics / beginners

author: kfitch42
author_rank: Frequent Contributor
timestamp: April 25, 2013, 06:02:33 pm
content: In the recent vid I was a bit perplexed as to why the "decade" resistance box had 11 positions on each knob. From what I can tell this seems pretty common for all sorts of 'decade' boxes. Is there any practical reason for this? Do you really need/want multiple ways to construct the same resistance value?

author: Excavatoree
author_rank: Frequent Contributor
timestamp: April 25, 2013, 06:18:14 pm
content: Because if you just have 10, you've got nowhere else to go. Ours have that one extra.Seriously, I imagine it's for convenience - if one is setting the thing for several values for some reason or another, having multiple options can reduce the number of switch changes. If you're adjusting from 1,2,3... and need to get to 10, you can get there without moving your hand to another knob. It's small, and one without the extra would work, but more is nicer.

author: mikes
author_rank: Regular Contributor
timestamp: April 25, 2013, 06:25:15 pm
content: When you're searching for the right value, you can move between 9 and 10 with one click. Otherwise, you'd have to go from 9 to 0 on one decade, and up 1 on the next. Of course, if you need to go even more, you need to do that, but having 10 on a decade allows for a quick and easy check before you commit to that large change. When searching for the right value, you often keep incrementing until you go 1 too far, and having a 10 makes that quicker.Less significant is that lower decades usually handle more current, so you get a bit more range. Also, it extends the range of the box (e.g. 11,110 vs. 9,999), so nothing is wasted.

author: oPossum
author_rank: Super Contributor
timestamp: April 25, 2013, 06:31:12 pm
content: Because 11 is one louder than 10

author: ejeffrey
author_rank: Super Contributor
timestamp: April 25, 2013, 08:48:15 pm
content: Also, decade boxes are not necessarily monotonic. 1000 ohms could be less than 999 ohms, since different resistors are in use. Alternately, there could be a large gap. Overlapping ranges allows you to always have a small usable region where the box is monotonic and without large gaps.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 26, 2013, 12:31:13 am
content: Well they makefridges, so why not....

author: 4to20Milliamps
author_rank: Regular Contributor
timestamp: April 26, 2013, 01:44:25 am
content: I'm buying a Marshall fridge but only if it has a keg tap AND I can plug my guitar into it.I won't give an answer........but I will give a clue:http://en.wikipedia.org/wiki/E48_series#E_series

author: cwalex
author_rank: Frequent Contributor
timestamp: April 26, 2013, 04:42:22 am
content: Quote from: mikeselectricstuff on April 26, 2013, 12:31:13 amWhen I saw the thread title I was sooo hoping no one had made the jokes yet. I should have known better.
Why do I have three of these transistors(?) - Page 1
Forum section: electronics / beginners

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: April 18, 2011, 11:56:11 pm
content: Hi.I've been sorting out my parts draws and discovered in a compartment near the ordinary bc548 and bc558 transistors, were three TO-92 parts and I cannot identify nor recall what they are or why I would have acquired them.There are three lines of characters, side on to the leads of the TO-92 package:Top line: An icon, looks like a wavy "T" or perhaps a "Y", then "EC"Middle line: "103B"Bottom line "H1M"I've hardly ever had the time to play with too many transistors, tbh, the only really the ones that have come with kits that I have bought. I built my parts draws twenty years ago and only very occasionally need to use them cos the kits satisfy all parts requirements, bar the odd resistor or capacitor.So my question, what is this thing?

author: bilko
author_rank: Frequent Contributor
timestamp: April 19, 2011, 12:00:32 am
content: Could be a BC108B with its print rubbed off

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: April 19, 2011, 06:41:23 am
content: Quote from: yachtronics on April 19, 2011, 12:00:32 amThanks for the reply.I'm not sure for two reasons. The numbers are not damaged in any way. They are pristine, unused parts. The other reason is the BC108B seems to be in a TO-18 case.

author: bilko
author_rank: Frequent Contributor
timestamp: April 19, 2011, 10:24:57 am
content: I've just googled it. It looks like its a EC103B SCR, seehttp://www.datasheetcatalog.com/datasheets_pdf/E/C/1/0/EC103B.shtml

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: April 19, 2011, 10:04:54 pm
content: Hmm. That must be what it is then. I still have no idea why I have these.Oh well. Case closed I guess. I'll apply the test circuit in the PDF to check and confirm.Thanks yachtronics.
Why do I have to touch the crystal to start oscillation? - Page 1
Forum section: electronics / beginners

author: cellularmitosis
author_rank: Unknown
timestamp: February 06, 2014, 02:59:20 am
content: So I had a bag of crystals lying around labelled "3G1431A". I assumed that meant "14.31MHz", but I thought a good learning exercise would be to actually measure the frequency to be sure.I googled for crystal circuits and came up with figure 1e from Jim William's AN12: Circuit Techniques for Clock Sources.I gave it a go on the breadboard and the circuit worked. I measured 14.29MHz on my DSO.However, what puzzles me is that I have to touch the crystal (one finger on the case, then tap the leg which connects to the 2N3904 base pin) in order to get the oscillation started.What's up with that?

author: Nerull
author_rank: Frequent Contributor
timestamp: February 06, 2014, 04:04:05 am
content: Crystals generally have to be 'started' to work reliably. This is normally achieved via load capacitors, placed between each terminal and ground. The spec sheet for a given crystal will give the capacitance required.A quartz crystal works like a tuning fork, vibrating at a given frequency. But only once struck. The capacitor values in that diagram aren't even in the ballpark for load capacitors, nor in the right places. I'm not sure if they're left out with the assumption that you will know to put them in, or this application simply doesn't use them, resulting in the startup problems you see.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2014, 07:36:12 am
content: He's got the loading caps.Probably just not enough gain. Change the 1k resistor to a nice RFC (radio frequency choke), something more than 1k impedance at the resonant frequency (i.e., more than 11uH, with less than 11pF parasitic capacitance, i.e., a self resonant frequency 14MHz or higher).Tim

author: Unknown
author_rank: Unknown
timestamp: February 06, 2014, 07:44:40 am
content: Quote from: cellularmitosis on February 06, 2014, 02:59:20 amWhich is essentially a Pierce oscillator design. It may help to look at other transistor-based Pierce oscillators designs & explanations (which can be a bit difficult, because if you Google "pierce oscillator" 95% of results are based on CMOS gates...)Quote from: cellularmitosis on February 06, 2014, 02:59:20 amWithout doing the numbers, the 100pF cap from base to emitter/gnd looks a bit high - particularly for one built on breadboard. I'd try 1/2 of that or less and see what happens...
Why do I need this resistor - Page 1
Forum section: electronics / beginners

author: tiltit
author_rank: Regular Contributor
timestamp: July 24, 2014, 08:44:29 pm
content: Hi everyone,I am currently playing around with nixie tubes and using this transistor configuration to turn on the anodes.What I do not understand is why do I need R4. The base of U$2 should be pulled low by U$1 regardless of it's presents.Thanks

author: neildarlow
author_rank: Newbie
timestamp: July 24, 2014, 08:56:29 pm
content: R4 applies negative feedback to limit U$1 collector current. R3 will only have U$2 Vbe across it and without R4 the current flow through the two transistors would be excessive and with U$1's relatively high Vce lead to excessive power dissipation.

author: tiltit
author_rank: Regular Contributor
timestamp: July 24, 2014, 09:12:45 pm
content: Thanks.So if I understand correctly without R4 I am basically shorting out the emitter of U$2 through it's base? I have tested this circuit many times and it definitely does not work at all without R4.

author: neildarlow
author_rank: Newbie
timestamp: July 24, 2014, 09:22:30 pm
content: Without R4 in place U$1 saturates when driven and its Vice will be near ground. U$1 collector current will only be limited by its hFE and limiting in the high voltage supply. U$2 will act as an emitters follower and pull the high voltage supply down. This is potentially destructive and R4 is a required component for correct operation of this circuit.
Why do I see 60Hz mains when connecting long wire to probe? - Page 1
Forum section: electronics / beginners

author: pwnell
author_rank: Regular Contributor
timestamp: December 13, 2020, 06:16:26 am
content: Another newbie question - if I connect a single 10ft wire that goes nowhere to the positive terminal of my 10x passive probe, and the ground is left disconnected, why do I see a 60Hz semi sinusoidal wave of 5Vpp on the scope? From where does it get induced and why such a large magnitude?

author: ataradov
author_rank: Super Contributor
timestamp: December 13, 2020, 06:31:24 am
content: It is a capacitive coupling. Wiring in the walls and your piece of wire form a capacitor, which passes AC current.You don't even need the wire, you can just touch the tip of the probe with your finger. Your body is full of water and salt, so it is a decent conductor. Again, good enough to couple some AC signal.

author: radiolistener
author_rank: Super Contributor
timestamp: December 13, 2020, 11:17:36 am
content: It come from mains, your wire works as antenna. Input impedance is 1 MOhm, so it is very sensitive.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 13, 2020, 04:32:42 pm
content: A simple way to test if your scope still works is to touch the probe with a finger.It's not unusual to measure 70Vac on your fingertip, especially when you hold your other hand near anything mains voltage related.

author: Gyro
author_rank: Super Contributor
timestamp: December 13, 2020, 06:29:00 pm
content: Quote from: radiolistener on December 13, 2020, 11:17:36 amA small correction, he's using a x10 probe, so 10MOhm - even more sensitive.

author: pwnell
author_rank: Regular Contributor
timestamp: December 13, 2020, 09:40:39 pm
content: Yup I verified that - touching the tip of the probe and then with the other hand wrapping it around the (insulated of course!) power cord of my oscilloscope causes a nice 10Vpp 60Hz (distorted) sine wave to appear. Very cool stuff - thanks for explaining.
Why do optoisolator circuits use a parallel resistor, but other circuits don't ? - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: June 26, 2025, 01:08:24 pm
content: In SMPS, every opto circuit I can remember seeing, will have a parallel resistor across the photo-diode side. I guess it makes the biasing more stable, and predictable.So why don't more circuits use this, for example, in a Push/Pull amp in a stereo, it's common to see diodes used to bias the BJT's but, but I don't ever remember seeing a parallel resistor across the biasing diodes. Or other basic BJT circuits, that use diodes for biasing, rarely do I see a shunt resistor across the diode (but the BJT base-emitter's, may often have a resistor in parallel)But pretty much every SMPS I've worked on, or any SMPS circuit I see w/ an opto, will always have that shunt resistor on the photo-diode.So how much worse would opto circuits in SMPS be without them, and how much better or different would stereo's be, if they used them ??

author: xavier60
author_rank: Super Contributor
timestamp: June 26, 2025, 01:45:20 pm
content: The TL431 doesn't regulate properly until it's made to draw more than 0.5mA or so. This current is needed to bias internal circuitry.Extra: A resistor across the LED ensures that enough current flows through the TL431 for it to regulate properly before the LED starts to pass current.It's best to understand this as the real reason.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: June 26, 2025, 03:35:46 pm
content: Many designers have personal preferences {*} when designing a circuit. One of them being for the reference to operate as close as possible to the manufacturer’s test current, usually 10 mA.But it also turns out that an optocoupler’s CTR degradation increases with increasing LED current, and 10 mA may be a tad on the high side for products with long lifespan expectations. As such, the paralleled resistance diverts a little current away from the LED and helps ameliorate the degradation.{*} Again these are personal preferences that I have been explained to by far more experienced designers than myself. There will be forum members that will disagree. But that is alright, they have their own preferences.

author: Seekonk
author_rank: Super Contributor
timestamp: June 26, 2025, 11:25:04 pm
content: The LED will light with very little current. A resistor insures full turn off.

author: Andy Chee
author_rank: Super Contributor
timestamp: June 27, 2025, 05:48:56 am
content: Quote from: MathWizard on June 26, 2025, 01:08:24 pmThe context is the TL431.The TL431 requires a minimum of 1mA bias to operate properly.LED voltage drop is about 1V. Resistor is 1k. Using ohms law, this becomes a 1mA current source.Christoph Basso explains it better (see page 15)https://www.powersimtof.com/Downloads/PPTs/The%20TL431%20in%20Switching%20Power%20Supplies.pdf(see page 5)https://powersimtof.com/Downloads/Papers/The%20TL431%20in%20loop%20control.pdf
Why do some displays operate at 3V and not 3.3V - Page 1
Forum section: electronics / beginners

author: alank2
author_rank: Super Contributor
timestamp: March 19, 2017, 01:53:52 pm
content: When looking through displays on eBay or at buydisplay.com, or here:http://www.newhavendisplay.com/specs/NHD-C12832A1Z-FSRGB-FBW-3V.pdfI noticed that there are a lot of displays that operate at 3V or other odd voltages. Why would they design it to operate at 3V with a max of 3.3V? Isn't 3.3V more or less a standard that a lot of pcb's operate with? Why not 3.3V with a max of 3.6V ?I've seen others that are 2.8V, etc., as well. Is there any logic to this?

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: March 19, 2017, 02:47:54 pm
content: I don't think designers design like that. They design from the inside out and are not overly concerned about stuff like interfacing the display with an arduino and ebay fixed voltage boost or buck converter.

author: dmills
author_rank: Super Contributor
timestamp: March 19, 2017, 03:16:43 pm
content: Logic voltages have been getting lower for at least 40 years now.A 1.8 - 3.3V (Abs Max) display is perfect for example for interfacing with a 2.5V or 1.8V io bank (Which is then a good fit for a DDR2 memory or something connected elsewhere on the same rail).3.3V logic is common but anyone designing with a modern ASIC or FPGA would really prefer lower voltages, and IO in the 1.8-2.5V region seems commonplace at the moment in the sorts of applications that buy displays in volume.I recently did a design where the ONLY use of the 5V rail was USB, time was 5V logic was everywhere.All the logic action in that design was 2.5/1.8/1V, I did not even bother with 3.3V except to deal with the USB port and some external IO.You design the PCB to match the device you want to drive, nobody designs a display to support an existing PCB (Every one of the stupid things has a different FPC pinout anyway!).

author: Ian.M
author_rank: Super Contributor
timestamp: March 19, 2017, 03:23:56 pm
content: Additionally, the ability to run down to 3.0V or slightly below vastly eases single cell LiPO powered applications as a simple buck converter can be used. Anyway the ST7565R controller of the display you linked to is rated for operation up to 3.3V, so as long as your supply voltage tolerance is tight enough it can be run very close to 3.3V in a nominally 3.3V (but fractionally under) system.

author: BrianHG
author_rank: Super Contributor
timestamp: March 19, 2017, 04:54:41 pm
content: If you are worried, just send you 3.3v VCC power through a Schottky diode. It will then be powered by around 0.3v less. The digital IO being fed only 0.3v more should be safe.
Why do some high current rated shunts have cuts in them? - Page 1
Forum section: electronics / beginners

author: steamedhams
author_rank: Regular Contributor
timestamp: November 05, 2021, 07:56:33 pm
content: Why do some high current rated shunts have cuts in them?

author: capt bullshot
author_rank: Super Contributor
timestamp: November 05, 2021, 07:58:18 pm
content: That's the final adjustment of the resistance value.

author: steamedhams
author_rank: Regular Contributor
timestamp: November 05, 2021, 07:59:56 pm
content: sehr gut.
Why do some TH passive componets have such long leads? - Page 1
Forum section: electronics / beginners

author: george graves
author_rank: Super Contributor
timestamp: September 26, 2012, 04:34:30 am
content: Totally random question - thought someone might know the answer.Why do some TH passive componets have such long leads? Seems like a waste. It is for machine placement?

author: poodyp
author_rank: Regular Contributor
timestamp: September 26, 2012, 07:10:41 pm
content: Because circuits used to be built likethis.

author: free_electron
author_rank: Super Contributor
timestamp: September 26, 2012, 07:58:58 pm
content: @poodyp : nope. has nothing to do with it.circuits in volume are assembled by machine and parts are typically deliverd either in rails ( like resistors : one strip of pper on either side ) or in ammo-pack : a cardboard holder tht traps the part pins. the cardboard holder is then folded and deposed in a box much like ammunition for a machine gun is stored ( those belts that hold cartridges ) hence the name ammo pack.now, this requires an amount of copper to securely hold the part down.Second the pick and place robo grabs the part and snips it off the rail. it is NOT pulled out , it is cut off.third : the legs need forming. for resistors they get bent downward 90 degrees and there is a stub between the actual 'pin' and the body' so you need more material there.for radial capacitors the pins are typically bent 45 degrees out and then down to fit a 100 mil pitch. ( if the cap pitch itself is less than 100 mils ) so you need more material again there.then the part is inserted in the pcb. the protruding legs at the bottom are slightly bent outward so the parts don;t fall or vibrate out of the board as it continues along the assembly line.after wave soldering the excess is snipped off.the pick and plce macine themselves always grab a part by the leads as they do not know about the physical size of the part. so you need a bit of material to hold on to it there.add all those 'bits' up and you end up with a leg that is between an inch and 1-1/2 long. simple becasue of automated handling requirements.

author: mariush
author_rank: Super Contributor
timestamp: September 26, 2012, 09:06:38 pm
content: Here's a lead forming machine (albeit the very simple forming, 90 degrees):and here's a machine that inserts axial and radial leaded devices:Radial 8XT UIC?? ??And there's loads of tools that you can use to manually bend the leads in different shapes, see here:http://www.olamefusa.com/e_products.htmAn example for why long leads are needed... in some cases, you want the resistors or diodes to stay further away from the PCB so you put a bit of bend on the leads which keeps the resistor or diode far away from PCB ... so you can use the 2nd or 3rd type of lead bending in the link above for such purpose.
Why do SPMS use base/gate driver transformer - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: April 20, 2018, 12:04:26 pm
content: I'm learning a lot more of these, from trying to repair 1, wish I had more to repair.http://www.smpspowersupply.com/ATX_power_supply_schematic.pdfIs it for a level of isolation between the base/gate of the main switching BJT/MOSFETs and V+?

author: Benta
author_rank: Super Contributor
timestamp: April 20, 2018, 12:35:28 pm
content: Quote from: lordvader88 on April 20, 2018, 12:04:26 pmYes. But also for cost and ease of use. The advantage of transformer drive is that you don't need to create a floating supply for the high side transistors. And in the case of bipolar power transistors, a transformer can easily deliver the relatively high base current needed.Today, I think that on most cases an IR21xx-type driver will be used.

author: TimNJ
author_rank: Super Contributor
timestamp: April 20, 2018, 02:56:34 pm
content: These driver networks are kind of non-intuitive at first glance, but T3sl4co1l has a good explanation from this thread:https://www.eevblog.com/forum/projects/half-bridge-bipolar-transistor-psu-base-drive-transformer-design-data/For half bridge circuits, base drive transformers give you a floating high-side supply in the absence of a "bootstrapped" high-side supply that are common in new controller ICs. Also, this chip was intended to drive BJTs, so the requirements are different. I suppose BJTs were the better choice 10-20 years ago, in terms of performance/price.Quote from: T3sl4co1l on January 26, 2018, 10:58:09 pm

author: lordvader88
author_rank: Frequent Contributor
timestamp: April 20, 2018, 06:33:09 pm
content: I know a lot of 1st year university/college EE stuff, but I've hardly calculated anything with more than 1 transistor. I have to practice on little linear PSU's 1st.
Why does a 555 timer need two resistors in Astable Mode? - Page 1
Forum section: electronics / beginners

author: uk.casmith
author_rank: Contributor
timestamp: June 17, 2020, 08:48:49 pm
content: A 555 timer in Astable Mode shows that you need two resistors. Please see attached image.My question is why does it need R1?I've tested this with just R2 and it works fine when first turned on but when the discharge happens the current spikes.Is it because when the discharge happens it's fighting against the current coming from VCC without a resistor there? Does it matter what the resistance is for R1 this purpose?. I understand having a higher resistance affects times, i'm meaning to push current in the right direction.Or have I got it completely wrong?Thanks,Cameron.

author: pqass
author_rank: Super Contributor
timestamp: June 17, 2020, 10:15:41 pm
content: You've got it right.Pin 7 is an open-collector NPN transistor output so connecting it directly to Vcc [without a resistor] will short the power supply resulting in a large current going through the internal transistor. You'll likely kill the 555 eventually.There are other 555 astable oscillator circuits without using pin 7. See: "50% Duty Cycle Astable Oscillator" here:https://www.electronics-tutorials.ws/waveforms/555_oscillator.html

author: uk.casmith
author_rank: Contributor
timestamp: June 18, 2020, 08:39:32 am
content: Thanks pqass!

author: xavier60
author_rank: Super Contributor
timestamp: June 18, 2020, 09:27:33 am
content: The CMOS version 555 works very well with a single resistor feedback from the output pin because its output swings all the way from rail to rail which results in a very symmetrical square wave.