2n3904 CE current when off? - Page 1
Forum section: electronics / beginners

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 01, 2020, 03:00:29 pm
content: I have a high side LED switch as per the arrangement in the first diagram on this page:https://learn.sparkfun.com/tutorials/transistors/applications-i-switchesMy Q1 is a 2n3904, my Vcc =10V, and I'm switching a white LED, for which I have no datasheet. However, I've measured the fully on voltage as about 3V at 20-25 mA, which is what I've used to choose the value of the collector resistor.I've noticed that when Q1 base is pulled to ground, the LED is visibly dark, but the off voltage is around 2V. I found this surprising, so I've measured the off collector current to be about 7 microA. I have two questions:1) Is an I-V characteristic of 2V at 7 microA usual for a white LED?2) Is the collector current quoted above unusual for a 2n3904 with a grounded base? It strikes me as high, but I can't find anything on the datasheets that allows me to confirm this.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 01, 2020, 03:52:49 pm
content: Fully on (closed) 2n3904 must have less then 0.5V CE voltage.Off state CE current is in "Off characteristics" on the second page of datashee from ONsemi. It is typical value at 25 C degrees, and can be higher with higher cristal temperatures.

author: tggzzz
author_rank: Super Contributor
timestamp: February 01, 2020, 04:09:18 pm
content: Your statements aren't particularly clear, in terms of what values are measured simultaneouslyHowever, AoE x-chapters 2x.1 has measured the collector current with Vce=2V and Vbe=0V, to be <1nA, often several orders of magnitude less. But it doubles every 10 degrees C rise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 01, 2020, 05:58:21 pm
content: Were you by any chance measuring by difference, and thus involving the 10Mohm (or whatever) resistance of your meter in the measurement?Also entirely possible that you have eBay/Ali special floor sweepings, and your "2N3904", isn't.Tim

author: BravoV
author_rank: Super Contributor
timestamp: February 01, 2020, 06:12:21 pm
content: Quote from: T3sl4co1l on February 01, 2020, 05:58:21 pm

author: Zero999
author_rank: Super Contributor
timestamp: February 01, 2020, 07:30:58 pm
content: Quote from: aneevuser on February 01, 2020, 03:00:29 pmYour post is very confusing. The first diagram in the link is a low side, not a high side switch.A white LED will light dimly with 7μA. The leakage current for the 2N3904 should be much less than that, with a collector-emitter voltage of 10V. Is it possible you've got the emitter and collector pins reversed?

author: iMo
author_rank: Super Contributor
timestamp: February 01, 2020, 07:40:08 pm
content: When electronics were your hobby I would recommend you to download the LTSpice and look at the results. The results are pretty good these days.

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 01, 2020, 11:06:47 pm
content: Thanks for the replies - however, I've been seeing some other odd behaviour that I can't currently understand, so I want to rule out strange breadboard gremlins by making up a soldered circuit on veroboard. If all the problems then go away, at least I'll know where to start looking.

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 01, 2020, 11:11:49 pm
content: Quote from: T3sl4co1l on February 01, 2020, 05:58:21 pmNot sure what you mean by "difference"QuoteThat's unlikely though not impossible, as I have two lots from different sources, one lot being at least 20 years old - transistors from both lots show the same behaviour AFAICS. The newer lot could be potentially be floor sweepings, though I've built several linear circuits from them without obvious problems.

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 01, 2020, 11:15:52 pm
content: Quote from: Zero999 on February 01, 2020, 07:30:58 pmNo, it's definitely not that - in a moment of desperation I flipped the device wondering if it was mispackaged or I was blind or something, with no improvement.

author: tggzzz
author_rank: Super Contributor
timestamp: February 01, 2020, 11:23:35 pm
content: Quote from: aneevuser on February 01, 2020, 11:06:47 pmWait a minute, are you using a solderless breadboard? If so, then weird behaviour is standard!Rule of thumb: you will spend more time debugging the solderless breadboard than your circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2020, 06:57:59 am
content: Well, unless you've spilled tea on it, it shouldn't be particularly leaky. I've measured nA on one before, only obvious problem really, is the obvious physical problems, namely the ~4pF between rows.Can you show where and how you have taken these measurements?Tim

author: iMo
author_rank: Super Contributor
timestamp: February 02, 2020, 09:25:38 am
content: The resistance between the slots (long or short) is not measurable here on a cheapo solderless breadboard (34401A). The capacitance is typically 3.5pF between short slots (my measurements from past).Double check - take a 9V battery and solder 3904 with shorted base-emmiter with 220ohm resistor in collector.You should see <<1nA current.PS: to see the 7uA current the resistance has to be 1.1Mohm between the slots = (10V-2V)/7uA.

author: tggzzz
author_rank: Super Contributor
timestamp: February 02, 2020, 09:34:52 am
content: Quote from: imo on February 02, 2020, 09:25:38 amThere certainly shouldn't be any measurable DC leakage on a decent solderless breadboard, but some aren't decent and there may be contamination.Your suggested test is sound. If done outside the breadboard it will also test whether the 2n3904 is working as expected.

author: iMo
author_rank: Super Contributor
timestamp: February 02, 2020, 10:00:06 am
content: I've done the measurement on a cheapo solderless - 9v battery 200ohm and 3904 inserted into 3 slots. With shorted base-emitter I see no current (34401A). With floating base I see no current.

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 02, 2020, 02:34:37 pm
content: Quote from: imo on February 02, 2020, 09:25:38 amYeah, I'll try this - I'm still baffled as to what's going on with the breadboard. I'll have to leave it till next weekend probably though - busy today and for a while.I've tried removing the LED from circuit and switching the resistor alone.When switching the resistor alone, I get V_BE(sat) = 0.8V and V_CE(sat)=65mV.When switching the LED+resistor, I get V_BE(sat) = 0.79V and V_CE(sat)=50mV.Both of these seem to be in spec. for a 2n3904.Anyway thanks - I'll remove the breadboard from the picture when I've got time.

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 02, 2020, 02:39:45 pm
content: Quote from: T3sl4co1l on February 02, 2020, 06:57:59 amI'll try to put up more details next weekend if I haven't got to the bottom of it by then.Regardless, I'm not sure what you mean by "where" and "how" - I'm measuring the voltages and currents on a breadboarded circuit, using a couple of cheapish but adequate hand held multimeters. What specifically do you think I could be screwing up measurement-wise?

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 02, 2020, 02:43:25 pm
content: Quote from: tggzzz on February 01, 2020, 11:23:35 pmYeah, it's the prototyping system of Satan, but you know, millions do the same. Anyway at some point I'll solder up a dead-bug circuit to remove possible breadboard evilness from the picture.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2020, 04:08:25 pm
content: Stuff like --- What meter, what is its resistance, and at what voltages? Most meters are 10Mohm, but some switch to Hi-Z at low voltages, some are... like, who knows what kinds of bullshit from those $3 yellow thingies? (Which IIRC weren't so much bad because of impedance, but they were completely, utterly unfiltered, unprotected, useless in any real world environment.)- Were you measuring from +V to LED, or LED to GND? Or anywhere else?- Uh, I suppose if you were measuring in reverse, that might be a concern as well but I don't know offhand any meters that just screw up so badly when they read negative numbers. The fixed range POSs just read OVL I think, and reasonable ones just say it's negative.I suppose the circuit itself could be picking up RF too, for that matter? Unlikely, but possible.Tim

author: David Hess
author_rank: Super Contributor
timestamp: February 03, 2020, 12:49:31 am
content: 7 microamps is way too high. The 2N3904 specification is something like 50 or 25 nanoamps maximum but this only represents the lowest value that the automatic test equipment can test for in a short period of time; in practice the leakage should be 10s of picoamps or even less in most cases.A small value capacitor across the base-emitter junction will suppress RF if that is a problem.To help with noise immunity, sometimes a diode is placed in series with the emitter which a high value resistor pulls up to 0.6 volts. Then the base can be pulled to -0.6 volts with respect to the emitter.

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 03, 2020, 12:27:57 pm
content: OK, a couple more comments:1) I can't reproduce the 7 muA that I originally measured. That may have been user error, or breadboard problems or something - the off current I'm now measuring is 0 microA, which is the best my cheapo meter will do. I'll try to make up a current source to check the accuracy of the meter at some point - I did this in the past, but I didn't record the details, and I can only remember that I wasn't unhappy with the results (not sure how low I took the current either). (I've also measured a burden voltage of about 10mV/mA at 20mA, should anyone be interested)2) I've replaced the LED with a resistive load of 100 ohms, which is roughly the dynamic resistance of the LED at 30 mA. When I do this, everything looks just as I would expect - the voltage across the resistor drops to 0V when the the transistor is off - with the LED present, the voltage across it drops to about 2VThat mean anything to anyone?

author: tggzzz
author_rank: Super Contributor
timestamp: February 03, 2020, 01:07:12 pm
content: Quote from: aneevuser on February 03, 2020, 12:27:57 pmNormally written µA or uA.QuoteThe LED isn't a linear resistor. Look at the LED's V-I curve to correlate the expected value with what you see.

author: fcb
author_rank: Super Contributor
timestamp: February 03, 2020, 01:10:25 pm
content: Is it possible that your LED is acting as tiny solar cell? Does the 7uA change if you subject the LED to more or less ambient light?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2020, 01:30:12 pm
content: Would have to be direct sunlight to get that much current from most LEDs, I think. PV yes, but awful efficiency.Could just be the I-D-ten-T effect, or the observer effect. Shrug.Tim

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 03, 2020, 02:33:08 pm
content: Quote from: tggzzz on February 03, 2020, 01:07:12 pmIf I had an I-V curve for the diode, I'd hardly need to post this thread. I don't have a spec. for the diodes, and I can't find a datasheet that specifies the I-V characteristics for white LEDs at the low currents I'd expect when the transistor is off. I guess it's not impossible that the 2-ish V that I see is expected, but without the I-V data, I have no idea.

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 03, 2020, 02:37:55 pm
content: Quote from: T3sl4co1l on February 03, 2020, 01:30:12 pmIf you're talking about the 7 muA, then I'd forget it - I can't reproduce that measurement now, so I'm assuming that was spurious.Quote"I-D-ten-T effect" ? You'll have to clue me in there.

author: fcb
author_rank: Super Contributor
timestamp: February 03, 2020, 02:53:28 pm
content: Quote from: aneevuser on February 03, 2020, 02:37:55 pmJust measured the C-E (with B connected to E) leakage on a similar transistor (2N4401) at 10V at <0.1nA (7000x less than your measurement). So something wrong with what you are doing, please show a schematic of how you are measuring stuff.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2020, 05:35:37 pm
content: Quote from: aneevuser on February 03, 2020, 02:37:55 pm"ID10T". Also, "error exists between user and keyboard", etc.Happens to us all, from time to time.Tim

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 04, 2020, 09:10:43 am
content: Quote from: fcb on February 03, 2020, 02:53:28 pmYes, as I mention in the text you quoted, I can't reproduce that measurement - ignore it, it's probably spurious.

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 04, 2020, 09:21:31 am
content: Quote from: T3sl4co1l on February 03, 2020, 05:35:37 pmAh, right - for some reason, I was reading it as "identity". The current measurement probably is indeed a fine example of that effect.But to return to the main issue, I've found a datasheet for a random white LED -https://www.vishay.com/docs/81159/vlhw5100.pdf- which presents an I-V curve. I note that the scale ends at I = 1mA with V = 2.6V, which goes nowhere near the currents that I'd be interested in - anyone know what the significance of these values are? I guess you could try to extrapolate the curve assuming that it remains exponential all the way down to, say, 1 nA, but that's 6 orders of magnitude - is that sensible?

author: fcb
author_rank: Super Contributor
timestamp: February 04, 2020, 09:45:49 am
content: There is literally no point in plotting an IV curve below say 1mA (or whatever puts out a useful amount of light).

author: tggzzz
author_rank: Super Contributor
timestamp: February 04, 2020, 11:48:20 am
content: Quote from: aneevuser on February 04, 2020, 09:21:31 amUnsurprisingly the manufacturer thinks users aren't interested in operating the LED in that regime, and they can't be bothered to test/guarantee it.I'd try extrapolating, and see what you come up with.However, "strange" things can and do happen, e.g. with leakage in BJTs and JFETs - download 2x.1 fromhttps://x.artofelectronics.net/the-book/sample-chapter/for some measurements

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 06, 2020, 11:16:19 am
content: OK folks, I've had some time to play around with this a bit more, and I think I know what is going on.1) I'm driving the base resistor of the 2n3904 from a sig. gen. switching between 0 and 5V.2) If I measure the voltage between ground and the bottom of the diode with either a scope or a multimeter, the off voltage across the diode is about 2.2V, as measured by another meter. The diode does not visibly illuminate. I have found that even with the scope turned off but the probe connected this situation holds.3) If I make no measurement from ground to the bottom of the diode (i.e. scope or meter disconnected), but measure only the voltage across the diode, the off voltage is about 0.1 mV at most.4) If I touch the bottom of the diode with a finger, the off voltage across goes up to about 50mV; if I simultaneously touch ground with another figure, the off voltage goes up to 2.2V.From this, I deduce that when measuring the voltage between ground and the bottom of the diode, the measuring device is allowing a small but in-this-case-significant leakage current which corresponds to the 2.2V measured but which is insufficient to illuminate the diode.Makes sense?

author: Howardlong
author_rank: Super Contributor
timestamp: February 06, 2020, 11:59:25 am
content: Quote from: T3sl4co1l on February 02, 2020, 06:57:59 amI concur, last week I was making some SMU measurements on a micorcontroller claiming 1nA sleep current, showing low hundreds of picoamps on solderless breadboard.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2020, 01:01:38 pm
content: Yup, that was my guess, measurement method -- if you're measuring across the switch, the meter's leakage (typically 10MΩ) biases the diode. It will be visible in a dark room, with dark-adapted eyes, but not really otherwise. Measuring across the diode, you measure the transistor's leakage, which is small.The true answer is somewhere inbetween -- you'd need a hi-Z meter to measure the voltage at that node without influencing its voltage. A nulling measurement with a potentiometer can be performed to get a closer answer, for example.You can then take the difference of the two measurements, and determine the impedance of the node.Tim

author: Howardlong
author_rank: Super Contributor
timestamp: February 06, 2020, 09:06:22 pm
content: Quote from: Howardlong on February 06, 2020, 11:59:25 amJust measured adjacent rows on one of these on an SMU:https://www.rapidonline.com/K-H-AD-102-Advanced-Solderless-Breadboard-456-Tie-Points-34-0676?IncVat=1&pdg=pla-301834875110:kwd-301834875110:cmp-757438067:adg-44804851896:crv-207912323492:pid-34-0676:dev-c&gclid=CjwKCAiAj-_xBRBjEiwAmRbqYgidQlCW4OlHqXF41tdvRWRvCuUjU5v1K1YE_D-S_p5YaaPHqr__WRoC9SwQAvD_BwE6.0pA at 100V, or 16 terra ohms.

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 07, 2020, 07:12:54 pm
content: Quote from: T3sl4co1l on February 06, 2020, 01:01:38 pmWell, the mystery seems to be solved, but it's a bit of a pain - I can't use my standard scope probes to look at the voltage on that diode - are there some fancy HiZ scope probes that I need?

author: aneevuser
author_rank: Frequent Contributor
timestamp: February 07, 2020, 07:16:34 pm
content: Quote from: Howardlong on February 06, 2020, 11:59:25 amI'm reasonably certain now that the transistor is not at fault, given what I have discovered - I've used them in linear circuits a fair bit, and I probably would have noticed if they were significantly out of spec. I don't have any meter fancy enough to measure currents low enough to confirm it for sure though.

author: tggzzz
author_rank: Super Contributor
timestamp: February 07, 2020, 08:34:57 pm
content: Quote from: aneevuser on February 07, 2020, 07:12:54 pmDepending on the specific ratios of parameters, you may be able to use a standard 10Mohm *10 probe, but you would have to factor in the effect it has on the measurement.But that is true for any probe, and indeed any measuring device and measurement.

author: aneevuser
author_rank: Frequent Contributor
timestamp: May 15, 2022, 03:36:08 pm
content: I've just spent about 2 hours being baffled by this phenomenom once again, with a different circuit (constant current mosfet LED driver) before déja vu kicked in, and I recalled this thread.Having investigated it a little further, I find that with 8 V at the top of a white diode:a) and nothing but a x10 scope probe at the bottom, there's a diode current of about 500 nA with 2.2 V across the diode, which is enough to illuminate the diode sufficiently to be just visible with normal room illumination, and clearly in a darkened room.b) and the mosfet (IRLZ34N) turned fully off, there's no measurable diode current, and no visible illumination of the diode even in a darkened room.which I guess tells me that the drain-source leakage current of the mosfet is much less than 500 nA when off. So at least I've learnt something.

author: rstofer
author_rank: Super Contributor
timestamp: May 15, 2022, 04:05:00 pm
content: Is your LED unique or is it likely to have the same IV characteristics of any other while LED?http://www1.futureelectronics.com/doc/EVERLIGHT%C2%A0/334-15__T1C1-4WYA.pdf

author: rstofer
author_rank: Super Contributor
timestamp: May 15, 2022, 04:16:11 pm
content: Quote from: aneevuser on February 07, 2020, 07:12:54 pmI don't know the status of Dave's Microcurrent Gold:https://www.eevblog.com/projects/ucurrent/It shows as Out Of Stockhttps://www.eevblog.com/product/ucurrentgold/The design info is readily available so you could build your own.

author: aneevuser
author_rank: Frequent Contributor
timestamp: May 15, 2022, 05:01:00 pm
content: Quote from: rstofer on May 15, 2022, 04:05:00 pmIt's unlikely to be unique but the datasheets don't give useful information for intensity/forward voltage at the low currents (sub micro-amp) that I'm seeing (the one above shows a forward 2.6 V at 0 mA, for example) so they're not particularly useful for clueless individuals who don't know when to expect to see the LEDs glowing, or what-seemed-to-me suspiciously high forward voltage when "off".Anyway, it's not really a problem, now that I've reminded myself of the cause, and the unmeasured circuit seems to behave correctly w.r.t LED illumination when off, so I'm happy for now. Anyway, I'm really using this thread to as an online note-to-self (and maybe other noobs) for when I run into the problem again, in a year or two.

author: aneevuser
author_rank: Frequent Contributor
timestamp: May 15, 2022, 05:03:24 pm
content: Quote from: rstofer on May 15, 2022, 04:16:11 pmThat looks to be pretty interesting - I'll take a look at it. Thanks.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 17, 2022, 06:21:13 pm
content: TLDnR.Some LED's also work a bit as photocells, and can generate a voltage close to their "on" voltage when a bright enough light shines on it.

author: Howardlong
author_rank: Super Contributor
timestamp: May 17, 2022, 07:25:38 pm
content: Quote from: Doctorandus_P on May 17, 2022, 06:21:13 pmCorrect, and of you use LEDs for status or debugging ultra low power applications, this can lead to some head scratching.eg, the DUT doesn’t work as expected under the microscope where there’s lots of light.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 17, 2022, 07:47:43 pm
content: Quote from: Doctorandus_P on May 17, 2022, 06:21:13 pmAll of them in fact; it's an intrinsic property of semiconductor junctions.It's just not usually very much, some ~uA in bright light even, I think? The LED chip is quite small, and the lens (let alone diffused) doesn't work well as a receiver. But yeah, still something.Tim