LED glows slightly without any DC being supplied -- do I have a ghost??!! - Page 1
Forum section: electronics / projects

author: gooseEL34
author_rank: Regular Contributor
timestamp: January 04, 2014, 07:39:05 pm
content: http://www.aronnelson.com/gallery/main.php/v/diyuser/GGBB/Multi-RAT.png.htmlHey guys... I am not an EE but I can understand a schematic and find the parts on a PCB and know a little about moving electrons....Have this guitar pedal effect. No official schematics out there but this one is close to exact based on my examination.This is a multi schematic. For our purposes, this is a RAT2 and the purple traces are not present on my unit.The GREEN section is essentially only for the LED -- to turn on the LED when the effect is engaged and FS2b switches the output of the volume pot (to the output jack).Apparently, it uses a JFET to either supply the voltage to the LED, or cut it off.It appears that when the Effect is switched IN -- the bottom of R13 is lifted from ground as well as the BASE of the JFET. That must allow Q2 to act as a ON switch (make, not break) and provide voltage to the LED through the 2 diodes and the dropping resistor prior to hitting the LED.When the effect is switched OFF it appears that r13 and the Bass of Q2 get a path to ground via either the volume knob or the 10k resistor R15I am getting a wicked weird thing.1. The LED switches ON and OFF as normal with the pushbutton2. HOWEVER, the LED does not completely go out when the effect is OFF. It is ever so slighly glowing. HOWEVER...when I use a VOM on the LED there is 0 VOLTS!!!! Maybe it is such a slight voltage (my meter sometimes flickers 0.0001 v ) but I cannot imagine that .0001 vDC would light a LED at a minimal glow. I know that it is glowing and not just a trick with my eyes, as the glow goes out when I pull the guitar cable out of the switching jack (which lifts the ground of the battery)CRAZY EH??It was $10 so I am not too worried about it.So.... Why would this LED be giving me an ever so slight glow even though I can read 0v (Fluke 87) supplying it.Is it some crazy capacitance thing where there is 7-9 volts on the PCB near it and somehow that is exciting the electrons WITHOUT voltage being supplied through the leads?

author: Jon86
author_rank: Frequent Contributor
timestamp: January 04, 2014, 07:41:23 pm
content: No. The LED will not glow without at least half a volt or so across it. I'm not sure exactly what the voltage would be, but you'd be able to measure it.Likely your meter is faulty, or you're not measuring correctly.

author: dannyf
author_rank: Super Contributor
timestamp: January 04, 2014, 07:56:13 pm
content: QuoteNot at all - your experience is consistent with the circuit. The jfet is (incorrectly) positively biased when FS2B is switched out.

author: dannyf
author_rank: Super Contributor
timestamp: January 04, 2014, 07:57:45 pm
content: And, leds can conduct current at very low forward voltage and they light up with ua-level forward current.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 04, 2014, 08:05:33 pm
content: I wouldn't be surprised if you can control the brightness of the led with the volume potAlso, the moment you measure 0 volts the led will be off.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: January 04, 2014, 08:06:49 pm
content: It might be some sort of oscillation, either picked up by the guitar lead or caused by the ground shift. If the oscillation is centered around zero it would read as such on your meter. Try reading volts ac, see if you get a reading. Also keep in mind ac voltage would appear dim because you are in effect pwm the signal so the led only lights up a fraction of the time.Sent from my EVO using Tapatalk

author: gooseEL34
author_rank: Regular Contributor
timestamp: January 04, 2014, 10:26:11 pm
content: Ok... I must have been reading my meter wrong.Here's what I found.When the EFFECT is OFF (the Gate shows 0.004 vDC, the drain shows 9.24vDC, and the Source shows 2.907vDC),After the drop through the diodes, and dropping resistor the voltage on the LED lead is 1.565 vDC (so I DO have some voltage on the LED that probably should not be there)When the EFFECT is ON (the gate now shows 3.04vDC, the drain shows 9.14vDCm, and the Source shows 8.52vDC),After the drop through the diodes and dropping resistor, the voltage at the LED is 1.714vDC)So here's what I assume as a non-EE. The JFET switch is not working properly ..... I am assuming that when you switch the switch and turn the effect on, you are now introducing a GATE voltage (3.0 v approx) on the Gate and the JFET allows current to flow (as evidenced by the 8.5vDC that can be read right before the first Diode prior to the LED).So my EE friends. In this circuit, with no Gate voltage (0.004vDC) should I expect to find and DC on the DrainShould I just replace Q2, or is there something else I should look at?ThanksMake sense?Sorry guys I am not an EE

author: PedroDaGr8
author_rank: Super Contributor
timestamp: January 04, 2014, 11:42:17 pm
content: My guess is you need a gate resistor (from gate to ground). mosfets are charge based devices do it's very easy to have a small amount of charge build up and turn on the mosfet.Sent from my EVO using Tapatalk

author: dannyf
author_rank: Super Contributor
timestamp: January 05, 2014, 12:04:48 am
content: That portion of the circuit isn't meant to be a "switch" - instead, it is a buffer so the led's brightness will change with the input signal.The use a jfet allows low turn-on of the leds but creats the problem you observed. A bjt would alleviate that issue but requires higher output levels to start kicking in.

author: gooseEL34
author_rank: Regular Contributor
timestamp: January 05, 2014, 01:16:51 am
content: Really? Seems odd. Most LEDs on pedals are not variable brightness items. They are usually ON/OFF to tell the guitarist that the effect is engaged when light is low (like in a bar).I have also read that JFETS can be used as an analog switch, and I am pretty sure that they are doing this here.Also, the bottom half of FS2b (the switch), means that this part of the circuit is not really "in circuit" when the LED is ON (when the top half of the FS2a is routing the guitar signal right from input jack to output jack.I don't mean to argue, but it does not seem that this is correct.When the EFFECT is on you are supplying 9v to the JFET, when it is OFF, the switch gives a path to ground through the volume pot and changes the setup of the JFET, no?Quote from: dannyf on January 05, 2014, 12:04:48 am

author: PedroDaGr8
author_rank: Super Contributor
timestamp: January 05, 2014, 01:44:16 am
content: You have a direct path to the gate, albeit through a 10meg resistor. The gate though is extremely high impedance so that 10meg resistor won't be a problem it won't be hard to get the voltage up enough to charge the gate and barely turn it on. Like I said before, try a high ohm resistor from gate to ground to dissipate this charge.Sent from my EVO using Tapatalk

author: gooseEL34
author_rank: Regular Contributor
timestamp: January 05, 2014, 02:59:55 am
content: http://www.muzique.com/lab/true_bypass.htmSeems to explain it. The LED is not part of the circuit and doesn't glow variably with signal.It appears that I have a leaky JFETTim

author: megajocke
author_rank: Regular Contributor
timestamp: January 06, 2014, 04:03:33 pm
content: It seems your JFET is functioning perfectly. It's just that its pinch off voltage is a little too high to completely extingusish the LED. To completely cease conduction the source needs to be more positive than the gate. That's the reason for the extra diodes in series with the LED. You could for example put in more of those diodes, change them to a suitable zener, put in a suitable pulldown right after the JFET or change it to a MOSFET to make the glow go away.What is the input impedance of your meter? When measuring the node fed by the 10 megaohm resistor you need to take care because the meter will load it down. 10 megaohm is quite a common input resistance and would cause the reading to be low by 50%!

author: gooseEL34
author_rank: Regular Contributor
timestamp: January 06, 2014, 07:07:12 pm
content: Meter is a FLUKE 87. Not sure on the spec, as I am a shade tree tinkerer, at best.Thanks for the info. Can you suggest a Mosfet? Also, would I have to mod the circuit?TimQuote from: megajocke on January 06, 2014, 04:03:33 pm