Author Topic: Question Regarding Diodes in Schematic  (Read 10147 times)

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Offline bumba000

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Question Regarding Diodes in Schematic
« on: September 26, 2016, 03:13:59 am »
Hi All,
         I am working on a project here, but I've just come across something in a schematic that I've not seen before.
It looks to me like there's an LED and a standard diode applied in opposite directions at the same location. Please help me understand this.

Thank You, John

 

Offline T3sl4co1l

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Re: Question Regarding Diodes in Schematic
« Reply #1 on: September 26, 2016, 03:19:43 am »
Does it have a part number?

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Online rx8pilot

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Re: Question Regarding Diodes in Schematic
« Reply #2 on: September 26, 2016, 03:24:03 am »
It's marked as 2 LED's? Maybe just an indicator for reverse polarity? Green in the correct direction, red in the opposite?
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Offline bumba000

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Re: Question Regarding Diodes in Schematic
« Reply #3 on: September 26, 2016, 03:29:02 am »
Thank you all for your super quick responses. I figured that the image I posted may cause some confusion because of that double mention of LED. Attached is the schematic I'm working from.

Again, Thank you.

... There are no part numbers mentioned. Only general ideas of what can be used.
« Last Edit: September 26, 2016, 03:30:55 am by bumba000 »
 

Offline eliocor

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Re: Question Regarding Diodes in Schematic
« Reply #4 on: September 26, 2016, 04:01:33 am »
 
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Offline bumba000

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Re: Question Regarding Diodes in Schematic
« Reply #5 on: September 26, 2016, 04:33:20 am »
Well I of course don't have a ton of experience with all of this. I'm learning as I go and so I greatly appreciate your input (all of your inputs). That being said, I don't see any reason for a standard diode in reverse direction, so I'll go with the ESD idea. Since this is an audio spectrum analyzer most of the LEDs will be on/off/on repeatedly, quickly and for long periods of time. If ESD can be an issue here ( sounds like it can be ) then the standard diode in reverse directly seems like the solution.

Thanks again,
John
 

Offline bumba000

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Re: Question Regarding Diodes in Schematic
« Reply #6 on: September 26, 2016, 04:42:55 am »
umm, any ideas on which diode should be used for the ESD protection?
 

Offline ataradov

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Re: Question Regarding Diodes in Schematic
« Reply #7 on: September 26, 2016, 04:58:48 am »
umm, any ideas on which diode should be used for the ESD protection?
The whole symbol (2 diodes) is a single part. The one that is not light emitting is there for ESD protection (potentially) of the one that is. Knowing exact part number would be helpful.

Schematic is drawn haphazardly, so it is possible that whoever did it, just picked a diode with required PCB landing pattern and it ended up being an LED with embedded back diode. There is no real need for them in this particular schematic.
« Last Edit: September 26, 2016, 05:01:27 am by ataradov »
Alex
 

Offline bumba000

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Re: Question Regarding Diodes in Schematic
« Reply #8 on: September 26, 2016, 05:06:41 am »
I don't have any part numbers, only recommendations on transistor bc547 and op-amps lm358. In regards to the LEDs and other diodes, I'm being left in the dark so to speak...
 

Offline Gary350z

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Re: Question Regarding Diodes in Schematic
« Reply #9 on: September 26, 2016, 05:12:13 am »
Since the schematic symbol of the diode across the LED is very small compared to all the other diodes in the schematic, and that it has no part identifier, I would think it is part of the LED as in the Osram reference posted by eliocor (http://www.osram-os.com/Graphics/XPic6/00102535_0.pdf/ESD%20Protection%20while%20Handling%20LEDs.pdf).

As for ESD protection, I don't see that you would need it for this circuit.

Most LEDs have a very small maximum reverse voltage (about 5V), and this diode could protect against this, but again I don't see that you would need it in this circuit.

Where is this circuit originally used? Could it be in a really electrically noisy environment?

Maybe they just had a bunch of these types of LEDs lying around and they used them.

I don't know.
 

Offline ataradov

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Re: Question Regarding Diodes in Schematic
« Reply #10 on: September 26, 2016, 05:12:26 am »
I don't have any part numbers, only recommendations on transistor bc547 and op-amps lm358. In regards to the LEDs and other diodes, I'm being left in the dark so to speak...
Then just use whatever diodes you like.

Reverse diodes will also protect LEDs from applying excessive reverse voltage, but it is not a concern in this schematic. It is quite possible that originally selected diodes were for some special applications.
Alex
 

Offline bumba000

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Re: Question Regarding Diodes in Schematic
« Reply #11 on: September 26, 2016, 05:22:18 am »
Hey Gary,
                        The application of this circuit is an audio spectrum analyzer. There will be a lot of amplified music going through the circuit.

Hey Alex,
               How about the shottky diodes as in D1, D11, D12, D13, D14, D15, D16, D10 and D17. What would I use here and why would the designer have gone 11, 12, 13, 14, 16, 10 and 17?

Thank you,
John
 

Offline ataradov

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Re: Question Regarding Diodes in Schematic
« Reply #12 on: September 26, 2016, 05:25:37 am »
How about the shottky diodes as in D1, D11, D12, D13, D14, D15, D16, D10 and D17.
They actually set voltage thresholds at which transistors will open. Those are required.

What would I use here and why would the designer have gone 11, 12, 13, 14, 16, 10 and 17?
Again, schematic is poorly drawn, so numbering is also poor, there is no specific meaning behind this.

Also, the whole thing can be implemented with way fewer parts using MCU.
Alex
 
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Offline bumba000

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Re: Question Regarding Diodes in Schematic
« Reply #13 on: September 26, 2016, 05:37:26 am »
I know about the MCU methods. I have MSGEQ7's and many tiny duinos. I just wanted to analog it. I made a couple myself , but couldn't seem to get the voltage meter part working, so I bought some AN6884 before I found the schematic I've posted here. Now I'm going to try building this circuit. The diodes are confusing me though...

So then D1 might be .3Vf, D11 .4Vf, D12 .5Vf and so on??
 

Offline ataradov

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Re: Question Regarding Diodes in Schematic
« Reply #14 on: September 26, 2016, 05:41:41 am »
So then D1 might be .3Vf, D11 .4Vf, D12 .5Vf and so on??
No, they all must be the same to get a linear response. They need to be on a log scale to get logarithmic response (what you typically want from an equalizer).
Alex
 
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Offline amitchell

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Re: Question Regarding Diodes in Schematic
« Reply #15 on: September 26, 2016, 05:47:57 am »
Wondering if it something like the littlefuse PLED series

http://m.littelfuse.com/products/led-protectors/led-protector/pled-5-qfn/pled5q12.aspx

I am using these with CREE WLEDs as ESD and open circuit protection.

 

Offline ataradov

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Re: Question Regarding Diodes in Schematic
« Reply #16 on: September 26, 2016, 05:51:10 am »
I am using these with CREE WLEDs as ESD and open circuit protection.
With 5V supply and BC547 driving them, they must be regular indication LEDs.
Alex
 

Offline eliocor

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Re: Question Regarding Diodes in Schematic
« Reply #17 on: September 26, 2016, 06:16:01 am »
« Last Edit: September 26, 2016, 06:24:20 am by eliocor »
 

Offline bumba000

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Re: Question Regarding Diodes in Schematic
« Reply #18 on: September 26, 2016, 06:17:59 am »
ok Alex. I see how my D1 .5Fv , D2 . 4Fv is incorrect. I guess using all the same Fv make much more sense. .5 to turn on the first, an additional .5 to turn on the second and so on.

I will leave out the ESD protection for now and see what happens. I'm still trying to wrap my head around why the LED anode would connect to VCC (5v) and the LED cathode via transistor would connect to VCC2 (10v). Sure doesn't seem like it's going to work, but then again - what do I know? not much apparently!  ;D
 

Offline ataradov

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Re: Question Regarding Diodes in Schematic
« Reply #19 on: September 26, 2016, 06:36:55 am »
would connect to VCC2 (10v).
It connects to VCC/2 = 5V. and VCC is 10 V, so voltage across the transistor-resistor-LED is 5V.

And it connects to VCC/2 because it is a virtual ground in this design.
Alex
 
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Offline bumba000

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Re: Question Regarding Diodes in Schematic
« Reply #20 on: September 26, 2016, 06:56:07 am »
LOL, I'm off to a great start ehh? Already confusing voltages. Well thanks for pointing that out.

Virtual ground huh? Ok. I'll be back. I've some research to do on this virtual ground thing...

Thanks, John
 

Offline ataradov

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Re: Question Regarding Diodes in Schematic
« Reply #21 on: September 26, 2016, 07:01:01 am »
Virtual ground huh? Ok. I'll be back. I've some research to do on this virtual ground thing...
That's just so you could properly amplify audio signal for filtering. The input signal is decoupled though C1 and biased at VCC/2 via R3. So VCC/2 is a "zero" level of the audio signal. This way you don't need a negative supply voltage.
Alex
 
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Offline bumba000

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Re: Question Regarding Diodes in Schematic
« Reply #22 on: September 26, 2016, 07:11:53 am »
I was thinking the circuit designer had made a mistake with the voltages and non-use of ground. Thank you very much Alex.
 

Offline bumba000

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Re: Question Regarding Diodes in Schematic
« Reply #23 on: September 26, 2016, 07:40:06 am »
This will likely be obvious to you all, but why is IC1B seemingly not connected to anything? Is it really that half of the IC isn't used?
 

Offline ataradov

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Re: Question Regarding Diodes in Schematic
« Reply #24 on: September 26, 2016, 07:41:37 am »
Yes, that's the second half from the IC1A. Also, don't leave inputs of IC1B floating, short them both to ground.
Alex
 
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