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CGS114 Serge DUSG LEDs and driver questions

CGS114 Serge DUSG LEDs and driver questions

2016-05-17 by guymccusker

I'm about to build my first CGS114 DUSG. I've seen a few people mention that the module runs better with low-current LEDs rather than standard ones.


First obvious question: is that true?


Second question: if it is a good idea to use low current LEDs, what changes are needed to the driver circuitry? Obviously a higher value current-limiting resistor would be in order, but what about the base drive resistor?


I am keen to get good values from the outset because I hate desoldering and I understand the boards can be delicate too. Any hints will be very much appreciated!

Re: CGS114 Serge DUSG LEDs and driver questions

2016-05-17 by Ken Stone

I prefer to use high-brightness LEDs at very low currents. The resistor needs to be selected via experiment, though, as not all LEDs are equal. To test, connect your +VE through two diodes to the LED, then the LED resistor. Select for a value that doesn't dazzle you. I find resistors are usually in the range of 4k7 to 10k.

Using a lower current for your LED puts less switching noise on your power rails, as well as reducing the overall current required.

The driver circuit should be okay as it is.


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On Tue, May 17, 2016 at 6:31 PM, guymccusker@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:

I'm about to build my first CGS114 DUSG. I've seen a few people mention that the module runs better with low-current LEDs rather than standard ones.


First obvious question: is that true?


Second question: if it is a good idea to use low current LEDs, what changes are needed to the driver circuitry? Obviously a higher value current-limiting resistor would be in order, but what about the base drive resistor?


I am keen to get good values from the outset because I hate desoldering and I understand the boards can be delicate too. Any hints will be very much appreciated!




--
Ken Stone otherunicorn@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>

Re: CGS114 Serge DUSG LEDs and driver questions

2016-05-17 by guymccusker

Thanks very much Ken. I will experiment a bit.

I'm surprised to hear that the driver circuit won't need adjustment. To take the example of a 10k resistor on the LED, with a 12V supply this will saturate at about 1mA (10V drop across the resistor). Assuming hFE of about 100, that means a driving current of 0.01mA to fully switch the LED on, which means a .5V drop across the base resistor, so the LED fully lights when the output hits just over 1V.

I am new at this so my calculations could be completely wrong! I will fiddle about and see what I come up with.

Is the idea to have the LED pulse along with the DC output, so that with a slow cycle you would see the light go from off to soft to bright and back? And, for experimental purposes, is the output supposed to be 0 - 5V?

Thanks again for any help you can offer.

Re: CGS114 Serge DUSG LEDs and driver questions

2016-05-17 by Ken Stone

As you say, it does depend on your transistor's gain, somewhat, but those are all over the place, even in a single batch. The original Serge design used a 47k instead of a 150k there. Personally, I prefer to use voltage followers in this sort of location, as they give a more gradual LED response. You could try a higher base resistor if you wish.
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On Wed, May 18, 2016 at 1:49 AM, guymccusker@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:

Thanks very much Ken. I will experiment a bit.


I'm surprised to hear that the driver circuit won't need adjustment. To take the example of a 10k resistor on the LED, with a 12V supply this will saturate at about 1mA (10V drop across the resistor). Assuming hFE of about 100, that means a driving current of 0.01mA to fully switch the LED on, which means a .5V drop across the base resistor, so the LED fully lights when the output hits just over 1V.

I am new at this so my calculations could be completely wrong! I will fiddle about and see what I come up with.

Is the idea to have the LED pulse along with the DC output, so that with a slow cycle you would see the light go from off to soft to bright and back? And, for experimental purposes, is the output supposed to be 0 - 5V?

Thanks again for any help you can offer.




--
Ken Stone otherunicorn@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>

Re: CGS114 Serge DUSG LEDs and driver questions

2016-05-18 by guymccusker

Thanks again, Ken -- I really am very grateful for your answers here.

I'm confused about the original circuit vs your current recommendation. The schematic on your web site shows a 150k resistor while the PCB I have and the BOM on your site indicates 47k. Which do you recommend, in conjunction with your choice of a low current LED and 4k7--10k current limiting resistor?

Many thanks once again. I hope I can get the thing working after all this pre-build planning...

Re: CGS114 Serge DUSG LEDs and driver questions

2016-05-19 by Ken Stone

150k for the high brightness LEDs. (which is what I was looking at when I said no change would be needed!)
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On Thu, May 19, 2016 at 12:06 AM, guymccusker@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:

Thanks again, Ken -- I really am very grateful for your answers here.


I'm confused about the original circuit vs your current recommendation. The schematic on your web site shows a 150k resistor while the PCB I have and the BOM on your site indicates 47k. Which do you recommend, in conjunction with your choice of a low current LED and 4k7--10k current limiting resistor?

Many thanks once again. I hope I can get the thing working after all this pre-build planning...




--
Ken Stone otherunicorn@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>

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