LED forward voltage tolerance

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

ruffrecords

Well-known member
Joined
Nov 10, 2006
Messages
16,152
Location
Norfolk - UK
I am messing about with some Vactrols and I noticed something unexpected. I fed the LED of each one from a 9V battery through a 1K resistor. With the expected 1.9V LED forward drop that would push about 7mA into the LED.. Of the five Vactrols I tested, three were very close to 1.9V forward voltage drop but the other two were close to 1.7V. I then check them all on my ATLAS component tester which pushes an accurate 5mA through them with almost identical results.

Is this normal or do I have a couple of dodgy Vactrols ? (btw they are all the Chinese copies from Xvive)

Cheers

Ian
 
Resistance of the vactrol is proportional to brightness which is proportional to current. So if you're trying to accurately and consistently control resistance, you should use a precision current sink and not care so much about forward voltage.

Note: If you have a bipolar supply, you can make a good enough current sink with only two transistors and two resistors.
 
Resistance of the vactrol is proportional to brightness which is proportional to current. So if you're trying to accurately and consistently control resistance, you should use a precision current sink and not care so much about forward voltage.
What I am really interested in is the tracking between samples. There is first the function of light out vs current in for the LED followed by resistance vs illumination for the opto resistor. I was doing some basic tests of this when I noticed the difference in forward voltage. I wondered if this was a further unknown or an indication of dodgy product. I know my sample of 5 is small but I was surprised hey clustered close to 1.9V and 1.7V
Note: If you have a bipolar supply, you can make a good enough current sink with only two transistors and two resistors.
As you may have guessed I am using tubes so I have some nice high voltages to play with.

Cheers

Ian
 
That sounds like different LED formulations. Are you sure the Vactrol devices were the same part number?
Absolutely, I purchased five at the same time but none of the parts has a part number on it so it could be a vendor error. On the other hand, the type I purchased is the VTL5C3/2 which has two opto resistors and the vendor only does this one type with two opto resistors so that makes it less likely to be an vendor error so may be a production error?. I have contacted the vendor.

Cheers

Ian
 
From the Xvive datasheet it does appear that 5mA is right in the "knee" section of the current vs. voltage curve, so maybe there is just that much variation between device batches at 5mA. Do you have a good way to test at 10mA or 15mA? I guess you could just use the 9V battery and resistor again.

Xvive VTL5C3-2 datasheet
 
From the Xvive datasheet it does appear that 5mA is right in the "knee" section of the current vs. voltage curve, so maybe there is just that much variation between device batches at 5mA. Do you have a good way to test at 10mA or 15mA? I guess you could just use the 9V battery and resistor again.

Xvive VTL5C3-2 datasheet
Yes, I can easily test at 10 or 15mA using my bench supply plus a resistor and that is a test I should do to be sure to be past the knee.

Cheers

Ian
 
I have carried out some further tests tonight using a bench supply set to 20V and the LEDs again fed via a 1K resistor. The current in each case is very close to 18mA. The LEDs that measured 1.7V at 5mA now measure 1.8V and the ones that measured 1.9V now measure very close to 2V. So it looks like both have increased, as expected, but the difference between them is unchanged. The vendor emailed me today to tell me they have contected Xvive about this. I will let you know what they say.

Cheers

Ian
 
I have carried out some further tests tonight using a bench supply set to 20V and the LEDs again fed via a 1K resistor. The current in each case is very close to 18mA. The LEDs that measured 1.7V at 5mA now measure 1.8V and the ones that measured 1.9V now measure very close to 2V. So it looks like both have increased, as expected, but the difference between them is unchanged. The vendor emailed me today to tell me they have contected Xvive about this. I will let you know what they say.

Cheers

Ian

Worth a think about temperature coefficient?
In the words of a popular comedian "I've done no research" 😳
 
I would join ccaudle and suggest different LED formulations.
After all, the most important parameter here is the R vs. I curve. No one in their right mind would base a design on the accuracy of the LED's V/I curve.
1.9V is typical of an Ultra Red LED, 1.7V would indicate an infrared type. I can easily imagine someone deciding to replace the UltraRed with an infrared if the former went to be hard to procure, as long as the R vs. I curve is more or less similar (notice the typical 2:1 tolerance).
 
I don't recall who first advised me against expecting LEDs to be reliable voltage references while over the years I have used them numerous times for crude DC bias sources where the exact voltage did not matter.

From a quick glance at multiple data sheets they spec "typical" forward voltages with tolerance ranges of a few hundred mV, they even spec the frequency of the LED light with a range of frequency. This suggests to me that the nature of the technology or process is not extremely precise, while you would expect consecutive samples from the same batch to be similar.

JR
 
Last edited:
Anyone found a way around the EU ban on vactrols?

/Jakob E.
It is unclear to me if Vactrols are specifically banned in the EU. Cadmium is banned but Vactrols do not contain Cadmium. They contain Cadmium Sulphide. Common salt (NaCl) contains Sodium, a very dangerous metal but nobody bans salt. So why should Cadmium Sulphide be banned?

Cheers

Ian
 
It is unclear to me if Vactrols are specifically banned in the EU. Cadmium is banned but Vactrols do not contain Cadmium. They contain Cadmium Sulphide. Common salt (NaCl) contains Sodium, a very dangerous metal but nobody bans salt. So why should Cadmium Sulphide be banned?
Oh no, CdS is toxic. But it's not offensively toxic when used in small quantities. A CdS cell has only a tiny amount of CdS in it. So it's not bad like lead solder which is very toxic in a bioacummulation way and accounted for significant mass of a circuit board when it was used.

Note that chemical analogies do not work as expected. If you look at a periodic table, Sulfur is in the same column and directly under Oxygen and Cadmium is in the same column and directly under Zinc but Zinc Oxide has very low toxicity (it's used in lots of house hold products and even as a food additive) but CdS is definitely toxic. You would not want to make a hand cream out of it or sprinkle it on tacos.
 
Sulfur is in the same column and directly under Oxygen and Cadmium is in the same column and directly under Zinc but Zinc Oxide has very low toxicity (it's used in lots of house hold products and even as a food additive) but CdS is definitely toxic. You would not want to make a hand cream out of it or sprinkle it on tacos.
Coincidentally I have started experimenting with zinc oxide. I saw a wingnut mention that zinc oxide was once used to shrink skin cancers... I spend a lot of time out in the sun so have started applying zinc oxide to my ears and neck.

I am well aware that zinc oxide was used as sun block back in the day.

JR
 
Back
Top