Vactrols have gone through a number of manufacturer changes. Vactec is the first manufacturer of vactrols and who originated the name. In 1983 they were acquired by Perkin Elmer. In 2010 this group in Perkin Elmer was spun out and became Excelitas Technologies who manufactured them until 2015. CoolAudio is manufacturing the VTL5C3 and Xvive is manufacturing a complete line. I find a number of vactrols in my repair work where the range is just too high for the circuit to operate properly. In some of my builds I match vactrols and notice that they vary wildly. I decided to document random samples of different brands I have available.
I only had two samples of the original Vactec and they differed at low currents but matched up nicely at higher currents and the resistance side to side was also well matched. Both the Excelitas and Xvive varied widely in performance.
For the Xvive, #1 and #3 are the closest matched. For the Excilitas, #2 and #3 are closest matched.
The Perkin-Elmer specification for the VTL5C3/2 is a bit confusing. The characteristics table shows a 2R on resistance at 40 mA which is extremely low for a CdS photocell. However, the graph shows a more reasonable ~1.5K. I don't understand why such a large difference unless they dropped a "K".
Both the Excilitas and Xvive varied widely in performance. For the Xvive, #2 and #4 are the closest matched although they all tended to converge at the higher currents. For the Excilitas, #3 and #4 are closest matched and likewise they tended to converge at at the higher currents. In general the Excelitas were lower resistance than the Xvive at currents above 5 mA.
The Perkin-Elmer specification for the VTL5C3 is likewise confusing. The characteristics table shows a 1.5R on resistance at 40 mA which is extremely low for a CdS photocell. However, the graph shows a more reasonable ~1K. Again, I don't understand why such a large difference unless they dropped a "K".
I didn't have a CoolAudio part to test. The characteristics table shows a 500R on resistance at 40 mA and the graph shows a more reasonable ~1K. At 1 mA the characteristics table shows a 10K on resistance and the graph shows ~50K. They still don't match but are more similar.
I hand match my vactrols by applying 5 mA of current and immediately taking the first resistance reading. I chose 5 mA because the turn-on and turn-off specifications for some vactrols is at 5 mA. I bin them according to the first two digits, so 2K4, 4K3, or 0.89R. Then I match trying for the same group or the second digit +/-1. There was a discussion on the net about building a LFO using a vactrol and a op-amp or comparator. I designed and measured such a circuit but it seemed overly complicated. I decided a better method was to drive the LED with a low frequency square wave and monitor the voltage using the vactrol in a resistor divider. I chose to do just one of each type as I was more interested in qualitative than quantitative data.
My setup consisted of my square wave generator driving the vactrol with a 1K series resistor to ground. I monitored the voltage across this resistor with my oscilloscope (yellow trace) and adjusted the generator output for a 5V drop for 5 mA which is what I hand match at.
I then powered the resistive side of the vactrol with 5V with a 10K to ground and monitored this voltage (cyan trace) on the oscilloscope. I had to change time base settings because of the different delays. I tested an Excelitas VTL5C3 and VTL5C4 and a surplus Silonex NSL-32SR3.
When hand selecting vactrols at 5 mA the resistance readings can vary from <1K to over 6K. The issue with selecting a resistance is not all vactrols may reach that value. Instead the specifications are to 63% of final value. This is the Excelitas VTL5C3 turn-on to 63% of 2.5K in 2.4 mS. The typical specification is 2.5 mS.
This is the Excelitas VTL5C4 turn-on to 63% of 500R in 80 ÁS. The typical specification is 6 mS.
This is the Silonex NSL-32SR3 turn-on to 63% of 640R in 26 ÁS. The typical specification is 5 mS.
The turn off measurements are more critical for most applications. I chose 100K which corresponds to 0.45V. This is the Excelitas VTL5C3 turn-off curve to 100K at 18 mS. The specification is 35 mS maximum.
This is the Excelitas VTL5C4 turn-off curve to 100K is about 180 mS. The specification is 1500 mS maximum
This is the Silonex NSL-32SR3 turn-off curve to 100K is 120 mS. The specification is 10 mS maximum.
I suspect the VTL5C3 turn-on time is a bit of an outlier. In all cases the turn-on time was much faster than the turn-off. The VTL5C4 which is specified slower than that VTL5C3 was indeed about 10X slower. The Silonex was between the VTL5C3 and VTL5C4.
I had a "bad" VTL5C3 that I had removed from a repair and tested it. The turn-on was much slower at 12 mS and it only reached an on resistance of 5.6K. Turn-off was fairly similar.
With a sample size of just one not a lot of conclusions can be made other that the VTL5C4 is about 10X slower for turn-off than the VTL5C3 and the Silonex is in between.