Vactrol Analog Opto-isolators

Oak Ridge Radio stocks EG&G Vactec type VTL3A27 analog opto-isolators. EG&G Vactec calls them by the trade name "Vactrols". They contain a CdS photocell and incandescent lamp encapsulated in an epoxy package.

Curve 1: 25 C - 24 hours @ no input
Curve 2: 25 C - 24 hours @ rated input
Curve 3: +50 C - 24 hours @ rated input
Curve 4: -20 C - 24 hours @ rated input

Typical resistance vs. voltage curve for VTL3A27.

Cadmium sulfide photocells are good RF resistors. They are fairly linear so they do not produce much intermodulation distortion. They have low parasitic capacitance and inductance. For the VTL3A27, at 466 ohms DC resistance, I measured the impedance at 10 MHz as 435 - j110 ohms using a General Radio 916A RF bridge. This is equivalent to a 463 ohm resistor in parallel with a 8.7 pF capacitor.

Solid lines: turn-on time
Dashed lines: turn-off time

Response Time for VTL3A27.

For Beverage antenna termination use, the desirable characteristics for the CdS cell are a low on-resistance, a low light-history memory, a low temperature coefficient, and a shallow resistance versus voltage characteristic curve in the region around 500 ohms and below. The maximum on-resistance needs to be below a few hundred ohms since the resistance of the ground system is in series with the terminator, and the necessary termination resistance is somewhere around 400 to 500 ohms. A low light-history memory and a low temperature coefficient are desirable to minimize drift in the termination resistance setting. A shallow resistance versus voltage characteristic below 600 ohms keeps the control setting from being too sensitive. The VTL3A27 meets these requirements best.

Electro-optical characteristics at 25 degrees Celcius for VTL3A27.

Package Dimensions for VTL3A27.

How to order

The VTL3A27 is priced at $6.00 each plus $2.00 postage and handling per order within the US and Canada.

Enclose a check or money order in US funds payable to Oak Ridge Radio and mail your order to