[HomeBrew] Help identifying a component

[Flowertime01 at wmconnect.com]

Larry,

Consider them SCR.

The terminals will read open in any polarity of meter check.
These look like good high voltage types.

They may have had numbers on them to identify them but the numbers got lost
as the gold plate on which the numbers were printed wears easily.

Ground the stud.
Select you favorite DC high voltage source (100 volts or less).
Select a resistor to limit that voltage to 1MA when shorted to ground through 
the resistor.
Make sure the resistor can handle the voltage and power drop for a minute or 
so.
Apply that voltage to the big terminal on the device.

Use a jumper wire and short the big terminal to the third terminal (small 
terminal)
for just a moment. Here again you may want to start with a large resistor to 
drop the trigger voltage and limit the trigger current. Start big at 470K and 
work down in resistor size until you find a circuit combination that will trip 
the device under test into conduction.

If the device is a SCR, when you apply a voltage to the gate terminal you 
will trip the SCR into conduction.
The voltage at the big terminal will drop to near zero.
The power from the supply will then be dropped across the resistor and it 
will heat up.

To reset the circuit you need to remove power from the big terminal.

This is just a test to see if the device is an SCR if it is not sparks may 
fly.
Try some small voltage first.

These look like robust devices. The voltage rating would be in line with the 
DC cap values you found in the unit.

There is some minimum DC voltage below which the devices will not work.
There is some maximum voltage that will destroy the devices.

There is a range of gate voltage that will trip the device into conduction 
without destroying the device.


SCR's enjoy use as variable power supply devices.
AC is applied to the device and it acts as a 1/2 wave device like a simple 
diode.

The AC is dropped through a variable resistor to the gate.
This lets the trip voltage on the gate be set to change the period of time 
during which the device will conduct. A diode will conduct through a 1/2 cycle. 
An SCR can conduct from none of the cycle up to 1/2 the AC cycle.

The output of the device is filtered to provide some DC voltage. Varying the 
amount of time the SCR conducts each cycle and filtering that output over time 
provides a varying DC output.

Rather than regulate the DC output of a power supply by using the unwanted 
power as wasted heat in a resistor, the SCR power supply switches the source on 
and off in each cycle to just supply the desired power output.  The SCR power 
supply is more complex but also more efficient.  You may have these devices in 
sets of one, two or four. The circuits follow standard DC rectifier circuits.

Roger AI4NI   </HTML>