Starting with code number (on or around) 7234, Lincoln Electric introduced the PC board or Electronic idler. It was a tremendous improvement over the "vacuum idler systems." This would control idle/weld speed upon demand. As long as you were close to the machine, it would automatically speed up on demand and idle down when needed. It was a step in the right direction, but if you strung your leads out or need to run an extension cord out there 100', you will need to kill the idler and let the machine run at full speed.
How the system works and why.
The PC board turns the idler solenoid "on," pulls it in, and the machine slows down to around 1000 RPM or less. FYI, there is no hard-and-fast low idle speed. The engine can run at 200 RPM, but it cannot accelerate to the welding speed of 1500 RPM. When the solenoid releases, the governor pulls the carburetor throttle plate fully open, and the engine goes "lean" (not enough fuel in the mixture to keep running). It "hiccups" until the inertia of the armature keeps the engine running until the fuel/air ratio provides enough fuel to keep it running. The stronger the engine, the lower you can reduce the idle speed (adjusted on the carburetor's low idle adjustment screw). I have seen remanded engines bounce up from 600 RPMs. What the engine needs is electronic fuel injection. It would make the engine last for 100 years.
When the solenoid is pulled in (low idle), it overrides the governor and slows the engine's speed. On Lincoln's PC board, there is a cheap transistor that is used as a switch. When it turns on, it completes the circuit, provides a ground, and the solenoid pulls in, idling the engine down.
What causes it to "Idle Up" go from idle speed to Weld speed?
This is where the magical "Reed Switch' makes its appearance. Somewhere in the welding circuit, there is a reed switch. This 1940s technology, in the correct application, one where there is no bouncing or shock, is very reliable.​​
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They have two problems;
1. They are contacts inside an easily breakable glass tube,
2. They are closed by an induced magnetic field. The farther you get from the magnetic sensor, the more voltage it takes to trigger the switch. This is why Lincoln installs an Automatic/ Off idler switch on the original machines. Lay your leads out 100 feet, and the automatic idler usually won't work.
How Does it Work?
After market reed
switch & concentrator
How does your grinder make the transition from idle speed to Weld speed?
Every welder out there has heard of a reed switch. Somewhere in one of the welding output leads, there is a "concentrator" with a plastic tube containing the reed switch. When you strike an arc, welding current surges through the concentrator, forming a magnetic field that closes and, hopefully, stays closed. This triggers the board, and a sacrificial transistor (used as an electronic switch) turns off. Most generic and Lincoln OEM boards use transistor control. This opens the ground circuit; the idler solenoid releases, the governor pulls the throttle open, and the governor controls engine speed (1500 RPMs) under varying loads.
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Image of classic transistor failure.
The two black wires in the middle of the board carry auxiliary power through the board. There is a copper coil on the board, with a second reed switch. When a load is applied, all the current (up to 15 amps) flows through the coil, activating the reed switch and triggering the idle board to idle up the engine.