TruePower 123 Variable Speed Controller for Duct & Inline Fans - Adjustable Dial for Precise Airflow Control - Perfect for Hydroponics, Grow Tents & Ventilation Systems

Very cool device. Allows me to slow my grinder down to a hum. Great for the Dremel multi tool, Sawzall etc. For clarity... This has a three way switch... Variable, OFF and FULL. FULL is straight power, with the device just a pass through circuit. On VAR it works exactly like a dimmer switch but the highest VAR setting is only about 90% of FULL. Because the devices are motors, some will require a full power start before swapping to VAR and the motors themselves make the slope a little choppy, but that varies by the tool. There is a lag between making an adjustment and the motor catching up or slowing down, from momentary to a few seconds depending upon flywheel or rotor size.I've had this one for two years and it works well for routers because they have "universal-motors". I have also used occasionally as a temperature controller for a wood burning tool without issue. It will not work with most other types of electric motors (e.g., the large ones you find in bigger tools like table saws and drill presses). If you see a lump on the side of the motor for a capacitor this type of speed controller will NOT work, if you try it will not vary the speed and could damage the motor or worse.This has the desired effect; it slows the device (I use it on a fan). The fan makes a bit of an odd noise, but it definitely is controlled. Curiously, the multi-speed control on the fan (the built in 3-speed) is obviated by the variable speed controller. But, again, it do work very nice.Works well in my greenhouse to control on/off function so I don't have to plug/unplug the fans every time, also works well as a speed controller. It performs as advertised, which is refreshing these days.I used this to control the speed of my bench grinder. It didn't do that very well. It is sort-of OK if you set it on the slowest speed before turning it on. But if it is on a higher speed whennyou turn it on, the speed control knob doesn't make any difference in the grinder's speed. Connecting it to my meter, I see that it he resistance isn't linear. All that being said, it was supposed to be to control a router's speed and I'm not using it for that purpose. All my routers are variable speed anyway, so I can't test it for that purpose.Item gets hot after extended use. It has a metal back that becomes too hot to the touch. I would use this item sparingly and not for projects that require a lot of running time.Bought two as finer speed controls on two large fans.Works great, the box does not get warm, as some cheaper models may.The long heavy duty cord is a HUGE PLUS.I bought this variable controller to control the Lasko B20725 Metal Box Fan, 20-Inch. This fan has the same problem that most fans have now-a-days, very little speed control. I simply plugged in this fan to the controller. Then the controller into the wall socket. Turned ON the fan to any running speed (HI-Med-Lo) and then leave it alone. Now control the fan speed with this controller. Perfect. Well worth the money!