2 (above) shows how to obtain higher wattage and current. If the bulb is bright after replacing say a blown fuse, then the likely problem most of the time is a short or problem in the switching power supply.įig.
Hit 'play' and watch a movie the lamp will glow some, that is normal. This "idle" current is very low and the lamp will be dim at best. There will be little current from the power supply inside the VCR being used. A 75-watt or 100-watt bulb would be fine.Īlso consider that the VCR in this example is simply powered on and not running a tape. In general I like to keep the lamp current versus in this case the VCR current at a ratio of at least 3 to one. If for example a VCR is rated at 30 watts that's. A 100-watt light bulb operates at a current of. To determine what wattage bulb to use the check the power rating of the device to be checked. Note: higher watt lamps have lower filament resistance than low watt lamps! The transformer could even be connected backwards and would still be protected while the lamp glows nice and bright. If the item being tested, in this illustration a transformer, has any shorts or draws excessive current the energy is absorbed by the lamp and doesn't cause further damage to the device or blow expensive fuses. When current begins to flow and filament heats up and resistance rises rapidly to act as a sort of current safety valve. How this works is an incandescent light bulb filament has a low resistance when cold, typically under 20 ohms. If one had an isolation transformer between the plug in and power I'd highly suggest using it. The item being tested is then connected to the brass screw side of the lamp socket and the switch (points A and B). The GRN - WHT wires (neutral and ground) connect together and form the common connected to the 'white' or shiny metal screw on the lamp base. The 'HOT' (black) would go to an on/off switch, which then connects to whatever is being tested. Assuming 120VAC and a standard 3-prong electrical cord. Let's look at and discuss some real world examples.įig. Here we will learn how to determine what wattage light bulb to use and what to expect when something is working right versus something being wrong. My purpose here is to give a more detailed account of how to use it. This is at your own risk and for information purposes only.Īlso see Build Autotransformer-Variac AC and DC Power Supply.Ī load lamp is simply a common light bulb in series with a device being tested. First and foremost read this safety warning.