220 Volt Appliance to 110V

[JeffDajos]

Does anyone know an easy way to change an appliance that requires 220V AC so that it can run properly off of 110? I have some old HP power supplies that I would like to do this to.

[Dean Huster]

There are several mail order places where you can buy 2:1 step-down/1:2 step-up transformers for just such a use. The ones I've seen will work either direction (stepping 240 vac down to operate a 120 vac appliance or stepping 120 vac up to operate a 240 vac appliance). All you need to watch out for is making sure that your converter will handle the load. Most are rated in watts or volt-amps (same thing for our purposes here), so multiply your appliances voltage rating and current draw together to obtain that figure and select a transformer that'll handle that much power or more.<p>However, now that I reread your request, you may not need anything special for your requirements. Nearly all lab-grade test and measurement equipment (the power supplies fall into this category), especially those made by Hewlett-Packard, are dual voltage instruments. Many have a simple switch accessible from the outside (check the rear panel) to change the unit from 240v down to 120v and vice-versa; others may have the switch on the inside of the case; still others may be convertable by removing/installing soldered jumpers on a printed circuit board. If you don't find a switch somewhere, consult the instruction manual. If you don't have a manual, get a manual for the supply from one of the many sources on-line. You should have the manual anyway if you don't have one already since most Hewlett-Packard supplies have lots of options like remote sensing, etc. that might be nice to use.<p>A final note. Watch out for the fusing requirements. Some instruments require changing the line (mains) fuse when switching voltages. An instrument set to operate on 120 vac having a 10 amp fuse may require a 5 amp fuse when operating on 240 vac for proper protection. Again, check the manual to see if this is necessary.<p>Dean<p>[ September 21, 2004: Message edited by: Dean Huster ]</p>

[Bob Scott]

Jeff,<p>The smartest way to do this is with an autotransformer. These differ from a regular power transformer in that they are smaller and lighter and do not have any input-to-output isolation. You don't need isolation because your HP power supplies probably have isolation built in.<p>Autotransformers do not use separated primary and secondary windings. Choose one with 110/220 capabilities. They are also bi-directional, doing 110-220 or 220-110.<p>Bob

[bodgy]

Although Bob mentioned it, it is worth repeating, and autotransformer is not isolated from the mains power supply. Touch the terminals and you, your life and your fuse box will not be happy or well.<p>if you hook one up, make sure it is not plugged in and if for some reason you have to maneuver it whilst on do the old valve radio/tv repair trick, keep one hand in your pocket.<p>Colin

[JeffDajos]

Thanks a lot for the help. There were just a few wires that needed to be switched on the transformers. <p>I got one working perfect. The transformer on the other one was so old and dry rotted that all of the leads crumbled off of it when I touched it so I might just trash it.<p>Thanks again,<p>-Jeff

[Dean Huster]

Jeff, don't be too quick to deep-six that other supply. It'll make a fine hangar queen for the odd parts that you might need to fix your working supply if it decides to give up the ghost. The only down-side is that you do have to store the thing somewhere. But just having the larger filter capacitors available for replacement could be a lifesaver.<p>Dean