You Can Teach Old Inverters New Tricks Bob Grater K6SUB c.1992 Bob Grater In these days of super efficient modified sine-wave inverters, the old power hungry square-wave inverters that we started off with have all but been forgotten. Yet there must be a lot of them out there sitting in corners gathering dust or being sold at flea markets and garage sales for ridiculous prices. Tripp- Lite still manufactures square wave inverters ranging from 100 watt to 2000 watt output. These reasonably priced, well- built units can take a lot of abuse and keep on ticking, and there are probably others that I am not aware of. Just because these older style inverters aren't as efficient and can't stand the surge loads the new Whiz-Bang Gyro-Stabilized Talking inverters can doesn't make them useless. As a matter of fact they can come very close to matching the newer inverters for efficiency in a lot of applications Ä and for a lot less money. Don't get me wrong, the new high efficiency inverters are great. I have a Powerstartm UPG-1300 and consider it one of the best things to happen since they started slicing bread. But even these new inverters can stand to benefit from a couple of tricks. Trick Number One: DC Output for Universal Motors Trick number one is an old trick, used first on the Knightkittm KG-662 and KG-666 inverters made by Allied Radio in the late '70s. These square wave inverters put out 120 watts or 240 watts Ä your choice by flipping a HI-LO switch. They also had a built-in battery charger using the same HI-LO switch. Their 'trick' was a 120 Volt DC outlet as well as the 120 volt square wave ac outlet. Why 120 Volts DC? Just try and run a hand drill on the square-wave output Ä you might be able to drill through soft butter. Plug the same drill into the DC outlet and you have a full power, full speed hand drill. Let's borrow Allied Radio's 120 volt DC outlet idea and take it a step further to the point where, with a large enough square wave inverter, it will start and run a brush type power hand saw. A full wave rectifier is all that's needed to change the square wave into a sloppy 120 VDC. But wait Ä we can do better. Did you ever notice when you pull the trigger on your portable hand saw the lights momentarily dim? It takes a lot of current for a short period of time to get that motor turning. This momentary high current drain will stop an older square-wave inverter in its tracks. Fortunately the solution is easy: add a couple of hefty capacitors to store the extra energy for the motor start up. Capacitors are a lot like a battery in that they can be charged and discharged. In this circuit they get charged up when the inverter is turned on, and then when the motor is turned on they add their stored energy to the inverter's energy, giving the required boost. They also smooth out the pulsating DC coming out of the rectifier. Direct Current Caveats Be careful. Direct current can only be used on "universal" or brush type motors, resistive loads, and incandescent lighting. It cannot be used on induction motors, or any equipment that uses transformers (most consumer electronics, microwave ovens, etc.), or a lot of other things. Most variable-speed power tools will handle DC but run at full speed (you lose the variable speed feature). If your saw or drill runs backwards, simply reverse the plus and minus leads at the output receptacle. Any device using a thermostat such as a crock pot should not be used on DC. This is because DC tends to prolong arcing when contacts open and close and reduces the life of the contacts. INSERT TRICK#1 Building the Circuit The values of components I specified for this circuit are super conservative since I expect things I build to last about as long as I do. I took a small metal box that would contain all the components and mounted a fuse holder, the bridge rectifier (using the metal box as a heat sink), and the two capacitors with some tie-wrap to keep them from flopping around. A hole was drilled in each end of the box to accommodate grommets for a short extension cord that I cut in half for the input and output to the unit. The MOV (metal oxide varistor or surge protector) is optional but a good idea Ä see Trick Number Three. The 100Kê bleeder resistor bleeds off the charge in the capacitors slowly when the unit is shut off and is an inexpensive safety item. Make sure you observe the correct polarity when wiring the capacitors in parallel and to the bridge rectifier. This handy little box can then be plugged into different inverters or even used with standard house current if you have a long extension cord run and you need an extra starting boost to your power saw. Trick Number Two: Buzz Buster Capacitor Square wave inverters and the appliances they run can make an annoying buzzing sound. You can get rid of some of that buzz by wiring a 3 æF AC oil-filled motor start capacitor across the secondary or output of the inverter to round the edge of the square wave. This is referred to as the "brute force" method of buzz removal, and there is a price to pay in the form of added power consumption. The sharp edges of the square wave see the capacitor as a very heavy momentary load. INSERT TRICK#2 The Heathkit model MP-10 used a 5 æF AC electrolytic capacitor across the transformer primary to smooth out the edges of the square wave somewhat. This wet electrolytic capacitor often dries out and blows up (literally) or shorts out, making the inverter inoperative. Remove or replace this electrolytic capacitor with an oil filled motor starting capacitor which will essentially last forever. It will have to be mounted externally since it is much larger than the original small electrolytic capacitor. Trick Number Three: the Surge Suppressor Surge suppressors protect sensitive electronic equipment from high voltage pulses and radio frequency interference (RFI). High voltage spikes are more common on utility power lines than in home RE systems. But they can be a problem with poorly designed inverters or voltages induced by lighting strikes. RFI, on the other hand, is a fact of life with all square wave and modified sine wave inverters. Abruptly switching current creates high frequency harmonics which can interfere with radios, TV, etc. You can buy a surge suppressor from your local electronic outlet store, or you can build a better one for roughly a tenth the price. The schematic below shows how. INSERT TRICK#3 Incoming line voltage goes through a fuse or circuit breaker, directly across the MOV, and into the line side of the RFI/EMI (Radio Frequency Interference/Electro-Magnetic Interference) filter. The MOV is basically two wires stuck in a ground up semiconductor and wrapped up in a little package. It acts like an insulator until the voltage rises above a level determined by the distance between the wires, and the semiconductor used. At voltages above this level the MOV quickly becomes conducting. In this circuit the MOV starts conducting at 150 volts, and drawing more and more current as the voltage goes up. This is approximately the peak voltage that comes out of your wall plug or modified sine wave inverter. The 120 volt figure you are normally familiar with is actually the RMS (root mean square) voltage. Another way of putting it is, the 120 volt figure is the average effective voltage of the sine wave coming out of the wall. The MOV is designed to handle very short duration, high voltage spikes that can ruin most electronic equipment. If such a spike of any significant duration occurs the fuse will blow, protecting the equipment. MOVs will take a lot of punishment, but if you live in lightning country I would suggest getting a spare or two because they will short themselves out when really hit hard. If the MOV is shorted, the fuse will simply blow again when you replace it. (Don't leave the surge suppressor plugged in permanently to an idling inverter . Idling peak to peak voltages can rise higher than the MOV's threshold causing them to conduct and, after a while, blow. This exact scenario happened to me with a POW200 inverter and a computer surge suppressor. The POW200 did not survive the experience Ä CG). Don't pay retail price for your RFI/EMI filter; you can get it much cheaper as surplus. Also, get the highest-current filter available. The RFI/EMI filter consists of a network of capacitors and inductors designed to remove electronic noise from the power lines. Both sides of the power line are worked against the case ground so it is important to have a good ground. Ground the grounding prong of your input plug or have the case tied to a grounding rod through your system ground. This combination of items can be put into a box much in the same manner that Trick Number One was except no heat sinking is required, just good grounding. If you are plugged into the grid no matter where, from FFL (Florida Flash & Flicker) to PG&E (Pacific Graft & Extortion), there is an unbelievable amount of trash coming in on those power lines. The closer you are to an industrial center or the farther out you are in a rural area, the worse it is. The suburbanites seem to have the cleanest power except in hot weather when the air conditioners are switching on and off. Part of a project I was on at one time had me taking pictures of what comes into your home on those power lines Ä suffice to say it's a great argument for getting off the grid! Fun to Run These old timers (meaning the inverters not the author) are practical! I keep the Knightkit in the camper shell on the pickup, wired in and ready to go. The Heathkits have been moved from car to car over the years. The only requirements are heavy wiring to the battery or fused ignition line (size depends on distance) and plenty of ventilation, especially around the transistor heatsinks. Did you ever want to drill a hole to mount a new gadget in the car, or solder it in without draping extension cords all over the place? Want to check out a small appliance at a yard sale? Just plug it in, then complain about the terrible buzzing but accept it anyway at half price. These old square waves will run a computer without any problem because of the switching power supplies used in computers. Some printers don't like the square waves and a few video displays have problems with them. Generally they can be used in an application where you don't need them to idle for long periods. Just turn it on, use it, and turn it off. Out buildings and remote sites are handy places to have them. The Crock Connection I get the feeling that whoever designed the crock pot had inverters and long auto trips in mind. The power drawn by a crock pot sits right about on the peak efficiency curve of these small older inverters. While you're cruising down the road your alternator is putting out all the power needed to run the inverter so there aren't any worries about draining your battery and it would take an awful long rest stop to cause any kind of battery problems. In the meantime, dinner is slowly cooking and will be hot and ready to eat when you arrive at your destination. You can put that New England boiled dinner together and start it in the pot a few hours before you leave and then just move the pot to the car. Or, you can plug the pot in after you arrive to continue cooking and eat after you're settled. A crock pot gives you a lot of leeway in cooking times. Making the pot carproof is simplicity in itself. Drill two small holes, one in each handle, large enough to accept the hooks on a bungee cord. Insert one side of a short bungee cord in one handle, take a quick wrap around the knob on the lid, and attach the other hook to the opposite handle. That lid is now guaranteed not to go anyplace! I had this idea many years ago when the pots were small and I elected to mount mine on a plate with rubber underneath so it wouldn't go anywhere. The new larger pots have a much bigger footprint and just gluing some faucet washers on the feet pretty much guarantees it won't go anywhere unless you are on some really rough roads. The pot is still just as useful at home as it is in the car. I appreciate it being back in the camper shell with the animals since I can get awfully hungry toward the end of a long drive. The smell drives the dogs bananas! The Cocker/Retriever has to alternate between watching the bird and the crock pot, the Beagle/Whippet stays glued to the pot, and the cats just wander around and pounce on one of the dog's tails now and then to raise a little excitement. In the station wagon I must admit I've been forced to test how tender the corned beef was long before we arrived. Be sure to look at the wattage on your crock pot verses the wattage of inverter. We usually cook everything on low, saving the high setting until we get where we are going and just can't wait. I hope this has resurrected some of those old inverters and started some fun ideas working along with them. Access INSERT GRATERPARTSTABLE Author: Robert Grater, 1595-176 Laurelwood Rd., Santa Clara, CA 95054 ù 408-727-2050 All Electronics Corp., P.O. Box 567, Van Nuys, CA 91408-0567 ù Order Phone: 1-800-826-5432 ù FAX 1-818-781-2653 (Minimum Order $10.00 + $3.50 S&H)