Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!rutgers!apple!bbn!gatech!emory!stiatl!john From: john@stiatl.UUCP (John DeArmond) Newsgroups: sci.electronics Subject: Re: DC-DC inverter Keywords: power supply Message-ID: <4382@stiatl.UUCP> Date: 26 Apr 89 02:59:21 GMT References: <10837@bloom-beacon.MIT.EDU> Reply-To: john@stiatl.UUCP (John DeArmond) Organization: Sales Technologies Inc., a teeny little company in the existance of creation... Lines: 89 In article <10837@bloom-beacon.MIT.EDU> tytso@athena.mit.edu (Theodore Y. Ts'o) writes: >I am trying to design an alternative power supply for a photometer (the >GR1501-A, if anyone carse). The problem is that it requires 3 45 Volt >batteries which are almost impossible to obtain, and when they are >available, they are extremely expensive. > >So what I'm trying to design is a DC-DC inverter that will take power >from a 6V latern battery (or something similar) and covert it to a +90 >and -45 volt output. I've been building this kind of inverter for years in order to satisfy my peculiar perversion with keeping old tube-type portable radios working. My circuit is very easy to understand and build and is fairly efficient. Rather than try to enter a schematic in ASCII, I'll describe the circuit in enough detail to reproduce. My circuit uses a toroidal or cup-type transformer whose center-tapped primary is driven by a square wave oscillator push-pull style. I apply power to the center-tap and take each end to ground through a power FET transistor of appropriate rating. In series with Vcc applied to the center-tap is another power FET series pass element used for regulation. The oscillator is built around a hex schmitt trigger in a feedback configuration. This connection uses 3 gates hooked end to end with the output fed back to the input via resistors and caps. See the National linear databook for a discussion of this circuit. I trim the components to generate a square wave at about 5 khz. Feed the output into another of the triggers in the package to clean the wave up. Connect the output of this gate to the gate of one of the FETs. Also connect the output of this gate to another trigger to generate an inverted signal. Connect this to the other FET. Take the last gate and connect a feedback resistor from input to output to bias it as a linear inverting amp. About 10k or so should do. Connect the output to the series pass FET. This is the regulation circuit. You will later develop a voltage from the output of the inverter that controls the pass element. You will have to play around with scaling resistors in order to establish an operating point for the pass element. This is not a very precise regulator but it works fine for most tube applications. Winding the transformer. I build up the previous ciruit before winding the transformer so I can use it as a test circuit. You can get design manuals from core vendors such as Feroxcube but for one-off construction, you can usually do as well characterizing the core experimentaly which is nice if you find some uncharacterized cores. What I do is make a guess as to the proper number of primary turns. Experience says that 30 turns per side is a good starting point. I then connect the core to the driving circuit and measure the idling current draw. Set the applied voltage to at least the minimum operating voltage pluse some margin. For 12 volts, I usually use 10 volts. I increase the number of turns until the idling current ceases decreasing. The core is saturating at this point. Record the number of turns and calculate the necessary turns ratio for each output voltage. I wind a separate coil for each voltage rather than trying to tap one coil. Greatly reduces the incidence of ground loops, a real problem with high impedance tube circuits. Put a convential full wave bridge rectifier and sufficient filtering as needed. I usually select the highest current winding for regulation. Establish a voltage divider from this output to ground such that a voltage of the proper level is developed for feeding back into the regulator circuit described above. The easiest way to find this voltage is to connect a variable power supply to the input of the regulator and determine what voltage is necessary to drive the pass transistor to the operating level. Since the transformer core is saturating, the other outputs should regulate very well. In fact, for most radio applications where I run the inverter from a gel-cell, I rarely need a regulator. I simply apply battery voltage to the CT of the primary and use the output without any active regulation. This circuit is very non-critical and easy to configure. A schematic should not be necessary. Just get some parts from radio shack and go to it. John -- John De Armond, WD4OQC | Manual? ... What manual ?!? Sales Technologies, Inc. Atlanta, GA | This is Unix, My son, You ...!gatech!stiatl!john **I am the NRA** | just GOTTA Know!!!