mA78HGA Positive 5 Amp Adjustable Voltage Regulator
| I have been intending to building this for 18 years. The heart of the supply is an adjustable voltage regulator IC made by Fairchild. I don't even know if these are being made anymore, I got one from Radar Electric, Inc. in 1984 when I worked there as a counter salesperson. I e-mailed Fairchild recently and they still had the datasheet in .pdf format. It is a 4 pin TO-3-type packaged regulator that is programmable with two resistors. I was impressed with it in that it can handle up to 5 Amperes. I am only expecting it to give me 1-3 Amperes in a pinch, any more and I'd have to heatsink it better and get a bigger transformer. | mA78HGA Positive 5 Amp Adjustable Voltage Regulator |
The other items I'd been collecting for this project over the past 18 years are a Simpson 0-15 vDC panel meter, an Archer Transformer (which is also no longer available) 120V primary to 9v-0-9v secondary. I am using it as an 18 volt, non-center-tapped source for a full wave bridge rectifier. The DC going in to the first filter capacitor is approximately 24 volts. The 21500 mF capacitor is probably over kill, but I had a few laying around at 80 WVDC.
One thing this schematic doesn't show is that I fused both the AC input and the DC output to the regulator prior to the DC indicator lamp. This way the lamps indicate which fuse has blown. I enclosed the regulator circuit in a cage I pilfered from an old Motorola Civil Defense radio that used to enclose the old selinium rectifiers.
To set the voltage for the mA78HGA the calculation for the two resistors connected between pins 2, 3 and 4 can be found in the datasheet. I used a 4.7K W fixed and a 10K potentiometer to obtain 5-15 volts. I have tested it at 1.2 amperes using a 10W resistor at 12 volts.
What inspired me to dig all this stuff out and finally put it together? I've been spending a lot of time experimenting and prototyping lately...I'll also come in handy for the Morse Code transmitter (Tuna Tin 2) I recently built. I haven't decided where a current meter would fit...
