|
|||||||
|
|
|
|||||
|
|
|||||||
|
|||||||
|
|
|
|||||
|
|
|||||||
Making a High Voltage Power Supply
Congratulations you just built a Vacuum Tube Regenerative receiver!!
Oh wait a minute! How are you going to test and use it? All you got for power supply is that little 9 volt wall wart Aunt Ruth gave you with that answering machine you broke last month. WHAT ARE YOU GOING TO DO?.
Well, you have several options:
Batteries
Ok batteries. How many? What type?
Heater or filament- "D" batteries can pump out about 250 mA for an hour or so. A typical heater takes about 200 mA at 6.3 volts. Wait a second, your radio has 2 tubes each taking a total of 500 mA!! There must be a better way of doing this....
Plate (s) and screen (s) voltages- Alright you need 150 volts and about 240 volts at 10 and 20 mA respectively. that means you need either high voltage batteries, but they're expensive. 9 volt batteries, yes you can get those easily. Let's see... 240 volts divided by 9, 26.66666 batteries needed. Whoops you can't get .66666 of a battery. Lets make it a total of 27 batteries. Alright 27 batteries * $1.50 each is 40 ½ bucks !!! I'm not going to pay that much for an hour of listening. What's a cheaper way of doing this??
AC powered High Voltage Supply
Now you're talking. Lets make something that will cost about 1 cent for 3 hours of listening!!! Go to a ham fest one Sunday afternoon, you need some parts. A power transformer, a bunch of tube sockets, a Gas discharge voltage regulator, some power resistors , a half wave rectifier tube, and any other little cheap things that might be needed.
Power transformer
OK you found a nice looking power transformer. You got to test it. How?
Simple. All you need is a power cord, switch, fuse, 2 resisters and a multi-meter.
How to test: Hook up the cord, switch, fuse and the primary winding. Hook up your multi-meter to the red wires and turn on the power. Write down the voltage you find on a piece of paper. Turn off the power and hook your meter to the green wires. Also write down the voltage you measure. Now hook up the resister to the red wires with the meter in series in a range of about 100 mA AC. Turn on the power and after about 5 seconds, measure the current and write this down. Kill the power and now set up the meter to read voltage. After you power it up again and read the voltage, repeat the same thing on the green wires except put the meter in a range that would allow a current reading of 2 amps.
We just done this to measure the winding impedance to figure out the approximate power we can pull out of it and to figure out the rest of the circuit.
To figure out the impedance, pull out the old calculator and get it working. Now find the voltage drop ( no load voltage - load voltage) and using Ohms law, R( Ohm)= E( volt) / I ( current in amps), find the impedance.
For the heater supply( green wires) you can get away with a voltage of 5.9-6.0 volts to light the tubes. To find the maximum current, find the difference between no load voltage and the minimum voltage needed. This should be about 0.4 volt. Using Ohms law, I= e /r , find the maximum current it can handle.
The plate supply( red- red yellow - red wires) is a bit more tricky to find the maximum current. A larger voltage fluctuation is often the case. The voltage can go down 10-15 % from no load to full load voltage. Figure out about 87% of no load voltage for maximum current. Do the math just like before for the maximum current.
The Basic Circuit
Ok, now you have a basic schematic diagram. What now? You got to figure you the resister sizes. R1 needs to be figured out so you got a voltage of about 230-240 volts coming out of it. The voltage regulator tube takes a max of 30 mA, so that's the number you need to figure out the resister size. What is the voltage you think is the transformer going to put out when 30 mA is being drawn? Well it should be (no load voltage)- ( impedance * 0.030 ). Now take this number and remove 240 volts from it. Take this value and divide by 0.030. Wattage = 0.030 * 0.030 * ( resister value) It's better to go 2 standard wattage sizes higher for this resister.
The Voltage Regulator: There are only 3 options for this regulator, 0d3, 0a2, and the VR150. A gas discharge voltage regulator is placed in the circuit between ground and a resister to power source. They need at least 5 mA of current and a maximum of 30 mA. You need to figure out the size of R2 to keep the voltage at the proper level. A resister of 2850 ohms at 5 watts is what's needed here.
The capacitors needed here must be at least 100 uF at 350 volts. More capacity would be better here, but the voltage rating is fine.
R3 is placed here for safety. Its job is to slowly use all the voltage from the capacitors when you turn off the radio. The capacitors can hold a lethal charge for several days!!! A resister value of 220Kohms at ½ watt would be perfect here.
Ratings:
Since the voltage regulator needs a minimum of 5 mA to keep the voltage at 150 volts, your radio can take about 22-25 mA from it. Your 230 volt output put out about 90% from what you figured out earlier minus 30 mA for the regulator. Now test out the supply with no load for 30 minutes at see if anything fails. If its still in 1 piece after 30 minutes hook it up to your radio and test out the radio.
Have Fun
