Copyrighted 1996 Any bench supply consists of a transformer with a secondary of about 18 volts AC, a rectifier block and a filter capacitor.With no load, the DC voltage across the terminals of the filter is going to be 18 to 30 volts.Under load the voltage will sag due to the impedance of the secondary of the power transformer.
If it sags too far, the supply will loose regulation and it will pass 120 Hertz ripple (hum) to whatever you are powering with it.
Since the voltage across the filter capacitor can vary wildly as the load changes 723 in a little more detail now.
Note that it has its own filter power supply made up from diodes D1 - D4 and filter capacitor C2.723 compares this reference voltage to adjustment pot RV1 sets the ratio of the reives the base of Q1.Q1 acts as a simple amplifier to increase the current available ltage from the wiper of RV1 drops as well.When this occurs, the 723 increases the voltage from pin 10 driving the pass transistors harder through Q1.
So, the motransistors, the higher the output voltage will be.Thus we have regulation, the 723 meet the minute changes in load on the supply.Remember, the 723 does the thinking, the pass transistors do the work!
our example, lets say or power supply uses the common or can pass up to 15 amps of collector ny device at its maximum rating.
Thus for a 25 amp power supply you would want to have more than two 2N3055's in parallel.For the last supply I built, I used four.If you used higher current rated transistors, you could get by with less of them.If you simply placed the multiple pass transistors in parallel, slight differences in gain between them would causetransistors would hog most of the current flow and most likely fail.Tosmall value resistors called "Emitter Ballast Resistors" are added in series with the emitter lead of each transistor.These can be between .01 to
.1 ohms and should be of at Southeast Iowa Technical Society
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Copyrighted 1996 bigger.
For example if the schematic calls for a 35,000 mfd filter cap, anything larger will work even better.The same principal applies with the voltage rating.
If the schematic calls for a 25 VDC capacitor, you can't go smAlmost any high current power transistor will work for the pass transistors.Years ago the old 2N3055 was the industry standard.Today the 2N3771 (ECG181 replacement) used
amp collector currAt most hamfests I have been able to find laballast resistors already attached for a couplsave you a lot of work and expense! Filter caps and full wave rectifier blocks are also common and cheap.If you see them buy a few as spares.
The same for 2N3771 transistors if you own an Astron power supply.
Someday you will need a set for your Emitter Ballast resistors can be homebrewed if need be.I've made my own by winding copper magnet wire on large value resistor bodibody of the large resistor simply acts as a form to hold the wire.You can also make ammeter shunts this same way nd look up the Copper Wire Specifications for each size of wire.For my last supply I needed .1 ohm at ten amps.The smaller the wire diameter, the greater its resistance.Going down the column for wire in bundles, the smallest wire I could use for ten amps was #18.#18 is rated at 6.38 ohms per 1,000'.That's .00638 ohms per foot (6.38 divided to give .1 ohm resistance.That's makes for a planned, I had them when I need them and they are damned hard to burn out!
Southeast Iowa Technical Society
Whats Inside your Astron?All rights reserved 6
Copyrighted 1996 To sum things up in this article, armed with a little information you can repair, modify or If you can find the right junk, building is certainly cheaper than bJust remember, as long as the power transformer is good, any junk burned out 13.8 volt The schematic referenced in this article can be found on the last page of this document.I also have the schematic for this project available for the ISIS Cadpak software package.
please contact me directly.David Metz, WA0AUQAstron is a registered trademark of Astron, Inc..
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