Hi, I'm trying to troubleshoot a small 13.8V/2.5A regulated power supply I picked up from Radio Shack awhile back. I don't think much of it, and I'm planning on snagging an Astron when I get the opportunity, but I thought playing with this peice would be a nice learning experience.
The problem is that this 13.8V regulated power supply puts out a very stable 15VDC! I cracked it open and mapped out the circuit, which I included below. V-in is a standard chunky transformer, bridge rectifier and large filtering capacitor. These components put out 22VDC. The regulator circuit is a linear regulator, this isn't a switching PSU or anything. I read the handy "Build your own power supplies" book published by Radio Shack, and this circuit doesn't seem to make much sense to me.
I understand that the Zener diode is designed to provide some manner of reference voltage with which to compare the regulated output and produce an error voltage. In the book I read, a secondary transistor is used to amplify this signal and feed it to the base of the primary regulating transistor. In this power supply, though, I see no way in which the regulated voltage is sampled at all! I could really use a brief run down of how regulation is produced in this circuit, and perhaps a suggestion of how to tune the output voltage to 13.8VDC. My first reaction is simply to replace the 2200 ohm resistor with a potentiometer and decrease the resistance. I believe this should leak more voltage to ground and away from the base plate of the regulating transistor and therefore decrease its output voltage. In practice, though, this didn't seem to work having only the voltmeter on the V-out poles (no load).
The regulating transistor is a Motorola branded TIP3055. The secondary transistor is a TIP31C. I'm guessing that the capacitors here are for filtering; correct me if I'm wrong. Also, I have no idea why they put a 27 ohm ceramic power resistor on V-in.
Thanks for any help/insight!
**broken link removed**
The problem is that this 13.8V regulated power supply puts out a very stable 15VDC! I cracked it open and mapped out the circuit, which I included below. V-in is a standard chunky transformer, bridge rectifier and large filtering capacitor. These components put out 22VDC. The regulator circuit is a linear regulator, this isn't a switching PSU or anything. I read the handy "Build your own power supplies" book published by Radio Shack, and this circuit doesn't seem to make much sense to me.
I understand that the Zener diode is designed to provide some manner of reference voltage with which to compare the regulated output and produce an error voltage. In the book I read, a secondary transistor is used to amplify this signal and feed it to the base of the primary regulating transistor. In this power supply, though, I see no way in which the regulated voltage is sampled at all! I could really use a brief run down of how regulation is produced in this circuit, and perhaps a suggestion of how to tune the output voltage to 13.8VDC. My first reaction is simply to replace the 2200 ohm resistor with a potentiometer and decrease the resistance. I believe this should leak more voltage to ground and away from the base plate of the regulating transistor and therefore decrease its output voltage. In practice, though, this didn't seem to work having only the voltmeter on the V-out poles (no load).
The regulating transistor is a Motorola branded TIP3055. The secondary transistor is a TIP31C. I'm guessing that the capacitors here are for filtering; correct me if I'm wrong. Also, I have no idea why they put a 27 ohm ceramic power resistor on V-in.
Thanks for any help/insight!
**broken link removed**
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