Transformer -> bridge -> cap -> linear voltage regulator

Ah, the all-discrete, positive linear voltage regulator; no LM3xx parts, but maybe a 723 at it's core. Built a few, repaired a bunch. Sticking to the basic regulator, without goodies like current limiting and regulation down to 0 V, there are two basic circuit topologies:

1. Fixed reference voltage, variable gain. This usually has the output adjustment pot in the feedback voltage divider.

2. Variable reference voltage, fixed gain. This usually has the output adjustment pot across the reference diode, and fixed resistors in the feedback voltage divider.

I'm asking for experience-based opinions on the advantages and disadvantages of each circuit type. As a sub-question, which is more advantageous for a bench-type supply with a large output voltage adjustment range, as opposed to an industrial or application-specific supply with a narrow adjustment range like +/-10%.

Thanks.

ak

 

I like the variable reference, fixed gain option because it is usually more stable. Usually the variable reference is the center tap of a precision multi turn pot connected across an internal/external reference.

 

Basic premise: If you don't vary the reference voltage, you can never get to zero because altering the feedback loop always starts with the gain equation, gain = 1 + (Rf/Rc). It is the "one" that bites you in the zero.

 

Basic premise: If you don't vary the reference voltage, you can never get to zero because altering the feedback loop always starts with the gain equation, gain = 1 + (Rf/Rc). It is the "one" that bites you in the zero.

Unless you have a negative reference and an inverting amplifier - which I've actually seen.

ak

 

I have always done the variable reference voltage topology. I am not saying that it is the best. But, I do think it is the most common configuration...