Circuit Fun: Auto-ranging analog voltmeter for variable power supply - Page 1
Forum section: electronics / beginners

author: w2aew
author_rank: Super Contributor
timestamp: June 16, 2014, 01:52:50 pm
content: A fellow ham and I were discussing his variable power supply project, and how he wanted to use a nice analog panel meter to indicate the voltage of the supply. However, a single meter that would be adequate for the maximum output voltage would give poor resolution at lower voltages. So, I suggested building a circuit that would automatically change the range/scale on the analog voltmeter when the voltage was low.This video shows a simple circuit that I came up with to do this:

author: nixfu
author_rank: Unknown
timestamp: June 16, 2014, 02:43:28 pm
content: When bench power supplies all had analog meters, it would be almost a given that they would have a range switch and multiple scales drawn on the dial.I would think maybe a voltage divider with a fixed ratio to switch in and out of the meter driver might be an easy solution.

author: w2aew
author_rank: Super Contributor
timestamp: June 16, 2014, 04:15:40 pm
content: Quote from: nixfu on June 16, 2014, 02:43:28 pmOf course, this is how a lot of variable power supplies were made. In this particular case however, my friend is building a single-range continuously adjustable power supply (no manual range change), and wanted a way to instrument it with an analog meter and still have reasonable setting accuracy for both low voltages and high voltages. We had also discussed the use of a log amp to drive the meter to expand the divisions on the low voltage end. However, we thought that the linear scale(s) were nicer to use when tweaking voltages up/down.The circuit is presented here mainly as fun exercise, and an interesting application for my favorite little analog IC, the LM10.

author: Shock
author_rank: Super Contributor
timestamp: June 16, 2014, 04:36:11 pm
content: Another great video.

author: M0BSW
author_rank: Frequent Contributor
timestamp: June 16, 2014, 05:16:52 pm
content: I agree another top video from Alan , this will be my weekend project, for a power supply sitting here waiting.

author: w2aew
author_rank: Super Contributor
timestamp: June 16, 2014, 09:21:17 pm
content: The LM10 was designed by Bob Widlar, making it extra cool....Designed during his return to National Semiconductor in the late 70s. Here's a link to the application note that featured the unique capabilities of the LM10 - features that went unmatched for a decade according to many...http://www.ti.com/lit/an/snoa638a/snoa638a.pdf

author: qno
author_rank: Frequent Contributor
timestamp: June 17, 2014, 05:24:26 pm
content: Did Forrest Mimms teach you how to draw your schematics?I liked his books and you make the same kind of simple and to the point schematicswith minimal but useful notes.Good job, thanks for sharing.

author: w2aew
author_rank: Super Contributor
timestamp: June 17, 2014, 05:32:24 pm
content: Quote from: qno on June 17, 2014, 05:24:26 pmWell, not *personally*!! But, of course, I used many of his "Notebook" series of books when learning electronics in the late 70's. I'll often re-draw a schematic several times until I feel it is clear enough, and helps to convey the circuit operation and signal flow clearly.Over the years you really appreciate the value of a well drawn schematic. There's nothing more frustrating than trying to understand a circuit from a schematic that is poorly drawn. Excellent examples for simple designs are certainly reflected in Mimm's schematics. The schematics in the old Tektronix oscilloscope service manuals were also terrific.

author: megajocke
author_rank: Regular Contributor
timestamp: June 24, 2014, 10:52:50 pm
content: Neat. And the schematic does indeed look very nice.I actually have a power supply, a 0-60 V 0-10 A unit by Oltronix, which has this kind of autoranging voltage and current meters. The switchover points are at 6 V and 3 A.The schematic is attached. There is not a single IC to be found inside the beast!The range switching circuits are built out of a bunch of BJTs and the switchover reference is the (separate) power supply for the meter autoranging circuit which is regulated by shunt zeners.They use somewhat unusal symbols for the BJTs though in the schematic...