dual-tracking power supply - Page 1
Forum section: electronics / projects

author: Fenichel
author_rank: Contributor
timestamp: July 21, 2016, 11:34:50 pm
content: :-//I have implemented the dual-tracking power supply shown as Figure 9.15 on page 608 of the Horowitz & Hill 3rd edition. The H&H design calls for a feeder supply of +(28 to 38)V, -(28 to 38V). The only feeder supply I have handy maxes out at +-15V, but I expected that this would be OK, and that the only effect of this would be the obvious limitation on the regulated output.With this feeder supply, the -1.24V rail is fine (-1.23V), but the range of the regulated output is only from (+6.38V, -6.58V) to (+7.98V, -7.73V) as R2 moves from CCW to CW. At the same time, the voltage at the adjustment terminal of the LM317 goes only from -1.23V to +0.398V.Is this what I should expect with the reduced feeder voltage? If so, why is that so?Or, instead of finding or building a high-voltage unregulated feeder supply, should I look more closely at some implementation error in the circuit I've got?All suggestions appreciated.

author: Dave
author_rank: Super Contributor
timestamp: July 22, 2016, 12:31:42 am
content: According to what you are saying, you now have about 7.4V between the output and adjustment pins on your LM317. Like noted in the schematic, you should have 1.25V there, which leads me to believe that your regulators are faulty. You didn't by any chance buy them from a shady seller on ebay, did you?

author: Fenichel
author_rank: Contributor
timestamp: July 22, 2016, 04:11:43 am
content: I didn't get the 317 on eBay, but I don't remember where I got it. I'll unsolder it and check it out in isolation. THANKS for your comment; I always have trouble, without outside encouragement, acting on the supposition that my problems are attributable to bad hardware.

author: Fenichel
author_rank: Contributor
timestamp: July 27, 2016, 05:01:58 am
content: I have this working now; the problem was not a defective LM317 (tested in isolation & within spec), but rather a careless assembly error (I placed 121K for R1 instead of 121).I have now discovered something about the design that I hadn't realized, so I record it here in case others are also implementing this circuit. I thought that reduction of the feed voltage from the specified (+28V to 38V, -28V to -38V) would only limit the maximum output voltages, but there's more to it than that.Fed with (+15V, -15V), for example, the output gets up to (+12.99V, -12.98V) reliably, and even tracks sloppily up to (+13.44V, -13.23V). At the other end, though, the minimum output is a little more than (+2V, -2V) [tracking is good to (+3.00V, -2.99V), but at the bitter end the output is (+2.03V, -2.11V)].A feed of (+20V, -20V) is almost enough. The negative output neg-mirrors the positive output to within <0.5%, from <+1V up to ~+16V; tracking is lost at the extreme upper end (+17.37V, -16.86V).I got the PCBs for this from OSH Park, so I had to get 3, when one was enough. The PCBs cost me $6 each, and I'm happy to pass my two spares on to anyone who'll pay that + postage. Warning: I put the LM317 & the LM337 too close together, so one of them will accept only a Dremel-trimmed heat sink.