Yeah, paralleling worked fine. I did something like figure 3, but not a bipolar drive. I do like that it has some built in current limit when shorted. One thing I worry about is that all the spec sheets I've read say I'm only allowed to short one opamp per package to ground.
I'm abusing some opamps now... I'll probably use the opa2192. It seems a bit of a shame to use such a wonderful opamp as just a heater drive... but at $3 each they are cheaper than some power opamp.
George H.
Yeah, I've already ripped up the test circuit.... but the LM395 worked perfectly in it's place. I could try again... it would be nice to have a cheap, thermally protected output drive.
George H.
OK I made a mistake, the first time I only had a 1 k ohm load on the LM317 And the output voltage (with zero input voltage) changes with the load.
10 k 4.5 V 1 k 1.3 V 0.5k 0.7V 0.33k 0.5 V.I used the lm317 as an emitter follower. Collector (in) to V+ Base (adj) to opamp output (with 1 k ohm series R) and emitter (out) to load and feedback.
George h.
I've used it as a buffer or low gain amplifier with the same result. The fix was to put a resistor about 200-ohms in the feedback path to limit the FB slew rate that the input sees. No more explosions!
Thanks Alan, I'm paralleling an opa2192. (outputs have
10 ohm resistors to FB, (master/slave) and then a fat diode to output.) I left it feeding a shorted output overnight.. it was happily oscillating away.George H.
The 317 can't sink current any more than an NPN's emitter can. Apart from its slow speed and weird offset voltage, it's pretty much equivalent to a power NPN.
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
I'm curious about Fig. 3 there: the circuit controls both sides of the load - why would they do that? I.e., why not just control one side? (I know, it's a naive question, but I am too.)
Yeah, I'm not sure. The LM395 worked as expected... maybe someday I'll play around more (there must be a spice model for the lm317.)
The dual/parallel opamps worked great, with the advantage that the max current is 120 mA. So it doesn't stress the power supply... I'm going to lay that out today.
George H.
Fig. 3 is just an example of extending the technique to drive a floating load.
piglet
OK, then why does one need to float a load?
de?
gI call it a bipolar drive. It's good for things like TEC's. You can get twice the voltage drive.
George H.
de?
gdouble voltage swing, no need for a bipolar supply.
Weird. It works fine for me. Did it blow up?
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Huh, maybe? The only naughty thing I did was -5V through
1k ohm into the base/adj. pin. Is that enough to blow it? (maybe I should have started with my fav 10k*.)George H.
*I use 1%, 1/4W TH 10k's for bus/ hook-up wire, and often throw away the 10 k ohm part. This practice increased once I discovered that the 10k's costs us less than 1" of bus wire... (we buy a lot of 10k's and not much bus wire.)
Perhaps someone sent a preview copy of the 82 book to select individuakls disguised as the 80 book. The page facing the title page probably has the gory details.
-- When I tried casting out nines I made a hash of it.
The closest th Linear Applications Handbook ---------------------------- National Semiconductor
The purpose of this handbookd is to provide a fully indexed and cross-referenced collection of linear ...
There is no copyright data on the inside front cover. Oddly enough the other side of the first page (the second page) contains Acknowledgments, and includes R. J. Widlar but excludes Pease. The remaining fourty-one people listed with Widlar are unknown to me. The book sort of has the vibe of a tome thrown together to hand out at trade shows, ergo the professional label and sloppy copyright.
73,-- Don Kuenz, KB7RPU
The closest th Linear Applications Handbook ---------------------------- National Semiconductor
The purpose of this handbookd is to provide a fully indexed and cross-referenced collection of linear ...
There is no copyright data on the inside front cover. Oddly enough the other side of the first page (the second page) contains Acknowledgments, and includes R. J. Widlar but excludes Pease. The remaining fourty-one people listed with Widlar are unknown to me. The book sort of has the vibe of a tome thrown together to hand out at trade shows, ergo the professional label and sloppy copyright.
On second glance, there's a tiny:
near the bottom left corner of the rear cover. (It earlier seemed extremely doubtful that corporate lawyers would ever allow something to escape the corporate confines without a copyright.) In the end, we're left with a book printed in 1982, which pretends that it's from 1980. It seems a corporate fubar one way or the other.
73,-- Don Kuenz, KB7RPU
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