You need a 'Shorting Switch' that makes contact with the next position, before breaking the existing connection. As usual, Maynard leaves out the important detail.
-- You can't have a sense of humor, if you have no sense.
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I know that it works on paper, but to need high wattage resistors is just poor design. That one watt pot would likely be wirewound, and very noisy when it's adjusted.
-- You can't have a sense of humor, if you have no sense.
The devil is in the often unspecified details. If you're only building a few and don't have to deal with component variations from different vendors, the job gets easier.
How many different current limits do you REALLY need? Sounds like there's no need to switch the current limit while attached to the DUT?? Sounds like the current limit is only approximate. So, why the series current limting IC at all? What's the advantage over a resistor and a 7805 equivalent?
7800s come in M and L versions with different current limits. Might be cheaper overall to use three regulators and configure the correct output with the wiring harness to the specific DUT. Sometimes, there's benefit in limiting the options of the production staff. 7800s are generally stable if you let the input voltage crash into the output voltage, but I'd check what happens with non-linear loads.My favorite example is a USB hard drive that draws too much current to spin up. Makes quite a racket and can't be good to have the heads banging into the stops.
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An LM395 pass element, and a bit of foldback on the current limit. That prevents the voltage and current regulation modes from fighting each other. I sort of like the Power One approach, where the current limit latches the output low until you turn off the supply or crank the voltage way down and back up again. (My general purpose bench supplies are a couple of 25-year-old Power One linears--nice and quiet, and stable stable stable.)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Polyfuses are good for fire prevention. The radial leaded ones work well; the surface-mount ones less well.
I like to use a polyfuse and a transzorb on DC voltage inputs to my boxes, for overvoltage/transient/reverse polarity protection.
I also have used an LM317L TO92 as a current limiter, with the adj pin floating. It works great, but loses about 1.5 volts. I wish there was an equivalent lower dropout on-purpose current limiter.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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Yup. If the internal current limit is good enough for the purpose, you can just leave the adjust pin open, and it works fine. The thermal limiting gives you some slow foldback effect as well, but of course you don't know if the limit is 1.5 A or 3.4 A, because the spec is so sloppy.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
The 78Lxx parts aren't the same as the 7800s--they have far worse ripple rejection. I gave up on them long ago. 317Ls are okay.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
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Multiply all the onesies and twosies times the number of Spehros, Joergs, Johns and so on :-)
It's like Milwaukee. Hardly anyone needs more than one Sawzall but a lot of people need one just like I do.
I have a classic example in my portfolio. A unit that I designed in the early 90's, still coming off a production line in Guangdong. Changes: Zero, it's essentially still Rev A. Like with this board I always try to design without boutique parts so unless Armageddon happens they can keep producing it nearly forever.
-- Regards, Joerg http://www.analogconsultants.com/
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For bench supplies most folks want another behavior though. I sometimes need to run mine in constant current mode and a foldback or latching mode would nix that.
-- Regards, Joerg http://www.analogconsultants.com/
With clean 12V going in, ripple rejection shouldn't be an issue? Line regulation might be with a series current limit.
Another way...
ILIM sets the output current limit without requiring a 2W potentiometer.
Design of ILIM is left as an exercise for the student :-) Caution, a negative auxiliary supply of at least -1.25V will be required to pull output all the way to zero volts.
A similar approach should work with the two-LM317-in-series presentation ...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Or add some diodes to drop the output, then the TL431 loop.
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
LM1086 - (I'm not sure how it goes with a floating adjust pin)
-- We have failed to address the fundamental truth that endless growth is impossible in a finite world.
Please refer to the figure in the datasheet labelled "Current Source".
Important: it only gives low dropout if you have an independent bias supply. Dropout is about 350mV from the main input and about 1.35V from the bias input.
See also LT3082, '3083, '3085.
Disclaimer: I haven't used these particular parts in any design.
Regards, Allan
On a sunny day (Sun, 01 Apr 2012 15:10:58 -0700) it happened Jim Thompson wrote in :
What I do not understand is why not just put a PNP in front of a LM117, and use the beta:
PNP out b | | | | | === 150 [ ] | === 470 [ ]
Am 01.04.2012 00:54, schrieb Jasen Betts:
One L200 from STMicroelectronics will do the same. Unfortunately it also requires high power resistors for the current limit.
Cheers, Alexander
I'm mistreating the pot. probably enough to destroy it, it probably needs a 12W part instead of the 1W shown.
but if (as it transpires) a switch can be used instead 100mA steps can be done with 1/4W resistors, (after the initial 2W)
if your prepared to program the current limit in binary it's possibly even cheaper, eg: some number of 12R in parallel, each is good for
100mA of headroom.-- ?? 100% natural --- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net
the brief (as I understood it) was a linear regulator with a 12V supply
every watt that's burned in a resistor isn't being used to warm the heatsink,
use 1 to 10 12 ohm 1/4W resistors in parallel for the current selection.
-- ?? 100% natural --- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net
That's a $4 part...
The trip voltage is 0.45V, so just exactly how would that require a high power resistor? That's only 1/2W at the 1 Amp limit.
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