That runs into money, though. Negative LDOs especially. I have samples of the laser/TEC combinations, so once I get back on my feet I'll do some measurements and see if I can afford an extra volt-and-a-bit for the TLV431/LM4041-ADJ version.
That might be interesting. The issue is that neither side of the TEC is grounded, and it's on the other end of a cable, which makes it somewhat vulnerable to having one side shorted to ground. That makes any asymmetrical on-board solution a bit problematic--the TEC can melt before the thermistor notices anything wrong.
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
hobbs at electrooptical dot net
http://electrooptical.net
On 01/08/2017 10:57 AM, Jim Thompson wrote: > On Fri, 6 Jan 2017 16:23:04 -0800 (PST), snipped-for-privacy@gmail.com wrote: > >>> I'm not totally following your scheme, but wouldn't you want to >>> _replace_Q2_ with the TLV431 (using drawing _above_)? Then your >>> current limit is 1.2V/R2 with a _very_low_ TC. A TLV431 is >>> transistor-sized... TO-92 pack, unless you're in love with SOT's. >> >> Yup. The LM385-ADJ is the same sort of thing except that it holds the 1.2V between ADJ and K, so it works with PNPs. (To make a 1.2V reference with a TLV431 you short ADJ to K, whereas with an LM385 it's ADJ to A.) >> >> Still costs an extra volt-and-a-bit of headroom, and another buck or so on the BOM. Might be worth it.
My previous followup seems to have gone off into the Howling Ether.
Yes, exactly--it's an IC-ified version of the normal 2-terminal limiter. The 431-style refs only work with NPNs and NFETs, though, because they hold Vref between ADJ and A. For PNPs you need something upside-down such as the LM385 or LM4041-ADJ, which hold Vref between K and ADJ.
Yup, that's the idea, except that mine is linear. The maximum current is +-250 mA, so the dissipation is modest, and this avoids problems with switching crap getting into the laser.
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
hobbs at electrooptical dot net
http://electrooptical.net
I didn't have your @gmail address white-listed :-( It is now.
(I have various @gmail/@google posters whitelisted followed by a blanket kill of all the rest :-)
If it's linear, wouldn't it be easier to merge limits into the "conveyor"? ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I'm considering having the joint in my right big toe replaced. I'm a "walker" and the arthritis is impending my speed ;-) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Make that impeding... spell-checkers :-( ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
There are 4 current limiters, because I need it to have asymmetric limits and also survive having one side shorted to ground. Because the laser is on a cable, the short would have a 50-50 chance of being on the wrong side of the current sense resistor. I could put a bipolar current limiter in series with each lead, something like this:
Nah, nothing so dramatic--just a bad cold that turned into a nasty sinus infection. I'll be back in the lab some time this week. 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
hobbs at electrooptical dot net
http://electrooptical.net
That's my usual Winter malady... sinus infection(s)... since it's so dry here in Winter... but it's rained a lot this year (giving you East coast types lots of snow :-), so none so far, knock on wood. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Why negative LDOs? You'll have to look for the LM317-kind of three-legged regulators. Resistor in series, ADJ terminal to other side, done. Since you need it to work in both directions you'll have to hang a Schottky across and then another such (positive) regulator in series, also with a Schottky. Except that one is reversed. Not much money. Sometimes diodes are already included, to prevent regulator damage if a large electrolytic wants to dump its charge back after turn-off.
TECs are fickle, they can die fast. Non-grounded is fine as long as you can guarantee that things remain within the common mode range of the comparators.
None here, the world is using too much antibiotics as it is. On a long flight to a client I must have contracted a bug and it hit me hard two days later. Their VC of Quality Control recommended a "guaranteed fix": A bottle of Southern Comfort. It worked. I didn't drink the whole bottle though.
I misunderstood--you're talking about putting them in series with the TEC r ather than using them directly as the pass elements in the bridge.
Problem is, there's a cable in between, so if there's a short to ground it has a 50:50 chance of being on the wrong side of the current limiter if the re's only one.
Thus I'd need one set on each side, which is more complicated than what I'm doing already.
Yikes, Southern Comfort. Vanilla extract is about the same price and probab ly a lot more palatable. ;)
Of course the prize here goes to Mary Hartman, Mary Hartman (1976). Her nex t door neighbour, Coach Fedder, was treating a cold with large quantities o f bourbon. She brought him a large bowl of chicken soup, and he promptly pa ssed out and drowned in it. (I'm not a huge Norman Lear fan in general, but he was undeniably a genius.)
Apply the Mad Man Muntz rule... does it have too many parts? Seems the whole deal should be safe if the drive is truly a "current conveyor" (I hate buzz-word-named circuits... just say what function it performs, properly ;-) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
Sometimes an antibiotic will save your life. Cipro has saved mine. Lots of people used to die from initially minor scrapes or sore throats that turned into deadly infections.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
Should be easy enough to convey to the "conveyor" what quadrant it's in... n'est-ce pas ?:-) ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| STV, Queen Creek, AZ 85142 Skype: skypeanalog | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.