Temperature compensating diode drops

The last one east of here a couple weeks ago was felt my most neighbors. I didn't wake up.

That is what I meant with the comparators. Commutate them so the current limit shifts gears with each quadrant change. Costwise it's all probably a wash and not expensive but doing it at the driver saves energy.

Maybe I am understanding it wrong but if the current limiters are directly in line with and physically at the Peltier it shouldn't matter. You could have one current limit for cool and another for heat. Of course, if there is a chance that a wrench falls directly onto the Peltier contacts that's bad.

Just select different resistor values. One LM317 is for heat, the other for cool, regardless of whether the TEC is driven ground based or from an H-bridge. As long as that limiter is right at the Peltier. Else you'd need four LM317 or similar and four resistors. They are cheap but that adds voltage drop.

A current conveyor is a current in/current out gain stage. This one is clas s B, so it's pretty simple.

The driver stage is common base, and the output is common emitter, connecte d to the supply rail with emitter degeneration.

Thus neglecting Vbe drops (which is okay since the application is low speed and there are op amps wrapped round), a milliamp in produces N milliamps o ut. You put NPN and PNP versions with their inputs and outputs in parallel, and you have a current conveyor--low Z in, high Z out.

It's the fourth member of the family: normal op amps (voltage in, voltage o ut), CFB amps (current in, voltage out), OTAs (voltage in, current out), an d current conveyors (current in, current out).

Cheers

Phil Hobbs

Right, except that they aren't. The current limiters are on the board, and the laser/TEC is at the other end of a cable. Otherwise I wouldn't be worried about shorts.

That's the point of the exercise, right.

If the laser assembly could be mounted on the board, I wouldn't be worried, but it can't. The application is an industrial tester for manufacturing other things, so reliability is key because down time is expensive.

Not all of the users are in the first world, so "expensive" has to take account of the BOM cost as well. If it were like Ford and GM, I'd throw a couple of bucks' BOM cost at it and move on.

Cheers

Phil Hobbs

We felt that one in Truckee, vaguely around midlight.

There is a fault almost directly below our cabin, but those dinky faults don't do much damage.

Ok, didn't know that, then you need four LM317.

However, power consumption probably isn't much of a concern and even if you'd need four LM317 they are around 50 cents. Depending on curent they'll need heatsinking.

Probably the best is to revise the driver ciruitry so that it contains intelligent current limiting. You might get away with just sensing and shutdown (OR-ing the comparators) because if a short occurred it's a service call anyhow where at least a factory technician has to come over.

Turns out I do have enough headroom to do the TLV431/LM4041-ADJ thing, which costs about 14 cents per corner (Micrel/Microchip LM4041DYM3-ADJ and Diodes TLV431AFTA) and saves four transistors and four diodes.

Interesting discussion, though!

Cheers

Phil Hobbs

Yeah I've been a bit confused by Phil's problem. If he runs the TEC feedback loop by controlling/sensing the current, then a current limit seems easy. (But I'm probably missing something obvious. For one I don't understand the four quadrant thing... why does the current matter if it's being sourced by the TEC temperature difference?)

George H.

It isn't difficult, but I was sick in bed and therefore bored stiff, so I wanted to talk about electronics. Kicking around this sort of stuff is always good for keeping the mental joints from seizing up, and not every design has enough headroom to waste an extra volt-and-a-half for the TLV431/LM4041 approach or three volts or more for the symmetrical LM317 approach.

The last one I did ran off +8 and ground, so I did two current sources fighting each other, and put voltage feedback around one of them to keep the bridge centred. Not that it would have mattered if I hadn't, but I had a spare op amp section and it's tidier that way--spreads out the power dissipation.

It needs two different current limits, not four, that's right.

The four-quadrant thing is why you need to drive the TEC with a current rather than a voltage. It has a large and poorly-controlled offset voltage due to being a big thermopile with a potentially large temperature drop.

Cheers

Phil Hobbs

"back in the saddle again"

_Appropriate_ use is highly beneficial. _Inappropriate_ use is a huge problem, as it tends to promote the development of antibiotic- resistant strains and thus limits further appropriate use of those same antibiotics. Using massive quantities of antibiotics as "growth promoters" in animal feed is one of the greater insanities of the last few decades.

I'm personally grateful for the availability of good antibiotics. My wife developed a nasty bacterial infection while we were traveling overseas last year. The proper antibiotics (which included Cipro), administered by some very savvy doctors at a small Greek hospital, saved her life. My biggest fear at the time was that she might have contracted something which was so antibiotic-resistant that it would be untreatable... in which case I would probably have ended up bringing her ashes home in an urn :-(

Now that you are well again don't stop the orange juice. My wife thought she was over it and three days later ... bam ... it was back. Worse than before.

In order to ward off any flu bug I just got a shipment of nine more brewing ingredient kits, including another Belgian Tripel, a Superior Strong Ale and a Sabotage Saison. Just for medicinal purposes, of course ...

My wife had a problem in a small town in Ireland so we walked into a local clinic. The doc saw us right away, did his thing, and wrote a prescription for antibiotics and said "oh, just give me ten pounds" which I suspect was beer money. The drugs, at a pharmacy, were expensive, $75 or so.

Jim, you might get some relief using a saline nasal spray, like the one which is called Fess over here. Many of this kind of infection associated with hot dry conditions follow on inflammation (rather than causing it). The saline increases the osmotic gradient, which keeps the nasal trachea moister, thus avoiding the inflammatory trigger. If you still get trouble originating in the upper sinuses (as opposed to migrating there after you start sneezing) then you might consider a neti pot (widely used in India for the same reasons) or some other more thorough saline wash.

In any case, it can't hurt to try it for a while.

Clifford Heath

I'm good, so far, this year. The mini-ice-age will be beneficial to us "desert rats" ;-) ...Jim Thompson

Not that you could remember.

Jamie