Convert 12-14 vdc to 9 vdc for a device only needing 0.3ma...

Good Grief! It was a strip of liquid crystal, which are famous for drawing well under one mA, more like microamps.

Hope This Helps! Rich

So, in other words, Lexi are crap? (assuming by "Luxus", you mean "Lexus")

Thanks, Rich

The liquid crystals themselves were just temperature sensitive and drew no current (same as the liquid crystal thermometers you stick on your forehead) -- it was the linear resistor underneath that acted as a heater that consumed power.

A tad touchy? NOT creating warm fuzzy feelings about your real competence by my reading.

Michael has given more to the world than you ass-ume. I have yet to see similar contributions from you. Not that i have much to brag about, but i have paid my freight.

I will personally notice Michael's demise, but i won't even deign to acknowledge yours, though it has already happened.

Excuse me?

I guess you must one of the sheep being herded.

I feel sorry for your incompetence to see between the cracks.

I don't try to lead any one, especially ways that a few do in this arena, but if that is what you wish for, so be it.

I stand on my original content of this post! Just because the present day design of those cheap meters no longer allow the measurement of its own supply rail as many do not. I have 3 in my possession that do and can confirm they are not the same board design as was posted from a poster here, which I can't remember at the moment who it was.

It's all about how the meter is design, using a virtual ground from a signal source.

Have you ever heard of differential and single ended systems?

One requires a VG which makes it inadequate to measure it's own rails and the other, does not.

Next time you're looking around for panel LCD's, pay close attention to the spec's that allow you a floating common and non floating. That mays all the difference..

Now let me get back to something worth doing and it's not talking to sheep like you.

P.S.

I don't step aside for those that think the world owns them something.

"

a

increasing

"Think the world owns them something."? you have just shown a new high in your ignorance. You are illiterate and most often wrong, yet you demand that people kiss your sorry ass. You are as bad as Eeyore for posting crap, then refusing to admit you are wrong. So what if you have something that is somewhat similar to what is under discussion. It is as meaningless as the rest of your crap replies.

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There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you\'re crazy.

You can externally generate the negative supply or reference voltage. Neither one is cost effective for cheap meters. By the way, my local Harbor Freight is out of "cheap meters". The new version with back- light is $9.99, so $8 for the LED back-light.

LCD panels don't care about absolute voltage, as long as value between segments and commons are fixed. The chip can do both ways. It's just a matter of cost in the meter design.

That seems an untenable position, so you may have more in mind that what was posted.

Your original post was: "yes, use a couple of diodes in series." in response to the question: "Is there an easy (read cheap) way to drop the 12-14 volts down to 9 volts without increasing the power consumption very much?"

1) It has been shown that there are meters that won't work properly when measuring their own source. Your post makes no allowance for that. 2) A couple of diodes will not drop 12-14 volts down to 9 volts.

You might have something different in mind, but people are reacting to what you posted, not what you have in mind (assuming it is different). I'm interested in what your specific thinking is, assuming your statement about a couple of diodes did not truly reflect that.

I'm in the (slow) process of adding metering to a physically (relatively) large cabinet that houses a 12-15V switcher. I have 3 different DC-DC converters breadboarded. The cmos 555 version I mentioned in this thread works best, so far. In any event, adding two of those Harbor Freight meters is the cheapest way to go, so that makes it attractive. Now, if you have strong reason to know that they will work properly and not be damaged by a source that is 42% higher than nominal, I may be able to omit a zener from the DC-DC design. The switcher is already complete, it's just the meter supply that prompts the question.

Ed

it was a tapered thick film resistor acting as a heater behind a thermal colour-change compound.

yes, that was in response to dropping the voltage supply from 12 down to ~ 9 volts cheaply..

Not all can measure their own rail, that's true. Those using virtual grounds will most likely not be able to do this.

Each diode will give you a loss of ~ 0.6 volts. 2 diodes will give you around 1.2 volt drop. so maybe more is needed, But the idea was put in place so a simply solution was at hand.

Diodes are and have been a simply solution for years to drop the voltage level a bit.

I recently saw a photo of the inside of a freight meter, it does not look like the one's I have. Those M-830B's have been around for years and i'm sure the internal board design has been changed several times. The 3 I have, can! and I repeat can measure it's own source. But mine are over 5 years old now. That does not mean the new ones are designed the same way.

Why can't you just get one of those cheap LCD panel meters?

"

That's why they are cheaper. They removed the external voltage references.

meters?

My plan is to install rechargeable batteries on them, and charge through the 10A plug (cutting the internal wire, of course). I can still use them as regular meters, without turning them off or replacing batteries. They were steals at $1.99, but not anymore. They were all stolen from my local Harbor Freight.

I installed the meter on the ignition wire tap in my wife's car. Knowing her, it will always be on or off. I prefer it always on with key in ignition.

You seem to need an excuse, but i will not grant you one today.

Non-sequitur.

The cracks in your garbage claims i do see through, but they are several feet wide.

You lead? Now that is a joke.

I do not recall any poster saying that not being able to measure its own source was a good design, not even yourself. I recall many reports of meters that could not do so.

Maybe before you were born, must have been 50 years ago.

I claim that "I have earned all that i have, some of it many times over."

Ok, thanks. I was hoping you might have done enough testing to be sure that running one of those meters at ~12.8V would not damage it and that it would work properly at that voltage.

I can do that, and have in the past for smaller things. But you can't beat a $2.99 (sale) price tag for the Harbor Freight meter, and I have plenty of space in the cabinet to mount the entire meters in the front panel, if I want. And the test leads will be used to connect the supply to whatever it is powering.

Ed

I was just at the local junk store that carries like 75% of goods like harbor freight does and they have a whole shelve of them. $5.99

The model number isn't exactly like my 5 year old ones but close. I think theirs is a DT or BT-830B, mine is M-830B. I couldn't tell you what the board design of that model is. I didn't feel it was necessary to buy yet another junk meter to have lying around.

"

Somebody said forward bias on 1N4001 diode would be .6V but I was taught that silicon diodes forward bias at .5.

I wouldn't quibble about the difference but when using a whole string of those for a voltage drop wouldn't that become a big deal?

Is forward bias on silicon diodes actually .6 v or .5 v as I was taught?

At some really low current it'll be 0.5V, although 0.6V is definitely closer to "typical" operating condtions. Actually, if you look at a data sheet -- fig. 4 here:

-- you'll see that

0.8V might be an even better number to use, but it all depends on how much current you'll be pushing through the diode.

It might, certainly.

"0.6V" is other bandied about in circuits classes, with the (often tacit) assumption that you're talking about small-signal diodes (like the 1N914 -- datasheet:

with currents in the ballpark of a milliamp.

Classes that discuss anything having to do with power often use 1V as the rule-of-thumb... this is a typical value in the ballpark of an amp.

I've never heard the 0.5V number, but that doesn't mean much. Possibly chip designers who are working down in the microamp arena use it? What class did you hear it in? (This is reminiscent of how "low voltage" in circuit classes is "a handful of volts" whereas in power classes it's "a handful of kilovolts...")

---Joel

Aside from structure and materials, the voltage drop depends also on the current. For base-emitter junctions on common silicon BJTs my experience says the drop varies at about 60mV per 10-fold change in current -- consistent with this model:

Vd(I) = n k T / q * ( ln(I) - ln(Is) )

Since Is is a constant, ln(Is) is also constantand drops out of the derivative. So you get:

d Vd(I) ------- = n k T / q d ln(I)

With q the electron charge, k Boltzmann's constant, T is temperature (Kelvin), and n is a fudge factor (okay, 'emission coefficient') that is mostly 1 for BJTs. kT/q at ambient temperature works out close to

60mV (just under.)

For diodes, my experience seems to be that 'n' is no longer close to 1 and is not infrequently significantly closer to 2 than to 1. So I'd expect more like 100mV change in diode voltage for a 10-fold current change. For the common 1N4148 diode, I mentally estimate 0.5V at

100uA and 0.6V at 1mA and work up or down from those.

For LEDs, yet another story despite them being diodes, too.

So the way I'd take all this is that it depends on the current under consideration and the part being used. You might see someone say 0.5V (and less) when they are talking about microamp ranges of currents and say 0.6V for low milliamps and still more if the currents are higher still. So... it depends on context.

Jon

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0.5V is about right for 1N914ish sized parts at a few uA. I'd expect a chip designer to be working with a smaller part if they are working with uA levels.