Go Down

Topic: 6v to DC input jack seems to work fine? (Read 4 times) previous topic - next topic

Docedison

Indeed. My offering was simply an opinion it either fits or it doesn't as to multiplexing I chose that example on purpose to reflect the effects of multiplexing, the current drain will still be very similar, Mux'd LED's were all we had for years as indicators. Long before LCD's were more that a laboratory fantasy, They've been used in one form or another for more than 40 years of my direct knowledge. that having been said. I guess the remaining opinion I hold is that it is possible that there is a driver on board the LCD, it is there on my graphic LCD's for PWMing the backlight. 3V3 / 330 ohms = 10 mA. Impressive efficiency or base bias current for an NPN transistor? measuring the drop across the resistor should tell. Again IMO and good fuel for a discussion?

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

dhenry

Quote
the current drain will still be very similar,


Let's think it through. 5ma per segment and you have 2 segments -> 10ma per digit when that digit is on, 0ma per digit when that digit is off.

Let's say that you "mux" them 1ms per digit.

ms 1: digit 0 is on, digit 1..7 are off. Total current consumption: 10ma (for digit 0).
ms 2: digit 1 is on, digit 0, 2..7 are off. Total current consumption: 10ma (for digit 1).
ms 3: digit 2 is on, digit 0..1, 3..7 are off. Total current consumption: 10ma (for digit 2)
...

Do you see a pattern now? What's the new math?


Quote
I guess the remaining opinion I hold is that it is possible that there is a driver on board the LCD


You guessed right. Many of those lcds have backlight rated at 20ma max and no big difference at 10ma.


Docedison

I was also going to recommend that the OP look up the ecact part number of the regulator, there are graph's of dropout voltage vs load current that will serve him well as would looking at the schematic of the regulator and noticing how the reverse voltage protection is done there is a silicon diode in series with the positive DC input lead. The symbol indicates it is a junction type diode so it will have between .5 and .7 V drop across it depending on temperature and load current I just looked at the attached data sheet and again something doesn't add up right. If we accept his measured 6V at the PSU connector and (oops I just realized I don't for sure know which board he has and how it is reverse protected) a .63V typical drop across a si junction then there is 5.36V left to the regulator and according to the attached data sheet it should be out of regulation for all but the lightest loads unless there is another 1/2 volt or so present but unaccounted for. The 7V lower limit is spot on and good engineering according to the data sheet. If it were my problem I'd take a switcher and use the highest "power output" supply I had and set the output of the switcher to either 5V and supply the board directly or take the much safer route of supplying 7V to the board and let the internal supply do it's job, this also allows the switcher to be noisier that usual with no side effects as the linear regulator and attendant filtering will clean it up quite nicely. I would also use a 2A fuse and a 1.5KE6.8 Tranzorb or a 6.8V 2W Zener and a silicon diode to on the output of the switcher and slightly lower the switcher output voltage for board protection should the switcher fail, I've used a grip of LT1170's with one failure and that was caused by the power transformer being wired across a leg of a 440V 3 Phase Pump service and neutral(about 277V If I remember right) the electrician was using a defective wiggie and didn't really know what he was doing. Most switchers (All I own {5} have an area that can be soldered to a copper strap. I would use a .5mm X 50 mm piece of copper sheet metal and solder it to the area just under the actual switcher all of mine and all I've seen have a great lot of Via's at that point for heat conduction to the bottom side of the PCB and I would use a 1.5 - 2mm screw and nut W/Washer to mount it to something that would help in removing whatever excess was developed by the switcher. Efficiencies run about 90% so with a 1A load you would have to get rid of a half watt, Easy. Again just my $0.02's worth or  IMO

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

dhenry

Quote
according to the attached data sheet it should be out of regulation


Those minimum voltage differential requirements are specified at pretty high current levels (usually at or close to the rated current levels). An arduino board will not draw anything close to that kind of current. So smaller voltage differentials will work - there are two charts in the datasheet that indirectly speak to that.

Plus, out of regulation doesn't mean it doesn't pass power anymore. It just that its ability to regulate output voltage is gone.

Docedison

Well it would be a nice world is that were something you could take to the bank... but "Out of Regulation" is undefined and as such is highly questionable and based more on wishful thinking than reality. The reality is that you have a very high gain DC amplifier with an unstable reference voltage. a more understandable comparison might well be that you have a really big car (The Regulator) moving across an area (output Voltage) without a driver (The Reference Voltage) High gain because the reference term is a current (Typ 100uA) controlling a regulator capable of an Ampere of current and it doesn't take a genius to figure out that 100 uA is a tenth of a milliamp and as the name implies a milliamp is 1/1000th of an ampere so we have a gain of 10 X 1000 and no stable input reference. The typical result for an NPN pass type (AKA 78XX series) is oscillation and for the PNP Pass types like the AMS1117 the reference term is a part of the load current... So there is a minimum current that must be drawn from the regulator for the regulator to be stable and a processor unfortunately doesn't fit that condition... again the result is typically oscillation. I first ran into this issue trying to shift the output voltage of an LM2931AZ-5.0 (Data sheet attached) from 5 to 6.2V. Normally the ground pin which carries the reference current for the regulator is connected to a resistor to ground and the output of the regulator is fed back to the ground pin creating a "new" Ground reference voltage and thus shifting the output voltage by the amount of voltage dropped across the resistor to ground. Simple? Right? NO because if the input voltage is slow to come to value even be as little as a hundred uS it causes the regulator to oscillate at about a MHz or so and since the output filter is a short circuit at that frequency the regulator gets hot, goes into thermal limiting, cools down and then does the whole thing again and again it can be simulated easily by placing a 10 ohm resistor in series with the supply lead and not bypassing the input... Just 10 ohms. An interesting side note is that a 78l05 plugged into the same circuit works perfectly because the 78XX series of devices uses an NPN pass transistor and the reference supply is different, internally. In short IF a Regulator isn't biased properly it's output is undetermined and really undeterminable. The Chip designer had some variables and some constants in his design, load current and Minimum reference supply voltage, If you don't meet those conditions then your circuit's response is undeterminable. This statement makes me want to ask where you buy your drugs from...
Quote
Plus, out of regulation doesn't mean it doesn't pass power anymore. It just that its ability to regulate output voltage is gone.

Now what can that mean? What kind of POWER DOES IT PASS... if at all. My intent here is to point out that just because it might work in basic testing is no guarantee that it will work accurately and to "Some" specification under varying input and load conditions. which is basically what you said when you said that it wasn't a "regulator" anymore the problem is that not meeting basic spec means literally that the "regulator" is more a liability than an asset. IMNSHO

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

Go Up