Author Topic: LCD backlight  (Read 4251 times)
Medved
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Re: LCD backlight « Reply #15 on: May 10, 2016, 12:33:59 AM » Author: Medved
Not a fan, but putting the regulator somewhere else, further away from the measurement IC's.
Anyway I just wonder, why the regulator should be heating at all: The illumination should not consume more than 15..20mA (there is no reason to not have all the LED's in series) and dissipate most of that power on the ballast resistor (there is no need to supply that from the regulator) and the 7106 consumes just few mA, so in the total we are speaking of no more than 20 to 30mA. Even with 10V drop this is barely half watt.
So the question is, whether the design is really so stupid (each LED having separate resistor), or something broke there (an electrolyte is quite common) Many times I met the electrolytes to be assembled backwards - the thing appeared to work for a while, but consumes quite high current and the electrolytes were hot. It passed the QC just because the electrolyte leakage when reversed was still below what the supply could deliver and the same reason meant the thing then works at least for some time...


And may you, please, post again some better identification of the make/model of the source? I'm really curious, why the regulator should be hot (the link you have posted some time ago does not work anymore - appears they reshuffled their web or so, "404 not found"...)
« Last Edit: May 10, 2016, 12:37:59 AM by Medved » Logged

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wattMaster
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Re: LCD backlight « Reply #16 on: May 10, 2016, 10:20:54 PM » Author: wattMaster
Here's a better link, It's just a slightly different PDF name, I wonder why they did not keep the same name:
http://www.extech.com/instruments/resources/datasheets/382200_382202_DS-en.pdf
The reason I did not post a photo inside is because it happens to be a big pain to take photos here(The reason my gallery is so small), And the construction of the vent holes is that the only the Eye with very special lighting can see it.
Could the heat be from the LCD driver chips?
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Medved
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Re: LCD backlight « Reply #17 on: May 11, 2016, 12:48:06 AM » Author: Medved
Could the heat be from the LCD driver chips?

When everything is working, no way. Their consumption is around 2mA include the display load, supplied from ~9..10V, that means barely 20mW, that can not heat anything.
The only cause for the chips to heat up in the same way would be a supply overvoltage. But then they will likely not work at all...
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Re: LCD backlight « Reply #18 on: May 11, 2016, 02:50:14 PM » Author: wattMaster
Could the heat be from the LCD driver chips?

When everything is working, no way. Their consumption is around 2mA include the display load, supplied from ~9..10V, that means barely 20mW, that can not heat anything.
The only cause for the chips to heat up in the same way would be a supply overvoltage. But then they will likely not work at all...
Maybe there is a hidden component causing the heat.
Or what if they put in a heater to keep it at a constant temperature, To ensure ambient temperature will not affect the readings.
Or what if the Voltage/Current regulator for the output terminals is causing the heat?
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Medved
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Re: LCD backlight « Reply #19 on: May 12, 2016, 09:42:44 AM » Author: Medved
Maybe there is a hidden component causing the heat.

For that I would like to get the schematics - the service manual or such documentation.

Or what if they put in a heater to keep it at a constant temperature, To ensure ambient temperature will not affect the readings.

Such features start on instruments worth of at least $10000 and only for those for really high accuracy lab use (at least 3 or 4 order more accurate instruments; then there would be way more digits on the display; even a 6 to 7-digit accurate meters use just temperature compensation and no heating).
Otherwise it is too expensive and too problematic to work with (requires hour or so temperature stabilization before you can use the instrument,...) Definitely such feature is not usable for a service and/or everyday development this instrument is intended for, it would lower the productivity way too much.


Or what if the Voltage/Current regulator for the output terminals is causing the heat?

These components are on the heatsinks on the back side... Maybe some predriver stages do emit a bit more heat, but even that should not exceed about 0.5 to 1W for such output power level instrument.
I would really like to see, what is inside...
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Re: LCD backlight « Reply #20 on: May 12, 2016, 01:01:53 PM » Author: wattMaster
Maybe there is a hidden component causing the heat.

For that I would like to get the schematics - the service manual or such documentation.

Or what if they put in a heater to keep it at a constant temperature, To ensure ambient temperature will not affect the readings.

Such features start on instruments worth of at least $10000 and only for those for really high accuracy lab use (at least 3 or 4 order more accurate instruments; then there would be way more digits on the display; even a 6 to 7-digit accurate meters use just temperature compensation and no heating).
Otherwise it is too expensive and too problematic to work with (requires hour or so temperature stabilization before you can use the instrument,...) Definitely such feature is not usable for a service and/or everyday development this instrument is intended for, it would lower the productivity way too much.


Or what if the Voltage/Current regulator for the output terminals is causing the heat?

These components are on the heatsinks on the back side... Maybe some predriver stages do emit a bit more heat, but even that should not exceed about 0.5 to 1W for such output power level instrument.
I would really like to see, what is inside...
I would love it too, But there is too much of a risk if it doesn't work if I put it back together.
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Re: LCD backlight « Reply #21 on: May 14, 2016, 02:38:52 AM » Author: Medved
Well, when it works, it works, so who cares if it was supposed to get warm or not.
The worst thing that may happen is it will fail at one moment. Only then will be the right time to open it, if that means any risk of damage (if it already does not work, it may not get much worse than still not working)...
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Re: LCD backlight « Reply #22 on: May 14, 2016, 08:51:37 AM » Author: wattMaster
Well, it will be a long time, because I only currently use it at about an average of about 1 hour per 6 months.
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