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> High Side Driver Voltage Drop? (udn2981)
veryevil
Posted: September 26, 2008 05:43 pm
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Hey, I'm using a UDN2981 to drive a large common cathode 7 segment display. It's connected to a PIC running on 5v and to a 10v supply. However I only get 8.7v out of the driver?

the path is PIC -> UDN -> 7-Segment -> 100 Ohm resistor -> collector of 2n2222 Transistor -> GND (connected to Emitter of 2n2222)

base of 2n2222 is connected via a 10K Ohm resistor to PIC.

Any ideas why I'm getting the voltage drop and any way to stop it?

Thanks
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MacFromOK
Posted: September 26, 2008 05:53 pm
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Try a smaller resistor on the 2N2222 base (1k or so).


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veryevil
Posted: September 26, 2008 06:06 pm
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Hey, i thought current limiting too due to the transistor but for a quick test i even connected the 100 Ohm res to ground to check and it isn't the transistor.

Cheers for the quick reply
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OutToLunch
Posted: September 26, 2008 06:14 pm
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look at the datasheet. that voltage drop is completely normal and expected.
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veryevil
Posted: September 26, 2008 06:37 pm
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Oh, cannot find it mentioned. never noticed it before.

If that is the case how do I get 10 volts output to run the 7-segment?

Does one exist that wont mess it up?
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OutToLunch
Posted: September 26, 2008 09:04 pm
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does it really matter? i mean, what you are going for here is driving the LEDs so they can be seen. If they are not bright enough then decrease the current limiting resistor. But with that particular setup, every time you turn another segment on each segment will keep getting dimmer. The resistor will only allow so much current to go through it, so the LEDs will have to share that current.

You should tie the 2n2222 directly to the common cathode and have a separate current limiting resistor for each segment tied between the high side driver and the segment anode.
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GPG
Posted: September 27, 2008 03:29 am
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QUOTE
Oh, cannot find it mentioned. never noticed it before.

http://www.allegromicro.com/en/Products/Pa...s/2981/2981.pdf
Collector emitter saturation voltage, p4
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veryevil
Posted: September 27, 2008 07:07 am
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Hey, unfortunately it does matter as i have to match the brightness of another set of 7-segs so i need the correct voltage.

Good point on the individual current limiting resistors didn't think about that.

unfortunately at the 8.7v even without the limiting resistor they aren't bright enough.

is there no other driver IC or am i going to have to do this with an NPN to PNP driver set up giving me another 14 transistors and 21 resistors?
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CWB
Posted: September 27, 2008 01:03 pm
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just a thought here ... maybe i am missing something ...

is the new display entirely separate from the original ? is it a strobed (mux) address/drive ?
if it is you may be able to increase the brightness by increasing the "on time" per individual 7 segment display .


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veryevil
Posted: September 27, 2008 02:47 pm
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Hey, the original product had a board with 4 large 7-segment displays controlled via serial with data, clock and latch. this runs on 10v

It also had a board with 4 smaller dot matrix displays on it which were connected in the interest of PCB layout and not coding order to a serial shift register latch similar to the 7 segments.

However the dot matrix board was only designed to show one of two things so must have been fudged in code to make it display right. what i mean is serial output 1 was like the 15th anode and output 2 was the 2nd anode and so on so sending a string of serial data would have to be massively rearranged to display correctly on it.

As I am trying to turn the device into a clock anyway i got rid of (kept for another day) the dot matrix board and replaced it with another 4 7-segment board from an old pin ball / arcade machine.

This new board has no drivers or anything on but the segments and a link on each cathode to the pin header which is where i put the 100 Ohm resistor. The segments on this display only light up with about 8v which is why i planned to run it from 10v. The segments are multiplexed with a common cathode for each.

when i connected it as mentioned above it only just came on and i thought that was strange so i checked the voltage and found 8.7v which i know realise is what it should be.

At the moment the code on the pic doesn't even multiplex them it but when complete the serial data on the larger segments will be updated on the change of a minute and the smaller muxed 7-segments will be constantly refreshed as part of the main loop.

There hope that explains it all. At the minute looks like ill be making an NPN / PNP driver although ill have to buy some PNP transistors.
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GPG
Posted: September 28, 2008 04:29 am
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Raise the supply voltage to 12V or better.?
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