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Topic: 12V 7 segment LED display (Read 11 times) previous topic - next topic

Docedison

There is a nice little Buck Mode Switcher that will handle 2A from Ebay @ <http://www.ebay.com/itm/260858526297?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649#ht_2466wt_1168>
It  Will take 12 Vdc in and put out whatever lower voltage you might require. @ 98% Eff. and at a 2A load @ 5V the switcher losses would be about 200 Mw... 2% of 10 W (5V x 2A = 10W) = .2 W
This and a 5V shift register or two (74C595's) chain up nicely with 3 wires to whatever driver you might choose. The nice part is they only cost $1.94 so if you are worried about total power dissipation then use several,
they are certainly cheaper and easier to find than the High Current level translating drivers (12V outputs to 5V logic).

Doc
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"The solution of every problem is another problem." -Johann Wolfgang von Goethe
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CrossRoads

I think you've missed the point Docedison.

Large displays, such as

http://www.sparkfun.com/datasheets/Components/YSD-1600AR6F-89.pdf

need 12V to drive the 5 & 6 LEDs in series that make up a segment.
(altho this sparkfun example is common anode, not common cathode).

So a 12V source needs to be controlled.
One way is to divert the current from segments to turn them off, as I drew up, and to let the current flow to turn them on.

Another way is to switch the voltage on and off at the anode using either as PNP transistor or a P-channel MOSFET.
P-channel transistors, such as those used in UDN2981, have a large voltage drop across them, so Source voltage of 12V + Vce needs to be available.
I don't know of P-channel mosfet arrays, thus you need individual parts - and an NPN transistor to drive the gate. The NPN base can go 0/5V to turn it on/off, allowing the P-channel gate to swing 0/12V to let it turn on & off.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

point5

Hi, I have now built a circuit based on the TPIC6B595 to drive the common cathode displays.  The first attempt worked in that it displayed the numbers correctly but the resistors goot too hot when the segments were not on !!!  Each digit, fully illuminated, drew about 125mA (each segment draws about 15mA when on) - when all the segments were off the current jumped right up to about 1.5A - lots of heat coming off those 100R resistors.

The next attempt was very similar but using the TPIC6B595 to switch 2N7000G mosfets - I guess the theory is right but something is wrong in my circuit because it just doesn't want to play.

I have seen some circuits with a 100nf ceramic cap across the TPIC6B595 (connected to VCC & GND?).... decoupling?  and a resistor on the common cathode/anode line on the digits - I don't know if adding either of these will help?

This is what I have done.....

Docedison

Yeah about 10 minutes after I posted that worthless reply I figured it out... I do apologize. You made a comment about resistors getting hot as well as drawing 1.5 A when all displays are off... Did you ever post a drawing of what you are doing/trying to do and have you checked your thinking about it... electronics equipment doesn't usually draw excessive current when quiescent, it is usually the other way around, that UDN29XX device has a good deal of open voltage across it but should drop to nearly 0 volts when on (Across the switch fet or transistor) the diodes and connection to Vsupply Led is for back EMF protection, diodes that are in effect across the load to Vsupply Led (They are for solenoids or relays usually) but should work well in your application. IMO

Doc
--> 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

CrossRoads

Your circuit is not correct. Try it like this.
TPIC6B595 can not drive high, need a pullup resistor to turn off the P-channel MOSFET that sources current into the anodes of the segment.
N-channel MOSFET is not the right part.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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