a simple doubt?!

[vineeshba]

Hi all,
It is said that if you connect two batteries of different values in parallel,them the lower value will be obtained as output.If that is the case then how is it that a 7 segment led connected in common anode configuration with single resistor work?..because no two diodes will have the same cut in voltage and you can still get a display with all 7 led's glowing eg:no.8 for a 7segment display

 

[Boncuk]

Hi vineeshba,

of course you might use a single resistor for all segments, including the decimal point there are 8 segments to supply.

Assuming they will be supplied from a 5V power supply the resistance has to 20Ω/0.5W.

Since the forward currents will be different your display will not look very good because of different brightness of the segments.

Why not use a current limiting resistor for each segment? Is the investment of 4Cents beyond your budget?

Bosco

 

[rezer]

Hi all,
It is said that if you connect two batteries of different values in parallel,them the lower value will be obtained as output.If that is the case then how is it that a 7 segment led connected in common anode configuration with single resistor work?..because no two diodes will have the same cut in voltage and you can still get a display with all 7 led's glowing eg:no.8 for a 7segment display

No, the higher value battery would be taken as the output. The lower value battery would draw current from the higher source.

 

[vineeshba]

a simple doubt?

Hi Rezer, if what you said is correct then could you please tell me what would be the output if you connect two batteries ie. a 5V and a 0V in parallel?

 

[rjvh]

Hard to say without aditional information as internal resistance of both power sources and external curent draw

something inbetween

Robert-Jan

 

[vineeshba]

Re:a simple doubt

Hi. Robert,
for the timebeing we can consider both as ideal sources.

 

[JimB]

for the timebeing we can consider both as ideal sources.

Two "ideal" sources, 5v and 0v connected in parallel would result in an infinite current flow.
As the sources are ideal there is no resistance, so there is no power dissipation.

In the real world, there would be a big flash and a bang and the power supplies would be reduced to a smoking wreck.

JimB

 

[rezer]

Hi Rezer, if what you said is correct then could you please tell me what would be the output if you connect two batteries ie. a 5V and a 0V in parallel?

Ideally, or should I say realistically, the voltage out will be 5V unless the charge current of the discharged battery exceeds the AH rating of the good battery, in which case, your good battery will discharge fairly quickly. But I don't really understand the question. You wouldn't charge a battery using another battery.
But back to your original post, to connect two batteries of different voltages in parallel would not be a good practice. The lower would be over-charged by the higher. I assume you were asking this as a hypothetical question as related to the LEDs, and not something you would actually do in practice?

 

[vineeshba]

Re: a simple doubt

Hi Rezer,
Indeed the question is a hypothetical one,but my original question was the case of LED display and i still do not understand why it happens like that.

 

[rjvh]

how is it that a 7 segment led connected in common anode configuration with single resistor work?..because no two diodes will have the same cut in voltage and you can still get a display with all 7 led's glowing eg:no.8 for a 7segment display

Back to your first question than

my idea is that the tolerances between the diodes are prety small that it would not be so disturbing although it is not a elegant solution.

the use of 1 resistor on the comon anode is not a good design technique.

as you will have the problem of light intensity differences with a different NR of leds burning.

this design technique is used in cheap products to minimize the part count.

Robert-Jan