BJT as switch...

[BioniC187]

This is epic stuff

 

[Ian Rogers]

This is epic stuff

Who would have thought.... Lighting an LED could be Sooooo tough!!!

 

[Electroenthusiast]

If you are using 9V then you can't connect all in series. I think, what you need to do is connected 5 in series and each combined 5LEDs in parallel with other 5 LEDs in series. Also use a (one) limiting resistor in series with this series-parallel combination of LEDs. Ain't i right?

 

[ericgibbs]

If you are using 9V then you can't connect all in series. I think, what you need to do is connected 5 in series and each combined 5LEDs in parallel with other 5 LEDs in series. Also use a (one) limiting resistor in series with this series-parallel combination of LEDs. Ain't i right?

hi,
There could be a problem with 5 LED's in series depending upon the Vfwd of the LED, also a 9V batteries are not 9V but lower depending upon the type.

E

 

[alec_t]

OK, for series tell me what to do for White LED ?

Make up your mind, Ritesh! In post 1 you specified red LEDs.

 

[Ian Rogers]

White LED's have a larger forward voltage than red LED's.... YOU need to sit and design something to fit YOUR needs...

While we are all here, second guessing your requirements, nobody is gaining anything. You aren't learning and we're getting cheesed off..

 

[Electroenthusiast]

hi,
There could be a problem with 5 LED's in series depending upon the Vfwd of the LED, also a 9V batteries are not 9V but lower depending upon the type.

E

Okay, I got you. But wouldn't it work if it's Blue/Red/green/Yellow?

 

[ericgibbs]

hi Electro,
Consider the sums, using the minimum Vfd's ie: 5 * 1.8V =9V !
It would not be possible using a 9V battery.
If you used 4*1.8V =7.2V you could have a series resistor.
E

 

[audioguru]

The voltage of a little 9V alkaline battery quickly drops to 7.2V when it is used. Then five red LEDs in series WILL NOT LIGHT!
Some Ni-Cad and Ni-MH rechargeable 9V batteries are actually 7.2V or 8.4V then five red LEDs in series also will not light.

 

[ericgibbs]

The voltage of a little 9V alkaline battery quickly drops to 7.2V when it is used. Then five red LEDs in series WILL NOT LIGHT!
Some Ni-Cad and Ni-MH rechargeable 9V batteries are actually 7.2V or 8.4V then five red LEDs in series also will not light.

hi agu,

There could be a problem with 5 LED's in series depending upon the Vfwd of the LED, also a 9V batteries are not 9V but lower depending upon the type.

I have already explained to the OP that 9V batteries will not initially give 9v

E

 

[audioguru]

A Name Brand new 9V alkaline battery initially provides about 9.3V, not 9V and not less than 9V unless it is overloaded. But the voltage quickly drops to 7.2V when it is being used.

 

[koolguy]

EDIT: Ok, maybe you can use a single resistor for many LEDs in parallel, but all the LEDs need to be close to "identical". If you have bunch of different color, size and brand LEDs then you are in trouble.

yes all are same..

 

[audioguru]

Look at the datasheet for an LED. A Fairchild MV8191 red LED at 20mA has a forward voltage from 1.5V to 2.4V so they are not the same.
A bunch in parallel will have the lower voltage ones extremely bright and maybe will burn out soon and the higher voltage ones will not produce any light!

 

[Electroenthusiast]

so whats the best thing he can do? parallel, series or the combination of parallel-series?

coming to BJT, only npn and pnp are the only two BJT transistors right?

 

[audioguru]

so whats the best thing he can do? parallel, series or the combination of parallel-series?

LEDs should never be directly in parallel unless they are tested and matched so that their forward voltages are exactly the same. The bunch of parallel LEDs needs a current-limiting resistor in series with the bunch.
If many LEDs are in series and one fails open then they all turn off. My LED Christmas tree lights have 35 LEDs in series and they have never failed but they are lighted only at Christmas Time.
There is a latching semiconductor thingy available to connect parallel with each LED that shorts an open LED to keep the remaining LEDs in the string lighted.

Use groups of a few LEDs in series and each group needs its own current-limiting resistor in series. The groups can be in parallel for powering them.

coming to BJT, only npn and pnp are the only two BJT transistors right?

Yes.
Jfets, Mosfets, SCRs, Unijunctions and PUTs are different.

 

[koolguy]

The project is to make this type of **broken link removed**sign board.
To avoid lot of heat/power diss. i will be using 5V 7805 9V to 5V and i think 1N4007 can be used to reduce .7 voltage to make less Diss across collector Resistor.
so, if i will do this and make all parallel will it be fine.
There is no such deep issue of making all exactly bright so, if there are some less/more brightness it will work..

 

[audioguru]

The voltage regulator will create exactly the same amount of heat as a resistor that drops the same amount of voltage so do not use a voltage regulator.
The diode will also produce exactly the same amount of heat as a resistor that drops the same amount of voltage so do not use a diode.

Did you measure the forward voltage of each LED? In parallel, LEDs with a lower voltage will be bright and LEDs with a higher voltage will be dim or will not light. The bright LEDs might burn out soon because their current is much higher than you think (they have their own current plus the current that is not used by the dim or turned off LEDs).

 

[koolguy]

The voltage regulator will create exactly the same amount of heat as a resistor that drops the same amount of voltage so do not use a voltage regulator.
The diode will also produce exactly the same amount of heat as a resistor that drops the same amount of voltage so do not use a diode.

I can use several 7805 regulator and i have found the all LEd 1500 are talking 400mA and giving good brightness at 9V the all LEd are solder so, i don't to make new connection and wire them all.
Please tell and way to make it work.

 

[koolguy]

I want to know connecting series Resistance at collector or at Vcc will effect something or not??

 

[ericgibbs]

I want to know connecting series Resistance at collector or at Vcc will effect something or not??

Hi Ritesh,
If you are asking if the 110R resistor and LED can be swapped over and will it make a difference, the answer is no.
E