Does the circuit give the transistors enough base current for a collector load of 60mA? I doubt it.
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Does the circuit give the transistors enough base current for a collector load of 60mA? I doubt it.
Uncle $crooge
I am can do some measurements on the circuit if that helps.
Your schematic does not have a transistor part number and does not have resistor values. So we cannot calculate how many LEDs each transistor can drive.
Uncle $crooge
with a 2N3904, with a minimum beta of 100, 600uA would be required for driving a 60ma load, and i think 2N2222s have about the same beta range.
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Beta is used when a transistor is a linear amplifier with plenty of collector to emitter voltage. Most transistors use a base current that is 1/10th the collector current when used as a saturated switch.
Uncle $crooge
Hope this helps
Many parts in the circuit are designed to light individual LEDs with 16mA.
It can light 3 LEDs in series at each output if the value of the 620 ohm resistors is reduced to keep the current at 16mA.
Uncle $crooge
Thanks, that's a simple solution to getting more out of the circuit.
what value resistor would you recommend for the job?
can the LEDs be in parallel?
what voltage LEDs will work best?
It is not simple to get 10 times the current from a circuit.
A resistor in series with 3 LEDs needs to be calculated properly with the total LED voltage and the supply voltage.
LEDs in parallel need to be matched because each lED is different. This circuit is designed for the current of only one LED not more.
10 red LEDs that are 1.8V each have a total voltage of 18V. 10 blue LEDs that are 3.5V each have a total of 35V. Plus additional voltage for the current-limiting resistor.
Are you going to re-design the circuit so that it works from a 38V supply?
Uncle $crooge
Thanks again for all your help, it is very much appreciated.
On the circuit information documentation it says the circuit is "designed for a standard input level 750mv. In case of a lower input level, resistor R1 shall be replaced with a higher rating resistor. Dynamics is determined by the C3 capacitor. Supply voltage: in the range of 9 - 15 v"
The PSU I was going to purchase is 9v, 12v, 15v & 18V (switchable) 1200mA.
Would it be necessary to change R1 because it is 1200mA?
Would I expect to see a difference in brightness between 12v and 15v PSU? (12v ones are much cheaper)
I have accepted that this circuit is not designed for my application so I am treating it as a conceptual design and will live with its faults (LEDs that are not as bright as the could be, inconsistency in brightness across different coloured LEDs)
If it is a successful prototype and the band likes it, I will try to design are better and brighter version from the ground up. At the moment I'm pushed for time and I have already purchased four of the level meter kits so I will have to make do.
1200mA is the max continuous allowed current from the power supply. It has nothing to do with this circuit that will draw a max current of only 150mA.Originally Posted by civvie
R1 sets the gain of the circuit. increase the value of R1 for more gain so the LEDs will respond to a lower input signal level.
The difference in brightness will be small.Would I expect to see a difference in brightness between 12v and 15v PSU? (12v ones are much cheaper)
Last edited by audioguru; 21st May 2009 at 05:48 PM.
Uncle $crooge
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