Hello all, so glad I found All-About-Circuits!!
What an incredible resource!!!
Thanks in advance to anyone helping me...


I am using a TIP31 bipolar junction transistor to take an incoming audio signal and have the decibel level drive an LED. I need to prevent the LED from exploding should someone accidentally plug in CV or a cranked signal. I think what I need is a clipping diode circuit however I am not sure how to figure out what values I need nor how to set it up. I am using a standard 3mm white LED and cannot find information as to what it's maximum V rating is. If I knew that I could set the clipping diode circuit up to clip the incoming signal down into safer territory.

Has anybody had any experience with using clipping diode circuits?

Thanks kindly and take care,
dawn

 

Connect a 100uF capacitor in series with the base of the transistor,and put a 100 ohms resistor in series with the leds.

 

Dodgydave's solution is the most straight-forward; I would take his advice.

If you still want to make a clipper you can do this: You can leave the voltage sources, shown as batteries, out but that would give less sharp cut-in of the liming action compared ot he maximum signal amplitude. That's probably not important when flashing lights, but it could be a problem if you were listening to the signal through a speaker or headphones.

Another approach is to use a pair of Zener diodes back-to-back:

But Dodgydave's solution is the best.

 

Not sure you need Dave's coupling capacitor. With a 100 ohm resistor the LED current is limited to around 40 mA peak (assuming Vf = 2 V). That's a safe value because the transistor acts as a half-wave rectifier, so the average current will be less than 20 mA. Signal loudness won't matter. If hooking up DC is a real concern, then the coupling cap is the simple answer, but now there is the issue of capacitor polarity...

ak

 

Hello, Dick,

In the lower schematic, I think the limits will be Vz+0.7 Volts, as the zener in forward mode will drop about 0.7 Volts.

Bertus

 

Agreed; not my diagram -lifted from the internet, and I appreciate your correction.

 

Thanks everybody, I really appreciate your opinions and research.
It looks like there are a couple of ways to do this.
But which one is best for my application?
The big thing is that the way the Tip31 performs now is ideal, I can plug in a low db audio source (ipod, computer, etc) with no preamp and get a blinking light. Throw any more resistance on the line and that flashing gets weaker and requires preamplification to be effective.
Which option creates the least resistance?

Once again, thank you kindly, this is all very helpful.

 

A current limiting resistor in series with the LEDs per Dodgydave's solution will give you the clipping action you desire without affecting the way the TIP31 works (until clipping occurs of course).