High Current LED flasher

schreef in bericht news: snipped-for-privacy@34g2000hsh.googlegroups.com...

I'm not astonished. It's a tricky circuit not dimensioned to work with even mediocre modern Si-transistors. Nobody can guarantee it to work with the original transistors either. You can try to raise the 100k to 1M or higher but even then the performance - if any - will be poor. You'd better Google for "LED flasher" to find a better design.

petrus bitbyter

I cant see The site you link to.

So I dont know what frequency or current your talking about but you might try something like this.

I built a similiar one with an inexpensive low power LMV116,using a NDS351AN FET. I get about 120nS rise/fall times good enough for me.If you use a better opamp with more drive ability you could improve that. This was for 1 Amp pulses at 2.6uS 10% Duty .

I dont know why they are using an NPN as a highside switch. I placed my LED's between Vcc and the FET drain (low side switch).The schematic is from the LT1800 DATA sheet.

You can also buy LED drivers ( SMPS controllers) constant current that have a strobe or pulse function. I found these too expensive as well as the pulse frequency was limited typically to 10kHz.

the circuit shown relies on the LED having resistance of about 10 ohms try putting a 10 ohm resistor in series with the LED.

current flows through the 100K resistor charging the capacitor when ther voltage reaches 0.6v the NPN begins to turn on, this begins to turn the PNP on which increases the voltage on the other end of the capacitor turning both transistors fully on

as the capacitor discharges the current through the base of the NPN decreases and that transistor begins to turn off again this begins to turn off the PNP and the voltage on the LED end of the capacitor reduces this causes the voltage at the NPN end of the capacitor to fall to turning it off.

the circuit is highly dependant on the performance of the transistors and will probably be grossly effected by temperature etc...

I must admit I couldn't get the circuit to perform in that java simulator, but in LT-Spice (Switchercad, free download from linear.com) I got it working using 2n2907 for the PNP and 2n2222 for the NPN and 10 ohms in series with the led.

you don't get the pretty moving dots in switchercad, but you do get nice zoom-able oscilloscope (like) traces.

Bye. Jasen

You're right. But that 10R resistor neglects the main objective of the circuit which is to push as much current as possible through the LED. One may try two or more LEDs in series to obtain the same effect producing more light for the buck.

Another point not mentioned in the article is the influence of the internal resistance of the power source. It's about the only current limiting factor when both transistors are open. One of the reasons a similation may work and the implemented circuit may not or vice versa.

BTW. It is not the basic circuit that is tricky. The principle is well known and can be found in (older) textbooks and with no doubt you can find it on the internet as well. Replace the LED by a lamp (or one or more LEDs and a series resistor like you did), put some current limiting resistor between the base of the PNP and the collector of the NPN and raise the 100k resistor to something like 1M for modern Si transistors. That last thing was mentioned in the original article already. Though neither rocket science nor state of the art, the circuit will work as advertised. Including being sensitive for temperature - and supply voltage variations.

Nevertheless, if you want some blinking light you can find a lot of more effective and reliable solutions on the internet.

petrus bitbyter

I was just looking through my new GE transistor manual (7th edition, copyright 1964) that I got for $1.50 on amazon.com, and they have a very similar circuit, using the same transistors. It is a 1.5V ham radio code practice oscillator. Figure 15.6. Fun stuff.

The resistor/capacitor pair charges up until the transistor pair is triggered. This resets the junction voltage at the RC node to a lower voltage, where it starts to climb up again. I was playing with the circuit, and found that it is much more reliable with a small resistor from the node connecting the two transistors to Vcc. There is leakage otherwise which can cause the transistors to not turn off when the rc node resets.

The code practice oscillator works from 1.5V, but Forrest needs more voltage, since the LED will use 1.4V or so, so he bumps it up to 3V+

I always wondered where Forrest Mims got his circuits. Now I know, he got them from the GE transistor manual... ;)

Regards, bob monsen

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Hi Bob,

The circuit you described started my electronics career. I didn't find it in the GE Transistor Manual (which I bought years later). Instead, it was used in a single board code-practice oscillator that I bought for $0.96. (I still have the receipt.)

I modified the circuit to make my first travel aid for the blind. Later, I used the same circuit to make light flashers for model rockets. This became the first product of MITS, Inc., the company that introduced for Altair 8800 computer and provided Paul Allen and Bill Gates a way to launch Microsoft.

Best regards,

Forrest

Forrest M. Mims III

Hi Bob,

The circuit you described started my electronics career. I didn't find it in the GE Transistor Manual (which I bought years later). Instead, it was used in a single board code-practice oscillator that I bought for $0.96. (I still have the receipt.)

I modified the circuit to make my first travel aid for the blind. Later, I used the same circuit to make light flashers for model rockets. This became the first product of MITS, Inc., the company that introduced for Altair 8800 computer and provided Paul Allen and Bill Gates a way to launch Microsoft.

Best regards,

Forrest

Forrest M. Mims III

----------------------------------------------------------------------- I have several of your 'cookbooks'. They always had lots of fun things to do and try. I think it is so cool that you hang out (or at least listen in) on sci.electronics.basics!

Regards, Bob Monsen