Hi Guys, this is my first post. I included a 2 led flip flop flasher circuit in one of my projects. In the beginning it used 5v as the power source. I would like to use 24v as the power supply. I changed the resistors in front of the LED from a 470R to a 2K2 so as not to blow the LED's. I thought that the 100K resistors were used as a throttle in the discharge of the caps, so I played around with the values there but when I put 24v on the input the frequency of the flash is very fast and does not change. I tried all the way up to 3M9. I would like to know if I should change the Cap value from a 10uf to a bigger one? and also how would I start to calculate the correct values in this circuit. I must state the original circuit was downloaded off the net and I just blindly followed it and it worked (with 5v). Looking forward to some help. Thanks

 

Alternatively, you could just use a 7805 voltage regulator and leave the circuit exactly as it is.

 

Hi Blocco thanks for you quick reply. I am trying to eliminate space and cost on the pc board. But if there are a lot of mods to do on the circuit I will go back to the original design with the 7805.

 

When the circuit toggles, a capacitor couples the transition of the transistor turning on into the other transistor's base to turn it off. But at 24V that transition is too large for the transistor's base-emitter junction to handle. Usually for a small signal transistor the reverse Vbe rating is something like 5 V, and you are hitting it with 22 V. The junction is reverse conducting like a zener diode. Place a 1N914 or 1N4148 signal diode or 1N400x rectifier backwards across each transistor, cathode to the base and anode to the emitter, and see what happens.

ak

 

thanks will try

 

Adding a base to emitter diode will significantly affect the frequency of the circuit since it clips the reverse voltage, which is what determines the timing.
Adding the diode in series with the transistor base should have minimal effect.

 

Yeah, I knew you would pick up on that, but adjusting the timing components afterwards will be easy once the fault condition is resolved.

Less effect, maybe. But at -22 V with the two diode junctions in series, there is no guarantee that there will be less than 5to 8 V across the transistor junction.

ak

 

Yeah, I knew you would pick up on that, but adjusting the timing components afterwards will be easy once the fault condition is resolved.

Less effect, maybe. But at -22 V with the two diode junctions in series, there is no guarantee that there will be less than 5to 8 V across the transistor junction.

You would have to adjust the timing component a bunch since, instead of a 22V exponential discharge, it would be only about 1.4V.

What happens with a diode in series with the base is that the base-emitter junction will zener at it's breakdown voltage due to the reverse leakage of the series diode absorbing the rest of the voltage. But since the reverse leakage current of the diode is so tiny, it doesn't damage the base-emitter junction, which can readily tolerate microamps of reverse current.

 

All true, and I've done that, but none of our current customers' reliability guys would accept it. Off the record they would understand it, but on the record they would want back-to-back zeners in series with the base.

ak

 

All true, and I've done that, but none of our current customers' reliability guys would accept it. Off the record they would understand it, but on the record they would want back-to-back zeners in series with the base.

Well, certainly it's a different story if we are playing by your customers' on-the-record, overly cautious reliability rules.