I built an astable multivibrator with blinking lights (much simpler than using a 556!), as a test for a circuit that will use a power transistor (or MOSFET) to pulse a transformer primary for future experiments.
Strangely, when I replaced the two 2N3904 transistors with TIP31As, the circuit would not oscillate unless I briefly disconnected then re- connected R2 or R3. (I bumped the voltage up from 3V to 6V during these tests.)
As a work-around, I'm considering just putting the 2N3904s back in, and connecting the positive end of C2 to the base of a TIP31A. Inelegant, but I think it will work.
I'm trying to figure out why TIP31As won't work, but it also doesn't help me any that the TIP31A data sheet does not specify a minimum V_BE_on.
> I built an astable multivibrator with blinking lights (much simpler > than using a 556!), as a test for a circuit that will use a power > transistor (or MOSFET) to pulse a transformer primary for future > experiments.
> Strangely, when I replaced the two 2N3904 transistors with TIP31As, > the circuit would not oscillate unless I briefly disconnected then re- > connected R2 or R3. (I bumped the voltage up from 3V to 6V during > these tests.)
> As a work-around, I'm considering just putting the 2N3904s back in, > and connecting the positive end of C2 to the base of a TIP31A. > Inelegant, but I think it will work.
> I'm trying to figure out why TIP31As won't work, but it also doesn't > help me any that the TIP31A data sheet does not specify a minimum > V_BE_on.
One possible reason it doesn't work with certain transistors is that the two transistors come on together and lock up the operation. It fails to flip-flop. Also, there might not be enough base current for TIP31's with R2 and R3 at 10K. Lower these resistors. The minimum beta is 25 at 1 amp. so you're likely not getting enough current to drive these transistors.
To insure that an astable won't lock up, disconnect R2 and R3 from the positive rail and connect the junction of the two resistors to the cathodes of two diodes. Connect the anodes of the diodes to the anodes of the LED's, one on each.
With this arrangement, if the transistors both come on together, the base drive is reduced and the circuit will always start.
> > I built an astable multivibrator with blinking lights (much simpler > > than using a 556!), as a test for a circuit that will use a power > > transistor (or MOSFET) to pulse a transformer primary for future > > experiments.
> > Strangely, when I replaced the two 2N3904 transistors with TIP31As, > > the circuit would not oscillate unless I briefly disconnected then re- > > connected R2 or R3. (I bumped the voltage up from 3V to 6V during > > these tests.)
> > As a work-around, I'm considering just putting the 2N3904s back in, > > and connecting the positive end of C2 to the base of a TIP31A. > > Inelegant, but I think it will work.
> > I'm trying to figure out why TIP31As won't work, but it also doesn't > > help me any that the TIP31A data sheet does not specify a minimum > > V_BE_on.
> One possible reason it doesn't work with certain transistors is that the two > transistors come on together and lock up the operation. It fails to > flip-flop. Also, there might not be enough base current for TIP31's with R2 > and R3 at 10K. Lower these resistors. The minimum beta is 25 at 1 amp. so > you're likely not getting enough current to drive these transistors.
> To insure that an astable won't lock up, disconnect R2 and R3 from the > positive rail and connect the junction of the two resistors to the cathodes > of two diodes. Connect the anodes of the diodes to the anodes of the LED's, > one on each.
> With this arrangement, if the transistors both come on together, the base > drive is reduced and the circuit will always start.
>> > I built an astable multivibrator with blinking lights (much simpler >> > than using a 556!), as a test for a circuit that will use a power >> > transistor (or MOSFET) to pulse a transformer primary for future >> > experiments.
>> > Strangely, when I replaced the two 2N3904 transistors with TIP31As, >> > the circuit would not oscillate unless I briefly disconnected then re- >> > connected R2 or R3. (I bumped the voltage up from 3V to 6V during >> > these tests.)
>> > As a work-around, I'm considering just putting the 2N3904s back in, >> > and connecting the positive end of C2 to the base of a TIP31A. >> > Inelegant, but I think it will work.
>> > I'm trying to figure out why TIP31As won't work, but it also doesn't >> > help me any that the TIP31A data sheet does not specify a minimum >> > V_BE_on.
>> One possible reason it doesn't work with certain transistors is that the >> two >> transistors come on together and lock up the operation. It fails to >> flip-flop. Also, there might not be enough base current for TIP31's >> with R2 >> and R3 at 10K. Lower these resistors. The minimum beta is 25 at 1 amp. >> so >> you're likely not getting enough current to drive these transistors.
>> To insure that an astable won't lock up, disconnect R2 and R3 from the >> positive rail and connect the junction of the two resistors to the >> cathodes >> of two diodes. Connect the anodes of the diodes to the anodes of the >> LED's, >> one on each.
>> With this arrangement, if the transistors both come on together, the >> base >> drive is reduced and the circuit will always start.
I tried simulating various forms of this circuit with both 2N3904 and 2N3055, and it always seemed to work, at least down to 2.5 volts or so. It seemed to woek better if I connected C1 and C2 directly to the collectors, which have a bit more voltage swing. I would suggest connecting a logic level MOSFET to drive a transformer, so you will have minimal loading. Without the LEDs, you will have plenty of voltage swing for the gate. And you can use an N-channel to sink a higher voltage on a transformer CT, or P-channel to source the voltage. You might even be able to make a full bridge, but you need to make sure there is dead time where both the high-side and low side are off. This is why they have dedicated circuits for that.
> >> > I built an astable multivibrator with blinking lights (much simpler > >> > than using a 556!), as a test for a circuit that will use a power > >> > transistor (or MOSFET) to pulse a transformer primary for future > >> > experiments.
> >> > Strangely, when I replaced the two 2N3904 transistors with TIP31As, > >> > the circuit would not oscillate unless I briefly disconnected then re- > >> > connected R2 or R3. (I bumped the voltage up from 3V to 6V during > >> > these tests.)
> >> > As a work-around, I'm considering just putting the 2N3904s back in, > >> > and connecting the positive end of C2 to the base of a TIP31A. > >> > Inelegant, but I think it will work.
> >> > I'm trying to figure out why TIP31As won't work, but it also doesn't > >> > help me any that the TIP31A data sheet does not specify a minimum > >> > V_BE_on.
> >> One possible reason it doesn't work with certain transistors is that the > >> two > >> transistors come on together and lock up the operation. It fails to > >> flip-flop. Also, there might not be enough base current for TIP31's > >> with R2 > >> and R3 at 10K. Lower these resistors. The minimum beta is 25 at 1 amp. > >> so > >> you're likely not getting enough current to drive these transistors.
> >> To insure that an astable won't lock up, disconnect R2 and R3 from the > >> positive rail and connect the junction of the two resistors to the > >> cathodes > >> of two diodes. Connect the anodes of the diodes to the anodes of the > >> LED's, > >> one on each.
> >> With this arrangement, if the transistors both come on together, the > >> base > >> drive is reduced and the circuit will always start.
> I tried simulating various forms of this circuit with both 2N3904 and > 2N3055, and it always seemed to work, at least down to 2.5 volts or so. It > seemed to woek better if I connected C1 and C2 directly to the collectors, > which have a bit more voltage swing. I would suggest connecting a logic > level MOSFET to drive a transformer, so you will have minimal loading. > Without the LEDs, you will have plenty of voltage swing for the gate. And > you can use an N-channel to sink a higher voltage on a transformer CT, or > P-channel to source the voltage. You might even be able to make a full > bridge, but you need to make sure there is dead time where both the > high-side and low side are off. This is why they have dedicated circuits > for that.
> Paul
Yep, that's why I thought pulsed DC would be easier.
>I built an astable multivibrator with blinking lights (much simpler > than using a 556!), as a test for a circuit that will use a power > transistor (or MOSFET) to pulse a transformer primary for future > experiments.
> Strangely, when I replaced the two 2N3904 transistors with TIP31As, > the circuit would not oscillate unless I briefly disconnected then re- > connected R2 or R3. (I bumped the voltage up from 3V to 6V during > these tests.)
> As a work-around, I'm considering just putting the 2N3904s back in, > and connecting the positive end of C2 to the base of a TIP31A. > Inelegant, but I think it will work.
> I'm trying to figure out why TIP31As won't work, but it also doesn't > help me any that the TIP31A data sheet does not specify a minimum > V_BE_on.
One of the nice things about 555s and 556s is that they always start if you keep within the specifications in the data sheet. The main issue folks have is that they forget to bypass the power supply, which often causes retriggering problems.
If you have a cmos 555 laying about, you can do this in a way that is pretty much guaranteed to start.
As a side note, the easy way to build a square wave oscillator from a cmos 555 is to connect the output pin to a resistor, then connect the other side of the resistor to ground through a capacitor. Now, connect both the trigger and threshold inputs to the junction of the resistor and capacitor.
Using this configuration, you can also use the discharge pin as an 'open collector' output, and use it to drive arbitrary bits of circuitry. For your application, you might use the discharge to directly drive one LED, and have the other LED driven by a power PMOS or PNP transistor.
If the resistor you use is a pot, you can then adjust the frequency without affecting the duty cycle by varying the resistance. You can work out the limits of the frequencies you can obtain, given your parts.
Note that the TTL outputs of NE555 variants don't easily lend themselves to this approach, due to the asymmetric current draw of the output for high and low output signals. CMOS versions offer nice rail-to-rail square wave output.
This is a nice way to do it, which I learned here from a post by John Fields a couple of years ago. (Yet another example of what a nice resource this group is). I'm not sure if it was his idea, or if he learned it from somebody else. I haven't seen this idea in any of the data sheets I've looked at.
>> >> > I built an astable multivibrator with blinking lights (much simpler >> >> > than using a 556!), as a test for a circuit that will use a power >> >> > transistor (or MOSFET) to pulse a transformer primary for future >> >> > experiments.
>> >> > Strangely, when I replaced the two 2N3904 transistors with TIP31As, >> >> > the circuit would not oscillate unless I briefly disconnected then >> >> > re- >> >> > connected R2 or R3. (I bumped the voltage up from 3V to 6V during >> >> > these tests.)
>> >> > As a work-around, I'm considering just putting the 2N3904s back in, >> >> > and connecting the positive end of C2 to the base of a TIP31A. >> >> > Inelegant, but I think it will work.
>> >> > I'm trying to figure out why TIP31As won't work, but it also >> >> > doesn't >> >> > help me any that the TIP31A data sheet does not specify a minimum >> >> > V_BE_on.
>> >> One possible reason it doesn't work with certain transistors is that >> >> the >> >> two >> >> transistors come on together and lock up the operation. It fails to >> >> flip-flop. Also, there might not be enough base current for TIP31's >> >> with R2 >> >> and R3 at 10K. Lower these resistors. The minimum beta is 25 at 1 >> >> amp. >> >> so >> >> you're likely not getting enough current to drive these transistors.
>> >> To insure that an astable won't lock up, disconnect R2 and R3 from >> >> the >> >> positive rail and connect the junction of the two resistors to the >> >> cathodes >> >> of two diodes. Connect the anodes of the diodes to the anodes of the >> >> LED's, >> >> one on each.
>> >> With this arrangement, if the transistors both come on together, the >> >> base >> >> drive is reduced and the circuit will always start.
>> I tried simulating various forms of this circuit with both 2N3904 and >> 2N3055, and it always seemed to work, at least down to 2.5 volts or so. >> It >> seemed to woek better if I connected C1 and C2 directly to the >> collectors, >> which have a bit more voltage swing. I would suggest connecting a logic >> level MOSFET to drive a transformer, so you will have minimal loading. >> Without the LEDs, you will have plenty of voltage swing for the gate. >> And >> you can use an N-channel to sink a higher voltage on a transformer CT, >> or >> P-channel to source the voltage. You might even be able to make a full >> bridge, but you need to make sure there is dead time where both the >> high-side and low side are off. This is why they have dedicated circuits >> for that.
>> Paul
> Yep, that's why I thought pulsed DC would be easier.
But, also remember that you must have very little DC current in any transformer winding, or you will get saturation. Pulsed DC can be used if you are building a circuit where you are alternately storing and releasing energy in the magnetic field, in which case you really have an inductor, which may have additional windings for various reasons (especially isolation, multiple outputs, and large differences in input to output voltage/current).
You might have a look at the Microchip PIC16F616, which has multiple on-board PWM outputs that can be programmed to drive various power supply circuits ranging from simple single inductor buck or boost converters, to center tapped transformers, and full bridge circuits (using MOSFETs, of course). I'm using that PIC (and its similar cousin, PIC16F684) to make a single inductor 100 kHz boost converter from 12 VDC to anything from 20 to 60 VDC at close to 1 amp, and it would only take a few changes in the power components (MOSFET, inductor, Schottky diode, and capacitors) to scale that up to several hundred watts. And some simple additional diodes and capacitors can also provide a negative output that tracks the positive output for balanced loads. I have posted the LTSpice file before. If you want to learn PIC programming this might be a fun project.
On Fri, 4 Jul 2008, Bob Monsen wrote: > This is a nice way to do it, which I learned here from a post by John Fields > a couple of years ago. (Yet another example of what a nice resource this > group is). I'm not sure if it was his idea, or if he learned it from somebody > else. I haven't seen this idea in any of the data sheets I've looked at.
If you're talking about the resistor from the output to pins 6 & 2, that was in "Electronics" over thirty years ago, albeit with a pullup resistor on the output since it was the original 555 at the time. It was either in "Designer's Notebook" or "Engineer's Casebook", I guess the former is most likely.
It's included in Walter Jung's "IC Timer Cookbook" (which is a lot easier to dig out than the original article) and credits it to the June 21, 1973 issue of "Electronics". It does not bother with the pull-up resistor, though it then suggests it as an improved version of the simple circuit.
When I saw it, I thought it was great, no fussing with two timing resistors, and no worry that the pulse width would get too narrow over a wide range. So any time I need a non-specific clock, I use the circuit, without the pullup reisstor. It works fine, it gives far less variation in output pulse width than the standard circuit with the two timing resistors, that require you to fuss with the resistors to keep the pulse width reasonably constant.
For a lot of breadboarding, and even finished circuits, "close enough" is good enough, so you don't need the pullup resistor on the output, or a CMOS 555.
