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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I was able to get 10uS on 200us off, and you can change to mS if you like. totally stable just choose a good polyester or similar low leakage cap. I was able to vary it from 0.5% to 85 or 90% i think.
this was the principle time base for the planar circuit.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
I was able to get 10uS on 200us off, and you can change to mS if you like. totally stable just choose a good polyester or similar low leakage cap. I was able to vary it from 0.5% to 85 or 90% i think.
this was the principle time base for the planar circuit.
but were you able to 'set' the on-time to, say 10uS, and then vary the frequency independantly (ie set the off-time independantly), without it changing the on-time?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Sorry about the double post, but new information.
Sulaiman, I'm assuming you made a typo, and that this is what you meant. Tying the trigger of the astable to Vcc doesn't seem to make sense, to me anyway.
EDIT: For anyone who is trying to work out how this is 'supposed' to work, R and R1 are variable resistors, R determines the 'on' time, and R1 determines the frequency, which also determines the 'off' time. The output of the second 555 (monostable) drives a MOSFET (or similar switch), which drives the primary of the flyback. For more info, see the link to the 'NE555' datasheet above, or the Wikipedia page on the NE555.
Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195
one disadvantage of the double 555 circuit is that while it offers constant ON time is not capable to offer fully adjustable duty cycle for all frequencies
at a certain frequency the range of the duty cycle would be maximum at lower frequencies the range of the duty cycle would drop linearly with frequency at higher frequencies the on time of the monostable would exceed the astable period giving weird output signals, the higher the frequency the weird the output wave form
lat say the maximum ON time of the monostable is 1ms then the maximum frequency of the astable should not exceed 1kHz to avoid overlaps, and as the frequency goes lower the maximum duty cycle also go down, so at 1kHz he maximum possible duty cycle would be 100% and at 500Hz the maximum possible duty cycle would be 50% and so on...
if you really need constant ON time and don't care about the reduction of the max duty cycle then this circuit is perfect
You can use a LTC6990 and LTC6993. One is a wide range VCO, and the other is a variable one-shot. Complete independent control of frequency and pulse width. I've worked with the VCO version before, it works beautifully.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
dexter wrote ...
one disadvantage of the double 555 circuit is that while it offers constant ON time is not capable to offer fully adjustable duty cycle for all frequencies
at a certain frequency the range of the duty cycle would be maximum at lower frequencies the range of the duty cycle would drop linearly with frequency at higher frequencies the on time of the monostable would exceed the astable period giving weird output signals, the higher the frequency the weird the output wave form
lat say the maximum ON time of the monostable is 1ms then the maximum frequency of the astable should not exceed 1kHz to avoid overlaps, and as the frequency goes lower the maximum duty cycle also go down, so at 1kHz he maximum possible duty cycle would be 100% and at 500Hz the maximum possible duty cycle would be 50% and so on...
if you really need constant ON time and don't care about the reduction of the max duty cycle then this circuit is perfect
Yes, I realise that you need to keep the 'on' time shorter than the switching frequency, etc.
The optimum 'on' time for any flyback is a function of 'volt seconds', regardless of frequency, and the optimum 'off' time is dependant on several factors (load, secondary inductance, etc). Once we know the characteristics of these flybacks it should be easy to use a TL494 or whatever to 'reproduce' the optimum 'on' and 'off' times. Ideally you want to 'almost' reach saturation during the 'on' time. There are different modes for driving flybacks (continuous mode, discontinuous mode), which complicates things still further.
It's obviously possible to drive these flybacks using any flyback driver (single transistor, 50% duty cycle, etc), but to get the optimum performance I think you need to control the 'on' and 'off' times independantly, at least until you know what these are, for any given 'volts per turn', etc.
At least, that's what I've learned about flybacks during the four years I've been using this forum
@Daedronus: I'm sure you could use the TL494, but I don't think you can vary the frequency without changing the 'on' time as well, because the duty cycle is a percentage of switching period. Once the optimum 'on' and 'off' times are known, for a given application, I'm sure these could be reproduced using a 494, although I think the duty cycle is only variable within certain parameters, if I remember correctly.
@Sigurther: Thanks for those suggestions, I'll have a look later, but first I'll have a look at the 556, which, as I'm sure you are aware, is two 555's on one chip.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
If you really want precise control over the on times, etc, you might want to consider switching to a small microprocessor (ATtiny, etc), even with an attiny85 you get a pair of 8-bit timers and plenty of A/D to connect to the necessary knobs for controlling it. If you upgrade to an atmega (which you can get pre-programmed with the arduino bootloader for $4, or an entire arduino nano clone for $10) you get a 16bit timer that will blow away anything you can hobble together using 555's. It would also give a clear upgrade path for adding things like overcurrent protection, closed loop voltage/current mode, etc.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
... wrote ...
If you really want precise control over the on times, etc, you might want to consider switching to a small microprocessor (ATtiny, etc), even with an attiny85 you get a pair of 8-bit timers and plenty of A/D to connect to the necessary knobs for controlling it. If you upgrade to an atmega (which you can get pre-programmed with the arduino bootloader for $4, or an entire arduino nano clone for $10) you get a 16bit timer that will blow away anything you can hobble together using 555's. It would also give a clear upgrade path for adding things like overcurrent protection, closed loop voltage/current mode, etc.
Interesting. I'll keep that in mind for another prohect I'm working on which requires 100nS on times, and I may look into it later for this, but I think I'll go for a single SE556 for now, with two control potentiometers, one for frequency and one for pulse length.
I think it just needs two variable resistors, two other resistors and five capacitors, plus a MOSFET and maybe a snubber/TVS. I am trying to keep this as simple as possible for now, but I do intend to take this principle further in the not too distant future
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Driving 'Fiddy's Flybacks' in flyback mode.
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