allanhurst:
An ingenious circuit insofar as it provides variable pulsewidth/ fixed frequency over a large range.
But I can't see any easy trick to make it softstart..
regards
Allan
I don't see if and why it has fixed frequency but anyway - is fixed frequency any good when driving motor? If you replace the 1k resistor with resistor and capacitor in series you should get fixed time when output is high and decreasing time when it is low - getting soft start but losing fixed frequency.
allanhurst:
If you put a capacitor in series with the 1k it wont work at all.
Allan
Are you sure? I didn't try it but if you make the 1k larger and the cap is large enough it should work... When the added cap fully charges the pins 6 and 2 will be low and pin 3 high leading to the motor turned fully on. Until it charges it will charge 100nF cap but slower and slower increasing motor ON periods. (I just realized it is other way I said in my last post). It is possible adding resistor to pin 7 or connecting the diodes elsewhere may lead to better results.
And intuitively - somehow you have to get current from the +ve rail into the 1k/diodes/pot/100n network for the discharge pin to sink. Add a series cap and you can't.
allanhurst:
get current from the +ve rail into the 1k/diodes/pot/100n network ... Add a series cap and you can't.
What?
allanhurst:
prove me wrong..
Until cap charges current flows through it. Q.E.D.
Seriously: challenge accepted but I am on my phone now so will write something more elaborate later. I am quite sure you can get "soft start" this way but it's characteristics may be unusable without hudge cap or more changes in circuit.
In effect you already have a "soft start" with your PWM "speed controller". All you need to do is have it automatically ramp up from slow to full speed.
Was that the plan with the RC combo to form a timer circuit?
This is what I am searching for.. How can I "automatically" ramp up from slow to full speed?
Prior byuing I thought it had a classic potentiometer=5V-SIGNAL-GND so I thought that using a capaictor and resistance I could slow down the inital signal so it will "ramp up automatically".. Then I realized that it works in an other way and so I opened this topic..
Still I am not able to do it, is there any suggestion I can try?
I think that may be a bit more involved than just adding a capacitor and a resistor you might need another 555. How quickly do you want it to ramp up? 1 second, 10 seconds?
There are hundreds of 555 timer circuits out there.
Or use an Arduino to trigger the transistors and ditch the 555.
The Arduino will be used in another program, still, I was planing to PWM the signal of the potentiometer and control speed via an Aruino: not applicable as well..
clubman:
Still I am not able to do it, is there any suggestion I can try?
Try to put large cap (hundreds of uF???) between pin 5 and GND. I have no idea if and how it will work but it may work with rigth cap size and pot setting. Just make sure it is discharged between each try. (Maybe you should place a diode between 5 and Ve so the cap discharges to Ve when is power removed).
It may or may not work but you don't have to change the circuit.
Large cap and resistor in series instead of the 1k resistor should work but I would try it on a breadboard to find correct values (and if it really works) before desoldering.
Did you tried hard enough? I tried both and both worked ... somehow. Just to make sure I know what you want: soft start means that after power is connected the duty of PWM generated on pin 3 will slowly (~2 sec) increase from 0% high to 100% high.
I tried it on breadboard trying to follow your diagram as closely as possible. I used 1/2 of TS556 (CMOS version of NE555 from ST; 2 in one package) - you didn't say if you have CMOS or bipolar version. I used 100k pot because it was closest I had. I used BAT41 Schottky diodes. The power come from either "9V" or 2 AA NiMH batteries - you didn't say operating voltage. To simulate the motor I connected a red LED through 10k resistor to GND and was looking at it's brightness. It should be the more bright -> the faster speed if there were motor in it's place.
When I connected 220uF cap between Control pin and GND it worked well. When 100nF was used as timing cap there was some strange glitch at start up but when I used lower capacitance (22pF or only capacitance of the breadboard) it worked flawlessly. I must admit I am not sure why it works but I suspect forward voltage drop of diodes may be very important. Also if you have bipolar 555 you need much larger cap: mine 555 has 100k resistance between Ve and Control but bipolar has only 5k IIRC.
When I connected 1k resistor + cap in series instead of only the 1k resistor it worked. But I needed 4700uF cap to get start up time ~2s. I think most of it's capacity is wasted into discharge pin instead of charging timing cap - it is probably possible to connect it in more clever way (discarding diodes + pot) to have it working with smaller cap. I tried this version only at 9V.
So there are 2 ways, the first one will preserve possibility to control max speed with the pot and is very easy to implement but I am not sure if it is not parts/temperature/star alignment dependent. It will either work with your setup or not and you cannot do much for it. The second way is cleaner but needs more tinkering on your part to make it work as intended. But you should be able to make the circuit in such way it will work reliably.
When trying any of those make sure the caps are fully discharged before trying it (as they will be before you add power after longer period). Also you should add protection diode(s) from cap to power rails to protect 555 from the cap discharging through it when main power is removed.