Any beefy coil driver chips?

That's how my '60's ignition system worked, ramp ignition coil up to

Trick: Timing (and dwell) was via a ramp. If 5A wasn't reached by firing time, turn-on moved down the ramp, if 5A was held, turn-on moved up the ramp... result _very_ low dissipation ;-)

...Jim Thompson

And you 24V is what? From a pump?

...Jim Thompson

That's the method I'd usually take. And might take on this one as well, but hard to fit onto the real estate.

Except I have the problem that there is no supply rail. So it would need to go from zero to "ready for program execution in less than 100usec (considering that the program start itself will require up to 50usec).

I might look into FPGA.

That's going to be a whole science until itself on this one, more than the usual diode.

pump.

It's the on/off control signal, coming from some beefy wiring closet contactor. So when the relay ain't engaged there is no voltage on this line and it is the only line going there.

On a sunny day (Fri, 31 Jul 2009 09:53:28 -0700) it happened Joerg wrote in :

Well I looked for 'macro' on the canon, but for some reason could not find the mode. It focussed well ... where ... I dunno where.

about 400 us,

I think there is no RC time involved when using the external osc, it likely just counts clocks, but the proof of the pudding is in the eating of course.

There are a lot of papers on the Microchip site, but it always takes ages to read all that stuff, I am sure they have it in more detail for different PICs too. Will your solenoid be saturated, or the current too high within half a milli second? This is what is needed right, hold full power, and then gradually decrease? Just add some nops to get the correct delay, or do you measure current too? I do the curent trip using the PIC internal comparator in an other project. That internal comparator has a resolution of about 10mV IIRC. There is also a *real* PWM output on some PICs (16F690 for example) that has the PWM switched to zero in hardware in the PIC, when that comparator output goes high. Faster is not possible, that feature is what I use in the SEPIC.

pump.

Ok, a bit different here. I must hold it at peak for a while and then at the hold value for another while (this 2nd hold phase being from very brief to essentially hours). I am all for analog but you can't do that analog or you could fry an egg on there after some time.

That's why I bristled at your notion that PWM is for sissies ;-)

so

pump.

Inductance and resistance of load? On time? Is a flyback diode allowed across the load? Can you heatsink?

Inquiring minds want to know ;-)

...Jim Thompson

Well, then we're back to the simple circuit like a 555 timer..

If you make a PWM generator, the 555 can be powered from the control voltage. Normally, the initial start of the circuit produces a wider width, which can give you the PEAK your looking for, and then once it gets started, it simply lessens the duty cycle. This is due to the way the timer works with it's 1/3 voltage comparators it has.

You may want to experiment a bit with LTspice to see this take place, it does a good job at representing the basic 555 timer.

One could also use a dual timer to have better control over the initial PEAK and then hold values.

I noticed the 555 in there and was wondering how good the model is for situations where you need to see what happens when the supply rail is yanked up hard.

so

pump.

All in flux at this point. Low tens of mH, about an ohm, total on-time msecs to hours, flyback diode yes but must play tricks for faster ramp-down (nasty ...), cannot heatsink, would melt plastic.

so

pump.

It is... at least the way you're approaching the problem.

Post your design, when finished. Then I'll post the easy way ;-)

...Jim Thompson

Back in the hay days, we used to have dual coil solenoids, one as the start coil that was in series with a contact on the relay.. It simply open when pulled in, allowing only the low current coil to stay energized. But I'm sure this doesn't fit your application.

go

so

driver

pump.

How "fast" a ramp-down?

Post your design, when finished. Then I'll post the easy way ;-)

...Jim Thompson

mode.

about 400 us,

just counts clocks,

I'll have to plough through some of that. Somehow most of my stuff end with "nobody has ever wanted that before" and then I am back to discretes ;-)

second?

Yes, potentially.

Usually on many relays you want to do that rapidly.

Not necessarily. The NOPs or timers aren't the issue, in uC it's the guaranteed time after power application by when they are fully ready. The classic answer there is "don't do that". But sometimes you have to.

the PWM

I always wish I could do SMPS with uC. Then you could have one core design and just modify as needed whenever a new design rolls around the corner. But they are too slow for MHz-class converters.

It is very handy as the CPU takes care of the soft startup/shutdown, under/over voltage and the other issues, and you can implement whatever control algorithm you like.

It is possible. The PWM runs at MHz speed, with the resolution imporoved by the pre-computed noise shaping pattern. The control loop runs much slower, at 100kHz or so.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

so

pump.

I respectfully disagree :-)

How do you want to hold a coil at 25% current or so without PWM _and_ while dissipating next to nothing?

Not at liberty to do that, I'm afraid.

Not enough room. If that was possible they wouldn't need me :-)

go

and so

too.

driver

pump.

Around 1/10th of a msec. Twice that would probably be accepted but not a lot more.

Mine will be the cool way, cool as in no blister when touching :-)

go

so

driver

pump.

So why do you ask for help ?:-)

I think you're turning left ;-)

...Jim Thompson