08M2 PWM Help to drive a magnet

[Grant666]

Hi

I need to use a 08M2 to drive a magnet on a pinball on a home made pinball. These magnets run extremely hot if constantly "on" cooking the playfield and eventually tripping the ,magnets protection thermal . So the comercial pinball systems use a PWM to help run them a bit cooler.

The cycle would be extremely slow, maybe 1 or 2 short pulses per second.

Is it possible to run the PWM at that lower a cycle rate? and if so what would the code be?

Can you please let me know what you think?

The magnet is run off an 8 FET array that can handle the current and wattage loading required for the magnet. The 08M2 will output into a buffer chip that then runs the FETS.

Regards
G

 

[hippy]

To get such low PWM rates the best way would might be to bit-bang the output using a separate task dedicated to that..

 

[Grant666]

To get such low PWM rates the best way would might be to bit-bang the output using a separate task dedicated to that..

What is the lowest I could get the PWM rate down too? Or could I high and low the output using pause commands?

 

[hippy]

The slowest PWMOUT would be 61Hz - You can use the PWM Wizard of PE6 to see what the min and max are for particular PICAXE operating speeds.

High, Pause, Low, Pause would be the bit-banging technique suggested.

 

[rossko57]

You may not have to duplicate what the original machine did. Are we talking relay based logic? It may use slow PWM because it has to, rather than because it is better.
Consider a peak-and-hold drive - you start off with a big pulse to get the mechanism moving, then reduce current with PWM to conserve power (reduce heat).

Of course part of the appeal may be the healthy buzz from slow PWM

 

[edmunds]

If you do not need to do anything particularly fast, you could also slow down the picaxe with down to setfreq k250.


Good luck,

Edmunds

 

[Grant666]

You may not have to duplicate what the original machine did. Are we talking relay based logic? It may use slow PWM because it has to, rather than because it is better.
Consider a peak-and-hold drive - you start off with a big pulse to get the mechanism moving, then reduce current with PWM to conserve power (reduce heat).

Of course part of the appeal may be the healthy buzz from slow PWM

The original pinny's ran a darlington pair transistor array off their custom CPU. So I guess I could chuck a CRO on their output to see exactly what they do. In my case I need to keep the magnet strong for quick pulses off the player ( Its never been used like this for a style of play in a pinball before ) So overheating is a major issue...so you are right the PWM would run the magnet cooler for me. The peak drive then reduced hold current is common practice on flipper coils in a pinball.

 

[Grant666]

If you do not need to do anything particularly fast, you could also slow down the picaxe with down to setfreq k250.


Good luck,

Edmunds

Ok might try that and the bit banging method and see what happens. Thanks for the advice guys.

 

[erco]

Yo G:

I own a 1977 Gottleib Jungle Queen game, have had it for 30+ years. The last year of all-mechanical pins, works beautiful. My daughter just got her high score of 140,190 yesterday (proud papa pic attached).

You're not trying to PWM the flipper solenoids, are you? That would lower their force. At least initially drive at 100% then maybe reduce after 1/2 sec with PWM if to want to stop & hold the ball. Edit: As rossko suggested, peak & hold.

Or maybe you're making a Magna-save. That could just be a 555 one-shot timer.