Simple 555 PWM - disappointing performance

Thanks, I'll try that. BTW, it was 14uH. (The Atlas LCR consistently read 14.2 uH on several readings with two motors.) So I'll try 35kHz for starters.

Lots more complexity to modeling a small DC motor than I'd expected!

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Terry Pinnell
Hobbyist, West Sussex, UK

Hadn't thought about those sort of implications - thanks.

Not sure I follow you; could you amplify please?. The idea of both those RC filters was twofold:

- reduce switch bounce (although, in theory, it shouldn't really matter, as the bistables should latch on only the very first pulse).

- reduce noise, as the wiring to the main unit was likely to be quite long.

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Terry Pinnell
Hobbyist, West Sussex, UK

Thanks for the follow-up.

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Terry, West Sussex, UK

that they do.

Initially, these capacitors have no charge. that means Vcap = 0. At the "instant" of turn on, the +13V supply rises (however fast or slow) to

+13V, but there is 0V across each of the debounce/filter caps. for the caps to 0V, this means a logic zero. for the caps to +13V, (open/stop and close/stop switches) the resistor is *initially* +13V and hence logic level 1. Then the cap charges (to +13V) thru the 100k resistor, time constant 10ms, resulting in an exponential decay to 0V, at which point (really depends on gate Vil) the logic level becomes the zero you expect..

If the +13V supply rises much slower than the RC time constant, this effect can be masked. its quite common when using lab supplies, especially if you turn them "off" with the current limit dial, like I do :)

I've shot myself in the foot just recently with exactly that - a TUSB3210 based system, the micro's oscillator *wont start* if the supply voltage ramps up too slow! I spent a few hours trying to figure out why my programmed micro lit up LEDs it shouldnt....doh.

Cheers Terry

Put this crash-bang suggestion on the shelf Terry, (just in case your BUZ11's lose their smoke). 5v +---------+ 0.3ohms 1.8V? Your 5V PC +---| Your |--/\\/\\--+----+-----+ supply? | Relay | _|_ _|_ | +---|Switchery|--+ \\_/ /_\\ |+ 0v +---------+ | | | [Motor] | _|_ _|_ |- | \\_/ /_\\ | | | | | +-----+----+-----+

Use a 5v supply, shunt diodes to regulate a lower voltage across the Motor, and a series resistance to limit the stall current to 10A (guaranteed).

You already seem to have a total loop resistance of about 0.2R, of which 0.08R is known to be the motor.

Resistance budget: Relays+cabling...... 0.07 New resistor........ 0.30 (50W) Motor cabling....... 0.05 Motor resistance.... 0.08 ------ Total= 0.5R ------

This gives the req'd 10A current at stall.

The diodes will be pairs of Schottky+Silicon to get about 1.8v shunt regulation, and they will need to carry about 7A when the motor is running off-load.

Stick a thermal sensing switch onto the 0.3R, as an emergency relay unlatcher if either of the limit switches fails to operate.

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Tony Williams.

Thanks Tony!

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Terry Pinnell
Hobbyist, West Sussex, UK