Simple 555 PWM - disappointing performance

[Terry Pinnell]

Tony Williams <tonyw@ledelec.demon.co.uk> wrote:

In article <s6do91l38rhl08opvd9f4jdr80j31ft27o@4ax.com>,
Terry Pinnell <terrypinDELETE@THESEdial.pipex.com> wrote:

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.

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.

Thanks Tony!

--
Terry Pinnell
Hobbyist, West Sussex, UK