[AMPS] How to control a vacuum C ??

Ian White, G3SEK Ian White, G3SEK" <g3sek@ifwtech.com
Wed, 31 Oct 2001 18:35:13 +0000

WC6W wrote:
>   When you tried PWM did you include a catch diode(s) across the 
Good thought, Marv - the motor will try to keep going until the magnetic 
field collapses after switch-off. A reverse diode in parallel with the 
winding(s) will provide a path for the back-EMF, but it can sometimes 
slow the collapse because the reverse voltage across the coil is clamped 
to only about 0.7V.

The field will collapse faster if the reverse voltage is allowed to 
rise. A resistor in series with the reverse diode will do this, but a 
zener diode in series will be even better. The zener breakdown voltage 
can be about the same as the supply voltage (if the switching transistor 
can stand twice the normal supply voltage).

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)

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