Am I the only one to notice that it is clearly shown as a B
LAKE solenoid?
During an unexpected power down the solenoids clearly are disengaged rapidly.
Without an extensive study of the mechanical parts in the manual I would guess that the solenoids either release the brakes or engage the motors when they are powered.
The transistor turns them off rapidly when power is lost via there negative supply. The switches select positive supplies to them as required for normal operation.
(03-06-2019, 09:40 PM)Refugee Wrote: [ -> ]During an unexpected power down the solenoids clearly are disengaged rapidly.
Without an extensive study of the mechanical parts in the manual I would guess that the solenoids either release the brakes or engage the motors when they are powered.
The transistor turns them off rapidly when power is lost via there negative supply. The switches select positive supplies to them as required for normal operation.
Yes, the transistor Q705 is in circuit between the -ve of the reservoir capacitor C801 and the bottom end of the solenoid but only when the common and NC contacts of the auto shut off switch S802 are open. The way I see it, that would lengthen the time before the brakes are applied due to the charge on C705, not shorten the time, for the shortest time Q705 wouldn't be needed because without it C801 is disconnected from the solenoid the instant that the common and NC contacts of S802 are open.
What's going on with Q705 etc?
Lawrence.
Below is a simplified schematic showing the circuit in rewind mode but at a point where the common contact and the NO contact of the auto shut off switch S802 are open, the only time Q705's collector/emitter is in circuit is when those contacts are open.
Note: Earlier on I referred to the common and NC contacts being open, that was an error it should have been the common and NO contacts open.
Lawrence.
Hi Lawrence, Q705 forms an arc less switch, inductances are notorious to arc when a switch is opened I.e. car ignition systems. Q705 therefore preserves the integrity of S802. The brake solenoid is on to release the brake it's a fail safe system. If the mains goes off then Q704 and Q705 turn off solenoid off no arcing. If S802 opens then C801 holds enough charge for Q705 to switch off properly and therefore arc less switching.
Dave
Cheers for that
Q705 used as a switch to preserve S802's integrity makes sense to me, the time constant of the components in Q704's base circuit being long enough to turn on Q705 long enough to prevent the formation of an arc when S802's common and NO contacts break open but short enough to enable a decent charge to be retained by C801 in order to supply the auto shut off solenoid.
Lawrence.
The wire from the NO of SW802 has no junction spot to connect it with the common wire to the brake and pinch roller solenoids. They are just shown crossing.
SW802 is shown rather close to the wire to the solenoids however on zooming the viewer it shows that there is a mistake in your diagram in post#15.
(04-06-2019, 10:15 PM)Refugee Wrote: [ -> ]The wire from the NO of SW802 has no junction spot to connect it with the common wire to the brake and pinch roller solenoids. They are just shown crossing.
SW802 is shown rather close to the wire to the solenoids however on zooming the viewer it shows that there is a mistake in your diagram in post#15.
Cheers for that , it looked like a dot at first....damned copies.
I'll do another schematic, showing the other switches this time.
Lawrence.
Redrawn simplified schematic showing rewind mode, function switch contacts etc included this time, hopefully panic over.
Lawrence.