Let's see, Q=It and Q=CV.  So It=CV, therefore I=CV/t

Pluggin in C=2200uF, V=12 volts and t=1 second gives a current of only 26mA.  Assuming that my calcs aren't total horse$hit, then the total current is less than 1/2 amp for all 15 clocks.  I would suggest a constant current circuit for each clock would be the prefered solution.  Not sure if constant current diodes are available in current ratings that high but a cheap and quick way of doing this would be to use a LM317 voltage reg in constant current mode (as described in the app notes)

Regards

Mike Rigby-Jones

-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Jinx
Sent: Tuesday, April 04, 2000 18:06
To: PICLIST@MITVMA.MIT.EDU
Subject: [OT] Constant-current cap charging




I'm revising an F84 factory clock driver and would like some advice
about dealing with the output stage. The gif shows the solenoid in
the clock and how it's tripped. Almost all of the instantaneous power
comes from the 2200u cap. This cap then normally has 30 seconds
to charge up to 15V. However, there are, as always, complications.

This o/p stage is fine when there are only 1 or two solenoids to drive
(each has its own 4050-TIP121-cap) but the revised one has 15. The
recharging of 15 caps causes a huge surge on the PSU, which causes
the AC tap on the transformer to dip below that which can be reliably
measured by the PIC so the clock will/does lose time. The quick fix
was to put a resistor in series with the isolating diode, but as the circuit
also has a fast-forward mode, the slower charging of the o/p caps
makes the f-f stepping unacceptably slow (not to me, to the factory,
even though it's likely to be used only once a year)

A bigger transformer I tried gave much better results, but that had to be
abandoned because of space restrictions. For 29 of the 30 seconds
the tx isn't doing much, but has to deliver the goods straight after the
trip. The IC's in the circuit are diode-isolated + reservoir caps, their
DC supply isn't affected by the surge.

So is there a simple constant-current charger that would lessen the
surge on the PSU that I could put between the PSU and the junction of
all the 1N4001's ? A FET something ? The caps are completely
drained by the solenoids tripping and need to be recharged to at least
12V as quickly as possible (1 second ?) without that initial current
surge to minimise the PSU droop. Also, is this power-dumping harmful
to the cap in the long term ? They're standard PCB 25V 85C types.
The original has been in service for four years with no breakdowns
so far.

TIA