Well, how about
1) a small battery between the rectified transformer output and the power
supply regulator?
2) a nice big super cap after the regulator with a very low value resistor
to the solenoid caps?
3) replace / improve AC wiring to the unit? (probably won't help and would
be expensive...)

---
James Newton mailto:jamesnewton@geocities.com 1-619-652-0593
http://techref.massmind.org NEW! FINALLY A REAL NAME!
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-----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