A couple of suggestions:

1. Is 2200uF the lowest cap that'll work reliably? Reduce it till the
solenoid still works (even when it's warmed up) and use the next higher value.

2. Don't allow the cap to fully discharge. Try putting a small capacitor
between the 4050 and the base of the transistor. The total reactance of the
solenoid and cap should let u retain a couple of volts in the cap. If this
works, the current surge in charging the cap again won't be as bad.

3. Use a switchmode power supply. If 800mA ~ 1A is sufficient you might be
able to use some cell phone battery plug packs. Small but packs a punch.

If all 3 options are workable, you might just get away with series
resistors for the caps and an acceptable fast-forward rate.

Good luck
Terry



At 01:05 PM 4/5/00 +1200, you wrote:
>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
>
>
>Attachment Converted: "C:\EUDORA\Attach\solenoid.gif"
>