See the attached PDF file for a very simple constant current source that might work for you. You have 2,200 uf * 15 = 33,000 ufd. That is .033 Farads. Let's assume you want to fully charge all 15 capacitors to 12 volts (from a 15 volt supply) within 5 seconds using a constant current source. Iconstant= (Emax*Farads)/Seconds so Iconstant = (12*.033)/5 which comes out .0792 Amps. To get to 12 volts in 1 second would require .396 amps. Note that the constant current source circuit goes between your existing Vcc and the isolation diodes. All isolation diodes then attach to the collector of the PNP transistor. As far as power dissipation goes, the constant current source only provides the constant current until the capacitors charge up to Vcc-1.8 volts. After that the current will trickle off to next to nothing. It will keep the capacitors charged, but that means just supplying the leakage current. How often do these things fire? If it is something like once every 30 seconds, then you can probably use a small heat sink. You mentioned fast-stepping. The heat sink must be able to keep the transistor cool for the entire time the fast- stepping is in effect. Iconstant must be chosen high enough that the capacitors can recharge back to say 12 volts within the fast-step time period. Fr. Thomas McGahee -----Original Message----- From: Jinx To: PICLIST@MITVMA.MIT.EDU Date: Tuesday, April 04, 2000 9:13 PM 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 > > Attachment converted: growth:Ccsource.pdf (PDF /CARO) (0000D0A5)