In many electromechanical systems like this the holding current for the solenoid can be quite a bit less than the pull in current. By correctly sizing the R and C in this circuit you can have enough charge in the cap to pull the solenoid in reliably, and then the solenoid is held in with the current that flows thru R. Bob Ammerman RAm Systems ----- Original Message ----- From: "Wagner Lipnharski" To: Sent: Friday, December 20, 2002 5:06 PM Subject: Re: [EE]: Solenoid information > That's the point. > > A simple larger capacitor connected to the lithium cells via a current > limiting resistor. > The resistor should be calculate to keep the cap charging current below the > lithium max current. > > All the solenoid energizing current will be coming from the capacitor > rather than the cells. > So, the capacitor should also be calculated to have enough stored energy to > supply the necessary power to the solenoid. Of course, during the solenoid > energize time, the lithium cell will also supplying some current via the > resistor, but most current will be supplied by the cap. > > Remember that there is a cap charging time, gross calculated as time = RC, > what will limit solenoid consecutive activations to be longer than the cap > recharging time. > > Wagner. > > > ----- Original Message ----- > From: "Dave Dribin" > To: > Sent: Friday, December 20, 2002 3:25 PM > Subject: Re: [EE]: Solenoid information > > > Hi Roman, > > I'm dusting off this old message you sent out a while back as I'm > gettting to play with this project again. I bought a 6V solenoid with > a resistance of 20.8 ohms, the 36 AWG version of this: > > http://www.solenoidcity.com/solenoid/tubular/s-63-38hp1.htm > > I also bought a 2-cell lithium battery pack, so that's my +6V power > supply, two BR-A cells: > > http://www.panasonic.com/industrial/battery/oem/chem/lith/cylin.htm > > You mentioned using a cap to trigger the solenoid, using a buck > regulator to charge it. What exactly is a buck regulator? Some part > searches on Digi-Key turned up ICs that required a bunch of other > components. Is there a particular setup you recommend? > > BTW, could I do something simpler, like this, using a Darlington > transistor for Q1: > > +6V (Battery) > | > Solenoid (with reverse diode) > | > C-------+ > PIC-----R1---B Q1 > E > | > GND > > The only issue I'm worried about is that the battery needs to source > 300mA. The battery data sheet mentions it can supply 1,000mA as the > maximum pulse current. This would seem within spec, but perhaps it > will still pull the battery voltage down far enough where it will > affect the PIC. I plan on sticking 1 diode between the battery and > the PIC to drop the voltage closer to +5V. > > As usual, thanks for the excellent feedback. > > -Dave > > On Sat, Oct 26, 2002 at 04:26:45AM +1000, Roman Black wrote: > > Dave Dribin wrote: > > > > > Anyone had experience with them or know of other solenoid sources? > > > Ok, this may sound a little weird, but I don't know or care about > > > these requirements at this stage. I wish I had a good explain of why, > > > but suffice to say, for this 'demo' if the solenoid pushes out into > > > thin air, it'll be ok. Yes, I know it's weird. :) We're just looking > > > to investigate solenoids and get a feel for how the work. > > > > Hi Dave, your suggestion of using (say) 2x AA > > cells and a low power solenoid for micro-power > > use is perfectly valid. > > > > I suggest activating the solenoid *only* from a > > charged capacitor, especially as you only need it to > > activate on rare occasions. I've used a similar system > > with mechanical numerical counters, charge a cap > > at fractions of a mA, then dump it into the solenoid > > 500mA coil to activate it and clock the counter. > > > > I have a number of solenoids here with the manufacturers > > charts, you will find coil "pull-in" current depends > > on the distance it travels due to the magnetic path > > etc of the solenoid. Proper mechanical design of your > > latching mechanism will give very small travel, ie under > > 1mm total, with much reduced capacitor size to get it > > to latch reliably. With a spring loaded drawer it WILL > > spring open when the solenoid is dumped. > > > > I suggest a solenoid rated for 5v or 6v, a buck regulator > > (constant current) driven by the PIC to charge the cap > > to your battery voltage and a low sat transistor or FET > > to do the dump and pull the latch. Best if you only > > charge the cap just before you use the latch so it is > > normally in the discharged state and not leaking current. > > :o) > > > > > > > and get the 6V continuous DC version. It's rated at 2W of power. > > > Does that mean this thing will always use 2W of power, under all > > > circumstances? Is the current draw just 2W/6V or 333mA? What if I > > > apply 3V, is the current draw then 667mA? > > > > No, what you do is charge a cap to your battery voltage. > > This is done by a buck system that uses around 1/3 the > > battery current as charging the cap from a resistor > > based on my results. Takes about 1/2 a second. > > Then the capacitor is dumped into the solenoid, and > > the latch activates. The solenoid needs to work at > > your expected minimum battery voltage, and the cap > > size only needs to be big enough to give reliable > > pull-in under all conditions. > > -Roman > > > > -- > > http://www.piclist.com hint: The list server can filter out subtopics > > (like ads or off topics) for you. See http://www.piclist.com/#topics > > > > > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.