This is really helpful! Thanks for explaining the subtleties... the more I know, the more I realize how much I don't know. But at least I can make it work with some confidence of safety/correctness. Speaking of which, I notice that pretty much all the simple examples for solenoid and motor control I've seen involve using an NPN transistor and switching the ground side of the circuit. In my case, with a bunch of solenoids spread out over a fairly large area, wouldn't it make more sense to use a PNP transistor and switch the +V side instead? Is there any reason not to do it that way? darren On May 27, 2006, at 6:21 PM, Spehro Pefhany wrote: > At 05:04 PM 5/27/2006 -0700, you wrote: > >> On May 27, 2006, at 4:48 PM, Spehro Pefhany wrote: >>> You can swap out the diode, in the current position, for a TVS or >>> beefy >>> zener, and the circuit will work as-is. For example, a 1.5KE62A >>> (JEDEC >>> 1N6290A) . The solenoid will drop out faster than if you used a >>> (cheaper) >>> 1N400x across the coil. >> >> Cool... I'll check into that as well. >> >>> If you don't care about the drop-out time, the 1N400x is easier on >>> both the transistor and the diode. >> >> So is it smart in this circumstance to have something on both the >> transistor and the diode? Or have I misunderstood? > > Okay, you can think of these three being in series: > 1. the supply, generally with a fat output capacitor > 2. the transistor (on or off, with or without a TVS) > 3. the coil (with or without a diode) > > (view with fixed-width font such as Courier) > > Diode > +-----|<-----------+ > | | > | ___ | > o--------+----UUU- ---------|------------------------+ > | | | > | Solenoid | | > | 31R | C | > | .----------|------------------. | > | | | TIP122 | | > | | | | | > | | | | | > | | +------+-----+ | | > | + | | | | | | > ### | | | | | | > --- B | |/ | | | | > | |----+---| | | | | > | | | |> | | | | > 50VDC | | .-. | | | | | > | | | | | | | | | TVS > | | 8K| | | | | | | > | | '-' | | | | \ - \ > | | | | |/ - | ^ > | | +-----+----| ^ | / \ > | | .-. |> | | --- > | | | | | | | | > | | | | | | | | > | | '-' 120R | | | | > | | | | | | | > | | +------------+-----+ | | > | '-----------------|-----------' | > | E | | > | | | > o---------+-------------------------+-----------------+ 0V > > When the transistor switches off, the voltage at the collector > rises from > about 0.8V typically (assuming forced beta of 250, so base current of > 6.4mA). If there is a diode across the coil, the voltage rises to a > diode > drop above whatever the supply voltage is at the time, and that is > your > maximum collector voltage. The current continues to flow at 1.6A > decreasing > exponentially toward zero, and most of the stored energy is > dissipated in > the copper of the coil. If there is a TVS in there, it will not > conduct > unless the supply voltage is higher than the TVS breakdown. BTW, the > 50V is a bit close to the 60V breakdown voltage for a TIP120, so a > TIP122 > would be a better choice for the few pennies more you get 100V > breakdown and (unlike MOSFETs) no worse "on" characteristics. > > If there is no diode in there, the voltage rises to the TVS breakdown, > about 62V for the TVS I suggested. It's guaranteed to have less than > 85V across it with 18A current, so it will clamp the voltage nicely > for a TIP122. With that option, more of the energy is dissipated in > the TVS, and the magnetic field collapses more quickly so the solenoid > drops out faster. A higher voltage TVS (well within the ratings of the > transistor) will be even faster. > > The TVS also protects the transistor if there is a high voltage > transient on your power supply caused by something else in the > circuit. > > Sometimes it's possible to use a low power C-B zener and cause the > transistor > to turn on partially and dissipate the power in the junction, but > that's > much harder on the transistor and has other potential issues. > It's sometimes done in power ICs. Some MOSFETs are rated to safely > absorb a > certain amount of inductive energy through avalanche, but that > tends to be > a rather small amount of energy compared to what an 80W solenoid > stores. > > So, the diode or TVS would be the same whether you use a MOSFET or > darlington. > > At 50V (unregulated?) operating voltage and 1.6A there are not a > huge number > of logic-level MOSFETs with low Rds(on) and good voltage margin, so > you might > need to factor a 10V-12V gate drive circuit into the equation, but the > heatsink savings might be worth the extra cost and complexity. > >> Hope this helps, > > > Best regards, > > > Spehro Pefhany --"it's the network..." "The Journey is > the reward" > speff@interlog.com Info for manufacturers: http:// > www.trexon.com > Embedded software/hardware/analog Info for designers: http:// > www.speff.com > ->>Test equipment, parts OLED displys http://search.ebay.com/ > _W0QQsassZspeff > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist