@ Frank: Are you talking about MOSFETs? They're essentially voltage-controlled transistors (as opposed to being current controlled). They can be had with an extremely low "on-resistance" which makes them ideal for very high current applications, since they dissipate much less heat than an equiv. BJT (if you can even find an equiv. BJT with the same current handling capability). Another useful feature is because they're voltage controlled, you can wire many of them in parallel to essentially make one huge MOSFET (this is how the open-source motor controller is able to handle several hundred amps of current draw and still stay cool) They certinally exist in through-hole packaging. You can check out Digikey, or Futurlec (where I get all of my components from). Here's a link: http://futurlec.com/TransMosIRF.shtml They can be a little pricey, but they have pretty impressive characteristics. LoL @ "new kids on the block"
@ Modena: I'm guessing that your problem has to do with the fact that you're trying to switch a 12v valve with a 555 timer that's running on 5v. You should place a small NPN transistor (like a PN2222 or similar) between the 555 and the valve. Connect your 12v source the collector and your 555 output to the base. Connect the valve to the emitter. Give that a try On Sat, Aug 22, 2009 at 5:26 AM, Modena <[email protected]> wrote: > > hrm, have I made an error of judgement in assuming that the SSR I > bought is in fact opto-isolated? > > > On Aug 22, 8:17 pm, Modena <[email protected]> wrote: > > Frank, so my basic understanding is that the opto-isolator you speak > > of is an SSR, if this is the case why do I need another relay in-line > > after the SSR? > > > > I tried today hooking up output to an automotive relay, which produced > > weird results, no doubt because the relay could not switch fast > > enough. Then I tried a 12v SSR, less weird, but it still stays "on" - > > again, reverting to just the LED and all was well. > > > > On Aug 22, 2:09 pm, Frank Pittelli <[email protected]> wrote: > > > > > > > > > Modena wrote: > > > > testing the circuit with an LED works fine, but when I hook up my > > > > actual 12v valve, it must be causing interference or something > because > > > > pressing fire and the circuit switches on, and stays on indefinitely, > > > > hence the valve stays open. > > > > > > anyone have any ideas how I might filter this? > > > > > You need to isolate the load from the logic. There are three basic > > > solutions: transistor, logic-level FET or opto-isolator/relay. > > > > > If the load isn't too big and you like ruining lots of circuitry trying > > > to make things reliable, a simple transistor can be used. There are > > > deceptive devices, however, and when you think you've got them working > > > properly, they bite you. There are dozens of circuits on the web, > which > > > tells you just how many people have tried to design the "perfect" > > > transistor switching circuit. > > > > > The new kids on the block are logic-level FETs (I think they have a > more > > > official name) have been designed to be fool-proof, allowing logic > > > circuits to directly drive a high current FET. They have all kinds of > > > safeguards built into them (designed to eliminate the need for all > those > > > fancy transistor circuits mentioned above), but I have yet to find one > > > with good old-fashioned solder leads ... they all seem to be tiny > little > > > surface mount packages. If you find one, let me know, because I'll use > > > them for everything. > > > > > The approach that I've used the most (along with Dr. Sommer) involves > an > > > opto-isolator IC and a relay. The logic triggers the opto-isolator > (5v, > > > very low current), the opto-isolator triggers the relay (usually 5v or > > > 12v, 100ma or so) and the relay triggers the load (any voltage, any > > > load). It sounds like a lot, but it's quite simple once you've wired > it > > > a couple times and it is one of the most reliable switching approaches. > > > > > Frank P. > > > -- ---------- Donald Shankin Computer Engineering Undergrad Blue Marble Security Member IEEE Vice Chair, MTU Chapter NSBE Telecommunications Chair, MTU Chapter CAEL Partner (507) 301-2499 [email protected] [email protected] ---------- --~--~---------~--~----~------------~-------~--~----~ You are currently subscribed to the "R/C Tank Combat" group. To post a message, send email to [email protected] To unsubscribe, send email to [email protected] Visit the group at http://groups.google.com/group/rctankcombat -~----------~----~----~----~------~----~------~--~---
