chris@zorin.UUCP (Christopher Nielsen) (09/14/90)
First, I want to thank everyone for their help. Here are the reponses I received, and a revised circuit I intend to build, (Thanks, Mr. Koch!)... >> From: uunet!mlb.semi.harris.com!jws (James W. Swonger) >> >> If you're going to all the trouble of putting discrete components >> together, why not use a good 'ole 555 instead? They're tough and can >> be set up for monostable (one-shot) operation. They also are cheaper than >> most transistors. Mr. Swonger, almost the whole circuit is for driving the relay. Only the first capacitor, and resistor are for the timing. By adding a 555, I would mabey need to add another resistor, and still need the power transistors... >> From: grege@gold.GVG.TEK.COM (Greg Ebert) >> >> You are using a common-collector configuration. The load is at the emitter, >> and is 3 base-emitter voltage drops below your input voltage, which is >> at most 5 volts (TTL). Thus, the maximum voltage across the relay is about >> 3 volts. Note: the 2N2222 causes 1 VBE drop, the TIP120 causes 2 VBE drops >> because it's a darlington. I'll bet you don't even need the 2N2222 because >> the DC beta of a darlington is very high, maybe 5000 to 10000. >> >> Your circuit will work correctly if you place the relay between +12 volts, >> and the collector of the TIP120, then ground the emitter of the TIP120. It >> takes an additional splice, but it will work. >> >> You will have to clamp the inductive load from the relay by connecting diode >> directly across the relay, with the CATHODE going to the + 12V supply. >> >> Caveat: You are running on an automotive electrical system which is known to >> have beastly voltage transients!! >> Mr. Ebert, Thanks for your insight.. A couple of points... I am aware that my circuit has a good chance of working if I ran the relay directly to +12, but I would rather leave it as it is. The reason is for safety (less +12 running around), I can put it back to original if I want to easier, and because I also want to connect things like parking lights, which would need the same configuration (since they are also grounded). I forgot to mention, that there is already a diode connected inside the relays, according to the schematic, as you say. As for the spikes, and all that... haven't had a problem yet! The Radio Shack model 100 has a very forgiving power supply that I run off another +6 regulator. Even when I drained the battery, and proceeded to make a very hostile environment with attempting to start the car (nearly a short, I would guess at that level) didn't phase it. I has an internal NiCd to keep it's memory. >> From: ico.isc.com!uunet!motcid!ivory!koch@mps.ohio-state.edu (Clifton Koch) >> >> Try this: Connect the emitter of the 2n2222 to ground and the collector >> to the 100 ohm resistor (remove the 1K resistor). Replace the TIP 120 with >> a PNP power transistor with the emmiter going to +12 and the collector to >> the relay. Insert a resistor (try about 10K) between the 47uf cap and the >> base of the 2n2222. >> >> What this will do: When the +5 signal is applied to the cap, the other side >> will go to +5 and decay from the current going through the 100K and the 1K [gone now ^^] >> and base of the transistor. The current through the 10K will turn the 2n2222 >> on which will allow current to flow from the PNP base turning that transistor >> on, energising the relay. When the 47uf decays to ~.7v the NPN transistor >> will shut off, shutting off the PNP transistor, shutting off the relay. >> >> I doubt that you'll need a darlington transistor, but it doesn't matter much. >> You'll probably have to play with the '10K' value to get the time constant >> you're looking for (or the cap value). Remember to put a supression diode >> across the coil (1n4004 is a good choice) or your output transistor won't >> last long and probably will burn out the door solenoids. >> >> I've used this circuit or something similar for quite a few automotive >> projects with no problems. >> >> Cliff Koch Thanks again Mr. Koch! Now, following Mr. Koch's recommendations, here is the modified circuit I will test when I get my next chance... I think I will use a TIP125. On the card the TIP120 came on, they say that this is it's complement. I know this may be overkill, but since I don't have proper equipment to check on such things, I'd rather be safe. One question... Is the 100 ohm resistor all I need between the 2N2222, and the TIP125? I seems to me that I might need a resistor here to +5 to supply current to the TIP125, when the 2N2222 is not grounding it. I may be wrong! Also, I may also want to use this circuit to power my parking lights... I assume that replacing the relay with about 5 incandescent lamps will work just as well... _____ ,-----. 47uF / \/ C | +| | B | | / | | >--| |---.--\/\/\/--|-|< | | |-------| E | | > 10K | | \| |E | B | |--- +12 existing relay < \_____/\__. `-/\/\/\/---| | ______ / > 2N2222 | 100 | |__________nnnnnn___ R1 < | |-------| C `---|<------| 100K > | TIP125 | | | (PNP darlington) ----- ------ ----- --- --- --- - - - >> From: uunet!motcid!violet!reichert (Charles H. Reicher) >> >> Try adding a series diode (1N4148, etc.) with the 47uf capacitor, >> cathode (Banded end, arrow pointing to 2N2222 Base) and a 1 to 10uf capacitor >> from 2N2222 Base to Ground, positive end at base. This will allow the base >> to hold a charge while the diode prevents discharge back thru the 47uf cap. >> I'm assuming you only want to activate the Relay a short time! >> >> 73's Chuck Reichert KD9JQ Thanks, Mr. Reichert. I may also try this good suggestion. Thanks again for everyone's help! Christopher A. 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