Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!emory!stiatl!john From: john@stiatl.UUCP (John DeArmond) Newsgroups: sci.electronics Subject: Re: How can I turn on this bulb with TTL? Keywords: NPN, TTL, bulb, 12 volts, bias Message-ID: <5769@stiatl.UUCP> Date: 13 Jul 89 23:54:20 GMT References: <4363@merlin.usc.edu> Reply-To: john@stiatl.UUCP (John DeArmond) Distribution: na Organization: Sales Technologies Inc., "The Procedure IS the product" Lines: 100 In article <4363@merlin.usc.edu> cyamamot@nunki.usc.edu (Cliff Yamamoto) writes: >Greetings, > I know this is a simple circuit, but I can't seem to get the bias >right or something. The circuit is as follows : > > C----bulb---> + 12 volts >TTL---1 Kohm----B NPN transistor > E---+ > | > GND > >The problem is when the TTL is high, the bulb barely lights up. This circuit >is for automotive use so the bulb is just an instrument panel bulb (not a high >powered spot light). Isn't 2N2222 or 2N3904 good enough for this? How does >one go about selecting a transistor for this? And lastly, how does one figure >out the bias needed to fully turn it on? > Your ciruit would work if the load was not a bulb. The problem is the characteristics of a tungsten bulb. The resistance of tungsten varys widely between ambient and operating temperature. The net effect is that the bulb draws much more current cold than after it is lit. While your dashboard bulb may only be a 6 watt bulb (indicating about 500 ma), it will probably momentarily draw 10 amps or so when you apply power. Normally this is not a problem. Most circuits have enough reserve, either from adequate capacity, stray capacitance or inductance to provide this boost. The problem arises when you drive a bulb with a transistor. It helps to view a transistor as a variable constant current regulator. The current passed from collector to emitter is related to the current flowing from the base to emitter. This relationship is roughly the beta or gain of the transistor. This relationship implies that the collector-emitter current flow is independent of the applied voltage. This, in practice is roughly true. So a 2n2222 biased via the base resistor to handle the aforementioned 500 ma could not supply enough inrush current to cause the initial heating of the filiment. Even driven hard, a small signal transistor probably cannot handle the initial or inrush current due to collector saturation. In your circuit, a 1k resistor with 4.3 volts (5 volts - .7 volt base-emitter drop) passes 4.3 MA. This translates into a collector current of 430 ma, assuming a beta of 100. In other words, not nearly enough. A more insidious problem exists in trying to drive this device from TTL. Assume a beta or current gain of 100. That means that 1 ma of base current causes 100 ma of collector current to flow. For a collector current of 10 amps (inrush, remember), a base current of 100 ma would have to be supplied to the base. TTL can supply (hmm, my memory is foggy here) at most, about 10 ma or so. In any event, a TTL output is current-limited. Thus, even if your transistor had the collector current rating to handle the inrush, your TTL gate could not drive it. There are several solutions here. A darlington pair transistor effectively multiplies the gain of 2 transistors. A gain of 1000 or more is not uncommon. With this kind of gain, you can safely drive from TTL. There are incadescent lamp driver transistors available which combine a high current die in darlington configuration in a small package. This device has the necessary gain and can handle the momentary inrush current. Sorry, can't remember any part numbers off the top of my head. I do software here at work and hardware at home :-) A second alternative is the Sprague lamp driver ICs. These devices combine 6 high current drivers with the necessary TTL interface in one 16 pin IC. I believe the number is UNL1007 but I'm not sure. This is a very slick device. Contact sprague for a catalog. The last alternative is to use a power FET. This is a transconductance device meaning that its drain current is proportional to its gate VOLTAGE. No current is drawn from the driver. TTL can drive these directly. Again, don't remember a part number off the top of my head but I do know you can get suitable devices from Radio Shack. For sizing the device, a good rule of thumb is to assume an inrush to operating current ratio of about 10:1. The ratio may be lower for rugged filaments such as headlights. If you want to actually measure the cold resistance, you must use a low current ohmmeter. No, your Simpson 260 won't cut it. The problem with VOMs is that the current from the meter will heat the filament. The temperature vs resistance curve for tungsten is fairly steep so no heating is tolerated. A DVM on the low current ranges (ranges designed not to turn a diode junction on) will do pretty well. I've played around with this a bit. I've measured the effect of self-heating by carefully breaking a bulb and measuring the resistance of the filament with a DVM while the filament is immersed in oil as a cooling media. The difference is not great but is measurable. Hope this helps.. John -- John De Armond, WD4OQC | Manual? ... What manual ?!? Sales Technologies, Inc. Atlanta, GA | This is Unix, My son, You ...!gatech!stiatl!john **I am the NRA** | just GOTTA Know!!!