Strange Facts and Amazing Articles
A25: Driving Automotive Injectors
COROLLARY THEOREMS: Mathematics is built on two basic operations only
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 The
previous article A24 deals with a true, practical problem, but it is still ... too general. We need to present you
something more specific, in order to prove we are not utopian idealists living in the realm of pure and unfounded
theorems (or SF). Sure, we do not have to prove anything to anyone, because those few who do understand our
theories do not need these minor and insignificant examples. However, if you look beyond technical aspects
presented here, you could see few, major, social-psychology implications.Particular to our days is the great desire for "dream jobs": the employees want new or better jobs, and the employers want to hire the best trained employees. Now, if an employer buys the latest machinery, say BH345T--just a fake name--he also wants to hire someone having 10 years working experience on that particular machine. Please note: the mentioned machine is only few months old! Anyway, while working with hardware and firmware we noticed an incredible fact: electronic engineers know and use electricity differently than electrical engineers do! We hope our words sound sufficiently absurd to stir up your curiosity. Things are this way. Few years ago we had to design an Alternative Fuel Automotive Injectors controller, and we decided to do it by-the-book; of course, that was, the electronic hardware books. So, an automotive injector may draw current between 1 A to 4 A--the ones we worked with required 4A. Particular to those injectors is, they need to open very fast, and they draw a lot of current while opening. For example, a 4 A injector takes about 16 A to open. After 1 ms (generally) the injector's current falls back to 4 A. Now, a good driver must supply all current the injector needs in order to open it as fast as possible. The total open injector time is a variable ranging between 2.5 ms to 30 ms. That particular current variation, when the injector is opened, is handled by professional hardware designers in two modes: 1. with PWM 2. with the Peak-and-Hold circuitry We decided on the Peak-and-Hold method because PWM was a dangerous source of EMI for the automotive environment. In consequence, we searched for a factory-built IC to do the job. There are many options available, because driving automotive injectors is nothing new. The most tempting was LM1949 IC built by National Semiconductor (we were told it had also been used sometime before by Ford). The NS recommendation of the circuit implementation was something similar to the following: |
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Injector
driver circuit - approximation of manufacturer's recommendation |
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| Everything
was fine, and the only problem we had was R1 (0.05 ohms, 5 W). It was a "sense resistor", both expensive
and very difficult to find. In fact R1 was so hard to procure that we gave up on using it, and we modified the
above schematic to: |
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Modified
injector driver circuit |
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problem with R1, the sense resistor, is mentioned in LEARN HARDWARE FIRMWARE AND SOFTWARE DESIGN. What we did was,
we used a lot higher value for R1 (1 Ω, 5 W) but a lot easier to procure and way cheaper. The new problem
was, LM1949 required lower voltages, therefore we divided the voltage developed on R1 with the help of a
programmable potentiometer MCP41010. As it is mentioned in LHFSD, this circuit worked incredibly well. Even more, by using a programmable potentiometer, MCP41010, the above schematic is able to handle a wide range of primary currents. Implicitly, it also handles various injector types without modifications on the PCB. We built the first version of the controller, and then we tested it: it worked perfectly well. However, the interesting aspect was, we understood that electronic engineers handle electricity differently than electrical engineers do. Here is why. Both methods to control Ip the primary current (this is PWM and Peak-and-Hold) are able to only REDUCE the primary current. In the injectors driver circuit case any reduction of the primary current is not wanted/needed! In fact, the electronic designers of the LM1949 IC try to follow injector's natural current curve with their IC. As for control, the only control they could implement with LM1949 is to reduce the primary current, and that is, again, not needed. To any electrical engineer things are very clear: the injector will draw its peak value, then it will drop down to normal values BY ITSELF, and no electronic circuitry is needed! Please understand this: we need no PWM, and no Peak-and-Hold circuits to drive the injector. To help you understand this, think of the electrical circuit used to wire the bulbs in your house. On one distribution line are connected a certain number of bulbs. Now, the electrical wires are designed to handle, say 20 A of current. When we switch the bulb to ON, it will draw 10..16 times more current than its nominal current for a short period of time, and that is named by electricians the "inrush current". Next, the inrush current drops by itself to the nominal value. All it takes to control that bulb (or coil, or a simple motor) is power lines to carry sufficient current, and a reliable switch. We do not need any PWM or Peak-and-Hold circuitry. Exactly the same thing happens with the automotive injector, and the entire process is just a basic electricity application. The primary circuit in all pictures above is an injector in series with a switch. Incredibly, the hardware designers manage to implement the most complex circuits possible. We have seen an injector "driver" built with "hardware logic": it had about 300 (three hundred) electronic components on a 4"x4" PCB area. It was so dense with surface mount components, on both sides, that it took us a long time to realize what we were looking at. That is just beyond any reasonable logic! NOTE This note was added later, because we received few objections from one reader. He said that by using PWM or Peak-and-Hold circuitry the electronic designers try to reduce the temperature developed on the switching transistor. That is not true. The transistor generates the minimum amount of temperature when it works in saturation mode. On the other hand, by using PWM or Peak-and-Hold we change in fact the switch (the transistor) with a resistor, as an equivalent circuit. That resistor will lower the primary current Ip, and that will result in a longer time for the Peak period. Implicitly, any reduction of the primary current will result in MORE temperature generated by the transistor switch. In addition, even the sense resistor is another source of heat; by eliminating it, we also get less dissipated heat on the PCB. We hope this helps. The ONLY schematic needed to drive any automotive injector is: |
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The
maximum circuitry needed to drive automotive injectors: |
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| The above
schematic works as follows. The ON/OF command signal comes from microcontroller on the "CONTROL" line as
+5 V or 0 V digital signal, and the Darlington pair, TIP121, will close or open the primary circuit accordingly.
That is all. As you can see, in order to drive the injector, we need only one good transistor (0.25 USD), and one
ordinary current limiting resistor Rc (0.01 USD)! The diodes D1 and D2 replace the "Flyback" Zener diode; this is another "issue" mentioned in LEARN HARDWARE FIRMWARE AND SOFTWARE DESIGN, and you can also see the simulation models for each case in our Diode page. Diode DZ in previous schematics is a very expensive component--about 2 USD--and its main "qualities" are: 1. it is incredibly inefficient 2. it heats a lot! By using two (0.1 USD) ordinary diodes, the protection function is greatly improved, and heat dissipation is minimum. Well, this is all "mystery" about driving injectors. Please experiment it for yourself, because we suspect everybody could afford: 1 transistor; 1 resistor; and 2 ordinary diodes. Use the best oscilloscopes you can find, and change the injector to any type you can get. The resulting conclusion is going to be that you do not need anything more than the above circuit to drive any automotive injector. ATTENTION Please be careful with TIP121, because it is not the best Darlington pair for all current ranges. TIP121 is manufacturer's recommendation for injectors working at 1 A .. 2A. Other transistors are a lot better suited for that, but you will have to discover them yourself--we worked with 2N6045 at 4 A only for testing purposes. Again, other transistors are a lot better. Try to discover an 8 A or 10 A continuous DC transistor in a TO220x package. In addition, the sense resistor in the above schematics needs to be carefully calculated. We used the value of 0.05 ohms for exemplification, only. The calculated value at 4 A is 0.07 ohms. Please be very careful when dimensioning your electronic components, and never consider schematic circuits as having correct values for all possible situations. Always check and recalculate the values yourself. Please be aware the transistor will heat up a lot: it could easily reach 250 Celsius degrees or even more. It could become so hot that it will unsolder itself from the PCB in an instant. It did happen to us, because we had a fault in the control circuit and one transistor remained ON for more than 100 ms without any heat sink. Please, do not hold your transistor ON for more than 10 ms until you have proper heat dissipation means in place. Once your transistor is well protected against heat, experiment gradually with increased ON times, starting with 10 ms. You could do yourself a lot of good if you take a look at few "professional" automotive injectors drivers available on the marked. You are going to be stunned of how "inventive" people could be, when there is absolutely no reason or need for that.  This example, and
many others, come to justify few topics in our articles. Incredibly, many still refer to them as being
"Science Fiction"!
*** First published on August 04, 2005 |
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