AMAZING ARTICLES #25: DRIVING AUTOMOTIVE INJECTORS

 

COROLLARY THEOREMS: "reality is never what it appears to be."

LHFSD5
SC ED5RV07

Our previous article, A24, deals with a true, practical, engineering problem, only it is still . . . too general. We need to present you something specific, in order to prove that we are not just utopian idealists living in the realm of Science Fiction theorems.

Of course, we do not have to prove anything to anyone, because the readers who understand our theories do not need these minor and insignificant examples. However, if you look beyond the technical aspects presented here, you could see a few, major, social-psychology implications!

 


Particular to our days is the great desire for "dream jobs": the employees want new, better-paid jobs, and the employers want to hire the best trained employees.

GREEN LEAF RNow, when an employer buys the latest machinery, say BH345T--just a fake name--he also wants to hire someone having at least 10 years of working experience on that particular machine. Please note this: the mentioned machine is only a few months old! Unfortunately, such incidents happen quite often, lately.
 

 


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.

RED LEAFA few years ago we had to design an automotive Alternative Fuel Injectors Controller, and we decided to do it by-the-book; of course, that was, by the electronic hardware books. So, an automotive injector may draw a current between 1A to 4A--the ones we worked with required 4A. Particular to those injectors is, they need to open very fast, and they draw a lot of extra current while opening.

For example, a 4A injector takes about 16A to open. After 1ms (generally) injector's current falls back to 4A. Now, a good driver must supply all the 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 hardware implementation, because PWM was a dangerous source of EMI for the automotive environment. Consequently, we searched for a factory-built IC to do the job we wanted. There are many options available because driving automotive injectors is nothing new. The most tempting one was LM1949 IC built by National Semiconductor [we were told the LM1949 IC had been used in similar applications by Ford]. The NS recommendation of circuit implementation was something similar to the following:
 

LM1949








Injector driver circuit - approximation of manufacturer's recommendations


Everything was perfectly 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 difficult to procure that we gave up using it, and we modified the above schematic to:
 
LM1949-modified




Modified injector driver circuit

The 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), only a lot easier to procure and way cheaper. The new problem was, LM1949 IC required lower voltages, therefore we had to divide 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.

GREEN LEAF RBoth methods used 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 current, then the current drops 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, electrical wires are designed to handle, say, 20A 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 a coil/injector, or a simple motor) are 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 electrical 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 were we looking at. That is just beyond any reasonable logic!
 

Automotive injector driver







The maximum circuitry needed to drive automotive injectors

 

NOTE

This note was added later, because we received a 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 the 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.
GREEN LEAF L
Implicitly, any reduction in 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.

The "injector closing time" is handled orderly by the the following:

1. by a sudden drop in current, naturally;
2. by a very strong mechanical spring, carefully and specifically built by the manufacturer inside the injector to perform this job.

The flyback diode employed (which has nothing to do with either the PWM or the Peak-and-Hold circuits) is needed only to reduce/annihilate the induced EMF parasitic currents.

By the way, the injectors do not burnout due to high currents or due to high temperatures; they are specially built to work at very high temperatures (around 250 C) generated by the running engine block. We hope this helps.
 

 

 


RED LEAFATTENTION

The PWM method has a certain specific advantage, considering the "electrical primary circuit", which is of the "inductive" type due to injector's coil. It happens that an inductive circuit forces the current to "lag" the voltage. This means, there is a specific time delay until the current reaches its maximum. As a result, electrical engineers use to "chop" an AC sine wave, using PWM, in order to bring the current a bit "closer" to the voltage (in phase).

Naturally, for each PWM pulse, the current still lags the voltage in inductive circuits; however, the resultant current-vector (as the sum of very many small PWM pulses) is almost identical to the voltage-vector.

This is the reason PWM is so efficient: it "pulls" the current-vector closer to the voltage one. However, in automotive environments that PWM frequency is a real danger, therefore it requires expensive shielding.

[By the way, in case there is some confusion, an inductive DC PWM circuit behaves very much like an AC one; this means, there is some circuit (transitive) inductance/reactance to consider.]

 

 

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The new schematic works as follows. The ON/OFF command signal comes from the microcontroller on the "CONTROL" line as a +5V/0V digital signal, and the Darlington pair, TIP121, closes or opens the primary circuit accordingly. That is all. As you can see, in order to drive the injector, we need only one good transistor (of 0.25 USD), and one ordinary current limiting resistor Rc (of 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 the previous schematic is an expensive component--about 2 USD--and its main "qualities" are:

1. it is incredibly inefficient and unreliable;
2. it heats up a lot!

By using two (0.1 USD) ordinary diodes, the protection function is greatly improved and heat dissipation is at a minimum.

Well, this is the entire "mystery" about driving automotive injectors. Please experiment for yourself, because we suspect everybody could afford: 1 transistor, 1 resistor, + 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 1A-2A. Other transistors are way better suited for that, but you will have to discover them yourself--we worked with 2N6045 at 4A only for testing purposes. Again, other transistors are a lot better. Try to discover an 8A or 10A continuous DC transistor in a TO220x package.

RED LEAFIn 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 4A is 0.07 ohms. Please be very careful when dimensioning your electronic components, and never consider the schematic circuits have correct values for all possible situations. Always recheck and recalculate those values yourself.


NOTE

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 may 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 100ms without any heat sink.

Please, do not hold your transistor ON for more than 10ms until you have proper heat dissipation means in place. Once your transistor is well protected against heat, experiment gradually with increased ON times greater than 10ms.
 

 


You could do yourself a lot of good if you take a look at a few "professional" automotive injector drivers available on the marked. You are going to be stunned by how inventive people could be when there is absolutely no need or reason for that.

This example, and many others, come to motivate a few topics in our Amazing Articles. Incredibly, many still refer to them as being "Science Fiction"!

***

First published on August 04, 2005 
© SC COMPLEMENT CONTROL SRL. All rights reserved. 

 
 
 
 
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