Once you get past the basics of volts and amps and start to electronically
modify car functions like boost and air/fuel ratios, you’ll probably run into a
bunch of new terms. That’s especially the case if you’re using any of the
Silicon Chip high performance kits for cars, which give easy
do-it-yourself access to a level of modification never before possible. So what
sort of terms, then? Well, stuff like trip-points, hysteresis, duty cycle,
analog, digital, frequency...
These days it’s not enough just to have a vague idea of what these things
mean – now you’ll be making modification decisions based on your
In this series we’ll be looking at what all these words mean, not just in
textbook terms but also in practical performance outcomes. Yep, soon you’ll be
able to say: "I reckon a variable frequency, variable duty cycle control
approach is better in this application that a fixed frequency, variable duty
cycle – and far better than a set-point-based on/off system"... and not only know
what it means, but understand the implications for (say) controlling a water
injection system. Or an extra injector...or an intercooler fan...or an auto trans
line pressure control solenoid.
It’s absolutely vital knowledge if you want to get the best results at the
This week we’ll take a detailed look at systems that switch on and off.
Sounds simple – it is. But there are also some hidden tricks in the best configuration of such a system.
The Silicon Chip magazine projects referred to in this story are all
low-cost electronic kits that can be constructed by the do-it-youselfer. They
include frequency, voltage and temperature electronic switches. These can be
tripped on the basis of engine revs, road speed, engine load, oil temperature,
oil pressure, intake air temp, boost pressure, etc. See AutoSpeed Shop
One of the simplest control engineering systems is one that switches on when
a measured parameter exceeds a certain level, and switches off when that
parameter drops below that level. An example is a boost pressure switch – it can
be set so the contacts close above 11 psi boost. (A use is to operate a water
pump than runs an intercooler water spray.) In this case, the set-point
is 11 psi. (The set-point is sometimes also called the ‘tripping point’.)
The set-point is the point at which the switch changes state.
If the switch is open until the set-point is exceeded, it’s called a
normally open switch. If the switch is closed until the set-point is
exceeded, it’s called a normally closed switch.
Therefore, a pressure switch which is described as a "normally open, 10 – 15
psi adjustable set-point" will close when the set pressure is reached, which can
be adjusted to being within the range of 10-15 psi. A temperature switch might
be described as being "normally closed, 85 degree C set-point" which means it
opens when the temp reaches 85 degrees C.
But these descriptions have something missing. And it’s bloody important. If
the normally-open pressure switch has a set-point of 11 psi, it closes at 11
psi. But as the pressure is dropping, when does it open again? If you said: "11
psi", you’re wrong. How can it be shut and open at the same pressure? Short
answer is that it can’t.
Instead, the pressure has to drop a certain amount below 11 psi before the
switch opens. If the pressure has to drop to 10 psi before the switch opens, the
difference between the switch-on pressure (11 psi) and the switch-off pressure
(10 psi) is 1 psi. This difference is called the switch
Hysteresis is the difference in value between the switch-on/switch-off
This graph shows it in visual form. The graph line shows the change in
pressure, temperature, etc. When the line is brown, the switch is turned off;
when it is green, the switch is turned on. As can be seen, once the measured
parameter exceeds the set-point, the switch turns on. It stays on until the
parameter drops sufficiently. The difference between the two levels is the
Most mechanical switches have a fixed hysteresis. In the case of a boost
pressure switch, it might be 1 psi, while in the case of a temperature switch,
it might be 5 degrees C. All of the Silicon Chip high performance car
electronic kits have adjustable hysteresis. Why? Because hysteresis is
absolutely critical to getting good on-car results.
Let’s keep working with the boost pressure switch. For this example it
doesn’t matter whether you’re using a basic mechanical switch or instead an
electronic pressure sensor working with the Simple Voltage Switch
kit. But let’s say the hysteresis is small. How small? Well, try 0.5 psi.
The pressure switch is running an intercooler water spray.
So we can really clearly see the action of the switch, we’ve also wired a
pilot light into the circuit. When the switch is closed, the pump is running and
the pilot light is on. When the switch is open, the pump and light are off. The
switch trip-point is set to 5 psi. In other words, it’s a system found in a heap
of modified cars.
You jump into the car, warm-up the engine and then nail it in first gear. The
boost rockets up, the switch triggers and on comes the dashboard light and the
intercooler spray pump. You back-off just as sharply, and both switch off
crisply. Fine - no problems.
But then you climb a long hill in fourth gear. This time, manifold pressure
varies only very slowly – it’s the kind of hill where you’re only j-u-s-t coming
onto boost. In fact, the boost level is hovering around 4 psi – sometimes a bit
above, sometimes a bit below. And this time, the switch chatters like a mad
thing – you can clearly see the dashboard pilot light flickering. The switch is
turning on and off maybe five times a second – and so the poor pump is also
trying to do the same. Because pumps take a big gulp of current at switch-on,
there’ll also be electrical arcing occurring across the switch (or relay)
Just this situation occurs with nearly all traditional mechanical boost
switches – those that don’t use a high hysteresis ‘snap’ action.
On the other hand, if hysteresis is set at 2 psi - so the switch turns on at
5 psi and turns off at 3 psi – the switch-over will be ‘clean’ with no
Therefore, an on/off system with too low a hysteresis will chatter
around the set-point.
So that means you always want lots of hysteresis, then? Nope. Let’s look at
another example, this time with a temperature switch.
You’re running a cooling fan on a car sound amplifier. The set-point is 45
degrees C and the hysteresis is pretty big at 20 degrees C. (So the temp needs
to drop to 25 degrees C before the amp switches off.) The amp reaches 45 degrees
and the cooling fan switches on – but then it pretty well always stays on
because the temp has to drop a massive 20 degrees C before it’ll switch off.
This diagram shows what happens with large hysteresis. The brown line
indicates when the fan will be off and the green line when it will be on. As can
be seen, as the temperature rises above the set-point the fan comes on – but
then stays on, even though the temperature has dropped well below the
With a much smaller hysteresis, the fan switches off once the temperature
starts to fall, coming on again only as the temp again rises to pass the
set-point. The fan runs for much less time and the temperature is kept within
tighter limits. So in the case of the cooling fan working on the amplifier, you
would normally want small hysteresis.
We’ll come back to hysteresis in a minute, but first one more idea.
Low-to-High and High-to-Low
As we’ve already indicated, mechanical switches can be classed as normally
open or normally closed - they change from one state to the other as the
trip-point value is met. But with electronic systems, you can configure the
"switch" either way around. In other words, you can have a system that is
either normally open or normally closed.
To make this easier to understand, the changeover action is normally
described as Low-to-High, or High-to-Low.
In a Low-to-High system, the switch clicks over when the rising value
reaches the set-point. For example, an alarm might sound when the engine oil
temperature reaches 120 degrees C. The switch is set for a Low-to-High
transition and clicks over once the temp reaches 120 degrees C.
In a High-to-Low system, the switch is looking for a decreasing value.
For example, you might want to switch on an intercooler fan when the road speed
drops below 5 km/h. The Frequency Switch kit can be used to
do this by monitoring the output of the road speed sensor. As the speed drops to
5 km/h, the fan switches on.
Selecting the Right Hysteresis
Let’s go back to hysteresis, because it’s a vital part of system set-up.
In a system with a variable hysteresis, the selection of the ‘right’ amount
of hysteresis depends on what you are trying to achieve – and sometimes that’s
not always immediately obvious.
Let’s again take the above example of an intercooler fan.
Using the Frequency Switch kit configured in High-to-Low mode, it switches on
the fan when the road speed drops below 5 km/h. Fine – the car slows and the fan
comes on, forcing air through the ‘cooler and so keeping the intake temps low
when you boot it again. Typically, the fan will start operating as you slow to a
complete stop – at traffic lights, for example.
Now what hysteresis do you set the system to have? If you set it for 5 km/h,
the fan will switch off when the car increases in speed to 10 km/h. There won’t
be any chattering around the switching point and so things are cool. But is
there an advantage in setting the hysteresis to a much higher value? What about
having a hysteresis of 30 km/h, for example? If that was set, the car would need
to reach 35 km/h before the fan switched off. So what’s the point of that? Well,
in slow-moving traffic, where typically you come to a halt first, the fan will
stay running as you trickle along. It’ll also help force cold air through the
intercooler for much longer as you accelerate away from a standstill, a time
when you’re much more likely than normal to be on boost.
Practical on-road testing has shown that having a high hysteresis in this
situation works very well.
Another example of where high hysteresis is a positive is in a system that
triggers a warning light. You might have the Silicon Chip electronic
Temperature Switch working a warning on the dashboard, say to indicate a gearbox
oil temp above 120 degrees C. If you set the hysteresis to be only 2 degrees C
and the temperature moves around a fair bit, it will be easy to miss the warning
because it might stay on for only a short time. However, if you set a hysteresis
of 20 degrees C, the warning will stay on until the oil temp drops back to 100
degrees C – which should give you plenty of time to notice it.
But in some situations a low hysteresis is a must. Say you have the Frequency
Switch triggering a high intensity LED shift-light to tell you when to change
gears. The setpoint is 7000 rpm – but what amount of hysteresis is wanted? The
answer is very little – perhaps only 100 rpm. That way the light goes off
quickly if you don’t shift gears, but instead just let the revs drop a bit. If
you’re driving around a corner on the throttle and holding the engine near the
redline, you can take advantage of this warning characteristic. But imagine if
the Frequency Switch was set to have 1000 rpm of hysteresis.....
In short, using the right hysteresis in each situation has a major impact on
how well the system works. Start to see the benefit of having adjustable
Here are some key questions to answer when thinking about any on/off
switching control system:
- What is the required set-point?
- Do I need Low-to-High or High-to-Low switching?
- What hysteresis will best suit the application?
Next week: variable duty cycle digital signals – they’re the ones that run
the injectors and turbo boost control solenoids
Porsche Rear Spoilers
A number of Porsche models have used active rear spoilers. These are
triggered by road speed, using a Low-to-High system that has a lot of
hysteresis. For example, one 911 model’s rear spoiler rises at 80 km/h and
doesn’t drop down again until the speed has decreased to 25 km/h. Think about
what would happen if the system had very small hysteresis....