Download BTM Issue 2 Transistors Part 5 Typical Circuits part 1

Typical circuits
AC to Pulse string converter.
If we have an AC signal and want to convert it to a string of pulses that go from
ground to VCC we need only a simple circuit.
Resistor R2 limits the current to a safe value for the Base. Since our Emitter –
Base acts like a diode we need to protect this junction when it is reverse biased or V IN
may exceed the maximum E-B voltage. D1 provides this protection. When the E-B
junction is reverse biased D1 is forward biased so the input of the transistor will go
between +600 mV and –600 mV.
Positive incoming signals turn
the transistor on and the output at the
Collector goes to ground. We select R2
to determine the wave form we want
out. A low value of R2 puts the
transistor into saturation early in the
wave form and out output will be a
square wave as shown. A high value of
R2 would give us a rounded wave form as the transistor spends more time between fully
on and fully off.
We have signal inversion. As the input signal goes positive our output signal goes
low. As the input signal goes negative the transistor turns off and the output goes toward
VCC.
Comments on drawings from gaming devices.
0106061101
We have a typical example of using an NMOSFET to drive some lamps in a Bill Acceptor
Bezel, for example. A positive voltage on the gate
turns on the MOSFET. The top example shows the
board as it is. We blow out a lot of lamps on these
boards. I might suggest a modification as shown in
the lower example.
When incandescent lamps are cold they have
a low resistance. When we first apply power we get
a surge of current. This is when the lamps blow out.
If we insert a warming resistor as shown in the
lower example the lamps keep current flowing
through them to keep them warm and the resistance
high. We avoid the initial current surge when the
lamps are turned on.
An even better idea is to use LEDs. LEDs
seldom burn out. Usually LED failures can be traced
to bad soldering joints not bad LEDs themselves.
Later drawings will demonstrate his.
0106061102
Often we need more current that can be
supplied by one transistor. To get improved
current capability we parallel two transistors. One
disadvantage of just wiring them straight together
shows up when we realize that no two transistors
are exactly the same. In this case the one with
higher conduction or lower saturation voltage will
take a larger share of the current, shortening its
life. When one transistor fails the remaining one
takes an extra strain and it fails soon also.
To minimize the difference between
devices you can insert a small resistor in series
with each transistor, as shown. The value of the
resistor should be about 10% of the transistors
resistance to minimize the difference in the
transistors themselves.
0106061203
Transient Overvoltage protectors can be made to the
voltage of your desire by using a transistor to beef up the current
capacity of a Zener diode. By tweaking the value of the resistor
you can control the Zener point of the diode, thus the precise
voltage the circuit will trigger at.
0106061204
Here is a typical
example of how to get a
quick start on stepper
reels. With no signal in
R3 pulls the Gate of Q1
to ground keeping Q1
off. With Q1 off Q2’s
Gate is at the same
potential as its Source
and Q2 is off. Under
these conditions we have
the +13 Volt line
powering the Stepper motor that drives the reels.
When the MPU starts to turn the reels it sends a positive voltage at the input, “V
IN”. The influence of C1 is to pass only a small pulse at the rising edge on V IN. Q1
pulses on turning Q2 on for a short while. When Q2 pulses on it feeds a short burst of
+25 Volts to power the Stepper motor, giving it a burst of high power and the reel starts
with a quick burst of energy.
The nature of stepper motors creates a lot of spikes on the power line and you will
find a lot of failures on diodes in the place of D1 as shown in this schematic.
0106061205 and 0106061206
A typical example of a Common
Base using the transistor as a variable
resistor.
These drawings show a simple
example of roll-your-own Voltage
Regulators suitable for supplying a low
current at the voltage of your choice. Just
change the voltage rating of the Zener to get the voltage you want out. Select a value
about 600 mV higher than the voltage you desire out to account for the loss of voltage
across the Emitter-Base of the transistor.
Tweaking the value of the resistor will allow
making minor adjustments in the output voltage to get it precisely where you want.
In designing these follow the basic design guidelines for a standard Zener diode.
The Base current is the regulated current. The current through the Zener should be five to
ten times that value for best regulation. The current through R1 is the total of Base
current plus Zener diode current. Base current is calculated as Collector (or Emitter)
current divided by the gain of the transistor.
If the transistor has a gain of 50 @ 100 mA we can expect a Base current of
around 2 mA. Zener current should be five to ten times that (10 mA to 20 mA). The
current through R1 will be around 12 mA to 22 mA.
The wattage of the Zener should be watched. In this case 20 mA times 5.6 Volts
comes to around 100 mW. Well within the capability of even 250 mW devices.
Voltage of the Zener should be set at the desired output voltage plus the E-B
voltage of the transistor. A 5.6 Volt Zener gives us a 5.0 Volt output.
The wattage of the transistor can be estimated as the Collector current times the
difference between V IN and V OUT. Three volts difference at 100 mA gives us a
dissipation of 300 mW. Well within the capability of a TO-92 case but maybe not an
SOT-23.
These designs have no current limiting and I wouldn’t use them for higher
currents. But with small changes the design can be used for higher currents by using
higher power transistors.
0106061208 and 0106061209
If you do go to higher
currents and want to add current
limiting to either of the above
circuits it only requires adding
two components as shown here.
As increased current flows in the
circuit the voltage across R2
increases. When this voltage gets
to around 600 mV Q2 starts
conducting shorting out the
Emitter-Base of Q1. Attempt to
draw a higher current turns off
Q1, drawing the output voltage
down and keeping the current at
the value set by R2. We control
this current level by selecting the value of R2. 600 mV divided by the value of R2 gives
us the current limit level.
This is a Constant Current circuit used in a Current Limit application and it works
but has its limitations. It can blow up under extreme over-current conditions.
The designs shown will be used in a later exercise to demonstrate how the
regulators behave. To keep the smoke and flames down we have limited the current to a
modest 60 mA, but the design can be easily modified for operation up around 1 Amp
using higher power devices.
One problem with the previous current regulator schemes is that the output
current stays at the limiting level while output voltage drops. As output voltage drops
voltage across the voltage regulating transistor increases. The high current level and
increasing voltage of the transistor can exceed the power capability of the voltage
regulating transistor. As the voltage across R2 increases the limits of Q2 are also taxed.
An over current condition for a long period can destroy both Q1 and Q2 if the circuit is
not designed properly.
0106061210
With a minor change to
the circuit we add “Fold Back”
current limiting and our circuit
becomes more a Current Limit
circuit instead of a Constant
Current. As we draw current
beyond set limit the voltage
across R2 increases driving the
output voltage down faster.
This protects both
transistors to some degree.
Driving an LED
0106061211, 0106061212, 0106061213 and 0106061214
A few simple circuits to drive an LED from a
TTL or CMOS circuit without killing the capability to
drive other logic at the same time.
0106061211 and 0106061212 are good examples
of driving a TTL logic output. A TTL output can drive
about 400 A for a High out (3.5 V or greater) and
around 16 mA on a Low output (close to ground).
Looking at 0106061211. Design considerations
are simple. Referencing the data sheet for the LED we
find that it needs 10 mA to 30 mA, and drops around 1.6
Volts in that current range. We have made up these statistics for the sake of the exercise.
When the transistor is fully on we will drop about a few tenths of a Volts between
Collector and Emitter. Again we reference a data sheet that is only typical for the purpose
of the exercise. We find the transistor has a gain of 50 at the 10 mA we want to flow
through the LED. So across the LED and Q1 E-C we will drop around 1.8 Volts. The rest
of the voltage will have to drop across R2. With a VCC of 5 Volts we find the voltage
across R2 is 5 minus 1.8 or 3.2 Volts. With 10 mA flowing in the circuit R2 must have to
be 3.2 V divided by 10 mA, or 320 Ohms. 330 Ohms is a close enough standard value.
We are not talking precision here. The LED can work fine over a range of 5 mA to
perhaps 50 mA so we can save power by driving the LED at 5 mA, or make it brighter by
driving it at 50 mA. All we have to do to change this is select the value of R2 to change
the current to the level we want.
If the transistor has a gain of 50 at this 10 mA we can expect a Base current of 10
mA divided by 50, or around 200 A. This is well within
the capability of a TTL device. When the output goes High
the LED turns on. If we want the LED to come on when the
output goes Low we can use a PNP transistor. The value of
resistor R1 is again simple Ohm’s Law. We will drop about
600 mV across the Emitter – Base of Q1. The output of the
IC is around 3.5 Volts. The difference must be dropped
across R1. At 200 A this comes to around 10K Ohms.
Again we do not have to be precise.
Design of Collector circuit is the same as before.
Design of the Base circuit has the same concerns with the
exception of R3. Since the TTL output only goes up to 3.5 Volts the Base of Q1 will still
be forward biased even when the output of the logic gate is High. To assure Q1 turns off
we add R3 to pull the Base all the way up to VCC and turn the transistor off.
To drive CMOS outputs we have to consider the
limitations of CMOS devices. CMOS logic (4xx series
devices) can only drive about 1 mA High or Low. We need
to draw as little current as possible from the circuit that
drives the transistor that drives the LED.
Here we have the benefit of MOSFET transistors.
N-MOSFETs that are Enhancement Mode devices work
similar to an NPN transistor in that we apply a positive
voltage to turn on the transistor. FETs are Voltage operated
devices where NPNs are current operated devices. To turn
on an N-MOSFET we need to apply a positive voltage.
Since the Gate is isolated from the junction we draw almost
no current from the circuit. Design of the Drain circuit is
much like the Collector circuit described earlier. Design of
the Gate circuit need only make a connection to the voltage
that drives the circuit. R1 can be anything in the range of
100 Ohms to 10 M Ohms. R3 turns off Q1 when no voltage
is applied just as R3 did in the previous circuits. R3 is
almost optional.
The P-MOSFET is typically connected with the
Source connected to VCC and the output connected to the
Drain. This is much like the PNP device described earlier.
A Low in turns on the P-MOSFET.
0106061215
Here is a typical circuit extracted from a coin comparator schematic. The input to
our circuit comes from our Sense coil. We have a signal coming in here as long as the
reference coin does not match the incoming coin slot. As a good coin passes through the
slot we get a Null signal. Here is how we turn that Null signal into a pulse.
While we are unbalanced the signal we just mentioned gives us a signal at the
output of U1, pin 7. This is the point we usually monitor when looking for the Null. As
long as we get a signal here we feed this signal through C7. D4, D5 and C9 form a
Voltage Doubler that builds up a charge on C9.
U1, pin 7, also feed to Q1 through C8 and keeps turning Q1 on at a high audio
rate. As long as Q1 keeps turning on it keeps discharging Q10 and the output of our
circuit remains a low voltage value.
When our coins compare we get the Null out of U1, pin 7. Q1 turns off. C9 dumps
its charge into C10 through R12. This gives us a positive pulse at C10 equal to C9 times
R12 seconds. In this case about 27 ms. We will only get this pulse if we have a short time
between coins and have been allowed to build up the charge across C9. We have to get
another imbalance condition, generate more signal and rebuild the charge on C9 before
we can generate another pulse.
If we were running U1 from a +V and –V power we would return the bottom side
of R10 to ground and our signal would be centered around ground. Since we are
powering U1 from only a positive power rail our incoming signal will be centered around
+6 Volts. We generate a Virtual Ground level by returning R10 to a +6 Volt Source.
We adjust the sensitivity of this circuit using R13, R14 and pot P1. This sets the
level where Q1 will turn off.
Component reference numbers are the same as those you will find in most Coin
Mechanism CC-40 schematics if you want to relate this drawing to the actual equipment.
0106061216
This shows the typical schematic we might find in the same CC-40. It is a good
classical example of using Zener Diodes as Voltage Regulators. The incoming voltage
may be AC or DC. The voltage level depends on the connector pin we bring in the
voltage on. Pin 4 for something in the 24 V range. Pin 5 for something in the 12 to 15
Volt range.
If we apply AC it is rectified to DC by diode D1 and builds up a charge on the
reservoir capacitor C1. The circuit runs off the charge across C1. If we put in DC D1 is
unimportant. So our circuit will run on AC or DC. R35 and ZD20 bring this voltage down
to +8 Volts to power U2 (Coin Direction Sensing Logic). C1 also feeds R5 and ZD2
which drops us down to +12 Volts to power U1, the op amp shown in the previous
schematic. ZD3 drops this +12 Volts down to +6 Volts for our Virtual Ground just
discussed. When troubleshooting CC-40’s these circuits are a good place to start.
The LED also gets its power from the charge on C1, but the LED should not be on
all the time. When we generate an Accept pulse this LED should blink off for an instant.
0106061217
Yet another extract from a CC-40, here is a typical circuit we would find in Coin
Mech circuits to take our 27 ms pulse we described a few paragraphs ago and generate an
Accept Pulse.
Normally Q2 is held turned off by R15. When we get our 27 ms pulse Q2 is
pulsed on. When Q2 turns on it turns on Q3 which gives us a strong positive pulse out to
drive our Accept Coil as long as we are not Inhibited. R4, D22 and C12 feel this strong
positive surge and feeds back a pulse to the Base of Q2 to keep it turned on. This circuit
gives us the length of our Accept Pulse. The 27 ms pulse is just a trigger.
Our Inhibit input comes from the MPU and must be a low voltage before the
Accept coil will actually trigger. We still generate our Accept pulse even if we are
inhibited, but the Accept Coil will not pull out and our coin will be rejected.
0106061218
CC-37’s have a small modification to the circuit we just described. We do not
have the Inhibit circuit. The MPU controls the Coin Comparator by controlling power to
the coin comparator. Our Accept pulse is generated in the same way. The Accept coils is
driven by an N-MOSFET transistor. C13 and R41 act to hold the charge from the Q3
pulse a little longer to allow for larger coins.
0106061219
An now for something different... A part of the brightness circuit from a popular
monitor. When power first comes on we have no charge across C870 and it takes a few
seconds to charge up. While C870 is discharged Q807 is held off which keeps Q808 off.
As Q870 charges up we allow the common point of R806 and R807 to develop the signal
from our Auto Brightness Circuit. This voltage is passed to Q807 and Q808 to become
our Sub-Brightness signal to the monitor. R870, R871 and R872 is our adjustment circuit
for brightness on the main board. R823 sums this voltage with the Auto Brightness
voltage to allow us to tweak the brightness level.
If C870 is bad we kill our Auto Brightness signal and our monitor appears dead
even though all our voltages and HV are present!
0106022509
Here we have a simple circuit to monitor an RS-232 signal line and tell us we
have a valid +V and –V levels on the line. The input must go to + and – 3 Volts before
the LEDs will come on.