POOR & BAD CIRCUITS
With acknowledgments to
Horowitz and Hill: The Art of Electronics
for the presentation idea and some of
the presentation contents
POOR & BAD CIRCUITS
• Poor circuits – circuits which do not work very
well or are unreliable.
• Bad circuits – circuits which do not work at all,
or are downright dangerous
Simple transistor circuits – bad circuits A, B & C
A
B
C
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Simple transistor circuits – bad circuits – A
incorrect
correct
The incorrect version will severely distort the signal waveform because the transistor
will only conduct when the input signal is positive – effectively a class C amplifier
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Simple transistor circuits – bad circuits – B
incorrect
correct
correct
The answer to this one depends on what configuration you wish to use,
For a Common Emitter circuit, the incorrect version has the Base and Emitter swapped over.
It is unusual to use an Emitter follower circuit referenced to +Vcc, but if this is the
configuration you wish to use then you need a PNP transistor (not a NPN transistor)
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Simple transistor circuits – bad circuits – C
incorrect
correct
Note the dual polarity power supply arrangement. The single resistor providing the Base
bias is fine with this power supply system, but use a NPN transistor instead of a PNP type
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Simple transistor circuits – bad circuits D, E & F
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Simple transistor circuits – bad circuits E
incorrect
correct
The incorrect version has several problems – firstly the transistors are not acting
as Emitter Followers but as Common Emitters, so the signal will be inverted.
Then, unless the input signal amplitude is ‘from rail to rail’ both transistors will
always be ON, which causes high current through them and there is no
limitation with respect to Base current, so the junctions will be damaged.
In the correct version the transistors are acting as Emitter followers and this
circuit is OK for logic type signals. If linear signals are being buffered then some
forward bias is required for both transistors to prevent crossover distortion.
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Simple transistor circuits – bad circuits F
incorrect
correct
correct
For the incorrect version, there is nothing to limit the Base current into the output
transistor, so this will be destroyed.
There are two correct versions – the added resistors in the middle solution limit the
current into the output transistor and also ensure that it turns off when it should,
whilst the right hand solution is what is known as a Darlington configuration to achieve
the high current gain needed. Note the dotted resistor to ensure that it turns off.
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Simple circuits – poor circuit
Going on to simple power supply circuits, this arrangement is not very
good – any ideas why?
Simple circuits – rectification poor circuit discussion
This is what is known as a half wave rectification circuit.
This configuration was very popular with the old valve circuits, where the rectifier was
either a valve or a selenium rectifier, so why is it a poor circuit?
Several reasons –
The transformer is only providing power on the +Ve half cycles – magnetisation.
The ripple current is at the input frequency, which is more difficult to filter.
The current flow through the transformer secondary and the rectifier has a higher
peak value, so only suitable for very low load current applications
Diodes are now very cheap to buy, so full wave rectification is not expensive.
Simple circuits – rectification improvements
Full Wave Bridge configuration
Centre tapped transformer configuration
Both these circuits provide full wave rectification, where the rectification frequency is
twice the input frequency – a lower capacitance smoothing capacitor can now be used and
the ripple current is reduced by a factor of at least 2 from the ½ wave version.
The 4 diode full wave bridge is very common and is used in the majority of cases, but the 2
diode (centre tapped transformer) version is very useful at high input frequencies and at
high power (for instance in computer power supplies) because the rectification losses are
lower and the transformer secondary wires are thinner (each winding is only conveying ½
the average current), making the transformers easier to wind in a compact manner.
Simple circuits – bad circuit
The input mains supply is transformed down to a lower voltage.
This lower voltage is then full wave rectified, smoothed by a capacitor and
the resultant dc applied to the load.
Hint, look at the way the bridge rectifier is configured
Simple circuits – poor circuit
OK, we have got the bridge rectifier correctly configured now, but it is still
not a good circuit – just a poor circuit – why?
Hint, look at the way the circuit is drawn and think about where the
currents flow – this circuit is very common and often has the ‘mistake’
shown present in its construction.
Simple circuits – improvement to poor circuit
The dc supply voltage and current output from the bridge rectifier is pulsating at
twice the mains input frequency. The smoothing capacitor is present to smooth
out this pulsating voltage and current.
However, the wires connecting the output from the rectifier to the capacitor
have a resistance of their own, which causes a voltage drop due to the pulsating
current flow. Connecting the load directly across the smoothing capacitor results
in the minimum ripple on the output supply to the load.
Simple circuits – bad circuit
Here we have the ac input from the transformer full wave rectified and applied to
the smoothing capacitor in the correct manner, followed by regulation down to
5V via a Zener diode, so what is bad about this circuit?
Hint – think about how the Zener diode functions and what current will flow in it.
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Simple circuits – bad circuit discussed
The problem with this circuit is that the Zener diode will blow up!
Zener diodes function by conducting when the input supply to them is above
the Zener voltage. Since the 8Vrms secondary from the transformer will be
about 11V and its source impedance will be low, the Zener diode will be
conducting all of the time, taking whatever current the transformer will
provide, so it will get very hot and then its ‘magic smoke’ will escape. Since
the failure mode of Zener diodes is most usually short circuit, then the
rectifiers and transformers will also smoke!
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Zener diode regulator – practical circuits
Low power /current
solution. The series
resistor limits the
current flow into the
Zener diode, so the
Zener diode is
protected from high
current flow.
Higher power/current
solution. Here, the
current available is the
Zener current multiplied
by the current gain of
the transistor, but
dissipation still has to
be considered.
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Simple transistor circuits – bad circuits D
incorrect
correct
This is essentially an Emitter follower circuit where the voltage present at the
Emitter of the transistor is the same as that at the transistor’s Base, less about
0.6V, but the Zener diode requires some current to pass through it to provide
the Zener potential and the transistor requires some Base current to turn it ON.
The added resistor provides both these functions.
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Simple transistor circuits – poor circuit
Poor arrangement
Better arrangement
Don’t forget to consider power up and power down – in this case, at input power
application a high surge current will flow into the decoupling capacitor on the
circuit’s output which will severely stress the transistor and could lead to its early
failure.
Adding the resistor in the input line ljmits the surge current amplitude and so
prolongs transistor life. It also reduces the dissipation in the transistor.
Using linear regulators – bad circuit
You require a dual voltage power supply, in this case +5V and -5V, for your
application, but there are two faults in the arrangement above, and also a
poor feature.
Can you identify both of the bad arrangements and also the poor feature?
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Using linear regulators – discussions
Bad circuits – the negative linear regulator in the previous circuit had a positive input.
It simply would not work and also is highly likely to damage the negative regulator.
The decoupling capacitor on the output from the negative regulator was connected
with the incorrect polarity – it would cook and also not function correctly.
Poor feature – the secondary of the transformer was providing 20V rms, which would
result in about 28V dc being applied to the regulators, resulting in high dissipation. In
the circuit above about 14Vdc is applied to each regulator input, which is more than
sufficient and also causes the circuit to run much cooler.
Linear regulator – bad circuit
The output voltage of 15V is divided down to 5.1v by the 10k and 5.1k resistors and
this 5.1V is compared with the 5.1V provided by the Zener diode to close the loop.
Since the voltage across the 2.2k resistor will be about 10V, then the Zener diode
current will be just under 5mA, which is fine.
C1 and C2 provide decoupling,
BUT there are several problems with this circuit. Can you identify them?
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Linear regulator – bad circuit discussion
The 741 input connections are connected the wrong way round.
Since the 741 is powered from the +15V rail it cannot be powered up at switch on
because the transistor is OFF and there is nothing to turn the transistor ON plus the
output from the 741 has to be above 15V by at least 0.7V to allow for the transistor’s
base – emitter drop.
Since the circuit cannot turn ON there will be no Zener voltage.
Overall – the circuit will simply not work
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Linear regulator – corrected circuit
The 741 and the Zener reference are powered from the input supply – circuit comes ON
The 741 input connections are corrected .
The added 1k resistor in the transistor’s base circuit protects the 741 from damage should the
output be short circuited (but the transistor will cook if the short is not quickly removed).
Higher output current can be obtained by making the transistor a Darlington type.
With acknowledgements to The Art Of Electronics by Horowitz and Hill
Inductive load circuits
Inductive load circuits
Inductor tries to maintain its current flow when switch is turned off.
This causes a voltage V to appear across inductor terminals.
V = L . di/dt volts = (inductance x change of current)/change of time
If a fast switch (dt) is very quick, then high voltage is generated.
High voltage eventually arcs over switch contacts, causing pitting
and erosion to the contacts, or destroys semiconductor.
Hugh voltage transient caused severe interference to nearby items.
Cure – absorb voltage through a diode or snubber circuit.
Diode simplest but slows down circuit response and is only
realistically useful in dc circuits
Snubber (Resistor and capacitor or Zener diode and capacitor)
provides faster response, but higher voltage level.
Inductive load circuits