POOR & BAD CIRCUITS

With acknowledgments toHorowitz and Hill: The Art of Electronicsfor the presentation idea and some of

the presentation contents

• 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

POOR & BAD CIRCUITS

Simple transistor circuits – bad circuits A, B & C

With acknowledgements to The Art Of Electronics by Horowitz and Hill

A B C

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

With acknowledgements to The Art Of Electronics by Horowitz and Hill

Simple transistor circuits – bad circuits – C

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

incorrect correct

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

With acknowledgements to The Art Of Electronics by Horowitz and Hill

incorrect correctThe 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.

Simple transistor circuits – bad circuits F

With acknowledgements to The Art Of Electronics by Horowitz and Hill

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.

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 applicationsDiodes 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.

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 – bad circuit

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

With acknowledgements to The Art Of Electronics by Horowitz and Hill

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.

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

With acknowledgements to The Art Of Electronics by Horowitz and Hill

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.

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

With acknowledgements to The Art Of Electronics by Horowitz and Hill

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?

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.

Using linear regulators – discussions

With acknowledgements to The Art Of Electronics by Horowitz and Hill

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?

Linear regulator – bad circuit discussion

With acknowledgements to The Art Of Electronics by Horowitz and Hill

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

Linear regulator – corrected circuit

With acknowledgements to The Art Of Electronics by Horowitz and Hill

The 741 and the Zener reference are powered from the input supply – circuit comes ONThe 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.

Inductive load circuits

Inductive load circuitsInductor 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 circuitsSnubber (Resistor and capacitor or Zener diode and capacitor)

provides faster response, but higher voltage level.

Inductive load circuits