Protecting electrical contacts in clocks

By Roger Castle-Smith

INTRODUCTION

I have been asked, several times, about the protection of electrical contacts in clocks across a broad range, for example Synchronome, pre-electronic bedside alarm clocks and old car clocks. Without protection they can soon become pitted through sparking which leads to bad time keeping or total failure.

ELECTRICAL TERMS

It is necessary to have a knowledge of the following electrical terms so that the following sections can be understood.

Volt (V) Unit of electrical pressure.

Amp (A) Unit of electrical current.

Ohm (Ω) Unit of electrical resistance. The relation between a volt, amp and ohm is given by Ohm’s law, namely:

Amps = Volts ÷ Ohms

Ohm’s law applies to all normal resistors which are usually made from carbon or wire. These are known as linear devices because, for example, doubling the voltage across a given resistance, results in a doubling of the current.

– but see Diode below for an exception.

Diode A diode is an electrical component which allows current to flow in one direction through it but not in the reverse direction.

A diode has an electrode called an anode at one end and a cathode at the other one. When the anode is positive relative to the cathode, then current flows through the diode. The diode is then said to be forward biased. When the voltage is reversed, back biased, no current flows. The cathode is usually marked with a band or a minus sign as shown in the picture. The symbol used to represent a diode in some circuit diagrams is also shown.

Ohm’s law does not apply to a diode, and some other devices, because they are non-linear; that is, for example, doubling the applied voltage more than doubles the current.

TESTING A DIODE

When testing a diode, it is necessary to remember that it has a non-linear resistance characteristic as described above; that is, for example, doubling the voltage more than doubles the current.

Ideally a meter with a diode test facility should be used which takes account of the diode’s non-linearity. However, if this is not available, any meter with resistance ranges can be employed.

Typically, a meter measures resistance by applying a voltage and measuring the resulting current. It then uses Ohm’s law to determine the resistance.

The measuring voltage used by different meters, and on different ranges, varies considerably. Hence, because of a diode’s non-linear characteristic, the forward conducting resistance of a diode measured using one meter can be quite different from that measured by another one !

As a rough guide, the forward-biased resistance of a small diode is likely to measure between around 1000 to 6000 Ω depending on the meter used. But the effective resistance in the clock circuit, when the contacts open, will only be a few ohms at most as the applied voltage is then around 5 volts of more. When the meter connection is reversed so that the diode is back biased, the reading will be very high, typically several millions of ohms (megohms) or out of range of the meter.

Tip If diode (D1), in the circuit diagram below, has become open circuit (ie it will not pass current in either direction) then if the clock is placed in total darkness and the gap between the contacts is closely observed, it will usually be possible to see a tiny spark when the contacts open.

REPLACING A DIODE

The type of diode in a clock is usually too small to have a type number written on it.

Any small rectifier diode with at least the following minimum ratings would be suitable:

Peak inverse voltage (PIV) – also known as blocking voltage 5 volts

Minimum current capacity 0.6 amp

The PIV is the maximum reverse bias voltage which the diode can withstand before being damaged. The maximum current capacity is the maximum current which the diode will pass in the forward biased direction without damage. A suitable diode for most clocks is type 1N4001 rated at 50 volts PIV and 1 amp. It sometimes has an S appended which indicates silicon. Such a diode is readily available from the likes of RS components.

HOW A DIODE PROTECTS CONTACTS

A typical clock circuit which utilizes a diode for contact protection is shown above. The magnet may employ 1 or 2 coils but this does not affect the way the circuit works.

For the moment, assume that diode (D2) does not exist and is replaced by a piece of wire.

When the contacts close the electrical supply, typically somewhere between 3 and 6 volts, is connected to the magnet and current flows as shown by the solid arrow. This current produces a magnetic field which attracts the armature to the magnet poles. Current does not flow through diode (D1) as it is back-biased with the positive supply voltage at the cathode end. At the end of the impulse period the contacts open and the magnet releases.

When the contacts open, the magnetic field associated with the magnet coils collapses; this collapse induces a voltage across the coils which is of opposite polarity to the originally applied voltage. A problem arises because the voltage produced is proportional to the rate of change of current. As the contacts try to break the current instantaneously, ie an infinite rate of change of current, then this would theoretically produce an infinite voltage across the contacts ! But of course this cannot happen in practice and the result is a spark across the contacts as the coil energy is dissipated. Such a spark would soon erode the contacts and cause migration of metal from one contact to the other resulting in unreliable operation.

In order to fix this difficulty, diode (D1) is included in the circuit. This provides an induced current path, as shown by the dashed arrow. The result is that:

  • The magnetic field energy is dissipated primarily as heat in the resistance of the coils rather than as a spark.
  • The voltage which appears across the contacts when they open is limited to around 5 volts which is harmless. In the absence of the diode the peak voltage would be some 70 volts.

This leaves diode (D2) – what does it do ? It does not affect the functioning of the circuit as described above but provides protection for diode (D1) in the event that the supply is connected the wrong way around. In such an event, in the absence of (D2), current would flow through (D1) when the contacts close. (D1) is then a near short circuit across the supply and the resulting current could destroy it.

I have ever only seen (D2) in one clock so as to provide protection for (D2) if the battery is inserted the wrong way around.

ANY OTHER WAY OF PROTECTING CONTACTS ?

In the early days of electric clocks, for example Synchronome and car clocks, small diodes were not available. In such cases (D1) in the circuit diagram is replaced by a resistor. This is not as good as a diode but was used extensively in early clocks. A small disadvantage is that it passes current as well as the magnets when the contacts close. A rule of thumb is that a good balance between providing contact protection and passing unnecessary current is a resistor which has a value of around 10 times the magnet coil resistance.

Roger Castle-Smith