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4QD-TEC: Electronics Circuits Reference Archive
Current sensing relay
driver

Almost all electronic circuits, unless they are Hall effect, actually sense
voltage - not current. So to convert the current you want to sense to a suitable
voltage you use a current-to-voltage converter. No, that's not an expensive
chip! They're usually known as resistors! In this one the sensing resistor is
bypassed by diodes (so the current sensed can be varied over a wide range.

The current to be sensed should flow from A to B. It flows either through R1
or (if big enough) through D1. There is an anti-parallel diode also present so
the circuit can be used equally well for d.c. or a.c. sensing.

With the relay off, Tr1 is not conducting so Tr2 must be. Therefore Tr1 must
be off. This occurs (surprise!) at zero current. The 12v supply line will cause
current to flow through R3 and R2 so a voltage {12x(R2+R1)/(R1+R2+R3)} will be
developed on the base of Tr1. This voltage is 390 millivolts - enough to bring
Tr1 near to turn on. Any voltage developed across R1 will add to this 390mV.

When the turn-on voltage gets to about 550mV, Tr1 will turn on, so only 160mV
is needed across R1, or around 160 micro-amps through it. Obviously you can
alter R1 or even R3 to reduce or increase this but do remember that transistor
base-emiter voltage reduces abour 2 millivolt for every degree centigrade
temperature rise. Make the circuit too sensitive and it will turn on if the
weather gets too warm!

Tr1 turning on causes Tr2 to turn off so Tr2's collector goes high. C2 now
charges up via D4 and R4, keeping Tr1 turned on during charging (about 300mSec
with these values). If the input is a.c. then, after this charging period, Tr1
will turn off at the next zero-crossing interval, so Tr2's collector will go low
again, discharging C2 quickly (about 3mSec) via D3. This 'pumped feedback'
technique is quite effective. Although the circuit does turn off, this so brief
that the negative dips can be smoothed out easily - that is the function of C3.

Domestic heating boiler control

The next circuit is a practical application of the current sensor. It was
used in the centeal hearing control system of an old house I 'did up'. Now the
walls were thin and poor insulators. Rather than keep the heat on permanently we
had fan assisted heaters in each room which were switched on as required. These
heaters have a thermostat in them which switches off the fan until the water is
hot. So you need to be a bit crafty to sense the difference in current between a
fan heater which is switched off and one which is on but not fanning because it
is cold!

Before I go any further - a word of warning. The following circuit is
directly connected to the mains. It is threrefore potentially extremely
dangerous if you are not familiar with operation on live circuits. You use
the circuit entirely at your own risk and claims from next-of-kin will not be
accepted!

Sw is the on/off switch in the fan heater. Th is the fan heater's thermostat
which stops it fanning when the water is cold. Note C3 - a 100n capacitor across
the thermostat. This has no affect on the fan but does alow enough leakage
current to flow (when Sw is closed) to operate the current detector.

The more astute amongst you may notice that D3 and D4 are reversed (compared
with the first circuit). To explain this I shall start at the output. For the
triac to be turned on, the SBS must conduct. It will ony do this if Tr3 is
conducting. (If you don't know about the SBS - you missed the explanation on the
first page off circuits!) So Tr2 must be turned on and Tr1 off. For the output
to be on when the input is off is reversed logic from the first circuit. This
one is biased with Tr1 normally on, the negative voltage developed across D2
turns Tr1 off and he pump holds Tr1 off.

I don't know if you can still get SBS devices. They were made by Motorola,
MBS 4991 and 4992, but it's probably easier to use the 4 transistor circuit.

The other thing I should cover is the power supply.. it's a little unusual.
Firstly I used a capacitor as a dropper, rather than a resistor. Capacitors
don't dissipate heat. You must use a class X2 capacitor: these are
designed for safe operation when connected directly across the mains. The 100n
across the thermostat must also be X2 class.

The capacitor feeds directly to the zener diode: this is an unusual use of a
zener since the current through it will be alternating! In one direction (the
one we don't want) it will simply conduct as a forward-biased diode, but on the
other half-cycle it will develop 15v across it. This 15v feeds through the
second diode to charge up the 100µ reservoir capacitor (C4).

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Page's Author: Richard Torrens

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