0200 - Voltage Monitor.pdf

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EPE Online, February 2000 - www.epemag.com - 95

This simple Voltage Monitordevice has two light-emittingdiode (LED) indicators thatswitch on if the monitored sup-ply voltage falls below separatethreshold levels. The obviousapplication is in battery oper-ated equipment where erro-neous results could be obtainedif the battery potential falls be-low a critical level.

Having twin threshold levelsis very useful, as one can be setslightly above the critical volt-age, and it will then give a warn-ing if the battery will soon needreplacement. The circuit canalso be used with mains pow-ered equipment to monitor theDC supply voltage, and it willthen give a warning if the supplyvoltage drops to an inadequatelevel due to a malfunction.

ON THE THRESHOLDWith the specified resistor

values the circuit providesthreshold potentials of 10V and12V, but by altering the valuesof four resistors these voltagesare easily changed. They canbe set at any potentials fromabout 3

5V to 30V, but note thatthe supply voltage to the moni-tor circuit must never exceed 36volts. The mathematics requiredto work out the modified circuitvalues is extremely simple more later.

For battery monitoring ap-plications the current consump-

tion of the monitor is a criticalfactor. There is no point in hav-ing a monitor that draws such ahigh supply current that batterylife is greatly reduced.

This circuit has a typicalcurrent consumption of around0

6mA under standby condi-tions. This should not greatlyreduce the operating life ofeven a low capacity batterysuch as a PP3 type. The currentconsumption increases by about4mA per LED when the circuit isactivated.

COMPARATORThe Voltage Monitor circuit

is based on the two voltagecomparators in an LM393N IC.A voltage comparator is very

Keep an eye on your batterys condition with thislow-cost starter project.

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Fig.2. Full circuit diagram for the Voltage Monitor. It is essen-tial that an in-line fuseholder, with a 100mA fuse, is included inthe positive supply input lead if the monitor is to be installed in

a car, boat, caravan etc.

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similar to an operational ampli-fier (opamp). Like an opera-tional amplifier, each compara-tor has inverting () and non-inverting (+) inputs. If the non-inverting input is at a highervoltage than the inverting inputthe output goes high, and if theinverting input is at the greatervoltage the output goes low.

This is again the same asfor an opamp, but there is asubtle difference in that the out-put stage of a comparator is anopen collector type. In otherwords, there is a switching tran-sistor at the output that is usedto control a load of some kind.The load in this case is a LEDindicator.

The basic scheme of thingsused in each of the voltage de-

tector circuits is shown in Fig.1.The inverting input () of thecomparator is fed with a highlystable reference potential of1

2V, and the non-inverting in-put (+) is fed from the supplylines via a potential divider (Ra/Rb). A certain fraction of thesupply voltage is therefore fedto the non-inverting input, andthis fraction is controlled by thevalues of resistors Ra and Rb.

Suppose that the potentialdivider provides one tenth of thesupply voltage to the non-inverting input. With a supplypotential of 12V or more therewill be 1

2 volts or more at thenon-inverting input, and the out-put transistor of the comparatorwill be switched off.

If the supply voltage falls

below 12V, the potential fed tothe non-inverting input goes be-low 1

2V, and the inverting inputis then at the higher voltage. Theoutput transistor of the compara-tor then switches on and activatesLED Da. Resistor Rc limits theoutput current to the requiredlevel.

CIRCUIT DETAILSThe full circuit diagram for

the Voltage Monitor appears inFig.2. The two comparators,within IC2, share a common volt-age reference, and this is a sim-ple shunt regulator that has resis-tor R4 as the load resistor andIC1 as the voltage stabilizer.

The ICL8069 used in the IC1position is a highly accurate andstable voltage reference chip, andnot a simple Zener diode. It willoperate efficiently at currents aslow as 50uA, which is important inthis application where low current

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COMPONENTSResistors

R1 82kR2 1MR3, R8 120k (2 off)R4 47k 5% carbon filmR5, R9 2k2 5% carbon film (2 off)R6 62kR7 820k

See also theSHOP TALK Page!

All 0.6W 1% carbon film unless noted

CapacitorC1 47u radial electrolytic, 40V

SemiconductorsD1, D2 5mm red LED (2 off)IC1 ICL8069 voltage referenceIC2 LM393N dual comparator

MiscellaneousStripboard, size 0.1 inch pitch,having 20 holes by 17 strips; 8-pinDIL socket; multistrand connectingwire, solder pins, solder, etc.

$8Approx. CostGuidance Only

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consumption is a definite asset.It operates at a current in theregion of 200uA in this circuit.THRESHOLDVOLTAGES

The threshold voltage of thedetector based on IC2a is deter-mined by the values of resistorsR1, R2, and R3. Two resistorsin series are used in the upperarm of the potential divider be-cause this enables the thresholdvalue to be set accurately usingordinary preferred values.

With resistor R3 at 120 kilo-hms (120k), the threshold volt-age is equal to one volt per 100kilohms of resistance throughthe potential divider. This resis-tance is fractionally more than1200k (1

2 megohms), giving athreshold voltage of 12V.

The switching voltage of theother comparator is controlledby the values of resistors R6,R7, and R8. With fractionallymore than 1000k (1M) of resis-tance through the potential di-vider this gives a threshold po-tential of 10V.

It is easy to work out theresistance values for otherthreshold voltages provided re-sistors R3 and R8 are left at avalue of 120k. Multiplying therequired voltage by one hun-dred gives the total resistancethrough the potential divider inkilohms. Deducting 120 fromthis then gives the total resis-tance through the upper sectionof the divider. In other words,this gives the required seriesresistance through R1 and R2,or R6 and R7 in the secondvoltage detector.

As an example, supposethat a threshold potential of 7

5Vis required. Multiplying 7

5 by100 gives a total resistance forthe potential divider of 750k.Deducting 120k from this givesan answer of 630k through the

upper arm of the divider.The required value is unlikely

to conveniently match up with apreferred value, and this is cer-tainly the case here. The nearestpreferred value to 630k is 620k,which is actually an error of undertwo-percent. In some applicationsthis margin of error is acceptable,and it would then be in order touse a value of 620k for resistorsR1 or R6, and a link wire for R2or R7. If an error of two percent isnot acceptable, a 10k resistor anda 620k component in series giveexactly the required resistance of630k.

Things will not always workout quite this well, and in somecases it might be necessary toaccept a small error even if tworesistors are used. However, theerror should only be a fraction ofone percent, which is insignifi-cant.

CURRENT AFFAIRSThe LED current and bright-

ness will be quite low if the unit is

used to detect low thresholdvoltages of around five volts. Tocompensate for this, resistorsR5 and R9 should be reducedfrom 2k2 to 1k.

Similarly, at high thresholdpotentials of around 20V to 30V,the LED current will become rel-atively high, although it shouldstill be no more than about13mA per LED. Increasing thevalue of R5 and R9 to 3k9 willkeep the LED current at about5mA.

The operating current of IC1varies enormously over the op-erating voltage range of the cir-cuit, but with the specified valueof 47k for resistor R4 the oper-ating current remains within theacceptable range for theICL8069.

CONSTRUCTIONConstruction of the Voltage

Monitor project is extremelysimple indeed, and it should bewithin the capabilities of com-plete beginners. The circuit is


Layout of components on the completed VoltageMonitor circuit board.

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built on a piece of stripboardand the component layout, to-gether with details of the breaksrequired in the underside coppertracks, are shown in Fig.3.

The stripboard measures 20holes by 17 copper strips, and aboard of this size is trimmedfrom one of the standard sizepieces using a hacksaw. Strip-board is quite brittle, so cutcarefully along rows of holesusing a minimum of force. Fileany rough edges to a neat finishand then drill the two 3mm di-ameter mounting holes andmake the five breaks in the cop-per strips.

The board is now ready forthe components and link-wiresto be fitted. With a small boardsuch as this it is not necessaryto worry too much about the ex-act order in which the compo-nents are fitted, but it is proba-bly best to fit the resistors andlink-wires first, followed by thesingle capacitor and the semi-conductors.

The four link-wires are quiteshort and they can be made us-ing some of the wire trimmedfrom the resistor leadouts. Becareful to fit capacitor C1 withthe correct polarity. Use single-sided solder pins at the twopoints where the supply will beconnected to the board.

Although it is not a static-sensitive component, it is still agood idea to mount the LM393Ncomparator, IC2, on the circuitboard via a holder. There is aslight complication with IC1,which is produced in both metalcased and plastic encapsulatedversions. The metal cased ver-sion (two pins), as used on theprototype, is shown in Fig.3.

However, it is actually theplastic cased version (threepins) that is available from most


suppliers these days. With theplastic version the flat side ofthe case should be on the leftas viewed in Fig.3 (i.e. facingtowards resistors R2 and R3).Ignore the pin marked NC(which stands for no connec-tion).

ASSEMBLYTo some extent the way in

which the unit is constructedand used will depend on theprecise application. It can bebuilt into a small plastic or metalbox and connected to the mainequipment via a twin lead. Inmost cases, however, it is morelikely to be incorporated into an-other project.

Either way, the circuit boardand LEDs can be dealt with intwo ways. Either the board canbe mounted on the case andhard wired to the LEDs on thefront panel, or the LEDs can bemounted on the circuit boardand then fitted into holders onthe front panel of the case.

Due to the small size andweight of the circuit board thissecond method works well withany LED holders of reasonablequality. If the LEDs are notmounted on the circuit board, fitsingle-sided solder pins on theboard in place of the LEDs. Thepins provide an easy means ofmaking reliable connections tothe board.

Make sure the LEDs areconnected with the correct po-larity. The cathode (k) leadoutof an LED is normally indicatedby a shorter leadout wire andthat side of the case being flat.

TESTINGIf a suitable variable voltage

supply is available, connect theoutput of the supply to the input

of the monitor circuit and varythe voltage around the thresholdlevels. With the supply potentialabove the threshold levels bothLEDs should remain off, but re-ducing the supply should resultin the LEDs switching on at theappropriate threshold voltages.

For highly critical applica-tions the threshold levels can befine tuned by tweaking the val-ues of resistors R1 and R6. Anincrease in value raises thethreshold voltage, and a reduc-tion in value decreases thethreshold level. Provided onepercent tolerant resistors areused in the potential dividersthe accuracy of the circuitshould be very good though,and for most applications no ad-justment to the values shouldbe needed.

In the absence of a suitablepower supply the unit can begiven a rough check using somebatteries. Use a battery or bat-teries in series to provide a sup-ply potential that is somewhathigher than the higher thresholdlevel. With 10V and 12V thresh-old voltages for instance, a 15Vor 18V battery supply could beused. Both LEDs should beswitched off when using thissupply potential.

Now try a lower battery volt-age that is slightly less than thelower threshold level. For exam-ple, a 9V battery could be usedwith 10V and 12V threshold lev-els. Both LEDs should thenswitch on.

If there is any sign of a mal-function, disconnect the supplyimmediately and recheck thecircuit board for errors.

IN USENote that it is essential to

wire an in-line fuseholder fitted

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with a low current fuse (about100mA) in the positive supplylead if the unit is used to moni-tor the supply of a car, boat,caravan, etc. Otherwise there isa risk of a fault causing a verylarge current to flow, whichcould result in a fire.

Experienced constructorsshould have no difficulty in us-ing the unit to monitor the DC

supply voltage of a mains pow-ered project, but this is some-thing that should not be at-tempted by those of limited ex-perience.

Make sure the monitor isconnected to the supply with theright polarity. The semiconduc-tors and C1 could be damagedif the supply connections arereversed.


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0200 - Voltage Monitor.pdf