ELV Dummy Load for Power Supply Testing -...
Transcript of ELV Dummy Load for Power Supply Testing -...
SSPJI05: ELV Dummy Load for PSU Testing
Salvage Server Project 'Junk Ideas' No.5:
ELV Dummy Load for Power Supply TestingProduced by the Free Range Salvage Server Project.SSP_JI05, version 1.0, January 2005 http://www.fraw.org.uk/ssp/
You build a mains or renewable energy power supply for a laptop orother sensitive device, plug it in, turn it on and BANG – smokecomes from the power supply, laptop, or both! Here's a junk idea toavoid making such an expensive mistake.
What is a Dummy Load?
An appliance of some kind is in technical termsa load – it draws power. We can simulate thisusing a device, a dummy load, that draws powerat a known rate. Dummy loads are usuallynothing more than a large resistor. The supplyvoltage is dropped across the resistor and theenergy contained in the supply current isdumped as heat – often an awful lot of heat!
Dummy loads are common in amateur radiowhere they are used for tuning transmitterswithout actually transmitting. They are lesscommon in other areas of amateur electronics.However, this project has been developed as away of testing not just mains power supplies, butalso renewable energy systems where you needto dump the energy whilst you are setting up.However, this design is only designed for use inextralow voltage systems, or ELV. Technicallythat's less than 50 volts, but this design was builtspecifically for use with 12V and 24V systems.
Current and Power Rating
If you short circuit a battery you develop a largecurrent. Large leadacid cells can create shortcircuit currents in excess of 500 amps. This isbecause the shorting wires have very littleresistance. If you connect a resistor into thecircuit the current will rise to a point that isdependent upon the value of the resistor (seethe table on the next page to see how muchcurrent a resistor draws). Then, providing thatthe resistor has a big enough power rating, theresistor will happily burnoff the heat created by
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The ELV Dummy Load
SSPJI05: ELV Dummy Load for PSU Testing
the current. Electric fires use the same principle.The bars of the electric fire have a resistancethat means one kiloWatt of heat will beproduced by the fire at mains voltage (and onlyat mains voltage).
You can calculate the maximum current drawnby a load resistor using Ohms law:
Current (A) = Voltage (V) ÷ Resistance (R)
However, this is the easy part. The mostimportant issue in the design of the dummy loadis that the power output should not exceed thepower rating of the load resistor. You cancalculate the maximum power that a resistor willdissipate using the power equation:
Power (W) = Current 2 (A) x Resistance (R)
Conversely, if you know the power rating of aload resistor you can calculate the current that itcan handle by rearranging the above equation:
Current (A) = √[Power (W) ÷ Resistance (R)]
The calculated power rating for a resistor usuallyrepresents the level that will cause problematicoverheating. Therefore the resistor used shouldhave a capacity about 1.25 to 1.5 times morethan the calculated figure. Likewise the maincurrent path in the circuit (discussed later) mustuse wire that has a capacity of 1.25 to 1.5 timesthe maximum calculated current.
Current Control
As noted above, if you have a fixed resistor thenconnecting it to a battery or power supply willcreate a fixed level of current or power load.Whilst useful for checking circuits at a singleload, it's not very useful for checking how thepower supply works at different power levels.Therefore we need to be able to vary the powerdrawn by the load without the hassle ofcontinually disconnecting and reconnectingdifferent values of resistor.
The dummy load uses a field effect transistor, orFET, to switch the circuit on and off. By varyingthe period that the FET switches on or off, usinga system called pulse width modulation (see thebox on the next page), we can vary the amountof time that the FET is on and how much loadthe resistor will draw.
FET transistors are able to switch very large
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Maximum Design Current
The current the load will draw is dependentupon the voltage at which it operates andthe resistance of the load. Increasing thevoltage will increase the current. Increasingthe value of the load resistor will decreasethe current. The effect of different voltagesand resistors is shown in the table below.
The current (A) is calculated by taking the supplyvoltage and dividing by the value of the load resistor
(R). The power dissipation (W) is calculated by taking the square of the current (A) and multiplying by
the value of the load resistor (R).
The power in Watts (W) that the resistor willdissipate can vary, and often the figure quotedfor represents a level at which the resistor willoverheat. Therefore it's a good idea to multiplythe power figure by 1.25 or 1.5 and use aresistor of this value. This will preventexcessive overheating of the load resistor.
The maximum current a resistor can handle,irrespective of the operating voltage, can becalculated by dividing the power rating of theresistor, in Watts (W), by the value of theresistor, in Ohm (R), and then taking thesquare root of this figure to give the maximumcurrent in Amps (A).
Another important factor in the design is thatthe wiring from the power supply, to the loadresistor, and then on to the transistor, mustalso be able to handle 1.25 or 1.5 times themaximum current – or it will badly overheat.
Load Supply VoltageResistor 12V 24VR A W A W
0.1 120 1,440 240 5,760
0.15 80 960 160 3,840
0.22 55 655 109 2,618
0.33 36 436 73 1,745
0.47 26 306 51 1,226
0.56 21 257 43 1,029
1 12 144 24 576
1.5 8 96 16 384
2 6 72 12 288
2.2 5 65 11 262
3.3 4 44 7 175
4.7 3 31 5 123
SSPJI05: ELV Dummy Load for PSU Testing
currents using a very small control voltage. Thatmeans we can avoid using very expensivepower components, such as high power variableresistors or rheostats. Instead the dummy loadcontrol circuit uses a single low power variable
resistor and a single integrated circuit to changethe switching period, or duty cycle, of the FETand hence how much current the load will draw(see the box below).
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Current Control and Pulse Width Modulation (PWM)
The power figures used to rate resistors arequoted in Watts. This is a measure of the powerflowing every second. Therefore if we turn off theflow for half a second, only half the power, andhence current, will flow. This is the theory behindpulse width modulation, or PWM. By turning offthe current flow for a greater proportion of thetime we reduce the current flow.
The dummy load control circuit (discussed on thenext page) creates a triangular waveform that isused to switch the load resistor on and off. Thetriangular waveform oscillates between 1.2 voltsand 6.8 volts. This is fed into a comparator alongwith a control voltage that is set by a variableresistor. If the control voltage is greater than thelevel of the triangular waveform the comparatorswitches the transistor off. If the control voltage isless than the voltage of the triangular waveform itswitches the transistor on. Consequently theinteraction between the triangular waveform andthe rising control voltage switches the transistoroff for a progressively longer period of time.Therefore we can finely control the width of thethe pulses by varying the control voltage.
The figures below, taken from an oscilloscope,shows the triangular waveform and the outputfrom the comparator. The comparator is eitherfully on or fully off. We measure the width of thepulses, or the duty cycle, in terms of apercentage – 100% is fully on and 0% is fully off.At 100% the comparator is fully on. At 95% thecontrol voltage rises just above the lower voltageof the triangular waveform and transistor isturned off for 5% of the time. As the controlvoltage rises to one quarter and then threequarters of the waveform voltage the duty cycledecreases to 75% and 25%. At 5% the transistoris switched off for most of the time and at 0%,when the control voltage is higher than thetriangular waveform, it does not switch on at all.
The effect of this is to reduce the current, andhence the load, the load resistor draws. If a loadresistor theoretically draws 6 amps, at a dutycycle of 50% it will only draw 3 amps, or at 25%1.5 amps. This is how the control circuit of thedummy load is able to finely control how muchpower the dummy load unit will draw.
100%
95%
75%
25%
5%
0%
SSPJI05: ELV Dummy Load for PSU Testing
The Control Circuit
The circuit diagram for the control unit is shownbelow. There are essentially three parts to thecircuit: The components around Q2 form an 8V
voltage regulator that powers the control unit. The components around Q3a to Q3c form the
oscillator that creates the triangularwaveform.
The components Q3d and R2 form thecomparator and pulse width control systemthat drives the FET.
There are two major limitations to the voltage atwhich the circuit can operate. One is the workingvoltage of the FET, but the other is themaximum working voltage of the 7808 voltageregulator. The 7808 was chosen because it wasspare – a 7806 (6 volt) or 7805 (5 volt) wouldhave worked equally as well. Most 78XX voltageregulators have a maximum input voltage ofaround 36 volts to 40 volts. The use of aregulator is important because it allows thedummy load to have a wide operating voltage.
Another restriction is the dropout voltage of theregulator – the minimum difference between theregulated voltage and the voltage input that isallowed to ensure reliable regulation. For most78XX type regulators this is around 2.5 to 3
volts. If you want the circuit to work with a 12volt battery system then it has to work down toaround 11 volts. Therefore a 7808 is the highestvoltage regulator that's suitable.
The oscillator is a little more complex. Q3a is aunity gain amplifier that works from the potentialdivider formed by R7 and R8. This will producea voltage that is half of the regulated output –4.0V. This is used as a voltage reference byQ3b and Q3c which form the oscillator. Q3bcreates a square wave signal oscillating at a fewhundred Hertz. This is fed to Q3c which turns itinto a triangular waveform.
R2 is a multiturn cermet trimmer. In order towork over its entire range you have to turn thescrew on the trimmer eighteen times. Thismeans that you have a far greater level ofcontrol over its resistance, and hence the loaddrawn by the dummy load. You could use astandard threequarter turn trimmer, but youwouldn't have the same level of control. IdeallyR2 should be a linear trimmer. However, youcan use a logarithmic trimmer (as was used inthis design) and it works fine, but you lose someprecision at the lower current end of the scale.
R2 acts as a potential divider to provide thecontrol voltage to the comparator Q3d. Thetriangular waveform is connected to the non
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Circuit Diagram of the ELV Dummy Load (see end of report for the component list)
SSPJI05: ELV Dummy Load for PSU Testing
inverting ('+') input of Q3d and the controlvoltage to the inverting ('–') input. This meanthat when the waveform voltage exceeds thecontrol voltage (or + > –) the comparator outputsit's positive supply voltage – around 8 volts.When the control voltage is higher than thewaveform voltage the comparator outputs 0V.This mean that the FET is either switched fullyon or it is switched fully off.
As an indicator an LED has been added to theoutput of Q3d. The LED is a standard red LEDwith a forward current of 15 milliamps (mA).Ordinarily, given that the voltage coming out ofQ3d is just under 8 volts, the series resistor forthis LED (R3) would only need to be a fewhundred ohms. However, in order to reduce thecurrent drain through Q3d a 1,000 ohm resistorwas used. This means that the LED lights a littledimly, but it does its job – it warns the unit isworking at a high current.
Finally, L1 is a few turns of wire on a ferritetoroid. The waveform produced by the oscillatorhas a very low frequency, but the harmonicsproduced by Q3d could theoretically be in theradio frequency band. The purpose of L1 is todampen the harmonic frequencies to preventthem being emitted from R1 when it is switchedby the FET.
The FET and Load Resistor
The critical part of this project is the FET and theload resistor.
As noted earlier, any resistor connected acrossa power supply will conduct current up to atheoretical maximum level. In selecting R1 youshould consider what will be the greatest currentthat the dummy load will be required to handle.This is because once you reach this level theload will not conduct any more current – for a
higher current you will have to change the valueof R1.
For this project a 2 Ohm, 300 Watt vitreousenamel resistor was used (see pictures above).This has the benefit that is doesn't need aheatsink. These resistors are popular as dumploads in renewable energy applications, andtheir tubular construction means that theydissipate heat well. In use it was hung from ahook using a small wire loop fixed to one end. Itwould be possible to use high power wire woundor metal film resistor, but these nearly alwaysneed a heatsink to dissipate the level of heat forwhich they are rated.
Note also that the maximum current will varyaccording to the supply voltage. That's not aproblem with a narrow voltage range, but if youswitch between different voltages you have totake the current load into account. For thedesign shown here the maximum current at 12volts is around 5.6 amps. But at 24 volts thatrises to nearly 12 amps (see the table on page2). Provided that the resistor has a sufficientpower dissipation working over a wide voltagerange is not a problem. However, it might createa problem with the FET.
For this project the FET chosen was aHUF75333P3. This is an Nchannel ultraFETthat has a very low internal resistance. Any Nchannel FET will do for this job, but the criticalfactor is the maximum working voltage andcurrent rating. The HUF75333P3 can handle upto 56 amps at 55 volts – more than enough forthe maximum 30 volts/15 amps that this dummyload was developed for. If using other FETs youwill need to check that the selection of loadresistor and operating voltage does not drive thedevice to more than 75% or 80% of its ratedcapacity.
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A vitreous enamel resistor Wire suspension loop
SSPJI05: ELV Dummy Load for PSU Testing
Building the Dummy Load
This is a “junk” project, and so as far as possiblethe components and hardware are salvaged.For this reason the project has been designedfor assembly onto a stripboard to eliminate theneed to manufacture a printed circuit board.
A suggested stripboard layout is shown in thediagram below. A picture of the prototype board(which differs slightly from the examplestripboard below) is shown on the right. Thedesign requires just twelve cuts to be made inthe metal tracks of the board, and twelve wirelinks are used. Other than that, assembly isquite straightforward. Q3 is placed in an ICsocket not just to protect it during soldering, butalso so it can be reused if you desperately needone. The fiddly bit is making L1, by putting twoturns of insulated hookup wire around a smallferrite toroid or bead.
In the prototype project all the parts, with theexception of the load resistor, were mounted ona large heatsink. The specification of theheatsink was far in excess of what is required todissipate the heat produced by the FET, but itwas the only large piece of metal to hand at thetime. A 20cmsquare piece of thick mild steelsheet would have worked equally as well.
The main current draw on the control board is
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The control unit built on stripboard
Stripboard mounted on the heatsink
Example of a stripboard layout for the control unit
The example shown here usesa piece of standard 0.1” strip
board (or Veroboard) 22 holeswide by 22 strips long (this is
slightly different from thatpictured above as the layout
has been simplified a littleduring the transposition from
the prototype).
What's important are the 12track cuts – especially those
near the fixing holes where themetal screws might shortconnections between the
tracks. There are also 12 wirelinks, in addition to the 17
identified resistors, capacitors,inductors and semiconductors.
SSPJI05: ELV Dummy Load for PSU Testing
the LM324 opamp. However, this draws so littlecurrent that the one amp voltage regulatordoesn't require a heatsink. In terms of theoverall energy efficiency of the dummy load,given that the load resistor might be dumping upto fifteen amps of current, the sixty milliampsused by the control board is irrelevant.
The FET was fixed to the heatsink with an M3nut and bolt. However, because most FETshave their drain connected to the fixing tab thiswould mean that the heatsink, or any othermetal object the FET was connected to, wouldbe connected to the positive terminal of thepower supply. For the design of this projectthat's not a problem, but if the negative leadwere to accidentally touch the heatsink it wouldshort out the FET and create a surge of currentup to the maximum theoretical capacity of theresistor – possibly damaging any low currentdevices connected to the load.
To prevent any accidental power surges theFET was mounted on an insulating pad, with aplastic bush around the fixing bolt, to electricallyisolate the drain of the FET from the heatsink.Note also that to prevent the legs of the FETcontacting the heatsink a piece of PVC tape wasstuck to the heatsink. Then, to prevent the legsof the FET being bent and broken, the wireswere fixed to the heatsink using more PVC tape(see picture, above left). Also, to preventmechanical stress, a long length of heavy dutywire was soldered to the FET and then coiledbeneath the control board before being fixed intothe terminal block.
The heatsink was drilled using a 3mm drill – twoholes for the control board, one for the FET, and
three for the 30A terminal block. For simplicitymost of the holes were tapped to accept 4BAscrews (which were in the junk box). M3 nutsand bolts would have worked equally as well.The stripboard requires some form of standoffspacer, and for this two 2cm length of the outerPVC insulation stripped from heavy duty mainscable was used (reuse and recycle!!). Threadedonto the screws, it compresses when the screwsare tightened to form a rigid support for theboard (see the lower picture on the previouspage).
Finally, when all the components have beenfixed to the heatsink, the various parts can bewired together. This is shown in the schematicdiagram at the top of the next page.
It would have been easier just to solder wires tothe control board. The reason for using a PCBmounted terminal block is that the control boardis a self contained unit, and could be borrowedfrom this project for building/testing otherdevices that required a pulsewidth modulationdriver (motor speed controllers, light dimmers,etc.). The use of the terminal block also makes iteasier to connect and disconnect the wires forother purposes – such as connecting anoscilloscope (shown in the schematic diagramover the page)
The light duty (thin) wire could be anylightweight solid or multistrand wire. This isbecause it's never going to carry more than onehundred milliamps. For this purpose some sixamp multistranded wire was used – way overthe specification required, but it was availablefrom a scrapped wiring loom. However, caremust be taken when selecting the heavy duty
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The FET mounted on the heatsink
The drilled heatsink
SSPJI05: ELV Dummy Load for PSU Testing
(thick) wire for the main current path (see circuitdiagram). The wire must be able to carry,without overheating, the maximum currentdrawn by the load resistor. Whilst solid coredtwin and earth cable (usually rated around thirtyor forty amps) is OK, 24 amp multistrandedhook wire was used. The main benefit is thatmultistranded wire is more flexible, and the wirecan be regularly bent and moved withoutthinning or breaking the metal conductor.
Everything is held together using a block of six,30 amp terminal blocks. It's important that thecomponents of the dummy load are connectedin the order shown. If the FET is connectedbefore the load resistor it will overheat and blow(Nchannel FET's should only be used on thelow side, or negative supply side, of a load). It'salso important that the positive lead for thecontrol board is connected before the loadresistor otherwise the board will not get enoughvoltage to operate correctly.
As shown in the schematic, two of the terminalsare used to connect a multimeter to measure theload current. The multimeter must have thecapacity to measure the maximum currentdrawn by the load resistor or it will be fried!
Rather than permanently connecting amultimeter the probes of the meter can be
screwed into the terminal block only when thedummy load is used. However, if you want tocalibrate the load to a specific current for routinetesting the multimeter is not required – you canjust connect the two terminals together withsome heavy duty wire (as shown in theschematic). To calibrate the dummy load all youneed to do is connect it to an appropriate batteryor power supply, adjust R2 until the multimeterreads the required current, disconnect, and thenreplace the multimeter with a wire link.
If you are using a very low value resistor in thedummy load there is always the risk that youmight produce a current that could damage theFET, the wiring, the multimeter, or mostimportantly the power supply that you aretesting. For greater protection you could install afuse between the positive terminal of the powersupply and the terminal block – which, to protectthe whole wiring circuit, would need to belocated at the power supply end. An automotivetype blade use would do, although they can be alittle unpredictable at lower currents. Thereforewhen using low currents, of less than five amps,20mm quick blow fuses would be a saferalternative.
Using the Dummy Load
An ELV dummy load is a very useful device.
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Schematic of how the dummy load is configured for use
SSPJI05: ELV Dummy Load for PSU Testing
You can use it to check power supplies, to testbattery packs, to test renewable energy powersystems without the need for batteries, or tocalibrate inline components like ammeters.Whereas before you might have had toimprovise using different lowvoltage devices tocheck the power supply at different loads, withan ELV dummy load all you need to do isconnect it up and twiddle R2 until you create therequired load.
Assuming you were to buy the components(except the heatsink, wire and bolts – which canbe easily recovered from other sources) thisproject will cost around £6. That's really cheapconsidering how versatile it is!
The main problem with using the dummy load isthat you might switch on with R2 adjusted to turnthe load resistor fully on. Therefore when usingthe load you should follow the procedureoutlined below:
Connect up everything except the loadresistor, and switch on the power supply.
If the power supply works only the controlboard will be using power – less than 100mA.
Adjust R2 until the LED indicator goes outfully, then disconnect the power supply.
Connect the load resistor, reconnect thepower supply, and adjust R2 until themultimeter indicates the required current.
Be aware that when the current goes above fiveamps, even a 300 watt resistor will heat upquickly – so don't touch it! For this reason youshould hang/fix the resistor in a position whereyou can safely leave it for a long period after useto allow it to cool down.
The Salvage Server Project has been developed by the Free Range Network to promote the use of redundant IT equipment as a resourcefor community and grass roots campaigning organisations. This report has been produced to support the work of the project, and is madefreely available to encourage the objectives of the project.
© Copyright 2004, Paul Mobbs/Free Range Network. Permission is granted to copy, distribute and/or modify this document under the termsof the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with InvariantSections being the document title and author identification, no FrontCover Texts, and no BackCover Texts. A copy of the license isprovided at: http://www.fraw.org.uk/_admin/copyright.shtml This document has been wholly produced using the Gnu/Linux operatingsystem and free software.
The information contained in this work has been obtained from sources that are believed to be reliable. However, We cannot provide anyabsolute guarantee that the information contained herein is wholly correct, or that the manner in which the information is used is correct,and consequently we cannot be responsible for any error, omissions or damages arising from the use of the information in this work. Thiswork has been created on the basis that the Free Range Network is not intending to supply engineering or any other professional services– the purposes of this briefing are merely illustrative.
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Parts List
Components
Q1 HUF75333P3 Nchannel FET – but anyNchannel power FET that can handlethe required current and voltage will do.
Q2 LM7808CT or similar 1 amp voltageregulator between 5V and 8V.
Q3 LM324 quad operational amplifier, orsimilar single supply rail opamp.
C1 10uF 63V radial electrolytic capacitor.C2 220nF ceramic capacitor.C3, C6 100nF ceramic capacitor.C4 1uF 16V radial electrolytic capacitor.C5 10nF ceramic capacitor.R1 Vitreous enamel resistor (see text).R2 100K 18 turn cermet potentiometer.R3 1K0 0.6W metal film resistor.R4, R5, R7, R8 100K 0.6W metal film resistor.R6 47K 0.6W metal film resistor.D1 Any LED with a forward current (If)of
around 15 to 20 milliamps.L1 2 turns of lightweight hookup wire on a
small ferrite toroid or bead.
Hardware0.1” stripboard – around 22 holes by 22 strips.4way PCB terminal block (if required).6way 30 amp terminal blockA heatsink or a thick sheet of mild steel
sufficient to mount the project on.6 screws or 6 nuts and bolts to mount the
dummy load onto the heatsink.An insulating pad and bush to mount the FET
onto the heatsink (if required).One 14pin IC socket for Q3.
The Honesty Box
An 'honesty box' is usually something that shops have. You can pick something and walk outquickly if you drop the correct money for the item in the honesty box. We're taking a slightlydifferent angle on the honesty box. We'd like you read this, and the other Free Range projectmaterials on the web site, and honestly answer this question:
“Did you find this information useful, helpful or interesting?”
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The SSP project, like the rest of the Free Range Network, does not have a membership,official funding, or staff. The human resources are donated by those taking part. Theexpenses for the development of new resources and information are usually begged fromdifferent sources, to support specific activities (or to put it another way, now you know whywe make things from a lot of old secondhand, scrapped and discarded equipment!).
If you would like to help develop the SSP Project please send a donation, by cheque orpostal order (in pounds sterling, if you live outside the UK), made payable to “Mobbs'Environmental Investigations (Free Range/SSP)”, to –
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Thanks! Bokbtarrk!!
The SSP Team