SSPJI-05: ELV Dummy Load for PSU Testing Salvage Server Project ©Junk Ideas© No.5: ELV Dummy Load for Power Supply Testing Produced by the Free Range Salvage Server Project. SSP_JI-05, version 1.0, January 2005 http://www.fraw.org.uk/ssp/ You build a mains or renewable energy power supply for a laptop or other sensitive device, plug it in, turn it on and BANG ± smoke comes from the power supply, laptop, or both! Here©s a junk idea to avoid making such an expensive mistake. What is a Dummy Load? An appliance of some kind is in technical terms a load ± it draws power. We can simulate this using a device, a dummy load, that draws power at a known rate. Dummy loads are usually nothing more than a large resistor. The supply voltage is dropped across the resistor and the energy contained in the supply current is dumped as heat ± often an awful lot of heat! Dummy loads are common in amateur radio where they are used for tuning transmitters without actually transmitting. They are less common in other areas of amateur electronics. However, this project has been developed as a way of testing not just mains power supplies, but also renewable energy systems where you need to dump the energy whilst you are setting up. However, this design is only designed for use in extra-low voltage systems, or ELV. Technically that©s less than 50 volts, but this design was built specifically for use with 12V and 24V systems. Current and Power Rating If you short circuit a battery you develop a large current. Large lead-acid cells can create short circuit currents in excess of 500 amps. This is because the shorting wires have very little resistance. If you connect a resistor into the circuit the current will rise to a point that is dependent upon the value of the resistor (see the table on the next page to see how much current a resistor draws). Then, providing that the resistor has a big enough power rating, the The ELV Dummy Load resistor will happily burn-off the heat created by Copyleft 2005, The Free Range Network page 1 of 9 SSPJI-05: ELV Dummy Load for PSU Testing the current. Electric fires use the same principle. Maximum Design Current The bars of the electric fire have a resistance The current the load will draw is dependent that means one kilo-Watt of heat will be upon the voltage at which it operates and produced by the fire at mains voltage (and only the resistance of the load. Increasing the at mains voltage). voltage will increase the current. Increasing You can calculate the maximum current drawn the value of the load resistor will decrease by a load resistor using Ohms law: the current. The effect of different voltages and resistors is shown in the table below. Current (A) = Voltage (V) ÷ Resistance (R) However, this is the easy part. The most Load Supply Voltage important issue in the design of the dummy load Resistor 12V 24V is that the power output should not exceed the R A W A W power rating of the load resistor. You can 0.1 120 1,440 240 5,760 calculate the maximum power that a resistor will 0.15 80 960 160 3,840 dissipate using the power equation: 0.22 55 655 109 2,618 Power (W) = Current 2 (A) x Resistance (R) 0.33 36 436 73 1,745 Conversely, if you know the power rating of a 0.47 26 306 51 1,226 load resistor you can calculate the current that it 0.56 21 257 43 1,029 can handle by re-arranging the above equation: 1 12 144 24 576 Current (A) = √[Power (W) ÷ Resistance (R)] 1.5 8 96 16 384 The calculated power rating for a resistor usually 2 6 72 12 288 represents the level that will cause problematic 2.2 5 65 11 262 overheating. Therefore the resistor used should have a capacity about 1.25 to 1.5 times more 3.3 4 44 7 175 than the calculated figure. Likewise the main 4.7 3 31 5 123 current path in the circuit (discussed later) must The current (A) is calculated by taking the supply use wire that has a capacity of 1.25 to 1.5 times voltage and dividing by the value of the load resistor the maximum calculated current. (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). Current Control The power in Watts (W) that the resistor will As noted above, if you have a fixed resistor then dissipate can vary, and often the figure quoted connecting it to a battery or power supply will for represents a level at which the resistor will create a fixed level of current or power load. overheat. Therefore it©s a good idea to multiply Whilst useful for checking circuits at a single the power figure by 1.25 or 1.5 and use a load, it©s not very useful for checking how the resistor of this value. This will prevent power supply works at different power levels. excessive overheating of the load resistor. Therefore we need to be able to vary the power The maximum current a resistor can handle, drawn by the load without the hassle of irrespective of the operating voltage, can be continually disconnecting and reconnecting calculated by dividing the power rating of the different values of resistor. resistor, in Watts (W), by the value of the resistor, in Ohm (R), and then taking the The dummy load uses a field effect transistor, or square root of this figure to give the maximum FET, to switch the circuit on and off. By varying current in Amps (A). the period that the FET switches on or off, using a system called pulse width modulation (see the Another important factor in the design is that box on the next page), we can vary the amount the wiring from the power supply, to the load resistor, and then on to the transistor, must of time that the FET is on and how much load also be able to handle 1.25 or 1.5 times the the resistor will draw. maximum current ± or it will badly overheat. FET transistors are able to switch very large Copyleft 2005, The Free Range Network page 2 of 9 SSPJI-05: ELV Dummy Load for PSU Testing currents using a very small control voltage. That resistor and a single integrated circuit to change means we can avoid using very expensive the switching period, or duty cycle, of the FET power components, such as high power variable and hence how much current the load will draw resistors or rheostats. Instead the dummy load (see the box below). control circuit uses a single low power variable Current Control and Pulse Width Modulation (PWM) The power figures used to rate resistors are The figures below, taken from an oscilloscope, quoted in Watts. This is a measure of the power shows the triangular waveform and the output flowing every second. Therefore if we turn off the from the comparator. The comparator is either flow for half a second, only half the power, and fully on or fully off. We measure the width of the hence current, will flow. This is the theory behind pulses, or the duty cycle, in terms of a pulse width modulation, or PWM. By turning off percentage ± 100% is fully on and 0% is fully off. the current flow for a greater proportion of the At 100% the comparator is fully on. At 95% the time we reduce the current flow. control voltage rises just above the lower voltage of the triangular waveform and transistor is The dummy load control circuit (discussed on the turned off for 5% of the time. As the control next page) creates a triangular waveform that is voltage rises to one quarter and then three used to switch the load resistor on and off. The quarters of the waveform voltage the duty cycle triangular waveform oscillates between 1.2 volts decreases to 75% and 25%. At 5% the transistor and 6.8 volts. This is fed into a comparator along is switched off for most of the time and at 0%, with a control voltage that is set by a variable when the control voltage is higher than the resistor. If the control voltage is greater than the triangular waveform, it does not switch on at all. level of the triangular waveform the comparator switches the transistor off. If the control voltage is The effect of this is to reduce the current, and less than the voltage of the triangular waveform it hence the load, the load resistor draws. If a load switches the transistor on. Consequently the resistor theoretically draws 6 amps, at a duty interaction between the triangular waveform and cycle of 50% it will only draw 3 amps, or at 25% the rising control voltage switches the transistor 1.5 amps. This is how the control circuit of the off for a progressively longer period of time. dummy load is able to finely control how much Therefore we can finely control the width of the power the dummy load unit will draw. the pulses by varying the control voltage. 100% 75% 5% 95% 25% 0% Copyleft 2005, The Free Range Network page 3 of 9 SSPJI-05: ELV Dummy Load for PSU Testing The Control Circuit volts.