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ATX Power Supply Tester -ELEKTORATX Power Supply Tester -ELEKTOR, zasilacze
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ATX Power Sup Ton Giesberts PC power supplies can often be bought cheaply at places such as computer fairs. But it isn’t that easy to check if such a (second hand) power supply still works properly. This dedicated tester makes that job quick and straightforward. 46 pply Tester Checks all voltages New ATX 2.2 specification ATX connector 20-pin 24-pin pin 1 pin 11 pin 1 pin 13 This tester was designed for recent ATX power supplies, but it is also ready for use with new power supplies described in version 2.2 of the ATX specification. These have a main connector with 24 pins instead of 20 (75 Watt extra for use by PCI Express cards). There is a curiosity in the new specification regarding the -5 V con- nection. According to version 2.2 of the specification it is no longer used and the pin in question (20) is marked as NC (not connected). However, according to the manuals of several motherboards with a new 24-pin connector the –5 V is still present. So keep in mind that when you test a power supply with a 24-pin connector the –5 V output may or may not exist. The –5 V should always be present on a 20-way connector. The change from 20 to 24-pin connectors is compatible with the older 20-pin connectors, with an extra +3.3 V, +5 V, +12 V and ground added to one end. An older ATX power supply with a 20- pin connector fits in a 24-pin socket and can only be inserted one way, so mistakes aren’t possible. +3V3 +3V3 -12V GND PS - ON# GND GND GND -5V +5V +5V +3V3 +3V3 -12V GND PS - ON# GND GND GND +3V3 GND +5V GND +5V GND PWR - OK +5VSB +12V +3V3 GND +5V GND +5V GND PWR - ON +5VSB +12V NC +5V +5V pin 10 pin 20 +12V +3V3 +5V GND pin 12 pin 24 040112 - 12 Figure 1. The pin-outs for 20 and 24- pin ATX power connectors. Apart from the power supply and this tester, you’ll only need a mains cable (and socket!). All outputs from the power supply can be tested under load and any deviations from the nominal values are shown on 6 LEDs. to the motherboard. A quick test would be very useful then. The true hobbyists may also want to investi- gate the exact fault in a broken power supply. But it isn’t a straight- forward job to test a PC power sup- ply with a multimeter. The power supply tester described here is a very useful and compact tool. We have to admit that you prob- ably won’t need it very often. But once you have acquired one, word will spread amongst your circle of friends and you shouldn’t be sur- prised when you’re called to ‘quickly’ check a PC power supply for them. put voltages. The percentage devia- tion of a selected output is shown on 6 LEDs. Two of these LEDs show whether the deviation is positive or negative and the other four indicate the percentage difference from the required output voltage. For output voltages that are con- nected to more than one pin only the first pin is tested. (A power supply generates only a single +5 V supply, even though it is made available on several pins.) There is a 26-pin header (K2) on the PCB that can be used to test each pin individually. The outputs are con- nected through 1 kΩ resistors to pro- tect them against short circuits. If you connect an extension lead to this header you can use a multimeter to take measurements from any pin. Although the power supply in a PC has little bearing on its overall speed, there are times when it needs to be replaced. This may be because the old power supply has simply given up the ghost, and sometimes the internal fan has become too noisy, or an upgrade of the PC has increased the power requirements above that what the old power sup- ply can deliver. ATX power supplies are available from virtually every computer shop. When you buy a new power supply it is obviously safe to assume it will be in perfect working order. But when you buy a (used) power supply at a computer fair or boot fair you want to be sure that it works before you fit it into the case and connect it What is measured? Our tester doesn’t require a separate power supply, as it takes its power from the PC power supply under test. All you need to do is plug the power supply into the tester and then use a mains lead to connect it to the mains. A rotary switch is then be used to quickly check all the out- A look at the circuit An ATX power supply has a total of 6 output voltages, which all have to be tested: +3.3 V, +5 V, +5 V for 1/2005 - elektor electronics 47 pply Tester +3V3 R28 3k3 R29 28k7 IC1 = TS922IN +2V5 +5VSB R49 R30 100k C1 +5VSB IC3 = TS924IN +5V 6 12 220n R45 R31 100k 5 11 +5VSB S1.A S1.B 100k D 4 4 3 13 14 10 R34 IC2 R32 15k R33 365k 9 5 +12V 6 6 5...10% 2 8 7 G3 IC1.B 1 R55 7 R35 200k R37 453k 7 IC3.B 6 -5V 5 R36 27k R38 100k C2 MDX 14 11 R46 R51 R50 12 13 -12V 2 + 3X1 3X2 1 M1 +5V +5VSB +5VSB 220n 2 1 15 10 D 5 D 3 R39 10k0 +5VSB R13 9 10...20% <+/–5% R59 4 5 3 8 9 R53 IC3.C +5VSB +2V5 10 R40 10k0 R47 R52 D 2 R60 12k 74HC4053 2 POWER ON 1 D 7 P2 IC4 IC1.A D 6 3 R42 1k R43 1k T1 R11 10k 1k C6 2 neg. IC3.A 1 13 >20% 100p R48 14 3 IC3.D BC547B R41 R12 D 8 R61 12k 12 +2V5 R44 1M +5VSB LM4041 DIZ_ADJ pos. +2V5 R54 P1 8 C3 16 C4 4 C5 PWR_ON PS_ON 250 Ω IC1 IC2 IC3 11 K1 K2 4 100n 87 100n 100n 1 2 3 4 5 6 7 8 9 13 14 15 16 17 18 19 20 21 22 23 24 +3V3_3 R14 1 2 R16 +3V3_3 +3V3 +3V3_2 +3V3 1k 1k R15 3 4 R27 -12V +3V3_2 1k 1k -12V 5 6 +3V3 +5V +12V +5VSB R18 7 8 R57 +5V +5V 1k 1k R10 1k 9 10 R56 +5V_2 +5V_2 R19 11 12 1k R1 R2 R3 R4 R5 R6 R9 13 14 D 1 -5V R58 15 16 R26 -5V 1k 1k R23 17 18 R20 +5VSB +12V +12V_2 +3V3_4 +5V_3 +5V_4 +5V_5 +5VSB 1k 1k +5V_3 10W 10W 10W 10W 10W 10W 5W 10 11 12 R24 19 20 R21 +5V_4 +12V 1k 1k STANDBY +12V_2 R25 1k 21 22 1k R22 +5V_5 R7 R8 +3V3_4 R17 23 24 1k 25 26 ATX S2 PSU ON 5W 10W -5V -12V 040112 - 11 Figure 2. The measurement circuit itself is fairly small. A lot of room is taken up by the power resistors (R1-R9), which load the power supply. standby, +12 V, –5 V and –12 V. The standby voltage (+5VSB) is always present as long as the mains is con- nected. This voltage is therefore used as the supply for the tester ( Figure 1 ). LED D1 is driven directly from the +5VSB supply and hence indicates that the mains is turned on and that the power supply has at least a working standby voltage. The power supply is turned on by closing switch S2. This pulls pin PS_ON sufficiently low via R56. According to the specification this pin should be <0.8 V at 1.6 mA. A value of 470 A at a minimum voltage of 2.4 V, a buffer stage consisting of R11, R12 and T1 has been added. Once the mains is turned on (and D1 and D2 are lit), S1 is used to select the voltage that is connected to the input of amplifier IC1b. S1 is a 2-pole 6-way rotary switch (it has to be a break-before-make type, otherwise you’ll introduce shorts in the outputs). The first switch selects the supply voltage to be tested. The common output of this switch is also connected to a PCB pin (via a 100 Ω resistor for protection). It is possible to connect a small voltmeter module to this pin, so that the absolute value of the selected voltage can be seen. Next to the connection for the meter (M1) is an extra PCB pin with +5 V for the voltmeter module. The selected voltage makes its way via the common of S1b to one of the potential dividers connected to the inputs of IC1b. µ Each resistor combination gives the right amount of attenuation to the chosen voltage such that the output of IC1b will be a nominal 2.5 V at every position of S1. There is no need for a symmetrical power supply to measure negative voltages because IC1b is a rail-to-rail type opamp. With positive voltages IC1b func- tions as a non-inverting buffer. The two negative supply voltages are inverted and attenuated. We now take a small jump to the tol- erance LEDs in the circuit (D3-D8). According to the ATX specification all voltages should be within for R56 achieves this. The PWR_ON output, also called PWR_GOOD or PWR_OK, is used by the power supply to show that the most important outputs (+12 V, +5 V and +3.3 V) are within their limits and can supply a nominal current. When this signal is active, D2 lights up. Since this output can only source 5%, with the exception of -12 V, which may be ±10%. We have therefore cho- sen four tolerance ranges that are covered by the LEDs: <5% (green LED D3), 5-10% (yellow LED D4), 10- 20% (red LED D5) and >20% (second red LED D6). The range division at 10% was used to give you the choice whether to accept that deviation or ± 48 elektuur - 1/2005 200 Circuit details The potential dividers for IC1b have been designed as accurately as possible through the use of resistors from the E96 series. Three of the dividers are made with a (large) E96 and a (small) E12 resistor to get as close to the theoretical value as possible. Since the value of the E12 resistor is much smaller than that of the E96 resistor connected in series, it only has a small effect on the total tolerance. Hence a resistor from the E12 series is suitable here. Although capacitor C6, which is connected in parallel to reference zener IC4, is not essential according to the data sheet, a little bit of HF decoupling never does any harm with a switched mode power supply. R41 reduces the effect of the input bias current of opamp IC1a, keeping any error limited mainly to that from the tolerance of resistors R39 and R40. A small amount of hysteresis is required around IC3a to make it switch cleanly. This does introduce a small error near the zero point as far as a positive or negative deviation concerns ( ± 0.1%), but this is very small compared to the tolerance levels we’re looking at. For IC3b-d, which are used as comparators, we have intentionally used opamps rather than real com- parators because these usually have open-collector outputs. These wouldn’t be suitable for this purpose. The reference voltages (via R45-R48 and P1) for the comparators are 5%, 10% and 20% lower than the main 2.5 V reference (2.375 V, 2.25 V and 2 V respectively). Resistors R45 and R46 in the potential divider should of course have been exactly 500 , but 499 is a difference of only 0.2%, which is much less than the tolerance of the resistors themselves. not. A difference of more than 20% is not acceptable in any case. These LEDs are driven by compara- tors IC3b-d, which have their invert- ing inputs connected to a potential divider (R45-R48 and P1). This deter- mines the tolerance ranges with respect to the 2.5 V reference volt- age. P1 is used to set the reference levels as accurately as possible. This just leaves the section that joins the output signal from IC1b to the LEDs. This output signal is nominally 2.5 V and may be a bit more or less when it deviates. But the comparator circuit built round IC3b-d can only indicate negative differences. To get round this problem IC1a inverts the output signal from IC1b. This is fol- lowed by an analogue switch that can be controlled using a digital sig- nal. This switch is part of IC2 (a triple analogue multiplexer). The out- put signal from IC1b and the inverted one from IC1a are con- nected to inputs Y0 and Y1 of an analogue switch (pins 2 and 1 on IC2). The output of IC1a is also con- nected to opamp IC3a, which acts as a comparator and compares the sig- nal with the 2.5 V reference voltage. The output of IC3a acts as the con- trol signal for the analogue switch. When the deviation is negative (<2.5 V), IC3a switches pin 2 of IC2 to the output (pin 15), which is con- nected to the comparators. When the deviation is positive (>2.5 V), the inverted signal (pin 1) is connected to pin 15. In this way LEDs D3-D6 always show the deviation com- pared to the nominal value. The out- put of comparator IC3a is also con- nected to two LEDs, which indicate if the measured voltage is greater or smaller than the nominal value. The yellow LED (D7) is lit when the volt- age is lower and the red LED (D8) indicates that the voltage is higher than the reference voltage. The 2.5 V reference voltage men- tioned a few times previously is sup- plied by an LM4041DIZ-ADJ (IC4) made by National Semiconductor. This voltage can be adjusted to exactly 2.5 V with preset P2. All outputs from the ATX power sup- ply are provided with a resistive load, where some outputs are loaded more than others. The +3.3 V and +5 V outputs often require a mini- mum load for the power supply to operate correctly, and are therefore loaded more heavily. To avoid exces- sive heat generation we haven’t taken the maximum power from the supply, but have limited it to some 45 W (R1 to R9). Construction The PCB designed for the tester is shown in Figure 3 . The dimensions of the PCB have been kept as small as possible and are not based on any particular enclosure. The ATX power supply connector is on the edge of the PCB, so that this can stick out through the side of an enclosure. 1/2005 - elektor electronics 49 COMPONENTS LIST H2 Resistors: R1,R2 = 2 2 10W R3,R4 = 3 3 10W R5,R6 = 22 10W R7 = 33 5W R8 = 33 10W 5W R10,R13-R27,R42,R43,R49,R51- R54,R57,R58,R59 = 1k R11,R12 = 10 k R28 = 3k 3 7 R30,R31,R34,R38 = 100 k Ω R33 = 365k R55 R35 = 200k R59 R60 Ω R37 = 453k Ω R39,R40 = 10k 0 R41 = 4k 99 R61 IC1 Ω R45,R46 = 499 R43 R47 = 1k 00 R45 R44 R42 R48 = 7k 87 IC3 IC2 R50 = 820 R55 = 100 R47 R52 Ω R60,R61 = 12k R50 R49 R51 H4 P1 = 250 preset P2 = 1k preset Capacitors: C1,C2 = 220nF C3...C5 = 100nF C6 = 100pF Figure 3. There is room on the PCB for all components. The power resistors are mounted on top of each other. Semiconductors: D1,D2,D5,D6,D8 = LED, red, low- current D3 = LED, green, low-current D4,D7 = LED, yellow, low-current T1 = BC547B IC1 = TS922IN (ST Microelectronics, Farnell # 332-6275) IC2 = 74HC4053 IC3 = TS924IN (ST Microelectronics, Farnell # 332-6299) IC4 = LM4041DIZ_ADJ (National Semiconductor, Farnell # 271-263) This makes it much easier to insert the connector from an ATX power supply. There are no ‘special’ parts on the PCB. As long as you take care with the polarity and values of all compo- nents, and solder neatly, you should- n’t have any problems with the con- struction. All the power resistors are also mounted on the PCB. Due to the heat these generate they should be mounted at least 2 or 3 mm above the PCB, otherwise the PCB will give off smells. (The resistors will do that in the beginning anyway). Resistors R1, R3 and R5 are mounted another 2 to 3 mm above R2, R4 and R6. This method of construction leaves enough air around the power resis- tors for ventilation. Before you mount the board into an enclosure or drill any holes, you should make a careful note of the dis- tance between the rotary switch and the ATX power supply header. The wiring for the LEDs and the on/off switch can be made with thin stranded wire. Since this circuit generates a fair amount of heat, it is advisable to use a metal enclosure with sufficient (possibly even forced) cooling. A miniature 5 V fan will be essential if you use a small enclosure. This can be connected to the +5 V pin for the voltmeter module. Make sure that you have enough ventilation holes in the enclosure. To give the tester a professional look, and make it easier to use, we have produced a front panel, which is shown at a reduced size in Figure 5 . Miscellaneous: K1 = 24-way angled ATX header, PCB mount (Molex 39291248, Farnell # 413-8508) K2 = 26-way boxheader (2x13) S1 = 2 pole 6 position rotary switch, PCB mount S2 = on/off switch, 1 contact Optionally: M1 = 3 1 / 2 -digit LCD voltmeter module, range 0-20 V (e.g., Farnell # 422- 0146) Enclosure: e.g., type 1455L1601BK (Hammond Manufacturing) Calibration and operation There are two presets on the PCB that can be used to set the tester up accurately, although the circuit works perfectly well when they are set to their mid-position. For those of you who want to set the tester up as accurately as possible we’ll explain the calibration procedure. PCB, order code 040112-1 , see Readers Services page 50 elektuur - 1/2005 R9 = 10 R29 = 28k R32 = 15k R36 = 27k R44 = 1M R56 = 470 [ Pobierz całość w formacie PDF ] |
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