Manual do usuário LEKTOR ATX PSU

[. . . ] An older ATX power supply with a 20pin connector fits in a 24-pin socket and can only be inserted one way, so mistakes aren't possible. +3V3 +3V3 GND +5V GND +5V GND PWR-OK +5VSB +12V pin 10 +3V3 -12V GND PS-ON# GND GND GND -5V +5V +5V pin 20 +3V3 +3V3 GND +5V GND +5V GND PWR-ON +5VSB +12V +12V +3V3 pin 12 +3V3 -12V GND PS-ON# GND GND GND NC +5V +5V +5V GND pin 24 040112 - 12 Figure 1. The pin-outs for 20 and 24pin 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. 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 supply can deliver. [. . . ] The two negative supply voltages are inverted and attenuated. We now take a small jump to the tolerance LEDs in the circuit (D3-D8). According to the ATX specification all voltages should be within ±5%, with the exception of -12 V, which may be ±10%. We have therefore chosen four tolerance ranges that are covered by the LEDs: <5% (green LED D3), 5-10% (yellow LED D4), 1020% (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 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 comparators 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 comparators IC3b-d, which have their inverting inputs connected to a potential divider (R45-R48 and P1). This determines the tolerance ranges with respect to the 2. 5 V reference voltage. 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. [. . . ] A 20-way plug is connected to the `bottom' of the connector on the PCB, i. e. It won't fit any other way due to the shape of the plug and socket. The power supply should then be connected to the mains, and the mains turned on. If that isn't the case then the power supply has a serious fault and is best discarded. [. . . ]