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Add load protection to your power supply Do you have a dual tracking power supply? If you switch it on or off while a load is connected, you may damage circuit components in the load. This add-on board prevents dangerous voltages from being delivered by any dual tracking power supply when it is turned on or off. By JOHN CLARKE This is one of those projects that you may not have realised you needed - until now, that is. Let us outline the problem - and then we'll present the cure, which is this add-on board. All adjustable regulated power supplies use one or more op amps to control their main regulating elements. Fig.1 shows the general scheme for an adjustable regulated power supply. Essentially, it consists of a reference voltage source, Vref, an error amplifier, and the 66 SILICON CHIP & LEO SIMPSON series pass element, Ql. For a dual tracking power supply, such as the ± 50V supply described in the April 1990 issue of SILICON CHIP, the circuit arrangement is quite a lot more complicated but, essentially, it uses two series pass elements and two error amplifiers. The problem The error amplifiers are the cause of the particular problem we wish to discuss. Most op amps will work normally while their own sup- ply rails are within normal design values. Most designers run op amps with supply rails of ± 15V but they will work more or less normally even when their supply rails drop to ±3V. So provided the error op amps in an adjustable power supply have their own supply rails somewhere between ± 3V and ± 15V, they will operate as they should and the output voltage dialled up on the meter will be delivered to the output terminals. But what happens when the supply rails to the error op amp drop below ± 3V? The op amp loses control, that's what. Instead of acting as an "error amplifier" and closely controlling the series pass transistor (Ql in Fig.1), it loses control. And since the behaviour of op amps is not specified and is therefore not predictable, when their supply rails drop to low values, they often do the worst possible thing, and turn the series pass element(s) full on. This can mean that when you Fig.1: general scheme for an adjustable regulated power supply. The error amplifier compares the output voltage with a reference voltage (VREFl and generates an error voltage which controls series pass transistor Ql. turn a power supply off with the load connected, the supply voltage to the load may increase markedly just before it drops to zero. This can be a real problem if the load voltage was set for say, 5 volts, and the supply voltage jumps to 9 volts at switch-off. If the load was a TTL circuit, the chips could all be damaged. The situation can be worse if your power supply is set for even lower voltages. So far then, we have seen how the error op amps in a power supply can lose control when the unit is Facing page: this view shows the load protection board installed on the rear panel of the SILICON Cl-DP ± 50V Dual Tracking Power Supply. The unit can be fitted to virtually dual tracking supply, however. turned off. But the same thing can happen in reverse, when the power supply is turned on. And here, the voltage delivered to the load may be much higher than the setting you used on the last occasion, before switching it off. You can avoid both of these damaging scenarios if you remember to use the "load" switch on your power supply. That way, you turn the power supply on, adjust it for the desired output voltage, and then hit the load switch. When you switch off, you should disconnect the load with the load switch, before turning it off. But if you don't have a load switch on your power supply or you forget to use it, it can cause damage, as we have described above. And that's where our "Load Protection Switch" comes in. It is a small printed board con- 33 + 56 01 BC640 k.,. D1 B J'j· 2.2 + 63VWI 2x1N4004 02 . C The circuit of the load protection switch is shown in Fig.Z. As shown, it has component values to suit the ± 50V Dual Tracking Supply described in the April 1990 issue of PARTS LIST 1 PCB, code SC04204901, 60 x 70mm 1 2V relay with DPDT 5A contacts (Altronics Cat. S-4190) 8 PC stakes 4 6mm standoffs 4 machine screws and nuts to suit standoffs Semiconductors 1 BC640 PNP transistor 1 BC639 NPN transistor 1 BC546 NPN transistor 3 1 N4004 1 A diodes Resistors (0.25W , 5 %) 1 1 MQ 2 22kQ 1 1 00kQ 1 1 0kQ 1 56kQ 2 390Q 5W 100k 40VAC FROM TRANSFORMER SECONDARY How it works Capacitors 1 33µF 16VW PC electrolytic 1 2.2µF 63VW PC electrolytic 1M ---------'IW..-----0+60V 16VWJ taining a relay and a few other components and it can be installed in almost any regulated power supply. It is wired in to delay the connection of voltage to the supply output terminals whenever the supply is turned on. And it disconnents the output terminals of the supply immediately it is turned off, so that those damaging jumps in the output voltage just don't get to the load. In effect, it is an automatic load switch. RLY1~ .,. .,. l~ +L0A0 OUTPUT -LOAD OUTPUT EOc BOE BC546 BC639 BC640 VIEWED FROM BELOW POWER SUPPLY LOAD PROTECTION SWITCH Fig.2: the circuit uses D1 & D2 to rectify the transformer secondary and forward bias Q3. However, before Q3 (and thus RLYl) can turn on, the 33µ,F capacitor must charge up from the + 60V rail via a lMD resistor and turn on Q2 & Ql. Miscellaneous Solder, heavy duty hookup wire. SILICON CHIP but it can be adapted to almost any supply. It works as follows: Dl and DZ monitor the voltage from the transformer in the power supply. When power is applied, Dl and DZ rectify the AC and produce a DC voltage across the 2.2µF capacitor [with 40VAC the DC voltage will be about 60V). This applies a bias voltage to the base of Q3 via the ZZkQ resistor. This would normally let Q3 turn on to JUNE 1990 67 time of switch-on of the relay, but Q3 controls the time it switches off. The two 3900 5W resistors act as dropping resistors so that the voltage applied to the 12V relay is correct. As presented, the circuit would be suitable for almost any power supply with main (unregulated) DC rails up to 60V. For lower supply rails, the 5W dropping resistors would have to be reduced in value to allow the correct voltage to be applied to the relay. The two 390!] 5W resistors should be mounted about 1mm proud of the PCB to allow cooling. Be sure to use heavy duty cable for the input and output connections. power the relay but before that can happen, Ql must also conduct. Ql can't turn on initially because it is turned on by QZ and QZ can't turn on until the 33µF capacitor is charged by the lMO resistor from the + 60V rail. It takes about a second or so until the 33µF capacitor is charged sufficiently to allow QZ and Ql to turn on. This switches the relay on and connects the output terminals of the power supply to the supply rails. When the supply is switched off, the 60V rail(s) will take quite some time to drop to zero but the supply derived by Dl and DZ will drop almost immediately, because it is stored in a very small capacitor (2.ZµF). So effectively, Q1 , QZ and their associated components control the Construction We have designed a small printed circuit board (coded SC 04204901, 60 x 70mm) to accommodate the components. Begin construction by installing the 8 PC stakes. Next, the transistors and diodes can be inserted with due consideration to the correct type and polarity as shown on the overlay diagram. The 5W resistors should be mounted 1mm above the PCB to allow cooling. Now install the capacitors and resistors, noting the correct polarity for the capacitors. Finally, install the relay. The PCB is now ready for installation into the power supply. We mounted the load protection board into the ± 50V Dual Tracking Power Supply mentioned above. The following installation instructions apply to this power supply and will have to be varied when mounting it in other supplies. The PC board is mounted on 6mm Fig.3: here's how to install the parts on the PC board. Be sure to use the correct transistor type at each location and take care with component orientation. 68 SILICON CHIP standoffs on the rear panel in the clear area between the heatsink and mains cable entry. There are four supply leads to be connected to the PCB plus the relay load contact connections. The 40V AC connections are made to the secondary of the transformer, while the earth connection can be at the centre tap connection of the transformer secondary. The + 60V connection is made to the spare PC stake near the positive side of the filter capacitors. The load connections to the relay are made at the plus and minus output terminals on the power supply PCB, using heavy duty hookup wire. The minus output connects to the - input of the relay and the - output of the relay goes to the load switch via the inductor and filter capacitor mounted on the load switch. Similarly, the positive output from the power supply PCB connects to the + input of the relay and the + output of the relay connects to the load switch via the inductor and filter capacitor located on the load switch. Testing Now the Power Supply Load Protection switch is ready for testing. Apply power and wait to see if the relay switches on after about one second. This should apply the load voltage to the input of the load switch. When mains power is switched off, the relay should immediately switch off. ~ Fig.4: this is the full-size PC artwork.