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Fig.1: the circuit is based on an LR8N3 3-terminal regulator. Power comes from an external high-voltage DC source – eg, the high-tension (HT) line of a valve receiver or from the 12AX7 Valve Preamp Power Supply described in November 2003 SILICON CHIP. A Reformer For Electrolytic Capacitors By RODNEY CHAMPNESS Simple electrolytic capacitor reformer is easy to build and has six switchable output voltages ranging from 25V to 400V DC. This simple circuit is based on an LR8N3 voltage regulator which has an input voltage rating of 450V DC and a maximum current output of 20mA – all in a TO92 package. Fig.1 shows the circuit details. The input to the reformer is powered from up to 450V DC and this can be obtained from a suitable valve receiver. Diode D1 provides reverse polarity protection, while a neon indicator in series with a 560kW resistor across the supply line warns users that a high, potentially fatal, voltage is connected to the device. The reforming voltage (ie, the voltage applied to the capacitor) is set by switch S2 which adjusts the resistive 102  Silicon Chip divider connected between the output and adjust terminals of the regulator (REG1). Switch S1 is selects between Reform, Off and Discharge. The output current is monitored by measuring the voltage across a 1kW resistor. In operation, each milliamp through the resistor registers as 1V on the meter. The voltage across the capacitor itself can be measured using a digital multimeter. When reforming is complete, S1 is switched to the Off position. This allows the operator to observe how quickly the capacitor discharges. The slower the voltage decreases, the less leakage there is in the capacitor. Finally, S1 is switched to the dis- charge position. This discharges the capacitor so that it is safe to handle. Note that the discharge resistor is only rated at 1W even though the peak dissipation in the discharge mode is around 16W. However, this is for such a short time that no damage is sustained. The high-tension (HT) DC input voltage can be obtained from a working receiver. This receiver MUST USE a mains transformer. Do not even think of connecting the reformer to a transformerless mains-operated set – not if you want to live, that is. The reformer should be connected to the receiver’s HT supply via high-voltage leads and an insulated terminal block. (Editor’s note: if you want to build a self-contained unit, the 12AX7 Valve Preamp Power Supply described in November 2003 can be used to provide the HT. As described, this delivers a HT voltage of 260V but you can set this higher by reducing the 47kW resistor next to trimpot VR1. Alternatively, you could modify the Valve Preamp Power Supply to do the complete job by having switchable resistors in the feedback network, so that various output voltages could be selected. Note that current limiting using a suitable resistor would be required and you would need to fit a discharge circuit, to discharge the capacitor after reforming). The author’s prototype reformer was built on Veroboard and housed in a small plastic case. If you build the device, remember that it works at high voltages, so keep a liberal spacing between the various parts. A plastic case is necessary because of the lethal voltages present in this device. For this reason, be sure to use Nylon screws to mount the board (no metal screws should protrude through the case). An external insulated terminal block was used for the metering points and I simply tighten down the screws to hold the probes in place. Safety improvements Editor’s note: instead of using a terminal block, we strongly recommend using recessed banana sockets for the metering points. These can be mounted on an internal bracket and suitably recessed inside the case to eliminate the risk of user contact with high voltages. It’s then just a matter of making up some high-voltage meter leads with matching banana plugs. siliconchip.com.au The author’s prototype used an external terminal block to provide the voltage and current metering points. A better (and safer) scheme is to use recessed banana sockets instead, along with some suitable test leads – see text. Similarly, use recessed banana sockets for the high-voltage output terminals and make up some output leads with banana plugs at one end and fully-shrouded crocodile clips (with high-voltage insulation) at the other end (see text). Where To Purchase The LR8N3 Similarly, we strongly recommend that recessed banana sockets be used for the high-voltage output. A pair of high-voltage output leads (one red, one black) can then be made up, fitted with matching banana plugs. The other ends of these output leads should be fitted with fully insulated (fully shrouded) crocodile clips (also called “safety croc clips”). You can buy fully shrouded crocodile clips with high-voltage insulation from RS Components (www.rsaustralia. com). WES may also have them. DO NOT use conventional crocodile clips with exposed ends (and minimal insulation), as shown in the photo. Remember – we are dealing with high voltages here. Using the device A HT filter capacitor in a receiver that’s being restored can be reformed in the following way. First, remove all valves from the receiver and check that there are no shorts or bleeder resistors across the HT line. Alternatively, you can simply disconnect one lead of the capacitor from circuit. That done, switch S1 to discharge, connect the reformer to the capacitor and select the appropriate reforming voltage (it must not exceed the voltage rating of the electrolytic that’s being reformed – or any other capacitors siliconchip.com.au connected to the set’s HT line for that matter). Now switch to the reform position and apply power to the reformer. Initially, the current will be about 12mA but will quickly drop as the LR8N3’s thermal protection circuit kicks in. If the capacitor is reforming, the voltage across it will slowly climb until it reaches the reforming voltage. Finally, when reforming is complete, turn off the power to the reformer and switch S1 to the Discharge position. This will discharge the capacitor and make it safe to remove the leads but you should always use a multimeter connected directly to the capacitor’s terminals to confirm that it has indeed discharged before touching it. Don’t simply rely on the discharge circuit – if the discharge resistor goes open circuit, the capacitor will still be charged. The LR8N3 featured in this article can be purchased from Wagner Electronics Services (WES), 140 Liverpool Rd, Ashfield, NSW 2131. Orders can be phoned through to (02) 9798 9233 or faxed to (02) 9798 0017. The part number is LR8N3-G and it is priced at $4.98 plus postage and packing. Payment may be made by cheque, money order or credit card. The procedure for reforming an electrolytic capacitor out of circuit is virtually the same. Make sure that the capacitor is securely located on an insulated surface, preferably inside a plastic container). The whole process can take up to around three minutes, depending on how much reforming is required and the size of the capacitor. One limitation of this unit has is that the reforming current isn’t very high but if the capacitor can be reformed, it will get to the selected voltage in time. It also can not handle 525V and 600V electrolytics but can only reform them to about 400V (depending on the SC applied HT voltage). WARNING! This electrolytic reformer circuit operates at lethal voltage. DO NOT build or use it unless you are experienced at working with high voltages and understand exactly what you are doing. Note that the leads to the capacitor operate at high voltage and that a fullycharged capacitor can deliver a potentially fatal shock. Always discharge the capacitor before disconnecting it from the reformer and use your multimeter to confirm that it has indeed discharged before touching it. October 2006  103