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Got a Boat, Van, RV, 4WD, etc? Need Light? Really Bright 12/24V LED Oyster Light By Ross Tester Here’s a low-cost, 1000+ lumen, attractive “Oyster” LED light fitting for when you don’t have mains available. Or even if you do! It runs from 1224VDC or even from 230VAC with an optional (low cost) mains adaptor. A few months ago (gad, was it really January 2013?) we presented a “LED Solar Skylight” from Oatley Electronics. It was (and still is!) very popular for introducing light into dark corners. Now they’ve come up with another low-voltage LED-based light fitting which is ideal for boats, caravans, RVs and mobile homes, trucks, 4WDs . . . in fact, anywhere you have 12-24V DC available. And it could also be used as a mainspowered LED light with a low cost, optional AC adaptor (albeit with a bit of fiddling). There’s virtually nothing to build with this one. The Oyster-style lamp housing is already fitted with a PCB containing 24 ultrabright, pure white LEDs in two concentric rings, connected in series/ parallel. In the centre of these is a driver PCB which supplies the 40V <at> 260mA required by the LEDs Overall current drawn from a 12V sup42  Silicon Chip ply is around 1.1A, dropping to about 550mA from a 24V supply. Light output from the Oyster fitting is more than 1000 lumens. Looking directly at it, you’d swear it was a lot more! Incidentally, for comparison a traditional 36W fluoro tube puts out about 3500 lumens but that’s over a much wider length. Oyster size is about 260mm diameter x 75mm deep, so it’s not too dissimilar to other domestic light fittings. An integral 235mm diameter aluminium backing plate/heatsink is designed to be screwed to a ceiling/ wall/bulkhead/etc and the Oyster diffuser mounts on that via a twist onto three pins. Now a quick word to the wise: the diffuser is made of quite thin (and somewhat fragile) plastic so if you go at it like a bull at a gate, you’re liable to put your thumb or fingers right through it (we speak from sad experience, don’t we boss. . .). When disassembling, which is simply a matter of turning the diffuser with respect to the base, treat with a bit of care! The LED driver The driver PCB circuit is shown in Fig.1. You don’t need to worry about polarity of the DC supply as this is taken care of by a bridge rectifier at the input. However, this results in a not-soinsignificant voltage drop across the two bridge rectifier diodes (2 x ~0.6V or about 1.2V). Therefore you’re effectively throwing away 10% if it is a 12V supply. Directly feeding the filter capacitor (C1) rather than through the bridge would be more efficient, albeit at siliconchip.com.au rent boost LED driver. Inside IC1 is a switching Mosfet which turns on and off at about 180kHz. Every time it turns off, the magnetic field built up in L1 collapses, inducing a higher voltage at the anode of diode D5. This is rectified by D5 and stored by capacitor C3/C4. Ordinarily, this voltage could be quite high but is limited to 56V by zener diode ZD1 (IC1’s maximum voltage is 60V). If the voltage across C3/C4 exceeds 56V, the Zener conducts and stops the inverter by applying a voltage to IC1’s feedback pin (pin 5). The output voltage is further clamped to approximately 40V by the 24 high-brightness LEDs connected in series/parallel at the output. Effectively there are 12 LEDs in series, each dropping about 3.3V (12 x 3.3  40V). The voltage at the feedback pin of IC1 (pin 5) controls the duty cycle of the Mosfet, while pin 2 (the enable pin) can be shorted to ground to stop the inverter working. We use this pin to allow it to “soft start” the inverter so it can be used with a switch-mode supply, as detailed shortly. 24 ultra-high-brightness white LEDs are powered by an integral 12/24V driver (ignore the DC12V sticker!). The backing hardware also doubles as a LED heatsink. the expense of the reverse-polarity protection provided by the bridge. If you wanted to, you could feed the circuit with AC (obviously via the bridge) – say from 8 to 18 volts. However, this would require a much larger “filter” capacitor – the existing one is 100F; you would need at least ten times this for AC (preferably more – 2200 or 3300F would not be too much if it woud fit). This capacitor is bypassed by a 100nF. Power is applied directly to inductor L1 and IC1, a 60V, 4A switching cur- Operating on 230VAC Oatley Electronics have a verylow-price switch-mode power supply (KC24) which can deliver 24V DC at up to 1A. On first glance, this would appear to be ideal for powering the Oyster LED light from the mains. D5 L1 A + K A C3 100F K ZD1 D1 – D4 K 12–24V DC OR 8–18V AC A K A K 4 A 100nF IC1 XL6005 Vin FB A A  1k  K K A A  260mA 50V 10F* LED OYSTER LIGHT DRIVER  K APPROX 40V C1 100F WHEN POWERED BY AC THIS CAPACITOR SHOULD BE >1000F  K 5 GND 1 EN 2  K A K A A 25–35V siliconchip.com.au C4 100nF 56V 3 SW A K 50V A  *SEE TEXT 1.2 (OATLEY ELECTRONICS) 2.2 – K  K 24 x 0.5W LEDS IN SERIES/PARALLEL Fig.1: here’s the LED driver which is contained in the white container at the centre of the photo above. It supplies around 40V DC <at> 260mA from a low voltage DC or AC source. April 2015  43 XL6005 D1 – D5, ZD1 (SW) Oatley’s KC24 230V to 24V <at> 1A switch-mode power supply. It’s suitable for use with the Oyster LED light but that requires a small modification, as described in the text to stop the power supply shutting down at turn-on. Unfortunately, appearances can be deceiving! By its very nature, the Oyster LED lamp has quite a high surge current at switch-on, which is enough to trip the over-current protection circuitry in the KC24 supply, which turns off almost straight away. The high surge current disappears, the power supply again tries to start, resulting in a high surge current . . . etc etc! The result is that the Oyster LED Lamp “strobes” - a neat feature if you’re having a party but not quite so good if you’re looking for light! The KC24 power supply is sealed, so no adjustment is possible there (not that we’d want you to because it is a mains-powered device and can therefore “bite” very hard). But it’s easy enough to open up the LED driver in the Oyster LED Light and modify it slightly so that it “soft starts”, eliminating that surge current. All you need do is insert a small (say 10F) capacitor in series with the “enable” pin of the IC, which introduces a delay of a few seconds before operation is enabled. In fact, if you buy the Oyster LED light and power supply kit from Oatley Electronics, they will include a 10F, 35V electrolytic capacitor for this purpose. You’ll need to lift the enable pin (pin WARNING: 230V LED Driver An alternative 230V AC LED driver (285MACC) is also available but WE STRONGLY RECOMMEND THAT YOU DO NOT USE THIS DRIVER . It is not an isolated supply so the whole LED mounting base can become live – indeed, we checked this driver with the Appliance Insulation Tester described elsewhere in this issue and it instantly tripped it (ie, isolation <700k). Stick to the SAFE 12/24V LED driver and if you want to run it from the mains, use the KC24 switch-mode supply as detailed above. Incidentally, we checked the KC24 supply with the same tester and it passed with flying colours. 2) off the PCB. It’s not hard to do this as it is a surface-mount device – melt the solder with your soldering iron and use a hobby knife to prise the pin up, ensuring you’ve removed all the solder underneath (Solder Wick is handy for this). Solder the positive lead of the capacitor to the pin (not the PCB!) and the negative lead to a suitable earth point – we used the top end of resistor R3, as shown in the photo. That’s all there is to it. Power supply connections Power wires, either 12-24V DC or 8-18V AC go through a hole in the back of the Oyster LED light, very close to the push-button terminal block. As we mentioned before, you don’t need to worry about polarity so simply connect either wire to either terminal. Mains supply The KC24 power supply from Oatley comes with a short length of mains cable, fitted with a “figure 8” plug on one end (the end which plugs into the supply). The opposite end is bare – you’ll need to fit it with a standard mains plug. Make sure you connect the two leads Here’s that modification required to the LED driver – a 10F capacitor is added in series with the “enable” pin of the IC to slow its start down. This prevents it overloading the power supply and shutting it down. The PCB first meeds to be removed from the white housing in the middle of the LEDs. 44  Silicon Chip to the Active and Neutral terminals in the plug – they’re either labelled “A” and “N” or in some cases colour coded, usually red (Active) and black (neutral). In newer plugs, the colour coding may conform to the IEC colours of brown (Active) and blue (Neutral). Do NOT connect anything to the plug’s Earth terminal (“E”, green or green-gold) and make sure that you slide the rear cover all the way onto the plug body. The DC end will probably have a 4-pin DIN plug on it which has to be cut off. Stripping the outer insulation back will reveal four wires – red and black (positive) and green and white (negative). Note that – the black wire is definitely positive – it’s a trap for young players. You only need to use one pair of wires (ie, there’s no need to parallel them), say the red and the green wires, for DC out. These go through the hole in the back of the Oyster LED light to the push-button terminal block. As we mentioned before, you don’t need to worry about polarity. SC Where from, how much? The LED Oyster Light is available from Oatley Electronics, PO Box 139, Ettalong Beach, NSW 2257; phone [02] 4339 3429 (www.oatleyelectronics.com). On its own, the LED Oyster Light as described here (with 12/24V LED driver) sells for $25.00, inc. GST, (Cat No K400). The KC24 230V AC to 24V DC switch-mode supply sells for $9.95 inc. GST. However, at the time of writing Oatley Ekectronics are selling BOTH the LED Oyster and switch-mode supply for just $28.00 (Cat No K400P1), inc. GST. And if you want two sets, the price reduces to only $50.00 for both inc. GST (Cat No K400P2). All prices are plus freight. siliconchip.com.au