Fullscreen HTML5Med Res (??MB)HTML4

MARKET CENTRE Cash in your surplus gear. Advertise it here in SILICON CHIP FOR SALE KIT ASSEMBLY & REPAIR PCB MANUFACTURE: single to multi­ layer. Bare board tested. One-offs to any quantity. 48 hour service. Artwork design. Excellent prices. Check out our specials: www.ldelectronics.com.au KEITH RIPPON KIT ASSEMBLY & REPAIR: * Australia & New Zealand; * Small production runs. Phone Keith 0409 662 794. keith.rippon<at>gmail.com LEDs, BRAND NAME and generic LEDs. Heatsinks, fans, LED drivers, power supplies, LED ribbon, kits, components, hardware, EL wire. www. ledsales.com.au tronixlabs.com - Australia’s best value for hobbyist and enthusiast electronics from adafruit, DFRobot, Freetronics, Raspberry Pi, Genuino and more, with same-day shipping. PCBs MADE, ONE OR MANY. Any format, hobbyists welcome. Sesame Electronics Phone 0434 781 191. sesame<at>sesame.com.au www.sesame.com.au ETI VALVE GUITAR AMP POWER TRANSFORMER, EA Valve Guitar Amp Output Transformers 60W, NOS RCA USA 6550, Luxman 6240G. Phone 0400 494 294. VINTAGE RADIO REPAIRS: electrical mechanical fitter with 36 years ex­ p erience and extensive knowledge of valve and transistor radios. Professional and reliable repairs. All workmanship guaranteed. $10 inspection fee plus charges for parts and labour as required. Labour fees $35 p/h. Pensioner discounts available on application. Contact Alan on 0425 122 415 or email bigalradioshack<at>gmail.com DAVE THOMPSON (the Serviceman from S ILICON C HIP) is available to help you with kit assembly, project troubleshooting, general electronics and custom design work. No job too small. Based in Christchurch, NZ but his services are available Australia/NZ wide. Email dave<at> davethompson.co.nz Where do you get those HARD-TO-GET PARTS? Where possible, the SILICON CHIP On-Line Shop stocks hard-to-get project parts, along with PCBs, programmed micros, panels and all the other bits and pieces to enable you to complete your SILICON CHIP project. SILICON CHIP On-Line SHOP www.siliconchip.com.au/shop KEEP YOUR COPIES OF SILICON CHIP AS GOOD AS THE DAY THEY WERE BORN! ONLY 95 $ 1P6LUS p&p A superb-looking SILICON CHIP binder will keep your magazines in pristine condition. * Holds up to 14 issues * Heavy duty vinyl * Easy wire inserts ORDER NOW AT www.siliconchip.com.au/shop ADVERTISING IN MARKET CENTRE Classified Ad Rates: $32.00 for up to 20 words (punctuation not charged) plus 95 cents for each additional word. Display ads in Market Centre (minimum 2cm deep, maximum 10cm deep): $82.50 per column centimetre per insertion. All prices include GST. Closing date: 5 weeks prior to month of sale. To book, email the text to silicon<at>siliconchip.com.au and include your name, address & credit card details, or phone Glyn (02) 9939 3295 or 0431 792 293. I found the issue. Values for R1 are provided for the currents of 200mA, 400mA and 500mA, 600mA. According to the data sheet for the LM317T, the formula for working out the current for R1 is I = Vref ÷ R. So for example, working it out for 200mA, for the given value of R1 = 1.2W, I = 1.25 ÷ 1.2 gives 1.041A and not 200mA. Shouldn't it be 6.2W, ie, I = 1.25 ÷ 6.2 gives 0.201A or 201mA? Please correct me if I am wrong. (R. M., Perth, WA) • The current limit is set by transistor Q1 and not the LM317T. So as current flows through Rcl, there is a voltage siliconchip.com.au produced across this resistor. When Rcl is 1.2W and with 200mA current, there will be a voltage drop of 0.6V across the resistor and so the transistor base voltage will be 0.6V above the emitter. The transistor will conduct, pulling the adjust pin of REG1 low. This has the effect of reducing the output voltage of REG1 to limit current to 200mA. The calculation of the current limit you provide for the LM317 is when the output is connected to the adjust pin via a resistor and current is drawn at the adjust pin. However, in the Float Charger circuit, the arrangement is different and the output of REG1 is set by resistor R2 in series with R1, between the output and adjust terminals. Overall output voltage above the adjust pin is set by the current through R3 and VR1. Resistor R1 in this circuit is only required to protect against a short circuit at the output. In the event of a short circuit, Q1 will switch on fully but this can only reduce the output voltage to around 1.25V. This voltage then appears across R1 which dissipates the excess May 2017  111