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MARKET CENTRE Cash in your surplus gear. Advertise it here in SILICON CHIP FOR SALE WANTED KEITH RIPPON KIT ASSEMBLY & REPAIR: * Australia & New Zealand; * Small production runs. Phone Keith: 0409 662 794 keith.rippon<at>gmail.com tronixlabs.com.au – Australia’s best value for supported hobbyist electron­ ics from Adafruit, SparkFun, Arduino, Freetronics, Raspberry Pi – along with kits, components and much more – with same-day shipping. Speaker enthusiast needs a copy of a book once sold by Jaycar entitled “High Power Loud Speaker Enclosure Design & construction”. It had a catalogue num­ ber BC1166. DAVE THOMPSON (the Serviceman from SILICON CHIP) is available to help you with kit assembly, project trou­ bleshooting, general electronics and custom design work. No job too small. Based in Christchurch, NZ but service available Australia/NZ wide. Email dave<at>davethompson.co.nz LEDs, BRAND NAME and generic LEDs. Heatsinks, fans, LED drivers, power supplies, LED ribbon, kits, com­ ponents, hardware, EL wire. www.ledsales.com.au PCB PRODUCTION MISCELLANEOUS VINTAGE RADIO REPAIRS: electri­ cal mechanical fitter with 36 years ex­ perience and extensive knowledge of valve and transistor radios. Profession­ al and reliable repairs. All workmanship guaranteed. $17 inspection fee plus charges for parts and labour as required. Labour fees $38 p/h. Pensioner discounts available on application. Contact Alan, VK2FALW on 0425 122 415 or email bigalradioshack<at>gmail. com 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 ASSORTED BOOKS FOR $5 EACH Selling assorted books on electronics and other related subjects like audio, video, programming etc. The books are relatively old in most cases and vary in condition. You'll need to come in person to see what books we have and what we're willing to sell: Silicon Chip 1/234 Harbord Road (up the ramp) Brookvale NSW 2100 (02) 9939 3295 KIT ASSEMBLY & REPAIR NEED A NEW PCB DESIGNED? Or need to update an old board? We do PCB layouts from circuits, drawings, photocopies or sample boards. Contact Steve at sgobrien8<at>gmail.com or phone 0401 157 285. Get a new PCB and keep production going! Will pay $50 (including postage) to the first person who has a pristine copy, i.e., little use but slight dog ears ok. Contact Melanie (on behalf of inquirer on 02 8832 3100) ADVERTISING IN MARKET CENTRE Classified Ad Rates: $32.00 for up to 20 words (punctuation not charged) plus $1.20 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, ad­ dress & credit card details, or phone Glyn (02) 9939 3295 or 0431 792 293. hotter than the Mosfets. Why is that? I will be using the circuit to power two type 895 24V motors (360W each) on my bicycle, so I need to make sure I keep these components cooled appropriately. I am planning to use three or four Mosfets. The multimeter testing I did on the Mosfet/diode cases suggest the Mosfets and diode D2 can’t be installed on the same heatsink, so how can I best keep the circuit cool? (C. V., Ballarat, Vic) • The speed pot varies the control siliconchip.com.au voltage from 0 to 5V, but this voltage is also affected by the feedback resistors from the motor negative (M-) terminal. This feedback is what is causing the motor to run faster at the lowest speed setting. You can alter the feedback resistor to reduce the effect. The 18kW resistor is located near REG1 on the PCB and can be increased in value. We suggest you try 47kW. Alternatively, if you don’t need the feedback, omit this resistor entirely. You are right that it’s probably just the Mosfets which have been damaged. Australia’s electronics magazine Check that the PCB tracks connecting to them are intact and not fused. Diode D2 can run hot; it depends on the motor speed setting as to whether the diode conducts significant freewheeling current. Changing D2 to a schottky diode with a sufficiently high voltage and current rating should significantly reduce its dissipation, to less than half that of a standard high-current diode. You can attach both diode and Mosfet to the same heatsink as long as you use insulating washers and bushes. SC July 2019  111