Silicon Chip – July 2019

siliconchip.com.au Australia’s electronics magazine July 2019 111

KEITH RIPPON KIT ASSEMBLY &
REPAIR:

• Australia & New Zealand;


• Small production runs.
  Phone Keith: 0409 662 794
  [email protected]

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 [email protected]

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@gmail.
com

MARKET CENTRE

Cash in your surplus gear. Advertise it here in SILICON CHIP

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 [email protected] and include your name, ad­

dress & credit card details, or phone Glyn (02) 9939 3295 or 0431 792 293.

ADVERTISING IN MARKET CENTRE

KIT ASSEMBLY & REPAIR

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

same­day shipping.

LEDs, BRAND NAME and generic

LEDs. Heatsinks, fans, LED drivers,

power supplies, LED ribbon, kits, com­

ponents, hardware, EL wire.

http://www.ledsales.com.au

FOR SALE

PCB PRODUCTION

PCB MANUFACTURE: single to multi­

layer. Bare board tested. One­offs to

any quantity. 48 hour service. Artwork

design. Excellent prices. Check out our

specials: http://www.ldelectronics.com.au

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 [email protected] or

phone 0401 157 285. Get a new PCB

and keep production going!

MISCELLANEOUS

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

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.

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)

WANTED

SC

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 ap-
propriately.
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 in-
stalled on the same heatsink, so how
can I best keep the circuit cool? (C. V.,
Ballarat, Vic)

• The speed pot varies the control

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 re-

sistor 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. Al-

ternatively, if you don’t need the feed-

back, omit this resistor entirely.

You are right that it’s probably just

the Mosfets which have been damaged.

Check that the PCB tracks connecting

to them are intact and not fused. Di-

ode D2 can run hot; it depends on the

motor speed setting as to whether the

diode conducts significant freewheel-

ing current.

Changing D2 to a schottky diode

with a sufficiently high voltage and

current rating should significantly re-

duce 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.