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Control your Meccano models (or anything else) from your Windows PC! One disadvantage of the simple Darlington pair motor speed control discussed in the previous article is that about 0.7V is lost in each transistor, so the maximum voltage that can ever be applied to the motor is Vs - 1.4V. The circuit shown in figure 1 partially solves this problem by reducing the voltage loss to 0.7V. It is still a Darlington pair configuration, but this time the second transistor is of a type known as 'PNP'. The first transistor is an 'NPN' type of transistor.
To increase the range of the potentiometer you may want to place a low value resistor (try 470Ω) between the lower end of the potentiometer and 0V. This should start the motor turning slowly as soon a the potentiometer shaft is turned. Building the CircuitA stripboard layout for this circuit is given in figure 2. The layout includes space for an optional 6-way screw terminal block to make connecting up the circuit easier. The power supply for this circuit should preferably be un-smoothed (i.e. directly from the power supply rectifier). This helps prevent the motor 'sticking' at low speeds. With the TIP32C transistor given, the maximum power supply voltage may be 60V and the maximum motor current consumption may be 3A. Pin-out diagrams of the two transistors used in this circuit are given in figure 3. The TIP32C transistor will get hot in use because the high current supplying the motor flows through it. You should therefore fit a heat sink to it to dissipate the heat - a suggested heat sink is given in the Shopping List, or you could make one out of Meccano! Space is provided in the stripboard layout for a heat sink, but the positions and sizes of the holes are not shown for they will depend on the nature of the heat sink you choose. Construction
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Part Description |
Quantity Required |
Rapid Code |
Unit Price |
Total Price |
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1kΩ 0.2W Miniature Linear Potentiometer | 1 | 65-0705 | $0.56 | $0.56 | ||||
1N4148 Signal Diode | 1 | 47-3416 | $0.01 | $0.01 | ||||
38mm Heatsink for TO220 Style Transistor Cases | 1 | 36-0226 | $1.02 | $1.02 | ||||
3-way PCB Mounting Terminal Block | 2 | 21-0473 | $0.20 | $0.40 | ||||
470Ω 0.25W Metal Film Resistor | 1 | 62-3442 | $1.40 (per pack of 100) |
$1.40 | ||||
BFY51 Medium Power NPN Transistor | 1 | 81-0122 | $0.40 | $0.40 | ||||
Stripboard SRBP (36 rows x 50 columns) | 1 | 34-0515 | $1.66 | $1.66 | ||||
TIP32C High Power PNP Transistor | 1 | 81-0822 | $0.72 | $0.72 | ||||
Grand Total | $6.17 |
Order the above items from Rapid Electronics via our Circuits Shop and help support the Electronics in Meccano website, without affecting the price you pay! Just click on an item for information about it and/or to add it to your Rapid order.
Orders can also be placed by telephoning Rapid on 01206 751 166 or visiting their website at www.rapidonline.com. Prices quoted include VAT at 20%.
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