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Old 4th April 2014, 06:24 PM   #1
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Default Help with a DIY leslie Speaker...

Hello! I could really use some help with a lesie style speaker i'm putting together, somewhat like this one: DIY Rotary Speaker - YouTube

or even at the simpler end of the spectrum, this one: DIY kompakt and leight-weight Leslie-type Speaker (1st prototype) - YouTube

I'm really quite new to DIY electronics but what I lack in knowledge I make up for with enthusiasm.

I have so far gathered most of the parts I need - A motor from a radio control helicopter and the helicopters rechargeable power pack, an on off switch, and a potentiometer from some old PC speakers.

The issue i'm having is I want to use the potentiometer to control the motor speed however it doesn't seem to like it very much, it works (just) but the speed control is bunched up all at one end and when I pull it right down and right back up again I get a puff of smoke out of the pot. Any ideas on what I could do to resolve this?

Thanks
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Old 4th April 2014, 06:31 PM   #2
Mooly is offline Mooly  United Kingdom
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Welcome to diyAudio

So, smoking pot

OK, the pot smokes because the motor draws infinitely more current than a pot can handle. What you need is a new pot and an NPN power transistor to make a simple speed controller. Actually there is one more part... a resistor... to ensure the pot doesn't burn under certain conditions. Dead easy to wire up.

You are going to want it drawing now I suppose
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Old 4th April 2014, 06:40 PM   #3
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Originally Posted by Mooly View Post
Welcome to diyAudio

So, smoking pot

OK, the pot smokes because the motor draws infinitely more current than a pot can handle. What you need is a new pot and an NPN power transistor to make a simple speed controller. Actually there is one more part... a resistor... to ensure the pot doesn't burn under certain conditions. Dead easy to wire up.

You are going to want it drawing now I suppose
Such an incredibly quick reply, thank you so much!

I hate to be a pain but would you be able to tell me what value pot, transistor and resister i would need and also how I would go about wiring them up?

I'm trying to find some simple projects to get my working knowledge up and running but its still quite a steep learning curve for me, so your help is greatly appreciated.

Many thanks
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Old 4th April 2014, 06:43 PM   #4
Mooly is offline Mooly  United Kingdom
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So like this. The pot goes across the battery. The wiper of the pot goes to the BASE of the power transistor. The COLLECTOR goes to the battery positive. The EMITTER goes to the motor and then on to the battery negative.

The pot gives a variable voltage from 0 volts to battery voltage at the wiper. That voltage is fed to the transistor which amplifies the tiny current from the pot wiper and allows a much larger current to be drawn from the transistor emitter. The emitter voltage will follow the base voltage.

Simples
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Old 4th April 2014, 06:49 PM   #5
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Originally Posted by Mooly View Post
So like this. The pot goes across the battery. The wiper of the pot goes to the BASE of the power transistor. The COLLECTOR goes to the battery positive. The EMITTER goes to the motor and then on to the battery negative.

The pot gives a variable voltage from 0 volts to battery voltage at the wiper. That voltage is fed to the transistor which amplifies the tiny current from the pot wiper and allows a much larger current to be drawn from the transistor emitter. The emitter voltage will follow the base voltage.

Simples
That is fantastic, i'll pick up the parts and report back.

Thanks again for your help.

James
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Old 4th April 2014, 06:50 PM   #6
Mooly is offline Mooly  United Kingdom
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Such an incredibly quick reply, thank you so much!
Your welcome.

In the simple diagram I have drawn, the pot needs to be "lowish" in value because of the finite current gain of the transistor. If you can get hold of a "Darlington" power transistor such as TIP 110 etc then the circuit remains the same but the pot can be higher in value and the circuit would deliver more current if needed. I suspect an ordinary power transistor will do just fine for this though.

There are countless suitable transistors available, the TIP41 is a common device though and that or similar should be available anywhere for pin money.
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Old 4th April 2014, 06:53 PM   #7
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That is fantastic, i'll pick up the parts and report back.

Thanks again for your help.

James
OK. If you are not sure on anything then just ask.

And one thing to bear in mind, the transistor (depending on the current you pull through it) could get hot so it might need a small heatsink. If the motor is only drawing a few tens of milliamps it won't... so see how it goes.
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Old 4th April 2014, 06:59 PM   #8
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Originally Posted by Mooly View Post
Your welcome.

In the simple diagram I have drawn, the pot needs to be "lowish" in value because of the finite current gain of the transistor. If you can get hold of a "Darlington" power transistor such as TIP 110 etc then the circuit remains the same but the pot can be higher in value and the circuit would deliver more current if needed. I suspect an ordinary power transistor will do just fine for this though.

There are countless suitable transistors available, the TIP41 is a common device though and that or similar should be available anywhere for pin money.
Sorry to be a further pain, i've just been on the site for my local electronics store and they don't appear to have a TIP41 or 110, would any of these do the job? :Search npn | Maplin

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Old 4th April 2014, 07:05 PM   #9
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The TIP31C is fine,
HTTP 301 This page has been moved

The BD237 would work too (although the TIP is preferred as its more rugged) but the base and emitter pin outs are reversed compared to the others,
HTTP 301 This page has been moved

Always check the pinouts for any device you use. You can get them all here, just type the device number into the blank box,
Datasheet catalog for integrated circuits, diodes, triacs, and other semiconductors, view
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Old 5th April 2014, 07:49 AM   #10
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A bit more info for you...

Be careful wiring it all up. Transistors can be damaged in an instant if the polarity is wrong or you overload them. Make sure itall looks like in the diagram before switching on. Don't stall the motor because that could draw more current than the transistor can cope with or at least make it run to hot.

And be careful with the model batteries. They have enormous current delivery potential and if shorted could easily "weld" any wires that accidently touch.

Final thought on the basic circuit...

As drawn it allows a 0 to say 12 volts output (whatever the battery voltage is. If you wanted finer control then you could add a resistor in series with the pot. For example a 9k resistor in series with a 1k pot would allow a control voltage of 0 to 1 volts. Just ohms law. 9 volts would be across the 9k and 1 volt across the 1k pot.

Further refinement. You could add a series resistor to feed the pot and use a "zener diode" to generate a stable reference voltage. The zener would connect across the pot and would ensure that the motor ran at constant speed as the battery voltage dropped.

Whatever zener voltage you chose would be the maximum voltage that the circuit would apply to the motor. The resistor could be a 1k for this. Typical zener values would be 3.3, 4.7 or 5.6 volts.
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