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Author Topic: M7G motor set up for Radar  (Read 1561 times)

david48

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M7G motor set up for Radar
« on: September 09, 2015, 05:29:44 PM »


I am using 12v battery with a voltage reducer, ( the Componant Shop one ). This is set to deliver 6 v . The motor is a M7G supplied with a diode IN4003 and  the resistor ( brown green brown gold ) for 6volt operation  . I have put these on a strip board ,as the drawing supplied with the motor ,also on the board are two connector blocks for power in and out. When I run the motor ,the diode gets hot and the reducer board get hot. Too hot to touch not just warm. The wire size is the same as is on the motor . I have remade all the joints ,the motor turns at the required speed and there are no tight spots.

 I understand that a diode is a one way valve , and the resistor is bringing the motor to the required RPM . As I am reducing current to the motor the remaining energy will be heat but I did not expect this amount . What  is the diode doing .
I feel as I am reinventing the wheel, all this must have been done before is the a definitive list of the parts required to make a driven radar scanner on a 1-76 model,it just has to turn between 24 and 36 rpm.
I thought I had cracked it when it all worked,but as said before it all gets to hot for me .

What am I missing or doing wrong .
David
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inertia

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Re: M7G motor set up for Radar
« Reply #1 on: September 09, 2015, 07:01:39 PM »

David
I wouldn't worry about it, m'duck. Our show demo board has exactly that setup, save that it runs from a 7.2v battery and not a 6v regulator. The resistor and diode are soldered to a thin piece of stripboard which is wrapped in heat-shrink and is suspended in the wiring underneath the top of the board. It runs happily for 6 hours continuously at every show and has never required any attention in eight years. If the heat bothers you then fit a fatter resistor and diode (i.e. components which will handle a bigger power drop - say a 0.6W resistor and 3A diode).
DM
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malcolmfrary

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Re: M7G motor set up for Radar
« Reply #2 on: September 10, 2015, 10:54:03 AM »

If it is a 1/4 watt resistor, dropping down to 6 volts from what is probably a bit more than 12 volts, it will be right on its power limit (12.5-6/150 gives about 40mA, 0.04A*6.5V gives just over 1/4 Watt).  If it has an insulating layer of heatshrink over it, it will be less efficient at passing its heat to the air, so will get hotter yet.  A physically bigger resistor of the same value but higher wattage rating will cure that problem.  Small high wattage resistors are usually made from material that will live through getting VERY hot, and need air space around them to avoid cooking their surroundings, melting plastic insulation and sometimes the solder holding them down while not doing the copper tracking any favours.  Bigger resistors have more surface area, so run cooler if the air can get away.
I haven't found the diagram, so just guessing, but the diode is a 3 Amp type, only passing about 40mA, and should not get warm unless the hot resistor is conducting heat to it via the wiring/PCB tracking.
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david48

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Re: M7G motor set up for Radar
« Reply #3 on: September 10, 2015, 01:06:33 PM »

Thank you Inertia and Malcolmfrary and Stan for the replays I have enough info now to resolve my set back . Will get back to it ,the pigeons are coming down on the Barley so there will have to be purge to discourage them .
Thanks again
David
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inertia

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Re: M7G motor set up for Radar
« Reply #4 on: September 10, 2015, 02:21:10 PM »

The 1N4003 is a 1A diode while the resistor is almost certainly just 1/4W. Maplin's standard resistors are 0.6W - and eye-wateringly expensive!
DM
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malcolmfrary

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Re: M7G motor set up for Radar
« Reply #5 on: September 10, 2015, 08:20:25 PM »

The 1N4003 is a 1A diode while the resistor is almost certainly just 1/4W. Maplin's standard resistors are 0.6W - and eye-wateringly expensive!
DM
Whoops yes - I forgot that the 1N4001 is a 50 volt 1A diode, the increasing number at the right hand end indicates the max voltage rather than increasing current ability.  1N4002 is 100v, 1N4003 is 200v, the entire series being rated for 1 amp.  If it is just protecting against reverse connection, it should only be passing motor current of about 40mA, so shouldn't get warm at all, unless something else is heating it.
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david48

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Re: M7G motor set up for Radar
« Reply #6 on: September 10, 2015, 08:39:16 PM »

It seem's as though it might be me ,regarding the heat build up when I get back to it I will remake all the joints again ,as DM says his set up is the same and has worked for 8 years but runs hot . I will get back at a later date with my results.
David
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