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Old 6th Nov 2020, 7:54 pm   #1
Backtoreality
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Default Valiant Turtle Pen Motor Circuit

Hi,
I have a Valiant Turtle where the pen motor doesn't operate. The driver IC is a uln2001a . I've attached the circuit below. R39 and R40 are listed as 22R but are actually thermistors. On my board one reads as 52K and the other 39K. The schematic as seen says STC thermistor YR115D14013220. I assume that originally 22R resistors were fitted and later boards had thermistors fitted instead. On a second board I have the thermistors read approx 50R. Can anyone suggest a suitable replacement for the thermistors which have gone high or should I just put some 47R resistors in instead?
Cheers,
David
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Old 6th Nov 2020, 8:18 pm   #2
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Default Re: Valiant Turtle Pen Motor Circuit

Do you have any data for that specified thermistor?

I'm wondering if it's PTC with a short-ist thermal time constant so the solenoids get a slug of current to get them moving, and then the current backs down to a more suitable holding current. If so, you not only need to get the resistance the slope and the right direction of thermistor, you also have to to worry about the thermal timeconstant.

This may have been a mod to reduce the heat getting dissipated in the solenoids or to make life less stressful on the PSU.

It obviously worked with 22 Ohm resistors, but got improved.

David
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Old 6th Nov 2020, 8:31 pm   #3
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Default Re: Valiant Turtle Pen Motor Circuit

If that's the aim then perhaps putting a suitable electrolytic capacitor in parallel with the resistor would have achieved the same result (an initial rush through the electrolytic as it charges, then after the capacitor has charged up, just the holding current through the 22R).

Are these thermistors physically placed is such a way that they can 'feel' the temperature of the solenoids, or are they not strategically placed next to anything else?
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Old 7th Nov 2020, 12:44 am   #4
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Default Re: Valiant Turtle Pen Motor Circuit

There are no solenoids involved, just a miniature motor with a cog.
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Old 7th Nov 2020, 12:56 am   #5
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Default Re: Valiant Turtle Pen Motor Circuit

As the temperature rises from room temp to finger temperature the resistance drops by 3 or 4 ohms.
David
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Old 7th Nov 2020, 1:38 am   #6
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Default Re: Valiant Turtle Pen Motor Circuit

OK, thete's stuff missing from the diagram. If pen1 and pen2 are the two wires to the motor, then the transistors can switch either wire to 12v, but there is nothing shown other than two lines going off leftwards that could pull either line pen 1 or pen 2 down.

I'd made the assumption that pen 1 and pen 2 meant two pens! and assumed the pure single-ended drivers had to be for solenoids to ground (The Calcomp plotters controlled coil current to set pen-paper pressure).

So I'm now assuming there must be the rest of a bridge driver somewhere?

David
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Old 7th Nov 2020, 10:34 am   #7
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Default Re: Valiant Turtle Pen Motor Circuit

the circuits for the turtle can be found here:

https://research.roamer-educational-...rets-revealed/

Cheers

Mike T
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Old 7th Nov 2020, 12:17 pm   #8
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Default Re: Valiant Turtle Pen Motor Circuit

Circuit showing the driver IC ULN2001A as well.
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Old 7th Nov 2020, 6:48 pm   #9
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Default Re: Valiant Turtle Pen Motor Circuit

Yeah, that's using the output transistors in two channels of the ULN2001A as the bottom parts of a full bridge driver, and the BC327 transistors act as inverting top switches for the bridge, driven by two more channels of the ULN2001A.

They could have economised by having a single thermistor in series with the motor, unless they were using a trick with the thermal time constant of the thermistor.

The ULN2001A also shows that tit's a bang-bang switch and not an analogue drive.

So I'm now wondering could the thermistors be something like a polyswitch, to limit motor current when it stalls into an endstop? It's a common technique to limit the force on electrically operated windows to reduce their capabilities in the guillotine directions.

It explains how the thermistors have had a hard life and have worn out.

David
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Old 8th Nov 2020, 12:01 am   #10
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Default Re: Valiant Turtle Pen Motor Circuit

Quote:
Originally Posted by Radio Wrangler View Post
So I'm now wondering could the thermistors be something like a polyswitch, to limit motor current when it stalls into an endstop?

It explains how the thermistors have had a hard life and have worn out.

David
Hi David,
I think you're right and that is probably the purpose of the thermistors. Can you suggest a replacement or do you think I should try 47R resistors?
David
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Old 8th Nov 2020, 12:18 am   #11
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Default Re: Valiant Turtle Pen Motor Circuit

I'd try with the original 22 ohm resistors (make them 2 watt) They should be good for functional tests so you can see what gets hot. If the heat is too much in the motor then you're looking for a 22 Ohm ish value thermistor at room temperature and a positive temperature coefficient. If the motor seems to have plenty of torque, then you can reduce the current with a higher resistance value. It's a trial and error job, I'm afraid.

Come to think of it, having two thermistors as the circuit does, means a change of direction gets to start with a cool thermistor which sounds a good idea.

David
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Old 8th Nov 2020, 1:21 am   #12
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Default Re: Valiant Turtle Pen Motor Circuit

The motor only runs for a fraction of a second while raising or dropping the pen so I wonder if 2W resistors necessary? A cam keeps the pen holder up once it's up and gravity keeps it down. Should I measure the current drawn by the motor?
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Old 8th Nov 2020, 1:53 am   #13
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Default Re: Valiant Turtle Pen Motor Circuit

It would be a good starting point

If it's only momentary, then I wonder what the thermistors are for. Is it likely the mechnism could jam and the motor/drivers could need protection?

David
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Old 8th Nov 2020, 1:07 pm   #14
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Default Re: Valiant Turtle Pen Motor Circuit

The motor does just over two revolutions to lift or drop the pen. I'll have to do some reading to see if any version of the software leaves the motor energised. I don't think the version I have does so.
David
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Old 8th Nov 2020, 7:57 pm   #15
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Default Re: Valiant Turtle Pen Motor Circuit

The motors appear to be Mabuchi 3V motors. Powering with a 3V battery they take between 220 and 250mA.
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