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Vixen MD-5 controller and step motor questions

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#1 erraph

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Posted 31 January 2012 - 11:40 AM

Good evening!

I have a couple of questions regarding a Vixen MD-5 Controller. I asked these questions already at another place, but I guess that the cloudynights-community is significantly larger which might increase my chance of getting this time all questions answered :D.
You can tell from the questions that I don't have any experience with step-controllers... but I'm very willing to learn :-)

Ok, lets start with the practical practical questions:
p1) Whats the voltage needed to make it run? Is it rather "flexible" (like 6V-9V?) or do I have to reach a certain precise voltage?
p2) Are these controllers really *that* damageable by reversing accidentally the polarity? [ok I actually already got an answer on this questions: "Yes". But what is "dying" within the controller when reversing polarity?]
p3) What current does a standard motor for the MD-5, together with the controller, "consume"?
p4) Can I run the controller using rechargeable batteries?

Theoretical questions:
t1) Whats the difference between the old motors used with an MD-5 controller and the new MT-1-motors? I heard they wouldn't be compatible.
t2) Can I reduce the risk of damage by reversing accidentally the polarity by simply putting a diode between battery and controller?
t3) If the voltage differs slightly from the design-value, does the motor rotate with a different speed, or will just its momentum be different?
t4) Whats the risk the controller will die in the near future, given its age? Are the components still available, so I could repair it? What about the motor -- as its not compatible with MT-1-motors, can I modify an MT-1-motor to work with the MD-5 controller?

Ok thats it for now... would be so great to get some help!
Have a nice evening!

Raphael

#2 erraph

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Posted 03 February 2012 - 07:36 AM

Good morning astronomers!
I'm sorry for bothering you; are these MD-5 controllers so uncommon that nobody knows anything about them?
Do you have any advice where else I might ask for help?

Have a good day!
Raphael

#3 EdZ

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Posted 03 February 2012 - 04:11 PM

I'd say you have rather unique questions. If it is not so very common a product, even though there may be thousands of astronomers on this site, you need to have one of them that knows your product happen to visit this forum, and then have the answers to your questions. That narrows the field down considerably.

I have two Vixen motors and controllers but don't know anything about the model you've asked about. In fact, I couldn't even answer those questions regarding the controllers I do have.

edz

#4 ccs_hello

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Posted 03 February 2012 - 08:32 PM

Raphael,

It's hard to dig out info from the past (1986) but I'll try.
An internal picture would be helpful.

p1) Whats the voltage needed to make it run? Is it rather "flexible" (like 6V-9V?)
A1: usually the circuit has a built-in voltage regulator thus is kind of flexible (up to some extent).

do I have to reach a certain precise voltage?
A1': not really.

p2) Are these controllers really *that* damageable by reversing accidentally the polarity? [ok I actually already got an answer on this questions: "Yes". But what is "dying" within the controller when reversing polarity?]
A2: don't know if the circuit has a built-in diode for voltage reversal protection or not. If not, reverse voltage will be a VERY bad thing. How bad? Depend on the circuit design.

p3) What current does a standard motor for the MD-5, together with the controller, "consume"?
A3: this type typically won't exceed 150mA in tracking and no more the 500mA in high speed slewing.

p4) Can I run the controller using rechargeable batteries?
A4: yes, so long as minimum voltage is maintained.


Theoretical questions:
t1) Whats the difference between the old motors used with an MD-5 controller and the new MT-1-motors? I heard they wouldn't be compatible.
A1: don't know. One is 1986 era and the other is newer (but probably a slightly different design.)

t2) Can I reduce the risk of damage by reversing accidentally the polarity by simply putting a diode between battery and controller?
A2: yes. A Schottky diode (low voltage drop) is recommended.

t3) If the voltage differs slightly from the design-value, does the motor rotate with a different speed,
A3: no. frequency is stabilized by the crystal.

or will just its momentum be different?
A3': most likely not

t4) Whats the risk the controller will die in the near future, given its age?
A4: Don't know. It's 20 or more years old.
All depends on the workmanship.

Are the components still available, so I could repair it?
A4': may be. Good news is that simple stepper driver is not a state of art, if you are very, very good in electronics and circuit design.

What about the motor -- as its not compatible with MT-1-motors, can I modify an MT-1-motor to work with the MD-5 controller?
A4'': don;t know. May be other stepper used in a similar mount would work better.

Many of your questions are hypothetical thus only fuzzy answers can be provided.

Clear Skies!

ccs_hello

#5 sky_dave

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Posted 03 February 2012 - 10:46 PM

Hi Raphael,

Vixen used two different stepping motors with the Polaris,SP, and GP line of mounts. The original motor has a 5 pin connector. The coils are rated for 9V with a gear ratio of 1:300.

The second motor Vixen used has an 8 pin connector. The coils are rated for 6.3V with a gear ratio of 1:120. This is what the MT-1 is. This motor also went by other designations.

OK, which motor do you have? You can remove the three screws from the motor and read the sticker on the backside.

I have repaired several Vixens controllers that are for use with the 8 pin type motors. It is always the 7805 5VDC regulator that breaks. This part cost two bucks at Radio Shack. Cheaper from online part stores. It is easy to replace. You just need basic soldering skills.

I've repaired an MD-6 controller. It works with the 8-pin motors. So let us know which motor type the MD-5 works with.

I've not worked on any controller that is for the 5 pin motors. I'm guessing that it will use a different voltage regulator since the motor has a different voltage rating. The stepping rate will be different since the gear ratio is different.

It is quite common for the voltage regulator to fail in the older 8 pin motor controllers. They originally came with a 6 C-cell battery pack that puts out 9VDC. People hook them up to 12VDC packs. The higher voltage overheats the regulator and it fails.

Dave

#6 erraph

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Posted 04 February 2012 - 07:23 AM

Good morning ccs_hello, hello Dave!

Thank you so much for your replies.
1986 -- I didn't know the drive was older than me...

>> A2: don't know if the circuit has a built-in diode for voltage reversal protection or not.

It does not. I found out about *many* people losing their controllers because of reversing polarity accidentally at night -- and because the vixen power plug is designed quite different from what is usually used: the central pin of the power plug is the GND, and the "shielding" is V+!!

>> A2: yes. A Schottky diode (low voltage drop) is recommended.

So I will put a diode in series with the V+ entrance. I found diods with voltage drop between 0.3V and 0.7V, so I'll go for the version with lower voltage drop.

>> They originally came with a 6 C-cell battery pack that puts out 9VDC. People hook them up to 12VDC packs. The higher voltage overheats the regulator and it fails.

So I will go for a 8* 1.2V Battery pack, which gives me 9.6V -- and as the diode will let the voltage drop for another 0.3V, we are at 9.3V which should be just fine I think -- as long as my eneloops really keep the voltage of 1.2V each.

>> A3: this type typically won't exceed 150mA

Perfect. I just use the drive for tracking, and thus with 2000mAh batteries it will easily run for a whole night.

>> OK, which motor do you have? You can remove the three screws from the motor and read the sticker on the backside.

Its the 5-pin-connector version as you described it, which makes sense as the drive is from the Vixen SP-age.
Thus the controller probably differs from the ones you have repaired.

I'm sorry for the hypothetical nature of some questions. I agreed on buying a small used vixen travel mount at a rather fair price, equipped with a MD-5 controller and motor. But before investing in technology that might fail in the future (because of its age) I wanted to make some research how to prevent this from happening.
I got quite a lot of information from your replies, thank you so much!!

Enjoy your weekend,
Raphael

ps: thanks EdZ for your supporting words ;-)






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