Building a GOTO Mount

[Gordon WParticipant @gordonw]

I have no objections at all about this thread, and similar, but can I put in a plea for them to be written in English ?

[Neil WyattModerator @neilwyatt]

> I thought the 2 links I posted might be of interest.

They certainly are, but over the weekend I had to put in about 12 hours of work on my 'other job' whilst juggling with various issues on here…

My problem for alignment is that LP, trees and buildings mean that my sky is like looking up a well. Getting two stars

I suspect that the best bet for me is either autoguiding, so PA is less important, or any system that only needs a couple of stars in limited locations.

The advantage of 100% homebrew is I can just add what I need, when I need it. Bear in mind I will enjoy working out my own software as much as enjoy using it – I even feel using a Polulu is a bit of a cheat

Very busy week now pulling MEW226 into shape. I am hoping that next week I can get some serious workshop time to at least line up my three priority projects ready to go: the mount, my 'ancient engine' and the Adept.

Neil

[AjohnwParticipant @ajohnw51620]

Posted by Gordon W on 16/11/2015 09:56:52:

I have no objections at all about this thread, and similar, but can I put in a plea for them to be written in English ?

If you are interested in this area you will have to make use of the web to find out what terminology means. Often wiki what ever it is will produce some information. Unfortunately due to the way some one wrote the software that runs this site I can't put the letter x m l together as one word as it will assume I am not posting plain text.

John

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[Michael GilliganParticipant @michaelgilligan61133]

Neil,

Referring back to KWIL's comment, and my follow-up [on page 1]

Thinking about it further: Your mount may well be used for imaging deep-sky with a camera rather than a telescope, and therefore I revise my recommendation …

"Building a GOTO mount for the Telescope Astronomy" would be an exemplary title for the thread.

MichaelG.

.

P.S. … I simply suggest this as a practical way of helping people to navigate the site.

[AjohnwParticipant @ajohnw51620]

When I entered goto mount into google I had 1.5m hits and all I can see is details of goto mounts. Even one pointing out that they might be called go to mounts as well.

Now I have added more trash to the thread that doesn't have any relevance to the subject..

John

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[Neil WyattModerator @neilwyatt]

OK, I have put together a schematic for the mount driver. It's not going to work as a picture, so if anyone is interested, here is a link to it as a PDF, produced in Eagle.

Goto_Scope_Schematic.pdf

Comments welcome.

Neil

[Ian PParticipant @ianp]

Neil

Curiosity has got the better of me, is the purpose of JP4 or the switch so that you can test the PSU?

IanP

[jason udallParticipant @jasonudall57142]

Neil.
I don’t have the ramps schematic to hand..
But from memory it (and is normal practice anyway) has a resistor to ground and in series with the gate of the mosfets…gate looks like a significant cap…so a bit of limit for the microcontroller outputs…
Also the ramps layout has one or to considerations for the care and feeding of the the stepper carriers.
..
Thats all I see..
..
again the ramps has decoupling and power planes …

[Neil WyattModerator @neilwyatt]

Well spotted Jason, I would normally put 100R resistors in there for Justin (Justin Case).

Also I used the order code for the mosfets instead of the part number! It's RFD14N05L.

> again the ramps has decoupling and power planes …

All my decoupling is sitting up at top left!

Will see what the layout looks like but will be sure to have some caps by the carriers and keep their 12V well away from the 5V bits – ah, just noticed none of my decouplers are 12V – moved one down by the carriers and changed it to the 12V net.

Next challenge is the layout, hope to be able to do it SS.

Neil

[jason udallParticipant @jasonudall57142]

Flyback/catch diodes for the mosfets loads. To appease Justin.
I might also suggest header/dip switch for ms0 -2..one never knows when it might be handy. . ( if you have the spare bits maybe micro controlled)..also ..vref on the stepper carriers..lead it out because accessing it on the polo is fiddly.

Edited By jason udall on 24/11/2015 15:52:16

[Neil WyattModerator @neilwyatt]

> Flyback/catch diodes for the mosfets loads.

Resistive loads only.

> To appease Justin. I might also suggest header/dip switch for ms0 -2..

I have it under s/w control at the moment, but if you switch from one step mode to another and back you lose your place. For this application lost steps are a big no-no and at 20 MHz I can stall the stepper at maximum microstepping anyway.

> vref on the stepper carriers..lead it out because accessing it on the polo is fiddly.

The onboard pot isn't too fiddly, set and forget.

Neil

[jason udallParticipant @jasonudall57142]

Well I find proding those carriers a bind..

[Neil WyattModerator @neilwyatt]

I'm good at fiddly bits.

Neil

[AjohnwParticipant @ajohnw51620]

They show drive currents up to 20ma per pin but some where in the data sheet there will be a limit on total drive current across all ports – well there should be but I can't see one so if at work I would ask.

A bit naughty on the mosfets Neil. The port pins are floating during reset and till you do something with them so there should be pull down resistors. Where I come from there would probably be a cheap inverter in the way back to the processor and a pull up resister on the front of that, The port pins do have clamp diodes on them though so should be ok without the additional gate. I'd looks and see how well the fets switch with 2k in the gate circuit and not go lower unless really needed.

The automotive industry use truck loads of IR avalanche rated logic level mosfets for solenoid driving, hence some being dirt cheap. No clamp diode needed for inductive loads. Increasingly high side drives are used from the same source so some of those should be dirt cheap too. You may well find these have other advantages ie pull ups and resistors in the way etc. Things have gone that way as it keeps the power source in the ECU rather than trailing about all over the place.

John

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[Neil WyattModerator @neilwyatt]

I'm not minded to build in extra components for the purpose of protection that isn't needed for the application in hand. I'm not switching motors and with a lead-acid battery a few hundred mA into the dew heaters at switch on isn't going to cause any woes.

I can find plenty of examples of no or much smaller gate resistors, and the transistors I've chosen are meant for direct drive.

Now I've wasted lots of brain power on trying to work pout what actually happens (I'm not blessed with SPICE).

With 100R a first estimate value for the peak current would be 5V/100R=50mA and a gate capacitance of 700pF means the time constant for the gate is 30nS, so after 30nS the current will be 24mA.

But the maximum current from the I/O pin also depends on the rise time and for an AVR that is about 20+(0.1 x load-capacitance-in-pF) or about 90nS.

Modelling it as best I can using excel, the gate voltage lags the output voltage and assuming the output voltage rises linearly, the maximum current through a 100R resistor is 15mA, around the middle of this graph.

, so the peak current would be limited more by the rise time of the gate than the resistor and is unlikely to approach the 40mA limit, even for nanoseconds.

Equally, pull down resistors are only an issue if the mosfets being switched on could cause problems – unlikely here.

[jason udallParticipant @jasonudall57142]

Gates do fail sc

But yeah a couple of R saved is well worth it.

[AjohnwParticipant @ajohnw51620]

It up to you Neil not me. As for instance looking at ab IR golden oldy

**LINK**

More expensive but I would be thinking in terms of something like this.

**LINK**

Max switching frequency about 3khz – way way to high, drive series resistance 4 to 10K. There has been some debate in astro circles about using low side drivers when these are available.

In TO220 they start at 5amps as shown here

**LINK**

In surface mount, some are easy to hand solder, there will be more available and some pcb copper can be used as a heatsink. There is info about on their web site about that, often in the data sheets but I have only ever needed to look at that on straight ordinary mosfets.

John

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[Neil WyattModerator @neilwyatt]

Ignore that. I made some wrong assumptions.

[Ian PParticipant @ianp]

Posted by Ian Phillips on 23/11/2015 21:35:25:

Neil

Curiosity has got the better of me, is the purpose of JP4 or the switch so that you can test the PSU?

IanP

Bumping my own post, but what does the switch do?

[Neil WyattModerator @neilwyatt]

I'm sure 100R will be fine, any more and all it will do is slow down the switching.

I don't want the dew heaters to be more than a few watts, so perhaps I should just use bipolar transistors. BC107 should be up to the job

Neil

[AjohnwParticipant @ajohnw51620]

Best not let you work on any ABS controllers then Neil.

Meant to add – a board like that with 3 stepper drivers and a couple of switched output may be of interest in other areas.

John

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Edited By John W1 on 25/11/2015 10:10:58

[Ian S CParticipant @iansc]

I can get you an OC70 from my junk box.

Ian S C

[AjohnwParticipant @ajohnw51620]

Posted by Ian S C on 25/11/2015 10:20:41:

I can get you an OC70 from my junk box.

Ian S C

Great idea as they have lower saturation voltages. Certain ZTX's are way way better than BC017's as an alternative. One of them can be used as a high side switch too – however ……………………

I have a few germanium bits about too. They came with an Xmas present when I was around 8 or 9. My electronic set. Had an electric one the year before but motors/dynamos and shocking coils etc get boring after a while. All of that one has unfortunately gone, most of the other too. I never thought of it as a career but strangely finished up doing rather a lot of it at times.

John

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Edited By John W1 on 25/11/2015 10:44:52

[Neil WyattModerator @neilwyatt]

Posted by Ian Phillips on 25/11/2015 09:54:11:

Posted by Ian Phillips on 23/11/2015 21:35:25:

Neil

Curiosity has got the better of me, is the purpose of JP4 or the switch so that you can test the PSU?

IanP

Bumping my own post, but what does the switch do?

Er.. turn it on and off?

N.

[Neil WyattModerator @neilwyatt]

I'd point anyone wanting to do this at the RAMPS shield, rather than copying mine – this one will need bespoke software, while the Ramps will be arduino compatible.

Neil

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