Building a GOTO Mount

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

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

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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

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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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Ignore that. I made some wrong assumptions.

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

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

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

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

Er.. turn it on and off?

N.

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

What I mean is RAMPS a shield for the Arduino mega (which sounds like compatible to me) and all the libraries for use with the Mega will be available as well as the Arduino IDE.

I'll be programming mine in AVR assembler from scratch using studio 6.2. I wouldn't recommend that to others - its the path I choose because it is one of my interests.

If the end product is of use to others I'll be willing to share it, but I'm not going to build in extra complexity because someone else might want to use it for something else in the future.

Neil

P.S. if you think hanging a FET of a port pin is dodgy, what about this from AVRfreaks:

So does the PSU sense that its output is being shorted so it shuts down?

Ian P

Freaks is the right thing to have on the end of the name. It's interesting to put a current sense on the supply on a circuit like that but few people have anything suitable - high frequency hall effect oscilloscope probe as both mosfets will be on for short periods as they are switched. They have sort of done something about the problems that is likely to cause on the supply almost certainly adding more parts than if the drive had been done correctly.

90nSec rise times are good at generating RFI especially with long lengths of wire on them. No need so why switch that quickly unless it's needed. The actual rise times might turn out faster that that. I'd hope that the data sheets have pulse thermal resistance info otherwise guess.

The absolute maximum rating of the i/o pins on the processor is 40ma however the max for the chip is 200ma. They point out that these aren't what might be called real absolute ratings as long term use might damage the device or cause premature failure and later reduce them to 50% as far as the pins are concerned.

Often on electronic items spec's are given in regions that they are expected to work in. As far as output is concerned that's 3ma but on this sort of part that can be exceeded providing the totals are looked at. The i/o conditions that the device can stand are given in condition 3&4 on page 317 of the full data. Essentially 100ma per port by the look of it and 20ma per pin - they don't mention the chip limit when driving which is likely to be 100ma max in practical terms with near 10 running the processor.

The reason for putting something in the way when driving mosfets is the capacitance in them. It's possible for the processor pin to get volts forced onto it back through the mosfet. The ports do have clamp diodes but it looks like they are for ESD protection only as they don't specify a max current through them. Processors turn into thyristors if the port input voltage range is exceeded. A puff of smoke, a nasty smell comes out and a small hole appears in the package. Not terribly likely in this case but it can happen. Pull down resistor can help a bit even the usual 20k because it may happen when they are inputs. Looks like high side drivers ideally need pull downs as well. I'd have thought that they would build them into the chip as no one wants to add more bits. It's annoying. Lots of things need them to define what happens during reset and before the post is set to the correct state.

John

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You are making this complicated.

Two wires from the pads go to a switch, and two more wires go from the switch to a battery.

Neil

Whatever the datasheets say, in practice AVRs are pretty bombproof. I think I've blown one up so far. They can do quite surprising things...

This:

Plus this:

Makes this (actually this is an earlier version, that ran out of flash for the interpreter and more esoteric display driver functions):

Which does this (amongst other things):

Later version isn't very pretty... but it is a work in progress.

Neil

Not trying to make it complicated, just that it looked to me as if the two wires went to the poles of a switch. If it had said 'From Switch' or 'from PSU' I probably would have twigged.

As the power supply is not shown and you (sensibly) use the same symbol for all the points connected to 12v I thought 'JP4' was used for either a switch or a jumper. Presumably the normal on/off switch would be on the input side of the PSU but putting the connector on the board gives you the choice later.

Now, studying your PDF in greater detail I realise that all your connectors have the JP prefix.

Ian P

LOL I missed that aspect and something else about it too. How the load is connected.

As to the rest automotive stuff is produced in rather large numbers and some one some where eventually finds the flaws. Not a problem if just a few but frowned on anyway as it could cost a fortune to put right and has at times. The other aspect is that if some one spends 24 years on them most things crop up one way or another. Personally if something means adding a couple of resistors or using a better drive arrangement that might cost a couple of quid more so what especially in the hobby area.

Unused pins on them have the usual suggestions. Activate the internal pull up resistors or better still in some circumstances use external pull ups and pull down but direct connect to a rail is discouraged.

If a board was ever available I would be happy providing that any spare pins could,be used if needed.

Atmel do have a zero crossing application note that shows that the input pin diodes can be used as clamps via high value resistors. A rather low current as I suspected. We have had mains spikes round here that have blown dimmers and bulbs. Not sure how it would cope with that. They can be in the Kv range, more than one of them at times. MOV's have killed that problem.

John

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To be fair on me, its really just a working diagram to generate the PCB, so I'm happy to flaunt conventions or use the wrong part with the right footprint, for example. If I use a 'pin header' to get a series of pads at 0.1" spacing, it calls it a 'jumper'. Often I wire direct into them, especially for power leads.

It took me a while to understand 'nets', if you use these instead of a wire to join up power pins etc. then you don't need to physically join them up on the circuit diagram. This is what I have done with +5, +12 and GND.

The board is a right dog's dinner at the moment.