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301  Using Arduino / General Electronics / Re: Measuring current & 'active' over current protection on: January 16, 2013, 08:48:22 am
Using the FETs on resistance is a possibility, if I use very high gain in the subsequent amplifying circuitry. However, given the FETs on resistance changes with voltage, current & temperature - it seems I'd probably have to use the controller to do the shutdown, not passives, if I want to cut power at an accurate current level.

I'll have to compare the variations between a FETs resistance, and a shunts resistance given the above changes. Thanks for the heads up thus far.
302  Using Arduino / General Electronics / Re: Measuring current & 'active' over current protection on: January 15, 2013, 06:35:28 pm
Yes I have heard they're not the best news - hence the thoughts to replace them! smiley

So, simply a shunt resistor in series with the load, measure the voltage before and after the shunt, feed these into an opamp, then feed the output into an ADC to measure the voltage drop and have the controller applying the appropriate action, i.e. switch off power.
303  Using Arduino / General Electronics / Measuring current & 'active' over current protection on: January 15, 2013, 05:49:39 pm
Evening all,

I've just had a thought - I'm currently protecting a design I'm working on from overcurrent by using polyfuses, so any component failures that lead to shorts to ground, or any external sensor and/or wiring faults that cause a short to ground will cause the polyfuse to trip and the power to be cut.

However, if I were to introduce a FET into the power supply (infact, I've already got a p-channel FET providing reverse polarity protection), and directly measure the current drawn then I could have the controller switch off the FET if a current threshold is exceeded for a certain amount of time.

How does one go about measuring current drawn by a device, and in such a way I can convert this to an analog voltage and feed it in to an ADC pin? I'd imagine I'd filter the signal first as there are plenty of current spikes in my application?

It seems a shunt resistor is my only real option?
304  Using Arduino / General Electronics / Re: Flyback diode configurations on: January 14, 2013, 04:24:11 am
Also, there is so much electrical noise in an automotive environment that a bunch of EMI generated by injectors
is probably miniscule compared to everything else, especially spark plugs and ignition coils and alternators, as
well as starter motors, air compressors, power steering pumps, on and on.

Therefore, the manufacturers will have gone to great lengths to add protection cktry on their ECUs, on both the
I/O pins, and the power and ground pins. I imagine an Arduino board would last about 3-msec in there.

Couldn't agree more. I'm not using an Arduino board, I'm in the process of designing my own PCB, and just happen to be using an AVR chip and have stuck with this forum as a number of people have been incredibly helpful.

The Zener info is great, thanks. Conducting to ground is what Texas Instruments recommend in a couple of application notes, using a ~33V Zener. However, that was an old app note. I'll test a number of different setups, scope it out and see what I get. It would seem to, if a Zener failed - there's a direct path to ground. Using a higher voltage Zener and recirculating sounds like a viable option.

On the subject of noise again however, my application won't be locating the board in an engine bay - and have been spending considerable time protecting parts of the design from noise.

Thanks for all the input! When I've got results, I'll post them up here with some scope shots etc and let you all know what I've got.
305  Using Arduino / General Electronics / Re: Flyback diode configurations on: January 13, 2013, 06:29:50 pm
Quote
Far, far too long! Check the movement speed of your solenoid. I suspect it's MANY msec
longer than 1-msec. Mechanical devices tend to have a bit more inertia than electrons.
Worse for bigger solenoids.

My solenoid in this case is a high impedance fuel injector, and of course - with typical injection pulsewidths ranging from 1 to 15ms, the mechanical movement time of the injector is very small indeed. That's the very reason I cannot avoid the long wire runs, but then again - it works successfully in everybody's vehicle on this forum.

Of course, if I could - I wouldn't have the long wire runs, but I simply cannot avoid it.

So really, my choice is - do I conduct the spike to battery voltage, or do I conduct to ground.
306  Using Arduino / General Electronics / Re: Flyback diode configurations on: January 13, 2013, 05:53:20 pm
Quote
I would place this across the coil as close to the terminals as possible!

Cheers for the device recommendation! I can't get close to the solenoid terminals unfortunately, these'll be a couple meters away.

Quote
You might look up actual data re "slows down closing of the solenoid". How slow is it?
2-msec? 2-sec? As far as effective goes, probably 99% of the time [guesstimate], people
just put the diode in reverse across the coil, so that should say something.

1ms extra opening would be far, far to long in my application - that's why I'm concerned. I'm in the 1% unfortunately. I can't put the device across the coil (I should have mentioned that before!) but I can feed it back into the power supply that feeds the board before the regulators etc, as that ultimately is the same supply that feeds the external solenoids.

I'm not sure if that, or conducting it to ground would be better...
307  Using Arduino / General Electronics / Re: Reccomend me a pcb package on: January 13, 2013, 03:09:38 pm
Why don't you just edit the library, clone the part, but make the smd pads bigger on the new clone.

Another technique, if there's nothing behind, you can put another heatsink pad in the bottom copper layer, and use several vias to connect/bridge the upper and bottom pads. 

Also order your boards as 2oz copper.

Yep - I've decided to do this, I've just cloned the packed with the suffix "HOT".

I'm making the boards myself by UV etching, so I'll have a play soon with 1 vs 2oz etches. Cheers smiley
308  Using Arduino / General Electronics / Re: Cheap small capacity SD Cards? on: January 13, 2013, 01:33:47 pm
Quote
but variable quality and shipping time may not be worth the savings.

Couldn't agree more - some of the real cheap cards are more than shocking...
309  Using Arduino / General Electronics / Flyback diode configurations on: January 13, 2013, 01:29:45 pm
Looking around, it seems there are a number of ways to place a flyback diode across an inductive device (such as a solenoid in this case) to protect against the voltage spike at switch off - namely;

- conduct the spike to ground
- recirculate the spike to the positive of the device
- or, either of the above with a resistor in series with the diode to dissipate more of the spikes' energy as heat

I've heard it's awful practice to conduct the spike to ground, but it would also appear recirculating it back to V+ isn't great, as that slows down closing of the solenoid.

I'm not looking for a "well, it works dosen't it?" type solution. I'm looking to build a robust, effective setup.

So, questions -

a) what are the advantages / disadvantages, other than those I've named - of each setup?
b) are there any other configurations other than those above?
c) ...it would appear, I'm also at a loss as to how to actually select the appropriate diode. With relays in the past, I've just used a 1N4007 - but I'm sure there must be better diodes for the job out there.

I'm switching typically 0.75 - 1.5A through the solenoids on a 12V power supply.
310  Using Arduino / General Electronics / Re: Creating heatpads in EAGLE on: January 13, 2013, 08:24:34 am
Not in the library editor no, the pads are just little squares on the IC, so I extended them out a tiny bit in the library editor and just want to connect to them in the board editor. I'll probably make a duplicate of the package in the lib editor, and just create the heat sink pads in there... much easier by the looks of things.

Pad info, page 25.

http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00043711.pdf
311  Using Arduino / General Electronics / Re: Creating heatpads in EAGLE on: January 13, 2013, 05:18:05 am
Quote
You said you named a pad to VCC. Why? Surely your heat sink wants to be on GND anyway?

I'm sinking all of the pads of an IC to get max dissipation.

Quote
I think you will have better luck just drawing rectangles in the .brd file. You will get overlap errors when you run the DRC, you can ignore them.


Cheers Bob, I've just done with - with some success, it seems one cannot move a rectangle about on the alternate grid, so I cannot line it up correctly...

EDIT - No, I'm just being thick! I can move it on alt, but I can't right click and type a position. That's not very useful, EAGLE!

A workable workaround, thanks. There must be a 'correct' way of doing it though...
312  Using Arduino / General Electronics / Re: Creating heatpads in EAGLE on: January 12, 2013, 06:05:44 pm
Thanks for the quick reply!

...but sorry, I'm not with you. I do mean device footprints, as I am trying to connect a heatsink to these, as the MultiPowerSO30 has the exposed pads as shown, I've just extended them out a couple mm in the package to allow one to connect to them.

A net name? You may have guessed, this is my first PCB design.
313  Using Arduino / General Electronics / Re: Need help identifying the use for circuit on DC motor on: January 12, 2013, 05:52:21 pm
Yep, they're to reduce noise, just as John says. Where it says 'soldered to the outside' - that means you solder one leg of the cap to a motor terminal, to other leg to the motor case next to the terminal. The noise absorption is superior to a single cap across the terminals.
314  Using Arduino / General Electronics / Creating heatpads in EAGLE on: January 12, 2013, 05:14:18 pm
Hi all,

I'm trying to work out how to create polygons in EAGLE that I can connect to IC pads and fill with copper for them to act as a heatsink.

I almost hijacked somebody elses thread with this question... here's what was said.

Heatsink pads - I fake those in eagle by giving the polygon pads the same name as the signal on the board drawing, then you can overlap them. Will get overlap errors when you run the DRC that you can ignore.

I have just tried it, no success. I've checked the pad I'm trying to connect my polygon to is named VCC (in this case), I've made an overlapping polygon onto this VCC pad.

I tried renaming it by Right Click > Properties > Name, but it told me to use the name command, so I did and have successfully changed the name of the pad to VCC. However, if I rename the pad to VCC and then try to fill it using ratsnest, nothing happens? The polygon just remains an empty space outlined with dashes.

If I leave the name unchanged, and click ratsnet - great, it fills it - but leaves a gap between the polygon and the pad - defeating the point of being a heatsink. Any ideas? Here's some screenshots.

1) Here's the polygon I've drawn, overlapping into the pad I want it to connect to.



2) If I don't rename the pad, and just click ratsnest - I get this, filled - but a gap.


3) If I rename the polygon to the same as the pad, click okay, then click ratsnest - and... it just looks the same. No fill.

315  Using Arduino / General Electronics / Re: Reccomend me a pcb package on: January 12, 2013, 02:55:17 pm
Heatsink pads - I fake those in eagle by giving the polygon pads the same name as the signal on the board drawing, then you can overlap them. Will get overlap errors when you run the DRC that you can ignore.

You're a star - cheers. I'll give it a try later.

EDIT - I have just tried it, no success. I've checked the pad I'm trying to connect my polygon to is named VCC (in this case), I've made an overlapping polygon onto this VCC pad.

I tried renaming it by Right Click > Properties > Name, but it told me to use the name command, so I did and have successfully changed the name of the pad to VCC. However, if I rename the pad to VCC and then try to fill it using ratsnest, nothing happens? The polygon just remains an empty space outlined with dashes.

If I leave the name unchanged, and click ratsnet - great, it fills it - but leaves a gap between the polygon and the pad - defeating the point of being a heatsink. Any ideas? Here's some screenshots.
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