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61  Using Arduino / General Electronics / Re: Arduino mega + GSM + LCD + SD Card + Keypad on: October 03, 2013, 03:19:33 pm
In what way do you mean mounted? You could create your own PCB with all of the appropriate hardware on, yes.

In terms of checking which port, what do you mean? You think that maybe one or more of your prototype shields for example may both be trying to access a serial port at the same time?
62  Using Arduino / General Electronics / Re: SD card doubt on: October 03, 2013, 03:15:25 pm
To interface to a 3v3 SD card using 5V signals, you should (or can...) use a bidirectional voltage shifter such as the TXB0104, which I've used successfully before with an SD card.
63  Using Arduino / General Electronics / Re: Design of Analog Temperature sensor on: October 03, 2013, 03:10:56 pm
Do you really mean you want to 'design your own', or are just looking for a solution? Texas Instruments produce many very easy to use analog temp sensors like the LM35, LM45, LM60 etc that cost next to nothing.

If you wish to do it with just basic passive components, simply set up a voltage divider using an NTC or PTC thermistor - a resistor whose value changes with temperature, with either a negative or positive temperature coefficient. However, you will not achieve the same level of accuracy with such a simple solution. Plus, precision NTCs/PTCs are probably nearing the price of a temp sensor IC anyway...

Hope that helps!
64  Using Arduino / General Electronics / Re: Ultra high reliability capacitors on: October 03, 2013, 07:46:15 am
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Do you mean just reliability or lifetime?

Well, I'd define reliability as intended operation over a long lifetime - what would you define it as?

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Aluminium electrolytics are very reliable when new, but suffer from age/heat related deterioration. Why do you need 470uF at 40V - for high reliability you might want to think outside that box?

The caps are the main caps in an h-bridge driving currents up to ~10A, the chip in use is recommended as having 470uF per 10A load current, and my supply voltage is up to 30V. However, I wonder if for reliability I simply have a TVS diode to absorb any spikes that switching may cause from any parasitics, and omit the capacitance.
65  Using Arduino / General Electronics / Re: Ultra high reliability capacitors on: October 03, 2013, 06:36:19 am
Hi Fungus,

Well yes - I would presume the failure mode would be referred to as open, technically - as a fuse blows - and surely, this fuse should blow before any fire!

However, some parts of my application require roughly 470uF, with a max voltage rating of ~40V - something that tants seemingly can't provide. What are my options - considering I don't want to use electrolytics?
66  Using Arduino / General Electronics / Ultra high reliability capacitors on: October 03, 2013, 05:49:46 am
Hi all,

I'm working on a design at the moment requiring very high reliability at elevated ambient temps, up to ~70degC. I know electrolytic have certainly increased in reliability over recent years - but are still commonly the failure point in any design that uses them.

It seems solid tant capacitors are now becoming the cap of choice in high reliability applications. However, two questions.

a) I believe the common failure mode of a tant cap is short, as opposed to open with an electrolytic. So, as a second safety layer - I suppose I could use a Polyfuse or similar in series with the bulk tant caps to prevent shorts IF failure occurs
b) Would potting a device (with a compound with high thermal conductivity, not cheap I know...) with electrolytic caps extend their life by preventing the caps from drying out?

Thanks!

EDIT - http://www.vishay.com/docs/40126/ds04053.pdf It would seem good quality solid tants have internal fuses in them already!
67  Using Arduino / General Electronics / Re: Positive input voltage to unpowered chip on: October 01, 2013, 04:00:41 am
Thanks for the sanity check, as I thought... the chip isn't actually an Atmel MCU, but I have clear maximum current limit values set out in the datasheet for the control input pins.
68  Using Arduino / General Electronics / Positive input voltage to unpowered chip on: September 30, 2013, 09:59:01 am
Hi all,

I've got a little bit of a (potential) problem in a design, under certain conditions - I have a chip that is powered down, however - some of the inputs to the chip may or may not still be high, so I've now got inputs way in excess of the chips supply voltage (which not surprisingly being unpowered is 0V).

However, i've noticed some IC datasheets don't actually list inputs as VCC +0.3V like some do, and instead just list a current. So if I use series resistors of the correct value to limit to current into these pins, then I shouldn't run into issues with latching up a chip when it's unpowered yet still got control signals going to it?

Many thanks in advance!
69  Using Arduino / General Electronics / Re: Car automotive "produces several hundred volts" on: September 29, 2013, 04:44:40 am
I had my tectronix (744a) scope on the fuel injector looking at the idle pulse width; and there was a significant shutdown spike, well over 60V, everytime the injector fired. IIRC the injector pulse width was about 3.2ms, and the shutdown spike was ~0.3ms(+/-).

Considering that my Arduino Mega doesn't function well withing 3 feet of a florescent light, keeping an eye our for electrical noise is not a trivial task

Thanks - this illustrates my point quite well. At the particular load side you measured, 3.2ms of fuel was required - if you then tried to suppress that spike with a flyback diode across each injector - you'll add approximately 1.5ms of fuel - well outside of any closed-loop fuelling strategy, at those sorts of extra fuel levels a fuelling error will simply be reported (not to mention idle fuel isn't often controlled closed-loop).

60V however is likely a clamped value, internally clamped by the ECU - injector spikes will often exceed 60V unclamped. The fuel tables will however be populated for that particular voltage clamp.
70  Using Arduino / General Electronics / Re: Car automotive "produces several hundred volts" on: September 29, 2013, 03:31:29 am
...wow, just managed to delete my own post on my phone - how annoying.

The transient experienced by disconnection of a charging alternator is known as a load dump, not a load drop. Load dump on a 12V system often lasts for around 200-400ms at around 100V, but is very much dependant on alternator speed at the time and load current. The full formula for calculating internal resistance of an alternator can be found in an Infineon app note somewhere...

Transients around several hundreds volts however are much shorter in duration, but much more common (i.e. occur in normal operation, whereas load dump is really a fault) and are caused by switching through the harness transients. They are however becoming less common through better harness design and local suppression. It must be noted of course however that transients due to switching injectors for example shouldn't be clamped, clamping an injector with a flyback diode for example can increase its closing time by a factor of around 5, effectively making large injectors unable to pulse small idle quantities of fuel, and therefore - one must just protect against these.

The reason your radio or any other device in your car hasn't blown up, is simply because they're protected against these transients. Take a look at ISO6737-2 which outlines the common transients and their sources - don't believe for a second auto transients are a wives tale, it's well known the automotive power supply is one of the harshest operating environments.

Note also, to 'view' these transients, the positive terminal isn't the place to hook up a scope - you need to probe a 12v distribution line within the harness itself, or the 12v supply to other devices such as an airbag controller or radio for example. However, depending on your vehicle - these may all be supplied by a conditioned source as opposed to a direct battery connection.

EDIT - Another challenge of designing for an automotive supply is the cranking voltage, trying to keep MOSFETs that are passing not insignificant current fully switched on with the hugely reduced voltage can be a significant challenge.
71  Using Arduino / General Electronics / Re: FIVE terminal current sense resistor, not four? on: September 17, 2013, 06:26:34 am
Exactly - the power rating is in terms of dissipated power. The maximum current for this device is 168A (IIRC, haven't done the calc in a few days), but it dosen't say with what type of heat sinking.

Either way, the shunt is heat sunk through the large copper bus bars and power cable and enclosure etc, which is pretty overkill for just a few Watts in a solid metal device as I'm sure you know.
72  Using Arduino / General Electronics / Re: FIVE terminal current sense resistor, not four? on: September 17, 2013, 03:24:43 am
I see - however, i'm not dealing with mains voltage. I'm passing 12DC through the resistor in excess of 100A continuous, more for shorter periods.

So the sense leads are going straight into an op-amp to be amplified, so the 'shield' pin will simply be no connection then in my application - or just used as a mechanical support.
73  Using Arduino / General Electronics / Re: Heat disspiation with Surface Mounted Resistors on: September 16, 2013, 09:35:09 am
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Why would it? You can add extra solder to both ends raise the resistor off the ground... are you claiming it would no longer handle 1/4th of a watt?!

SMD parts are rarely hand soldered, as i'm sure you know - so the 1/4watt is given with a strictly defined pad layout, on certain laminate size and thickness, with certain copper thickness etc. I'm sure there is also a recommended solder paste layout which would give rise to a constant amount of solder.

I'm sure if you can somehow keep the temp down, yes you probably could drive it harder than 1/4watt.

But that's true of anything, submerge a through hole 1/4 resistor in liquid nitrogen and i'm sure it'll go much furthur than 1/4watt.
74  Using Arduino / General Electronics / Re: FIVE terminal current sense resistor, not four? on: September 16, 2013, 08:20:04 am
But you could just use the voltage sensing terminal on the battery side to sense voltage, a high impedance ADC around high meg ohms or gig ohms isn't going to throw out a current sense reading.

Or perhaps it's just not 'technically' Kelvin connecting if you do the above...
75  Using Arduino / General Electronics / FIVE terminal current sense resistor, not four? on: September 16, 2013, 06:43:44 am
Hi all,

I've got the need for a very low resistance current sense resistor - and have settled on the Vishay WSM series due to availability, cost and physical form. Here's the datasheet.

http://www.vishay.com/docs/30173/wsms3124.pdf

However, I've used four terminal resistors before for Kelvin connection - but these seem to have FIVE? It seems logical that the 5th (the third small copper through hole tab) is just there for increased mechanical support onto the PCB, but then it seems strange if that's the case to list is specifically as a "5-terminal connection design" in the features on the first page of the datasheet.

I'm not missing anything here no? 2 power connections, 2 for kelvin connection - but why the 5th?

Cheers!
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