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Test Equipment / Re: Fluke 187
« Last post by Svuppe on Today at 08:22:06 PM »
No need to check the current draw. The 187 doesn't have a supercap, only 189.
I've had my 187 for over a decade, and I've changed the batteries once or maybe twice in all that time.
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Beginners / Choosing The right Oscilloscope
« Last post by MotoDC on Today at 08:17:42 PM »
Hi  I am an automotive repair student at lvl 4 and we just started using oscilloscopes and I wanted to get one , so found this one on ebay ( Fluke 123 )  bought it  and it wont start phoned them and they said 360 GBP for repair LSS ( I cant afford that ) , so I would like to know what to look for when buying a scope and what should I get to suit my needs as motor-vehicle Tech
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Ok, Vgs, why is it listed as +- 20v?

Also it's coming off the transformer as ac. It it's all rectified to dc.  There's no need for triacs.

most mosfets will withstand up to 20 or 30V of either polarity before they break, so the +/- spec is an absolute maximum. You want to look at the Vgs "test voltage" which usually is 10-16V, that is as much as you need.
 in the case of an N channel mosfet: source to ground, drain to negative of load, gate swithed to 10V. If you want to maintain isolation you need to step down your 48V to 10V-15V to drive the mosfet gate through the traic. You an use a zener or a voltage reg or in fact you may need a bit of both as most regulators don't like over 35-40V
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Other Equipment & Products / Re: FTDI driver kills fake FTDI FT232??
« Last post by cypherpunks on Today at 08:13:57 PM »
Those sparkfun meters deserved their death.

WTF?  They are sold as Digital Multimeter - Basic, i.e. "cheap but basically functional".  AFAIK, that's exactly what they are.  I have a pile of even cheaper Harbour Freight multimeters precisely because they're basically disposable.  I can stash one anywhere one might be useful.  There's one in the car with the jack and spare fuses just in case, one in the drawer with the spare batteries to ensure I don't get a dead one mixed in, and so on. 
Quote
there is no way to grant a license (temporary or not) because that will kind of mean endorsement, which fluke, in good conscience, just couldn't do.

Utter rubbish.  They are required to police their trademark or lose it.  But just like a simple defense to adverse posession of real estate is to grant permission, you can do the same with a trademark.  SparkFun had already sold thousands of the things in yellow, and was going to seel thousands more after changing the rubber.  Letting you off with a warning isn't endorsement.
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Makes no difference even with an account, you still pay through the nose. We use a few couriers at work, and there the minimum cost for international is in the order of $120 for the first kilo, and slightly lower after that. Local door to door I can get under $20 per parcel under 20kg, including POD and tracking. I even have gotten 60kg motors for work via courier, on our account. You just have to pack really well, they broke a few vacuum pumps by dropping them on the shaft end and destroyed the end clearances.
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Projects, Designs, and Technical Stuff / Re: Crumbling Capacitors
« Last post by fcb on Today at 08:08:34 PM »
It's likely to be (as others have stated):
1. Faulty parts (I haven't heard if Xicon).
2. Board flex (are they near the edge, is the substrate thin, how do you break our the panel,etc).
3. Mounted pressure to high (faulty machine, bad programming, etc..)
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Quote from: TonyStewart on Today at 04:32:38 PM
2 beers says it is lack of grounding in a laptop powered by a SMPS to USB port and the poor CMRR causes differential noise on the preamp.  Ground the USB to earth ground or connect VGA to an external monitor which is already grounded.


It still looks like common mode noise to me which due to inbalance gets converted into a differential noise.  Touching it only susbtantiates it for me. Grounding it, may not be an option but it will work. The fix is to raise the CM impedance with a large ferrite high mu core around the mic. cable.


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Any water vapour within the box would condense when the temperature drops past the dew point.  You said the box isn't completely sealed, so it's entirely possible that moisture will get in.  Are the PCBs suitably protected from moisture, e.g. conformal coating?   This is just a suggestion to rule out though, not saying this is the problem.
Moisture.
Moisture indeed.
I was able to reproduce the problem reliably by bringing the thing cold indoors for a minute, to let moisture condense, then getting it back outside.
It freezes, things stop working.

Nothing actually breaks, the radio just gets detuned.

First time it broke after an hour, i closed it indoors, so all the humid air condensed inside.
Second time it still works, i closed it outdoors after an hour sitting there open.

As far as i understand it, the box, not being airtight, would accumulate moisture inside of it with variations in pressure, temperature and humidity - easier to get in than out.
So, after some time it would start to condense and freeze.
Sounds plausible?

If so, would a coating of polyurethane help?
Or perhaps putting some silicagel desiccator into the box?

That module looks like some junk with a blob on the pcb. It might be that some bonding wire from that IC to the pcb became loose.
RFM12B, not the best, but i haven't heard much bad about it either.
Datasheet says it's rated to -40.

The battery at its voltage is one of the things that you must check as already pointed out.
It's one of the parametres being read out, and it's stable.

Is the current consumption constant or do you have some periodic, rapid increase of current draw?
Near-zero for most time, about 17mA bursts (1/4s) every 5 to 10 seconds.

Your capacitors could also be freezing, as they might only be rated for -10c to 70C operation. You probably have issues as well with the battery getting cold.
Aluminium polymer capacitors, rated down to -55*C with 1.25 Z ratio to +20. Should be good enough, i guess?

One solution would be to insulate the electronics and battery very well with polystyrene shehting so that the heat they generate in use keeps them above freezing.
Less than a milliwatt average?
Don't think that would do much.
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Projects, Designs, and Technical Stuff / Re: Pull-up or direct connect?
« Last post by Gizmo on Today at 08:02:59 PM »
Hmm, I've had a look at the datasheet but don't really know what I'm looking for. Would it be "Control inputs" listed as 1uA max? In which case that's all good.

I'm less bothered about testing as I know it's all setup correct and it's a dead simple board (only 7 IC's, most of which 245's). I'm more interested in unnecessary current draw as this is battery powered.  :D
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The factor for choosing a diode for high speed switching is called the reverse recovery time.  Since this is not sub-nanosecond speeds, the 1n4148 is best suited.


I would be concerned about the burden load for the CT to ensure the cap does not cause false readings. CT's are design for a fixed Resistive burden load, not nonlinear bridge rectifiers. It would be better to use a precision OA rectifier..
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