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61  Using Arduino / General Electronics / Re: Need help with a power supply... on: May 25, 2013, 10:48:18 pm
... and last but not least, the backside of the SL-HUB. Not too many connections!
62  Using Arduino / General Electronics / Re: Need help with a power supply... on: May 25, 2013, 10:46:53 pm
Here is the schematic that I believe I have traced correctly:
63  Using Arduino / General Electronics / Need help with a power supply... on: May 25, 2013, 10:45:29 pm
... but not one I designed or use with an Arduino... apologies!

Background: My weathermatic SLW10 rain sensor stopped working last year after a good season of helping contain the water bill. The sensor system consists of a hub (SL-HUB) that is placed inside the irrigation controller that communicates with the weather / wind / etc. sensor over two wires. The sensor platform is a 9V-block battery-powered unit. Based on my research, I ordered a replacement SL-HUB, and it works like a charm. Being curious re: the failure mode of the original unit, I decided to dissect the old one. Here is what I think I found and what has me scratching my head.

The SL-HUB appears to be powered via the same 24VAC bus as the rest of the weathermatic unit (have to confirm this via measurements). As you can see in the circuit I traced below, the incoming power runs through a half-wave rectifier before it gets to fill a 100uF 50V cap. Then it is run through a 12kOhm resistor before being regulated down to 5V with two de-coupling caps to keep things smooth. The LM7805CZ is made by WST and the maximum allowable voltage is 40V per its data sheet. I have measured the voltage coming into the weathermatic controller to be as high as 27.3VACrms. In other words, the peak voltage is around 38V. Doesn't this seem a bit close to the edge re: the maximum ratings mentioned on the datasheet?

Additionally, is it just me, or is this a weird way to deal with  24VAC incoming power to make 5VDC? All the circuit appears to be powering is three transistors and a DS1487 RS485 transceiver. Perhaps even a drop-cap supply could have worked  

Another nice touch is the diode that was hand-soldered from one of the interface pins to the 5VDC output pin of the 7805. I presume this is to prevent the voltage on said pin to go higher than 5VDC?
64  Topics / Science and Measurement / Re: Why use bridge vs. other measurement circuits for thermistors? on: April 21, 2013, 06:49:15 am
Hi and thanks for your answers!

I understand the need for common mode noise elimination in the context of very small voltage measurements for things like strain gage's. They require serious amplification so eliminating as much noise from the circuit is a good thing.

On the other hand, the only times a thermistor will feature a resistance remotely close to the 'opposite' resistor in the bridge is within a very narrow temperature band. For example, the NTCs I have been using feature a range from >10k to 150 Ohms for the temperature range I am interested in covering. Not sure how a bridge circuit can have impedances that are close to balanced unless you employ a variable impedance resistor in the bridge? Or perhaps a ladder of resistors in series that are shorted by transistors as needed?

FWIW, I got that second circuit 'inspiration' from LTC with its easy drive ADCs. See this design note as an example.
65  Using Arduino / Networking, Protocols, and Devices / Re: ICOMSat Sim900 / Mega2560 on: April 14, 2013, 11:24:40 am
IIRC, the Seeeedstudio shield is as beefy as it is re: capacitors (470uF vs. OEM recommended 100uF) for precisely that reason, i.e. to allow the shield to function even if it's 'just' using USB power. The maximum power draw is around 2A, though only in very short bursts. Hence, the big capacitors.

The Seeedstudio shield has worked fine for me in a GPRS application, using a PC as well as a Mac USB port as a power source, though I realize that not all USB ports are created equal (i.e. some allow more than 500mA to be drawn). Similarly, a GPRS / Arduino board I built is happy with just USB power. Both the Seeeedstudio as well as my board use the Simcom 900 chipset.
66  Using Arduino / Project Guidance / Re: How can I get the 5v outputs to switch 12v outputs on this project ? on: April 11, 2013, 06:46:54 am
PWM at any speed is not good with relays. The contacts are usually rated for so many closures, which PWM would quickly wear out, and the chattering would drive one mad as well.

Crossroads, thank you for the help but please allow me to quibble re: the above. Many appliances feature relays being used in a PWM manner. Whether its cooktops, ovens, clothes dryers, or microwave ovens, large loads are still predominantly switched using relays in the white goods industry. However, that's not to say that Triacs and similar solid state devices that allow faster PWM without the chatter you describe are not used, far from it.

The electric cooktops of a client of mine started using solid state switching devices in 1999, there is a Triac for suction control in our Kenmore canister vacuum cleaner, and variable-speed drives are also quite common for motors / compressors in clothes washers, dishwashers, mini-split AC systems, and even refrigerators.

These days its not uncommon to find appliances that feature both solid state as well as electro-mechanical switching systems. Some things like LEDs get dedicated LED driver chips to enable dimming, multi-color displays, etc. while larger loads that do not require speed control still predominantly use relays. Some appliances like microwave may feature a couple of small Triacs for small loads in addition to larger electro-mechanical relays for the magnetron power supply and other loads.

I expect more and more solid state relays being used in appliances as time goes on, driven by the dropping costs for solid state options and the features that can be added to appliances through their use. Years ago, a variable-speed motor in a vacuum cleaner would have been realized with multiple speed taps and a bank of relays or a high-power selecting switch, today it's done some pushbuttons, a Attiny, and one Triac. The soaring cost of commodities (such as the additional power lines required for the old approach) vs. the dropping costs of and greater familiarity with solid state devices presumably had something to do with that.

Thus, both types of power switching technologies currently find their application in high- and low-power switching situations, with and without PWM. As I see it, the most applicable solution depends on many factors and it wasn't clear to me just what the application was going to be, i.e. the magnitude of the load being switched, the speed requirements / duty cycle for the PWM, etc. Thanks again for all you do for the community!
67  Using Arduino / Project Guidance / Re: How can I get the 5v outputs to switch 12v outputs on this project ? on: April 10, 2013, 06:20:13 pm
How'd relays come into this?

All a function of current, no? Isn't it typically cheaper to use a electromechanical relay for higher currents as long as you don't need to PWM quickly? Hence my recommendation to have a look at the datasheet vs. the expected power draw of the LED string. The ULN2003a is usually good for up to 500mA, which typically gets you about 25-50 small LEDs. However, larger LEDs can draw 1W-3W, which limits the number of such LEDs you can drive with a ULN2003 to less than 5.

Remember, the OP gave no indication re: the power of his LED string...
68  Using Arduino / General Electronics / Re: Wheatstone bridge on: April 10, 2013, 09:48:59 am
Review the sites of Analog Devices, Microchip, Linear Technologies for the ADCs they offer - these (16+ bit) ADCs typically include a drawing in the data sheet on how to use these chips with a Wheatstone bridge - especially when you're dealing with very high-resolution ADCs like the LTC2485, for example.

Theoretically, you could also use an op-amp with an Arduino to build a crude wheatstone bridge. However, I wouldn't simply because the resolution of the ADC inside the Arduino isn't that great (about 10 bits).
69  Using Arduino / Project Guidance / Re: How can I get the 5v outputs to switch 12v outputs on this project ? on: April 10, 2013, 09:40:45 am
Yup, the ULN2003 is used in just about every appliance I have torn down for work to drive relays. It's a great little chip and the only additional advice I'd give is that there are current limits for Darlington-arrays like the 2003A.

So review the data sheet and your LED-driving current requirements carefully to ensure that you are not exceeding the current limits of the 2003A or switch to a beefier transistor or perhaps even a electromechanical relay. It all depends on what you are trying to do - for example, dimming via PWM will not play well with most/all electromechanical relays, while simple on/off may be perfect for a relay and not so perfect for the solid state relay (heating).

You will still need a chip like a 2003A to drive a electromechanical relay, however. And, unless the chip has a built-in snubber, add a snubber diode to limit inductive kickback from the relay from damaging the 2003A.
70  Using Arduino / Project Guidance / Re: Simplest way of providing 50khz clock pulse? on: April 10, 2013, 09:35:27 am
Another simple option is to use a CMOS 555 chip to provide the square wave in case you don't want to burden the Arduino with providing it. Similarly, a Schmitt-trigger chip can provide a very nice square wave using just a resistor and a capacitor. Either option doesn't cost much and provides a reliable square wave signal, allowing your arduino to focus on other tasks.
71  Using Arduino / General Electronics / Re: Recommendations on affordable lead free solder? Confused by all the formulas. on: April 09, 2013, 08:23:04 pm
I'm using the NXG33 solder paste, whose metal composition is 96.5% Sn, 3% Ag, and 0.5%Cu, the same as my through-hole solder
72  Using Arduino / Microcontrollers / Re: Making My Own Board With Atmega2560 from Scratch on: April 09, 2013, 01:22:26 pm
Apologies, Crossroads. I have corrected my post above. Thank you also for the long list of potential shops to source things from.
73  Using Arduino / General Electronics / Re: Power Requirements on: April 09, 2013, 11:11:43 am
Well, you could simply attach the rig to a fully-charged battery pack and see how many hours you get out of each configuration using a high-quality voltage meter monitoring the battery. Once the battery voltage reaches a certain level, shut the test down and note how many minutes, hours, or days you got. Many high-quality digital multi-meters (DMM) also allow you to measure ampere draws, which could be a more direct way to measure the flow of power. In this case, the DMM would be inserted in series with the battery pack to measure the current draw - see your DMM manual on how to do this.
74  Using Arduino / Microcontrollers / Re: Making My Own Board With Atmega2560 from Scratch on: April 09, 2013, 09:48:53 am
No you don't need a very expensive programmer, you just need the right one. For example, the AVR ISP MKII programmer works just fine and costs about $30. Some of the programmers on e-bay retail for less than that and work also. You may want to search around here for posting from folk like crossroads re: the e-bay programmers they have used.

[corrected to reflect Crossroads post further down, apologies!]
75  Using Arduino / General Electronics / Re: Recommendations on affordable lead free solder? Confused by all the formulas. on: April 09, 2013, 09:46:07 am
I switched to Kester lead-free solder paste for my boards because I couldn't discount the possibility of some lead going airborne as part of the reflow process. Lacking very good ventilation to the outdoors or reliable Pb filters, its the better thing to do, IMO. I have found the Kester paste a joy to work, even very fine pitched-components come out fine and no re-work is required (unlike some of the leaded pastes I have used in the past).

I have also switched to the Kester SnAg3.0Cu0.5 formulation for the through-hole work. I agree that it is harder to work with than eutectic solder, especially if you have to remove it later. My recommendation for lead-free repair or disassembly work - use ChipQuick! That stuff really works at getting all the solder out of the hole and into your solder-removing braid. Another problem area for Pb-free solders is soldering large components like 0.25" fast-on tabs - my 50W Weller simply lacks the 'oompf!' to do so cleanly - between my wide copper paths and the thermal radiation from the faston connector itself, lead-free solder (with its higher temperature requirement) has a hard time coming to temperature and flowing nicely.
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