## How to Explain It...

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
cpprioli
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### How to Explain It...

Hi folks... I'm a longtime reader and a subscriber with a long-running self-taught electronics hobby. One of my grandsons came to me with a question that I am having a hard time explaining to him so that he understands my answer. The schematic pictured below is the PSU for a summer-school extra-credit STEM project he is building -- an alarm system of some sort -- and I have checked that he has assembled it in accordance with the schematic. Pretty hard not to, as the kit included custom PCB's for the various sections.
The question that he had for me is "Why does the LED illuminate at full brightness whenever the power cord is plugged into the wall, regardless of the power switch position? However, the other sections (amplifier and output) don't power up until the POWER toggle switch is put into its "ON" position." When the power switch is "ON", the LED appears to be be the same as when the switch is "OFF". Measured voltage at the LED anode, with the switch either "ON" or "OFF"", is +11.99V (measured to chassis GROUND). The LED cathode measure 0V when measured to chassis GROUND.

I have asked him to measure the voltage there (at the LED) and at the DC OUT header with his scope. He hasn't gotten back to me yet with those results.

How do I make this make sense for him?
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Getting there is half -- hell, make it three-quarters -- of the fun... but don't let the magic smoke get out!

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Janitor Tzap
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### Re: How to Explain It...

Take a look at this schematic.

Here the S1 ON-OFF switch, and the F1 Fuse are on the (Line-HOT) Primary winding of the Transformer.
So, there is no AC flowing when the S1 switch is open.

In the schematic you provided.
It shows the S1 ON-OFF switch, and the F1 Fuse connected to the Secondary windings of the Transformer.
Thus, AC is making it to the one side of BR1 {Bridge Rectifier}.
This is causing the BR1 {Bridge Rectifier} to rectify part of the AC Sign Wave to the DC voltage.
Supplying the +12Vdc for the "Pilot light" LED at all times.
Only when you close S1 Switch, does the other side of the BR1 {Bridge Rectifier} get the other part of the AC Sign Wave.
Giving you the -12Vdc.
So, unless the circuit requires +12Vdc for a remote ON-OFF receiver.
I'd move the S1 ON-OFF switch, and the F1 Fuse to the (Line-HOT) Primary winding of the Transformer.

Signed: Janitor Tzap
CeaSaR
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### Re: How to Explain It...

The simple explanation is that the transformer is a center tapped unit - ie GND on the CT - and only 1/2 of the transformer is switched. the side that has the fuse goes toward the positive side of the full wave rectifier, which is ultimately fed through the 7812 to the LED. In order for this to be completely off, you'd have to have a DP switch, 1/2 controlling each side of the transformer secondary and another fuse to do the same, or move the SPST and fuse to the primary side.

This was a pre-designed kit? Hmm, makes for a good safety analysis starter to bring up to the "teacher".

-CeaSaR

***Yeah, what JT said.
Hey, what do I know?
cpprioli
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### Re: How to Explain It...

OK guys -- I just got done with going through the "PIG Alarm Kit Student Guide" -- the manual he was given with the project kit. It is from a company I've never heard of and for which I cannot seem to find anything useful on the web -- STEM Labs Educational Systems Ltd.. Among other things, there is an ""analysis"" area for each kit section. In the PSU section, I find this:
4) Refer to the illustration above.
1. Describe the way(s) this circuit operation would differ from the original design.
2. Explain the pros and cons of each circuit design.
3. If you were the design engineer, which circuit design would you choose, and Why would you make that choice?
Having gone over the circuit and your replies with him, and with the kit and 'manual' on my workbench, he now has a better understanding of the circuit. Once he saw the waveform on the scope, it immediately became a little bit clearer for him. His only other question to me was why anyone would have that design as the final design. I pointed out to him the fact that the TV in my shop has a red LED lit all the time that the TV is plugged in but turned off -- a loose parallel at best, but it made the point to him. I suggested that the designer may have meant it to be a hot PCB tell-tale indicator to reflect the fact that the line voltage/current are present on the PCB any time that the unit is plugged in. In the production design, the power switch is switching low voltage rather than line voltage...

Anyway, thank for helping me to get the point across to him. Your replies helped as much as the scope did. Funny how the same words coming from strangers seems to carry more weight than those of old Pop-Pop do...
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Getting there is half -- hell, make it three-quarters -- of the fun... but don't let the magic smoke get out!

-ChrisP
haklesup
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### Re: How to Explain It...

the new circuit is better because it switches power at the primary side of the transformer turning off all power while the first circuit had a design defect allowing half of the circuit to stay powered. in the first, the switch effectively cut the power in half but it was always sufficient to light the LED but if you had a motor load, you would probably have noticed a loss of power or torque for example. More specifically the regulator would see half wave rectified signal instead of full wave rectified but the regulator and caps were able to keep it alive anyway. Even with the new ckt, I'd expect the LED to stay on for up to a few seconds as the large capacitor discharges.

FYI. https://www.multisim.com/pricing/ still has a free version for students and is a good way to learn how to simulate these circuits and probe nodes without building it first. Frankly, I'd give extra credit to a kid who finds a simulator and correlates predictions with actual measurements. You are using a voltmeter but with out an oscilloscope, its difficult to see the difference between full and half wave rectification, the simulator will have a virtual oscilloscope but I have also seen some very low priced ones on amazon that fit in a pocket.

Why would one use the first design. Well, you nailed it, even when some things are off, there are still live circuits like remote control receivers that may need power. In any case, the design is nearly obsolete in new products with new switch mode power supply (SMPS) modules being very popular and available on ebay and amazon often for only a few dollars. The LDO regulator is still heavily used and pretty easy to design around but it has limitations like it wastes power in portable battery operated circuits and should not be operated when the input is much greater than the output or it will get quite hot. New SMPS modules (buck-boost type) take 3V to 35V at the input and regulate on nearly the same range at the output while wasting less energy by getting hot. SMPS are much harder to understand how they work, I struggle with it each time I look.
cpprioli
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### Re: How to Explain It...

Yeah... as I said in my last post... once I got him and the kit to my shop and showed him the difference in scope waveforms with the switch on and off, the light, so to speak, went on for him.

I'm not so sure that this kit was such a great teaching tool for a Junior to Senior STEM class, as it does use what I would consider to be a non-standard design. At the same time, I can see why the manufacturer might not want to be switching line voltage...

Thanks for your feedback. I'm waiting to see what his instructor will have to say about his "analysis" of the circuit...
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Getting there is half -- hell, make it three-quarters -- of the fun... but don't let the magic smoke get out!

-ChrisP
haklesup
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### Re: How to Explain It...

Maybe not applicable to classwork because they lack a detailed lesson plan but there are some great electronics kit companies out there, either PCB level assembly kits or assembled modules you can piece together, SMT and thru hole versions. Most sell on ebay and amazon then you can figure out their website to see the whole line. Covers the gambit from flashing lights to uController + firmware kits, robotics, power modules etc.
CeaSaR
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### Re: How to Explain It...

You know, this whole thread reminds me of the early home computers that had 2 power switches:
1) on the back on the power supply itself - switched line voltage in and out.
2) main motherboard power-up switch on the front (or elsewhere) that would start up and shut down the computer.

No, it's not quite the same, but the ideas behind the 2 different power supplies above share similar design goals.
One cuts power to everything, providing safety and ZERO current use when not being used, while the other controls partial standby power to then further control the rest of the circuitry, allowing for things like instant on functions and remote control functions with the side effect of always drawing some current, albeit most likely very little.

Safety wise, the most safe one is switching the AC line in and out. Down side is the higher voltage and current can shorten the lifespan of the switch if it is not built/rated for the load(s). It completely cuts the hot from coming into the circuit. Well, it should. That schematic really should have the switch first then the fuse second on the hot (or line as it is labeled). Reason being is that if you disconnect that line via either the switch or a blown fuse, you will only be in contact with the Neutral and ground, which SHOULD be at the same potential - around 0 volts +-, not 120 or 240.

So yeah, both schematics have their questionable elements.

What STEM class is this? I applaud them for trying to stimulate the kids minds, but I wonder just how much they themselves know or if these kits / curriculum are/is provided by 3rd parties without much review. It's better to teach them right from the get go than to try to undo poor habits that have been unwittingly taught. Now, if they go into the explanation of the downsides during the class, then I can understand why. I just hope they do.
Hey, what do I know?
cpprioli
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### Re: How to Explain It...

From what my grandson tells me, it's a "STEM Enrichment" class that is commercially offered but is arranged through the auspices of the regional technical high school that he attends. He has just completed his Junior year and is trying to boost his GPA to make college entry easier. His intent is to become an electronic engineer at some point. From what I can see though, there is NOT enough time spent on theory, and that makes the hands-on activities simple rote behaviors with little understanding. That's why he came to me, though I am not an expert -- just a simple hobbyist. It is through exposure in my shop that he got the electronics bug to begin with.

I will say this -- the "analysis" area of each project section presents questions and hypotheticals that require circuit understanding in order to answer correctly. I just hope that the instructor -- it IS an online class -- spends enough time and is able to answer the questions that will surely arise. Not ebery kid in such a program has someone in the family or neighborhood to go to with questions.

I'm still not sure why I could not get the point across to him on my own, but at least he understands this circuit now. Thanks to all who contributed...
-----------------------------------------------------------------------------------------------------------

Getting there is half -- hell, make it three-quarters -- of the fun... but don't let the magic smoke get out!

-ChrisP
haklesup
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### Re: How to Explain It...

"from what I can see though, there is NOT enough time spent on theory, and that makes the hands-on activities simple rote behaviors with little understanding."

That's not surprising, schools have to cater to the least common denominator (the barely average student) in many cases. The object in high school is to get them interested or hooked and show them how to find resources they can use to self learn and to grow the hobby and decide if they are to go on to college and in what major. If he truly has the mind of an engineer, he will find that and begin climbing the learning curve to what should remain a stable and profitable career. I found electronics approx my sophomore year in HS but before that I was always interested in mechanical stuff.

"switching the AC line in and out. Down side is the higher voltage and current can shorten the lifespan of the switch if it is not built/rated for the load(s)."

Not necessarily true, switching voltage only matters with respect to terminal separation, Switching current is what will wear out a switch or relay. For the same current, an AC switch will survive longer because the arc is always extinguished within 1/4 of a 60hz cycle while it returns to the zero crossing while a DC arc can persist longer and can vaporize more of the switch contact in the process. Note the ratings on the side of the switch almost always have a lower DC rating than AC. Hot switching is an important consideration when selecting relays for example. Yes, one should always select a switch with rating above what it is switching, if you do, you should be OK.
jwax
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### Re: How to Explain It...

Looks like the polarity of the 12 VDC at J2 are reversed.
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