Home school circuit analysis class suggestions needed

dl324

Joined Mar 30, 2015
16,839
I gotta admit, I don’t understand flux. I’ve soldered for 25 years as a hobby-ist, just never used it.
The solder you used probably had a flux core. Older solder had rosin based flux. Newer solders tend towards less corrosive compositions (e.g. no clean).

I went for 30-40 years without buying any liquid flux. It's handy where you need more cleaning activity. If you've ever tried to solder things that seem to have an oily film, you'll appreciate having the extra flux. The solder wets better.

https://www.mclpcb.com/blog/prevent...,to the component for an optimal solder joint.
 

MrSoftware

Joined Oct 29, 2013
2,188
Soldering Iron Kit - Soldering Iron 60 W Adjustable Temperature, Solder Wire, Tweezers, Soldering Iron Stand, Soldering Iron Tips Set, Desoldering Pump, Solder Wick, Heatshrink Tubes [110 V, US Plug] https://a.co/d/8x0n0iz

I didn’t really want to spend this much (14 kids), but it has a lot of good stuff.

Id like to keep it to roughly $50/kid. So $19 for the soldering kit leaves about $30 for any build kit and miscellaneous supplies. Seems reasonable.
In reality, any of those kits will likely work for through-hole soldering. Every kid doesn't need wick, flux, etc.. but it would help to have some in the class as you're inevitably going to need to clean up some mistakes and messes. I guarantee 5 minutes in some kid is going to fill one of the holes with solder before the put the part in, with a little wick you can suck it right out and they can start again.

Flux makes hot solder flow like water. Watch about 30 seconds of this, starting at about 3:30:
 

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rpschultz

Joined Nov 23, 2022
416
Flux makes hot solder flow like water. Watch about 30 seconds of this, starting at about 3:30:
wow, I never knew it could work like that. I’ll have to get some and try it. But it does add a messiness factor both during and afterwards with cleanup.
 

dl324

Joined Mar 30, 2015
16,839
So what’s the easiest type of solder for beginners? 60/40? 63/37? Flux core? Rosin core?
63/37 is better for people new to soldering because it's harder to make cold joints.

I doubt that you can buy solder wire without a flux core. A no-clean flux will let you get away without removing residue after soldering.

I think the risks posed to adults by lead solder are overblown. I held solder with my lips for years when I was an R&D technician and needed an extra hand and didn't always wash my hands after handling solder (I was a nail biter back then). There will be no lead in fumes from soldering at normal temperatures. Some people will be sensitive to flux fumes, but I find fumes from rosin to be pleasant. I still try not to breath in a lot of the fumes.
 

Thread Starter

rpschultz

Joined Nov 23, 2022
416
I think the risks posed to adults by lead solder are overblown. I held solder with my lips for years when I was an R&D technician and needed an extra hand and didn't always wash my hands after handling solder (I was a nail biter back then). There will be no lead in fumes from soldering at normal temperatures. Some people will be sensitive to flux fumes, but I find fumes from rosin to be pleasant. I still try not to breath in a lot of the fumes.
I agree. I think it was interesting in Halt And Catch Fire how Gordon got cancer and ultimately died an early death. they attributed it to solder fumes. Back in the 80s. Someone like that, doing it every day for years, seems possible.

But now, better quality control, for hobbyist… occasional soldering, seems highly unlikely.
 

dl324

Joined Mar 30, 2015
16,839
But now, better quality control, for hobbyist… occasional soldering, seems highly unlikely.
I read a report that stated soldering posed little risk when appropriate safeguards were in place. It mentioned some people being irritated by fumes from flux.

Just use common sense. Don't breathe the fumes, don't touch molten solder, wear eye protection, etc. I burned myself with a soldering iron once. It only takes one time to make you not want to ever do that again.

The eye protection thing isn't something to be taken lightly. I save all of the bits of solder from tip cleaning and removal from circuit boards. I melt it when I get enough and one time, I had a lima bean sized blob of molten solder and a bit was ejected from the blob while it was cooling. I wear eye protection more often now.
 

MrSoftware

Joined Oct 29, 2013
2,188
wow, I never knew it could work like that. I’ll have to get some and try it. But it does add a messiness factor both during and afterwards with cleanup.
Rosin core 60/40 will work fine. Don't worry about splitting hairs on the lead content, if it's got a good bit of lead it will work fine.
The flux will come in handy when they burn off the rosin that is inside the solder roll. Once they have the hang of it, the rosin inside the solder will be plenty 95% of the time when doing through hole work. But when they're new, they're going to use too much heat, or take too long and the rosin will burn off leaving only solder, and this is when it gets frustrating and difficult. It is now that adding extra flux makes things nice again.

For cleanup, if you clean right away it will clean up super easy with alcohol and a brush. If you leave it on, or if they really cook the devil out of it, then it can get crusty and take some work to clean, but if they're going to be soldering in the future it's a skill they'll need anyway. :)

You've got a noble cause there, I'm rooting for you. :)
 

Thread Starter

rpschultz

Joined Nov 23, 2022
416
Been thinking through the curriculum for the semester. The ultimate goal will be for them to build a circuit of their choosing. But they also need to understand at least parts of it. So part of each class will be lab time, soldering, breadboarding, measuring, etc. but I’ll also have short teaching times to teach them about various topics and then quiz them on it the following week. Keeping it simple, not too complicated, I want them to know the basics well, not necessarily all the variations. Some of that will be review from last semester.

Off the cuff probably 2 week on each of the following: resistors/pots, caps, diodes/LEDs, transistors, op amps.

So transistors is not my strong suit. My basic understanding is that BJTs are low current like most electronics, whereas FET are higher current like amps and power supplies. Yes?

I’d like to find a very simple transistor lab, not that many components. Looks like a simple timer circuit might work. BC547.
Im learning, please advise
 

MrChips

Joined Oct 2, 2009
30,706
So transistors is not my strong suit. My basic understanding is that BJTs are low current like most electronics, whereas FET are higher current like amps and power supplies. Yes?
Not so.
You can have both kinds, low current and high current BJTs, low current and high current FETs.

You are thinking of high current MOSFETs used in high current switching applications such as DC motor drivers.
 

dl324

Joined Mar 30, 2015
16,839
So transistors is not my strong suit. My basic understanding is that BJTs are low current like most electronics, whereas FET are higher current like amps and power supplies. Yes?
There are many types of FETs, so it's best to use a more specific name (JFET, MOSFET, IGFET, MESFET, etc). JFETs were the first to be made, so that's what I think of when I see FET. Because they were the only FET available at one time, it made sense to drop the J.
 

MrSoftware

Joined Oct 29, 2013
2,188
Think of it like this. BJT is a current controlled device, a FET is a voltage controlled device.

With a BJT, the more current you put into the base, the more current it will conduct between collector and emitter. It amplifies the current that is put into the base, and the amount of amplification is called the gain. What goes into the base is added to the output.

With a FET on the other hand, the gate is high impedance, it's like a tiny capacitor, current doesn't flow through it. And like a capacitor, you have to pull it up if you want it high, then pull it low if you want it low. If you disconnect it, it will stay at whatever voltage for a long while. Changing the voltage of the gate controls the resistance between the source and the drain. There are a bunch of different FETs, some you raise the gate voltage to conduct more current, some you lower the gate voltage to conduct more, etc.. but the point I'm making is it's the voltage of the gate that controls the current through the drain and source, as opposed to the BJT where it's the current into the base that controls the current through the collector and emitter.

Does this make sense?
 

Thread Starter

rpschultz

Joined Nov 23, 2022
416
so here is an NPN. If enough current flows to the base then the led turns on. I get the different types. I guess I need to dig into WHY transistors are used

1673916526204.gif
 

MrSoftware

Joined Oct 29, 2013
2,188
For a transistor lab, try controlling a small motor with an Arduino. Put an N-channel MOSFET on the negative power lead to the motor, connect the gate to the Arduino. Put a diode across the drain and source so the motor doesn't kill the FET. Now using super simple code on the Arduino, you can put the pin high to turn the motor on, low to turn the motor off, and you can pulse the output to make a speed in-between on and off. Change the frequency and duty cycle of the Arduino pin and see how it affects the motor. Or choice B; instead of a motor, use an incandescent light bulb, and control the brightness using PWM on the Arduino pin. A bulb bigger than what the Arduino could run directly, just to make it obvious that the power is going through the FET.

Head over to Spark Fun. They've got easier to understand descriptions of some parts. For example, here's a big beefy N-channel FET (hard for the kids to kill), that can be controlled with a logic level device like an Arduino (not all FETs can so this is important): https://www.sparkfun.com/products/10213
 

MrSoftware

Joined Oct 29, 2013
2,188
so here is an NPN. If enough current flows to the base then the led turns on. I get the different types. I guess I need to dig into WHY transistors are used
Transistors are used to control big power using little devices. The LED project doesn't make this obvious because many microcontrollers can control an LED directly, so a transistor is entirely optional. But replace that LED with something beefier, like the bulb from a car headlight, or a motor, etc.. now you can use your delicate little Arduino to control that big power hungry device by using a FET.

Edit --> They're used for lots of other things too, but this is a common use case.
 
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rpschultz

Joined Nov 23, 2022
416
Side note. 25 years ago I bought a radio shack project book, jumped the the end and built the most complicated project in the book: a 0-30vdc (1A) power supply. Only it didn’t work. Few years later, my father-in-law (EE) figured out that I fried the power transistor. He fixed it and to this day I use it in my workshop. So yeah, it’s easy for rookies to fry transistors!
 

MrChips

Joined Oct 2, 2009
30,706
The transistor will conduct when the base-emitter voltage exceed 0.65V.
We can calculate the base current given VBB = 5V, R1 = 1kΩ, VBE = 0.7V.

IB = (5V - 0.7V)/ 1kΩ = 4.3mA

If we assume current gain β = 100, IC = 4.3mA x 100 = 430mA.
But that cannot happen because the load is greater than R2 = 100Ω
If we were to ignore the diode LED1 and transistor Q1, the max Ic would be 5V / 100Ω = 50mA.

The voltage across the LED1 is significant, about 2V.
So we can expect the collector current IC to be about (5V - 2V)/100Ω = 30mA.

Conclusion: the transistor is used as a current switch between Vcc and GND. The current that is switched depends on the load.
If the load resistor is too low, for example, 5V / 0Ω = ∞A, you blow the transistor.
 

Thread Starter

rpschultz

Joined Nov 23, 2022
416
Another father who has offered to help teach is more of a software guy, I’m hoping he can teach arduino for a couple weeks.

I like the idea of using the arduino and transistor to control a light bulb. That’s a great application. Arduino has enough power for an LED, But not much more. So to get more power, let’s use a transistor!

I might need a little help putting that all together…
 

MrSoftware

Joined Oct 29, 2013
2,188
Sure, post here what you want it to do, we can help. If you get the Arduino with the big DIP chips on them, the chips are just plugged into the socket. If (when) a kid fries one of the chips, just pop it out, pop in a new one, write the boot loader to it and you've now spent maybe less than $1 instead of however much an Arduino costs. You'll need to get setup to write the boot loader ahead of time, and order some extra chips from a place like Digikey or Mouser.
 
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