Short and Current Protection Circuit

Wow. I’m looking at this literally 50s after you posted it.

Nice circuit. Interesting design. Works well.

Wow! That's crazy!

I just updated it too. Thanks.

It takes 10-15uS to trip. Much faster than the LED can burn out even with the 300mA spike caused by the short.

Yeah, much better response than the relay. But it's a lot easier to draw a lot of current through a relay, than it is a transistor, because a transistor overheats a lot quicker. That was the reason why I was using a relay in my first design... well that, and I only have transistors that can handle up to 600mA max.

Adding a 100uH inductor in series with the load, shortens the spike to about just 60mA.

But inductors make potentially damaging voltage spikes in an attempt to maintain the current going through them which might blow the transistors of the protection circuit making it more like a fast-acting fuse.

Adding an inductor to the power supply probably isn’t recommended, especially if it’s as big as 100uH. Even with 300mA short current 10-15uS isn’t that long.

If it’s shorted out with a 0ohm short, (close the switch on the right) it maintains 400mA-350mA of current for about 250nS-300nS before the current starts to reduce taking just a few uS to fall to 0A.

Once the current starts to drop it takes about 1.5uS to fall to 10mA. How much wattage can your highest power BJT handle?

With a full short while the button is held down so that it won’t trip, only about 400mA flows, although that’s very dependent on the gain of the transistor, which is very different for different models of transistors and increases with heat. 400mA*5V=2W. If your highest power PNP can handle 2W, then this can short out and not trip and your transistors will be fine, assuming the 2W of heat generated doesn’t create a thermal runaway situation where more heat leads to more gain which leads to more current which leads to more heat. If a thermal runaway happens, then you might need a current protection device for your current protection device.

My origional idea was that, using an inductor would allow for a larger current to pass through for a more demanding load without overheating like most resistors would... and by using an inductor, it should shorten the 300mA current spike caused by the short, because it has to have enough time for the current to rise/fall. I know they can cause large damaging voltage spikes, but a diode should prevent that with no problems, right?!

My transistors are rated for 40v @600mA, or 60v @200mA.

Voltage and amperage ratings I think are different from wattage. I think it means that it can handle up to 600mA at 40V, but doesn’t necessarily mean that it can sustain that 600mA. It can only handle short pulses of 600mA.

Yeah I don't know how they rate it for sure, (either way, it's probably not that accurate, but more of just a guide as to how much it could potentially handle), but it depends on the application I'm sure... usually a good design will never approach the full max power rating of any component though.

Do your data sheets say the wattage?

Closest thing I can find to that for my 2N3906 transistor is... Total Device Dissipation: 625mW

Which is rated at a constant temperature of around 30°C I think

Hmm. Not quite sure what that means as I’ve only used this simulator.

Don't have any comments to experiment with at home?

*Components*

Only an EP-130 Electronic Playground, with two PNPs, one NPN, a few capacitors of various values, a few resistors of various values, a dual op-amp chip, four NAND gates on a single chip, a button, a switch, a couple inductors on a common core with an unknown inductance (used for an AM circuit in the manual), a step-down (high voltage low current to low voltage high current) audio transformer for driving a speaker, a speaker, a quartz earpiece (presumably piezoelectric), a variable capacitor with a knob for tuning into stations in an AM circuit provided by the manual (never got it to work, just provided a slow geiger counter like ticking when turned all the way in one direction and heard from the quartz earpiece since you had to press your ear against the speaker to hear it when it is connected to the speaker), a 7 segment display with resistors built into the kit that cannot be bypassed, 3 red LEDs with resistors that can be bypassed with one of the LEDs blown because I didn’t know what I was doing, and a battery holder for 6 AA batteries (with terminals that split it into 2 independent sections where each section has a center tap with one battery on one side and two batteries in series on the other side). I only have a couple resistors and capacitors of the same value, I think two 10kOhms and 2 10uFs, but I’d have to check.

The only circuit I’ve ever made with it that wasn’t provided by the manual was a PNP astable multivibrator since I didn’t even have a 555 timer. And even then, it was slightly lopsided since I think I used a large capacitor with a somewhat large resistor on one side, and a somewhat large capacitor with a large resistor on the other side since I didn’t have enough components to make both sided the same. I managed to make the pulse width of each LED round 50%, but one faded at the end of it’s pulse a bit differently than the other one. The only other thing I did with it that wasn’t in the manual is make one of the LEDs glow faintly with some home made batteries (some zinc-copper potato batteries, which managed to work, and some zinc-copper batteries with an electrolyte of vinegar, although the vinegar dissolved the zinc coating on the galvanized nails so I never got it to work because steel-copper has a much low voltage than zinc-copper).

I think I actually have the same kit, it has a bunch of terminals made of springs for bending them back to hold a wire in place, and it says the age limit is 108 right? And it's got one really really long green wire for an antenna too I think

I've probably had that thing since I was 12, probably 1 or 2 years after, I got another one called snap circuits or something for a birthday present. Nice little kits, but they've only got a small variety of components, it's much better to just order your own online... I found like two different transistors that came in a 50-100 pack on Amazon for only like $2.00, I think their 2n3904 and 2n3906. You would need a breadboard too, but I'm not sure how expensive those are, I've also bought a 1000 pack of metal oxide film resistors for like maybe $5.00 I think. They have 5 bands, and I think their heat resistant to snow extent (they'll heat up some, but won't pop)

*Some*

Age limit is 108? That’s old. Maybe you meant 10. And yeah, it’s the one with a bunch of springs and that really long green wire, but you might have a slightly different model. Mine is EP-130. I had another one that was like EP-80 or something, but it’s down in Mexico from last time I vacationed there. What number is your kit?

I guess all I need is just some wires, a breadboard, possibly a wire stripper (scissors work too, but you have to be careful to not end up cutting the copper inside) a bunch of PNPs, a bunch of NPNs, a bunch if resistors of various values, a bunch of capacitors of various values, a few ICs (like some dual op-amps, perhaps even some 741s if I feel I’m gonna be abusing them since I heard they’re quite resistant to most everything other than over voltage, and some 555 timers, and maybe some logic gates if I want to get into logic circuits), some LEDs, maybe a speaker and a few other miscellaneous parts like a cheap laser diode and photo resistor if I want to make an optical audio transmitter.

I made an optical audio transmitter circuit. It’s my most recent two circuits (two circuits because one is with a 100kOhm resistor for photo resistor and the other has an emulated photo resistor).

No I meant 108, crazy huh! I'm not sure what the model number is, I'll have to check later. Did you use those red LEDs for the optical audio transmitter? Do you have a schematic of the circuit?

By the way, I tried building this protection circuit with some of the components I have, and it didn't work. But i didn't spend a whole lot of time trying to troubleshoot it either, so I suppose there could have been something else I missed, but really, I think the problem is that this circuit doesn't sense the actual current flowing through it at all. I built it in such a way, that shorting it out would draw all of the current away from the transistor, and thus no current should flow through it after that (eliminating having to sense the current at all)... but I believe the problem could also have had something to do with the way it latches on/off as in EC.

Never mind, I saw it on your posts.... at first, I thought they were projects at home you built with that kit

Nope. They were not built in real life, just like every other circuit I’ve EVER made in EC.

My model EP-130 lists the age as 12 and up. Not sure what’s up with your model. Maybe your reading the wrong number?

Oh my bad, I've got the same exact one as you, but the one that says for ages 8-108 is made by electronic snap circuits, model: SC-300.

Ok. Got any ideas about what I can do with it other than dimly light one of it’s LEDs with a potato battery? I’ve also made a circuit with a description containing my knowledge of possibilities for homemade batteries and my attempts at making the homemade batteries.

Sure! 1. You could make a touch switch out of just one or two transistors, where a small current flows through your finger between two points in the circuit. That's pretty easy, and a fun addition to any project (and a nice option if you don't have any push buttons) 2. A capacitor timer, that automatically shuts off power to something after it's been on for a variable amount of time.... could be used as a power saving feature.

Here's one I've been thinking of for a long time: Could you make a very simple push button LC transmitter. (Don't worry about making it oscillate indefinitely, and ot only needs a simple amplifier). I was trying to make a little experiment that would simply light up an led momentarily when you pressed that push button on the LC transmitter on a separate circuit (a few inches to a couple of feet away). Just to prove that it works, and that a capacitor and inductor in parallel really does oscillate.

And if you can get that to work, then my next plan is to make a pwm transmitter and receiver for a wirless multi-function push button circuit that I could use with anything, that would be pretty cool!

Hmm, finger touch would be pretty cool. Capacitor timer, well, I’m not too interested in that. LC transmitter, well, that’s a bit complex and would be horrible to troubleshoot seeing as I don’t have a scope. Maybe a finger touch powered astable multivibrator. That would be cool.

Yeah it is pretty neat. I'd love to be able to actually make something like that one day though, the transmitter

Shouldn’t be too hard if you know what your doing. You could just make an oscillator (in whatever frequency band is available for hobbyist use) and send that through an amplifier and to the antenna, and have the power supply modulated by a push button or finger touch switch. Possibly useful for morse code. You could also have a circuit in parallel that is a buzzer, and it will buzz when the power is on, and also you could have an LED that lights when the power is on. That way, the “1”s that you would transmit would create a buzz on the transmitter end and also have a small light on the transmitter end light up, and with “0”s, all power would be off. Maybe useful for sending SOS.

To send a "1" or a "0" though, you actually have to transmit at least 2 different signals. If you can only turn it on or off (transmitting a 1, or nothing at all), then how would you send 11 or 00? You can't, you could only send 10101010...you would have to send a separate signal

I mean by “1” I hold down the button, and by “0” I don’t.

This wouldn’t be used for digital data, but more for morse code or something. For example, SOS would be 3 short presses of the button separated by short pauses, then a little bit longer pause between the letters, then three longer presses of the button separated by short pauses, then another pause between letters, then three short presses of the button separated by short pauses, then a long pause between “SOS”s.

Mmm, I get that, but a long and short press can be easily mistaken from one another

But that's how they did it originally I guess

Yes, that’s how morse code works. If long and short get mistaken for each other, than hire a new operator to listen to the incoming signal.

I use to know a machinist that home built a h it ge Morse code machine, it involved intense soldering on 3 or 4 different pcb's about 1 square foot, in layers on each other. He could fine tune lots of things on it, (I'm not sure what, but it was definitely a big project, he even made a giant high power pcb antenna with a slideable core to tune into probably a small range of frequencies). Anyways though, he had a little lever on it that he could rock back and forth with his fingers as quickly as he wanted, sending one signal as a low with a leg swing, and another signal as a high with a right swing... and it sounded pretty cool too, cause it would beeb two different high pitches, and he could do it lightning fast! He could send morse code making that thing beeb as fast as he could fling his fingers back and forth, and he could interrupt it just as fast too. He use to teach at a school

Wow. Nice.