Backyard Aquaponics
A place to discuss aquaponics

This is just a general enquiry about how this electricity stuff works, but sometimes things like transistors say they are for large currents, but have tiny little pins.

So far, I've only powered small electric "hobby" motors (for my fish feeder), or lights through a transistor, but if my 12v electric outboard needs jumper cable sized cables to handle 40amps, how can little transistor pins cope with it?

Or the tracks I might solder them to?

Or for that matter, how can whatever is inside the little package cope with it?

If you have a transistor that's going to conduct 40A, you'll need to have it mounted on a large heat sink, with good thermal contact. The 3 connecting pins are only very short, so not too much resistance in them, but they need to go into a reasonable cross section of conductor if you dont want things to get too hot.
Power = current squared X resistance, so with high current you need very low resistance if you don't want too much power to be dissipated.

Thanks Gunagulla

I thought resistance would still be high even in something only a couple of mm long because the thing was thin.

So, thin AND/OR long add to resistance, or does only length add to resistance?

I still don't understand what's going on with a resistor.

Almost everything else makes some kind of sense.


[Gunagulla]"but they need to go into a reasonable cross section of conductor if you dont want things to get too hot."


Does that mean you cant just mount them into a normal circuit board?

Looking inside a computer supply power supply, I see nice big heat sinks (so I figure they are dealing with high amps), but they are still just soldered onto the circuit board like any other component. (although I guess the circuit board is a little industrial looking compared to say, that on the back of a hard drive, but none of it looks like jumper cables for my car)

Good question. I've been out of the electronics game for a few decades so with the rapid changed in the tech I may be wrong. Back then, we used relays to do high amperage switching. The benefit was isolation from the circuit. I did a search for "transistor capable of 40A" https://books.google.com/books?id=1bkJBAAAQBAJ&pg=PA14&lpg=PA14&dq=transistor+capable+of+40A&source=bl&ots=ACOojW3YQt&sig=X2FsXA0QCbRro00ApbRfHA2RZ5I&hl=en&sa=X&ved=0ahUKEwjq4PPLh5bOAhWJ1IMKHebaCR8Q6AEIUjAJ#v=onepage&q=transistor%20capable%20of%2040A&f=false
Again good question, I too have a difficult time figuring out stuff like this. Remember fuses are literally short circuits. In my electronics service center, I replaced large heat sinked transistors in amplifiers all the time. Most were socketed for easy replacement.

boss, do you have an invisible image or something that is greater than 800pixles wide?

I see your message as breaching the borders by 30% to the right.

Thin or longer will increase resistance, just think of a water pipe- the flow of water, equivalent to electrical current, will be less if it is thinner or longer. The heat sink will get rid of most of the heat, but don't expect normal circuit board tracks to carry 40A, they might vaporise.

Do relays do the same thing as a transistor?

I was given some by a kind local that took me under their wing and gave me some lessons when I first joined the PICAXE forum.

I have no idea what to do with them. They have 6 (or maybe pins.

I always thought a relay was like a fire alarm bell, but I cant see why you would need all those pins. My older brother had an electronics set with a relay as part of it that was in a clear plastic box so you could see a reed being attracted from one side to the other, but I cant see it lasting in an application like pulse width modulation (I think that's what I mean) to control the speed of a pump or something.

For instance...

As I understand it, the hotter your water gets the more it dumps oxygen into the atmosphere. But luckily their tends to be a bit more sunshine around when it gets hotter.

So I thought a solar backup air supply/pump system might run more vigorously when there is more sun/heat, and charge batteries when there is less.

I'd use a chip controlling when to send pulses of power to an air pump or water pump based on probable need.

If I was sending on/off commands to a relay every 100th of a second, wouldn't it wear out in no time? I figure there is a little arc every time it switches that would burn it out after a while. I doubt my household light switches could deal with being switched every 100th of a second for more than a few minutes.

Agghhhh! so much to learn. Why did I watch that first season of big brother. I could have had 6 months of a degree in electrical engineering in that time.

If you wanted to switch that fast, you would need a solid state relay, SSR.

6 pins will be a DPST relay, 8 pins will be DPDT (double pole double throw).

Yeah ok... (Do'h!)

Of course the circuit tracks are only carrying the data to the transistor to switch it, not the actual transmission of large current to the device.

Only 8091 questions to go folks.... hang in there

Thanks Gunagulla

Just for clarification, I'm just pulling numbers out of my head as far as how fast I would want to switch. I used PWM on some small motor projects with a PICAXE chip, so it just sprang to mind as an example.

This kind of device...

http://www.jaycar.com.au/solid-state-re ... g/p/SY4086

looks more like the kind of thing I thought would be required to carry 40 amps. Something with big screw terminals rather than the little pins on components that looke the same as the 5v voltage regulator I use to supply the tiny current to power the chip in a project.

Is there an advantage in using a solid state relay compared to a transistor?



(and I think I might be confusing MOSFETs and transistors. I think the things I'm looking at in old power supplies are MOSFETs, but I just looked at wikipedia and I think they do the same thing)

When do you need the extra pins?

Is that just for something like wanting to run a second circuit to light an LED or something to tell you the state or the switch?

I've never really understood why my switches have all those pins.

Even a little on/off toggle switch can be bristling with pins.

It must be that link that sent the width into crazy wide mode. Sorry. Another cool thing transistors in chips like the SCR can do is slowly change the voltage until it is all the way on or off. Extra pins or contacts as they are known in relays are to keep the circuit open or closed when powered, as Gary said.It depends on your design. My Wife's Son built a fabulously designed pump house power system connected to two separate photovoltaic systems; the whole setup is processor-less; totally logic driven using relays and float switches! Incredible system, magnificently done by a master electrician! Pump will only come on if several conditions are present. For example: If it is night time and the float switch in the top of the IBC storage tank is open or closed depending how he wired it, the power is derived from the batteries. That is one of a dozen scenarios which will turn on or off either of the two pumps, one in the well the other for pressurizing the system for the house. The second PV panel will run the well pump whenever the sun is shining and the float in the IBC says it needs water.

The T in MOSFET stands for transistor It can handle high currents.

re relays, if one has 8 pins, or a toggle switch that has 6 pins (you do the switching instead of the small coil), you will have the centre pins to connect to and the pins either side of it will be normally open and normally closed, for 2 poles, ie + and -. You can use that for reversing the polarity by crossing wires from the outer pairs of pins, to, for example, drive a DC motor in reverse or forwards.

What voltage and rated power is your trolling motor?

I only have readings from my 10 amp max multi meter that showed 16 amps (:)) on power level 2 of 5. (1=min)

12v motor, no idea what amperage it draws because I cant remember what model (thrust) I bought as all the watersnake black and white ones all look the same to google photo recognition.

From the forward force it generates at the other speeds, and the rate the battery is consumed (guessing all numbers because I only have vague notions about c numbers on batteries) I'm guessing it maxes out at around 60 amps plus whatever surge motors do. Although there are probably capacitors inside the motor controller for that.

But it's not just the motor, I've wanted to control a few other things that will require bigger motors, and haven't really understood any of the tech. I've only ever used small transistors to control little motors or power a nightlight on my dolphin torch - that kind of thing.

eg. I'd like to landscape my large back lawn (nice green weeds that thrive despite never being watered) so I can power the wheels of my mower, stake a wheel barrow wheel full of rope to [EDIT - the center of the lawn - attach the rope to the mower, so the rope unwinds as the mower drives in circles] , and let it go while I drink beer - that kind of thing.

Aquaponics wise, I would love to make a heliostat to point some sunlight onto my system which can currently only grow wild rocket (awesome btw) and a few herbs that don't seem to mind the shade it's in. And also a heliostat to point the solar panels to track the sun.

I can see you are going to have to spend a lot of money on a battery for the boat, and it should be a Lithium battery if you want 60 amps out of it on any sort of a regular basis. There are plenty of off the shelf speed controllers that will handle that sort of current, such as for electric bikes, RC planes etc. I'd go with one of them before trying to make one, they have proven reliability.