A! Thank you for your exhaustive explanation DVDdoug! Great, my calculations seem to pan out.

Also great point MarkT! I was just thinking of possible applications, but you're quite correct, that would be a poor design, haven't really thought of that

Thanks everyone for your help. Cheers, Val

Hm, ok, thanks, that helped.

I have sort of a followup question, hope this can go into the same thread. So if I know the dc current gain (designated as beta i think), could i potentially use a transistor to limit current that goes on through the emitter to the device? I mean, by placing a correctly valued resistor in series with the base of transistor, to control the saturation current?

For example, take the pic If I control the value of Rb to control the current the transistor lets through when saturated, could i omit the 62 Ohm resistor wired in series with the LED?

Maybe a stupid question, sorry Cheers, Val

Hey

I have a BJT NPN transistor, and i would like to know how do i calculate the maximum current that transistor "lets through" (collector to emitter) given the current to the base of transistor. I have a 1K resistor wired to the base of transistor, 5V Vcc. I don't quite understand the electrical characteristics in the datasheet. Something about "Collector cut-off current", then there's also Base-Emitter On Voltage, and Base-Emitter Saturation Voltage. I have a vague idea of what that means, but could someone pls explain..

So what I really need to know is, how to calculate the max value of the series resistor that I can wire to the base of transistor, to still allow maximum current through collector-emitter?

I know the question is a bit vague, sorry about that, I can't explain it much better.. Thank you for any help you can give! Cheers, Val

Hey

Thanks for replying. Yes, i've considered that too, unfortunately, i also have an ethernet shield, which draws about 150mA as it is, so using 12V power supply is out of the question. So yes, i'm stuck with a 5V or 7V relay. I would like to avoid bypassing the regulator altogether, since apparently it's not recommended, no idea why.. Also, i need a 20A relay, so 5V might be out of the question too, since they're hard too get from what i've seen

Either way, i have another problem. Sometimes when i switch off the relay, my LCD gets all wonky, displaying all sorts of weird characters.. Any ideas why that might be? I figured it might have something to do with the back current induction from the coil when it pops back off.. But i have it wired as the schematic shows, the power diode is supposed to make sure that doesn't happen heh. I'm also concerned that if i increase the voltage to control the relay, the back current surge might be larger, not sure about that tho..

Thank you again for all your input, probably couldn't have done it without you. So i'll let you know how it works out, hopefully i won't fry my board in the process

Cheers, Val

Hey

Well i've decided to go with a 7V, 1A adapter, should be good enough to not fry the voltage regulator, haven't tested it yet. Tho if i have two relays and with the ethernet shield drawing a low of current, that's aproaching the limit. Arduino can draw 800 mA max, according to specs, but i wouldn't go much higher than 400, maybe 500.

So could i wire the relay through the Vin pin, to power the relay's coil (100mA or so)? As far as i know, Vin avoids the voltage regulator altogether. But if i wire it up this way, i'm guessing i would have to use some sort of a resistor, so the relay's coil wouldn't suck too much current, yes?.. Or at least for Vin voltage to drop from 7 to 5V, since 7V is too high for the coil probably. Can someone please confirm this, i'm not good with electronics, yet

Thanks for the help. Cheers, Val

Ah yes, the voltage regulator I'm having issues with that too, I actually have another thread opened about that. Took some effort to diagnose the problem, since i'm a computer engineer and i'm a bit new to electronics. I used a 12V adapter and with the eth shield and a reed relay drawing a lot of current, i almost melted the damn thing hehe. Have to switch to a 7V adapter now, hopefully it'll be enough and i won't have to bypass the regulator altogether, since i don't know exactly how to go about doing that.

Thanks for the heads up, i really do have to watch out for that regulator

Cheers, Val

Hmm ok, thanks Jack, i'll think about it! For starters i can at least put the temp sensor on one of the edges of a pcb, and mount it on a socket to lift it a bit, i'll see what that does, tho if i put a box around the whole thing, i might have to lift the sensor so that it sticks out from the box

I'll see what happens when the project is finished (my BSc thesis actually lol) and i'll let everyone know if this is an issue or not

Cheers, Val

Hm, ok.. Thanks for the advice. Let's say burning down my house isn't a likely outcome

I know this isn't the right thread for this, but still the problem exists with using a temperature sensor, the heat from eth shield will set it off.. I'm thinking of isolating the temp sensor as much as possible on the pcb, but if i wrap everything up in a box, the heat will accumulate over time. Also, i just want to point out that for example ARM's ethernet controller gives off little to no heat, neither does the MCU and the prices are comparable now. This points to bad design of Arduino. Sorry, i don't wanna be a jerk and i know it's noone's fault here, but i just wanted to point it out..

Thanks again for your help. Cheers, Val

Hey

Thank you for your replies!

@spcomputing
I've tried your suggestions to no avail. Nothing seems to be touching, SCL and SDA pins don't seem to be shorting with DFU header. Thank your for your advice!

@SirRich
I have not tried my R3 shield with a R3 mega board. That's not really an option, since i would have to buy another board, which in my banana republic costs 60€ And there's no guarantees that it would work.. That was my first thought too tho, maybe i'll get a chance to try it out.

The chip gets hot very quickly, half a minute or so, after a minute it's pretty much untouchable..


I'm guessing i can't just wire up the IOREF pin and SCL and SDA pins with appropriate pins on my board? I'm guessing those SDA and SCL on mega board aren't the ones as the additional ones on R3?..

As far as i've been reading up on this issues, the eth shield seems to be drawing A LOT of current, 150 mA or so, maybe there's a way to limit that current with a resistor or something so the shield would still work? I'm not sure how i would go about doing that tho..

Thanks again for all your help. Cheers, Val

Hey

Sorry for the late reply. Hm, i didn't know i could just wire it up directly to these pins, if the voltage is right, have to read up on that, thanks. I'm guessing those are the pins on the POWER  header of the board?

I think i'm gonna start by buying a 7V, 1A adapter, should be enough to output 5V and the regulator should heat a lot less (even if drawing so much current), am i correct? Also, i think i'm gonna buy a solid state relay instead of a coil based one. As far as i understand, they use up a lot less current.. Maybe it will be enough for starters, what do you think?..

Thank you very much for your help, you have been very helpful, i probably wouldn't even be able to diagnose the problem in time without your posts Cheers, Val

PS: I thought about using cooling fans as well, but that seems kinda lame for such a simple application (it's a web controlled thermostat, and i don't want it to hum like a full blown barebone server machine)

Hi

Thank you for your reply! That was it. The voltage starts dropping very fast and the regulator becomes very hot.. I'm also using the Ethernet shield, and i've been reading that regulator overheating is a known issue, since the shield draws too much current, which really sux, so i'll try switching to a lower voltage adapter, maybe 7 or 9V.

The adapter is a bit old, i used an adapter i got with my previous router. I'm a computer engineer and a bit of a noob when it comes to electronics, so would you be as kind and suggest any solutions? I'm guessing wiring the relay through external power source, like a battery, would solve the problem. But i would like to avoid using an additional power source if possible..

Thank you very much for you help! Cheers, Val

PS: I've also meassured the current draw from the relay, it's a bit lower than 100mA, at times "only" 50mA, but you're quite correct, it draws quite a bit of current.

Hi

I have a problem when wiring a 5V controlled reed relay. I wired it using this schematic http://www.arduino.cc/playground/uploads/Main/relays.pdf.

The problem is, when the relay's coil is active (so the switch is on), something weird starts happening with my LCD. It takes a while, but the LCD starts loosing contrast, then the displayed characters get all mumbled up, it's like the lcd is getting too much current or something, i'm surprised i haven't fried it yet. Now when this starts happening, if i deactivate the relay, everything get's back to normal (if i do it before the chars get mumbled up) and if i reconnect the relay, it starts happening again immediatly.

This only happens if my board is connected to an external power supply. It doesn't happen if powered via USB. My adapter is 12V, 500mA. I just don't get it, is it possible arduino'ss GND is being overwhelmed? Or some current leaking somewhere perhaps..

Any ideas would be greatly appreciated. I can post schematics if neccessary, but lcd is completely separated and has nothing to do with the relay.. Also, the LCD was wired by this tutorial on adafruit http://learn.adafruit.com/character-lcds/wiring-a-character-lcd.

Thanks. Cheers, Val

Hi

I have an Arduino Mega 2560 board (R2) and Ethernet Shield R3. My problem is, the Wiznet W5100 chip on the Ethernet Shield gets insanely hot, it actually burns after touching for 3 seconds.. And that is running from USB power supply.

So my question is, what the hell?? Is that supposed to be normal?.. I don't want it to burn down my house.

Any suggestions would be greatly appreciated. Ow and btw, i bought the mega board from dealextreme, so is it possible the problem is with the arduino board instead of the shield? Or maybe because the shield is meant for R3 instead of my R2 board?

Thanks. Cheers, Val

Hey!

SOLVED!

It was a soldering problem, still no idea where lol. Anyway, i resoldered the header pins and everything is working fine, finally! I think the header pins might have been a bit too short for the protoboard before, causing unreliable connections.. Tho that is really weird, since i did continuity checks and everything seemed fine..

Either way, mastanley, great library and thanks for your help and i'm really sorry for misleading, i just didn't know Many thanks for your help too Grumpy_Mike.

Cheers, Val

Hey

Thanks for the explanation. I ought have been more clear, i connected 5V to a pin on the keypad (the rest of the pins are disconnected - floating) and that was through a resistor also. I'm a computer engineer and a bit new to electronics, but i know the current draw of directly connecting 5V to an input will probably fry it

Anyway, to be more clear. 5V is connected through a resistor to a random pin on the keypad, the rest of keypad's pins are floating and they all have 5V on them lol. Cheers, Val