I think you have got things a little confused here. Yes, the current matters but there is a threshold voltage aspect as well. Generally speaking, handling cables with 24 volts or less is considered safe. Just think; a car battery has several hundred amps available at its terminals but car batteries are generally regarded as non-lethal! Provided our enquirer is only switching 12 volts then there should be little risk at all apart from the obvious one of causing a fire by short-circuiting such a current in a non-fused circuit. I would go with Goeytex on this one and switch it with a big MOSFET. I would look at a IRLU7843PbF which has a continuous drain current of 100 amps or so at 30 volts peak. A 5 volt PICAXE should be able to fire this up to allow 15 amps through quite nicely; the datasheet specifies a very low RDS(on) at 4.5V VGS with a gate threshold of 1.4-2.3 volts.fairly wire hopefully, you need to make sure you completely comfortable with what you are doing, the sort of current you are using is very dangerous - it can take as little as 100mA is enough to kill a human, at 30A you are 300 time more likely to be killed - well give or take
Eh, I think we are losing the plot here a little... The OP stated in post 3 that he wants to switch "a TEC1-12715 possibly even two and i believe these are about 15A and 12V"That spec link you gave us has no coil rating, in other words, how much coil current or what is the coil resistance. That will determine what you need for a driver.
See Attached for diagram
Goey, I notice that you are using a 180R on the serial out from the PICAXE. I have seen some legacy diagrams showing this in place but none recently. Why do you choose to put a 180R here? I can see that it offers shorting protection, but any other reason? I can never make up my mind about putting it in or otherwise. Do you have any experience that shows it to be prudent or is it just a case of a belt, braces and a nail through the navel?
I use it for protection of cheap USB/TTL adapters that typically have 3.3v I/O circuitry. It makes these "5V tolerant" when the Picaxe is operating at 5V. I use an AXE027 where it is not necessary, however there is no way of knowing what other folks are using. The 180R resistor adds an extra level of protection. I have used values as high as 1K with no ill effects. Consider that the serin uses 22K and still works fine.See Attached for diagram
Goey, I notice that you are using a 180R on the serial out from the PICAXE. I have seen some legacy diagrams showing this in place but none recently. Why do you choose to put a 180R here? I can see that it offers shorting protection, but any other reason? I can never make up my mind about putting it in or otherwise. Do you have any experience that shows it to be prudent of is it just a case of a belt, braces and a nail through the navel?
With oddball USB/TTL adapters I also add a 10K resistor from the serout Pin to ground. This prevents a garbage character from appearing in the PE Terminal when the terminal opens the virtual serial port. I think the AXE027 includes this resistor, however a genuine FTDI adapter from another source required this resistor.
I have done no calcaluations for heatsinks im using these for the cold side and a lga775 heatsink for the hotside(similar to a stock size cooler) do you think these will be ok ?You ought to give jpacman some advice on heatsinking.
For kitchentable-tronics it may be easier (mechanically) to choose MOSFETs with a tab with an 'ole for mounting.
I've noticed people talking about PWMing; the MOSFET could get hotter without a proper driver.
(They could get hotter with a proper driver but not as bad as direct driving).
(And there is some dubiosity about PWM and Peltier chips but I can't remember what it is, someone else can search).
Talking of heatsinks, jpacman have you considered how much heatsinking you'll need for the Peltier?
Have you done any calculations?
would this heatsink be sufficent for the FET and alot of other people i have seen are useing LGA775 heatsinks for this peltier unit and have been getting some good results.Re: your heatsink link; I don't know. Where does it give degrees/Watt?
Guesswork could result in damage/destruction of the Peltier element. (Or you could be dead-lucky and smug )
Firstly, you MUST do some first-order (aka 'ball-park') calcs to save the Peltier and save your money.
For example, how many watts (worst case) are going to be generated by Peltier when run flat out?
The physical and ambient conditions will affect the figure.
Have you read the Datasheet for TEC1-12715?
I only had a quick look at one (2 pages) and it is woefully inadequate to design from it.
You won't know some of the parameters for the hefty equation posted earlier.
So, you are into partial trial-and-error territory.
The total worst case power will be something along the lines of:-
I2R + Cooling power
If you are making a small, well insulated, fridge then you can reduce this as the teperature reduces.
If you are making a dehumidifier for a big room the this power may remain high.
Hot-side ambient temperatures will affect things.
So, it depends on the application.
Other than preventing destruction another aspect is the Delta T.
If your hot-side is 70oC then the cold-side will never get colder than HotT-DeltaT oC.
And that's at Vmax and Imax with ideal hot-side heat-sink.
It'll be worse as you PWM it and with a weedy heat sink.
If you don't use it properly the results WILL disappoint you.
In my own first experiments (ah those were the days) I used a 75W Peltier with a fan cooled 0.8oC/W Heatsink.
For prototyping I added some thermal bimetallic cut-out switches on the hot-side heat-sink... and I'm glad I did.
My first tests I drove from a bench PSU (Thurlby-Thandar with V & I limiting) to slowly ramp it up.
This also allows your knowledge to be slowly ramped up
Diving in head-first can end in tears. (I'm turning into my Father).
If I drivvel on for much longer this post will be un-readable.
SO, may I strongly suggest that BEFORE you destroy your TE Device or waste money on unsuitable heatsinks go to the RS Website.
Search on 'Peltier'. [EXAMPLE]
1. Read the Design & Application Notes.
2. Read the Melcor Application Notes.
3. Read anything else - that's why the info is provided,
Once these have been read and absorbed anyone can be an expert and post sound advice