By my calculations, A 100mm piece of 14 AWG = about .0008 Ohms, PLUS Solder or terminal Connection Resistance.
And both these will increase in resistance with Heating.
Resistor could be .07-.1 because kanthal has an annoying tendency to change resistance once it's been heated, somtimes you need to start slightly under to get yor target, somtimes it'll overshoot.
I'd also like to vary the resistance (by just swapping the resistor) to maybe as much as .2, it depends on the heat level I require. It'll only be operating for a few seconds every few seconds, hopefully not enough time to heat the wire much.
Resistance is not critical at all. .08 will be the maximum I'll use, and likely most often, it occurred to me that I should build a circuit to accommodate.
I think the circuit is free of gross errors- one of the advantages of ETO is that you get your circuits checked by the other members.
As has been said, with a current sharing, high current circuit the layout is important. I didn't have time to add notes to the circuit but I will add them.
Some resistances, especially the RDss of the MOSFETS are in parallel, so will reduce the overall resistance from battery to load. That is why four PMOSFETs are used. Also to reduce the PMOSFET drain current and dissipation.
Pretty much all circuit parameters change with temperature, but it is a matter of degree. Can you show your calculations to quantify the difference in the readings.
Don't forget that the battery terminal voltage and internal resistance will vary with state of charge and with temperature and the battery is a given by the OP.
Yes, good idea. I did consider using an auxiliary battery to increase the PMOSFET gate drive voltage. That would almost certainly allow a single high-current low RDss PMOSFET to be used. A CR123 LiIon battery would give a duration of approx 1,000 hours. Also considered, were a dedicated high side gate driver or a simple cheap 12V boost converter from ebay for about £1UK. The other option considered was a switched capacitor supply line booster, either a dedicated chip or 555 timer based.
But I didn't want to complicate the circuit too much because of the OPs declared lack of experience in electronics, although arguably four PMOSFETs could be considered complex compared to one PMOSFET and a battery
I think it's time to get my parts on order. I'm confident I can put the circuit together quite neatly.
I will bump this thread with a picture once it's done, I think it deserves it
Id like to give everyone a massive thank you for help.
And spec.. holy $#!t, you practically built it for me! Kudos my friend.
I think it's time to get my parts on order. I'm confident I can put the circuit together quite neatly.
I will bump this thread with a picture once it's done, I think it deserves it
Id like to give everyone a massive thank you for help.
And spec.. holy $#!t, you practically built it for me! Kudos my friend.
So this project went on the back burner for a while..
Pleased to say I have managed to piece it together slowly and now I'm almost done. I've tested it a few times in its raw state and I'm pleased with the result heres a pic during a trial fit in the enclosure. Rough, yes, functional, he'll yes! Another big thanks for the help I received at the start of this project.
Couple of things that I need to attend to.. the display screen I used (http://www.modmeter.com/v1.php) has a 10sec time out that doesn't seem to function, is it possible that the circuit is keeping it alive? I guess it would mean there would be a permanent live even if the trigger switch is open??
And, I have a built in 2a micro usb charger board.. is it possible to have it automatically cut the main circuit off when I plug in the charger? I don't want to be able to operate the device while it's being charged. I figure there's some diode or transistor that I can use?
Any help very much appreciated.