Hi and alternative mosfet help

[chemelec]

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.

 

[gentlywiringit*]

I'm sorry, I don't fully understand the impact that has on spec 's circuit.

I'm not sure my readout screen won't short the p to n circuit posted last, one sensor wire is connected next to the .08 resistor

 

[chemelec]

All Circuit Resistances are ADDITIVE, including the Mosfets internal resistance.
So your Total Resistance will be Higher than that .08 resistor.

Just depends on How Critical the actual resistance needs to be.

And for Longer Tests, Heating will increase resistances, giving different readings.

 

[gentlywiringit*]

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.

 

[spec]

I have 12g or 14g silicone wire for the heavy load, itll be around 100mm long.


spec if there are no errors, you've told me the mosfet's, would you have the part numbers for the other components?

Hy GWI,

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.

Yes, I will do a parts list for you

spec

 

[spec]

Hy chemelec,

All Circuit Resistances are ADDITIVE, including the Mosfets internal resistance.

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.

So your Total Resistance will be Higher than that .08 resistor.

Hmm, interesting- can you show your calculations for this?

for Longer Tests, Heating will increase resistances, giving different readings.

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.

spec

 

[spec]

Possibly you can use the Enclosure as a Heatsink.
However it may need Electrical Insulation.

That is why I asked about the case

 

[spec]

Additionally if the Gate Voltage goes below -3.3 Volts, the Internal Resistance of the Mosfets will RISE considerably.

Can you show the basis of this?

Might be a good idea to use a Separate Battery for the Turn on Voltage.

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

spec

 

[spec]

Parts list and sources for circuit added to post #35

spec

 

[gentlywiringit*]

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.

 

[spec]

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.

No probs GWI

I hope your circuit works OK

spec

 

[gentlywiringit*]

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.

Mike.