Costing transistors

A project log for Stick / TIG converter

An budget add-on box to convert a stick welder to a TIG welder. This project aims to produce a low current TIG welder for <£100

DannyDanny 07/09/2014 at 16:500 Comments

this will be the third time I've started this project update.
stopping due to it getting far too late in the night, needing to go to the work or wanting to go to the pub.

the simple fact of this project is it can't be built with off the shelf parts at the target budget!

to get 300 amps of handling at 4 legs of an H bridge is no easy task, and buying new can cost well over the £100 I've projected for everything.

As it is I've got a simple cost analysis spreadsheet.

the first coloumn is the target current handling capabilities.
in this case I've got a 250Amp max transformer.
but it won't be run from an industrial supply. running from a cooker socket will get about 180 amps at the torch, and from a domestic 240v 13 amp supply will give about 120 amps.

so why did I go for 300amps? well. I want the chance to use the box on an industrial supply, so if the opportunity ever comes to go to 415 volt, this box will handle it.

second, I want a comfortable margine of safety, 250amp max handling would leave me scared to use 250amp supply on full.
a 300amp max handling capability is a comfortabl 20% margin, it's almost double what I can use at the moment, and well over double 150% margin above what most people will have to work with.

The fact is that these things are expensive enough to buy the first time. I don't want to need to buy them again!

Though happily I'm getting my max current capabilities by running the devices in parallel, (e.g. 10 x 30Amp capacity IGBTs) to get to that 300Amp capability -you don't need to stack these if you don't want to!

the second column is the current handling capabilities of the transistor in question.
that's Maximim DC, (not AC where it gets a chance to cool!)
the third column says you'll need that many. (simple A/B)

the fourth column is that amount rounded up. (add more safety margin rather than take it away.

so what's the price, how many per pack, and so the total price.
that's either D * E / F to give me my total price.

then I have four pathways on the H bridge, so I need four times that amount. -lastly I've got a link to where I've seen the transistor. -mostly on ebay and other end of line surpluss type places.

This does mean that each design will be slightly different.

to compensate for this, I'll make the ADC/Amplifier stage that drives the H bridge adjustable, -that way, the Maps used in the uC can stay standardised, and the transistors can change.

At the moment costs are looking to break down like this.
Auto coil for driving HVHF - £5 - £10
uC £3 - £7
LCD screen £5 - £20 (depending on size)
small components (op amps, thermistors, resistors capacitors £5 - 10)
Rectification of welding transformer £10
IGBT for H Bridge £30 - £60
Wires (big big wires) £5 - 10
small wires (I'll allocate £5, but mostly re-use wires I have)
Gas Solenoid - £5 - £10

giving a total cost of £73 - £122

On top of this there will be costs of Case, connecttors, and heatsink.
I'm toying with the idea of adding optional water cooling, as it stands I can currently use currents approaching the need for a water cooled torch. and at the very leave water cooling the heat sink used on the rectifier and IGBT bridge would reduce the heat sink costs.

Above I said I'd allocate an amount but mostly use what I have, this will be broadly true for the whole project, I'll re-use parts, but produce a BOM "as if" I hadn't spent half my life collecting junk.