Zealot Posted November 4, 2007 Share Posted November 4, 2007 I wouldn't have thought a standard DMM could damage a transistor... most of them can handle up to 500mA fine, and I can't see a DMM putting out more than that. But yes, depending which pins you connect, you could easily get 0 resistance. Remember there are two functions for transistors - either as an amplifier, or as a digital switch. Depending on how the IC is designed, you could have two pins connected in the power off state. Quote Link to comment Share on other sites More sharing options...
fishtailfred Posted November 4, 2007 Share Posted November 4, 2007 they can handle that current across emitter to collector, but not through the gate. Quote Link to comment Share on other sites More sharing options...
Zealot Posted November 4, 2007 Share Posted November 4, 2007 That's true. I still wouldn't have thought a DMM could fry one. The current would be tiny surely. It'd still be better to be safe than sorry however. Quote Link to comment Share on other sites More sharing options...
Zealot Posted November 4, 2007 Share Posted November 4, 2007 Actually... you're assuming they're BJTs... surely they'd be MOSFETs rather than BJTs, which means you'd need a pretty large voltage to damage the insulator under the gate... Plus remember that some MOSFETs are depletion mode, not just enhancement... so basically it's meaningless unless you know what it actually is. Quote Link to comment Share on other sites More sharing options...
fishtailfred Posted November 4, 2007 Share Posted November 4, 2007 true, but better safe than sorry right. the man was asking questions that implied he didnt know toooooo much about them, so urring on the side of caution. mosfets you can kill without a meter just by not being carful about which socks you put on that morning most (cheap) meters run on a 9v and thats enough to put a fair bit of current through something like that. just being too careful Quote Link to comment Share on other sites More sharing options...
fishtailfred Posted November 4, 2007 Share Posted November 4, 2007 btw, by memory, the only fets on there at the big to220 ones, the rest are bjt. EDIT there is also an IGBT on there. Quote Link to comment Share on other sites More sharing options...
Zealot Posted November 4, 2007 Share Posted November 4, 2007 Yea, I'd never considered that a multimeter could actually nuke a transistor - will have to be more cautious. Oh and I also hadn't looked at the schematic for MS - strange, I would have assumed mosfets would be preferable to BJTs. Quote Link to comment Share on other sites More sharing options...
V8Pete Posted November 4, 2007 Author Share Posted November 4, 2007 ZOMG WTF? LMAO ...and that is why I don't like cricket. TTYL Quote Link to comment Share on other sites More sharing options...
fishtailfred Posted November 4, 2007 Share Posted November 4, 2007 Yea, I'd never considered that a multimeter could actually nuke a transistor - will have to be more cautious.Oh and I also hadn't looked at the schematic for MS - strange, I would have assumed mosfets would be preferable to BJTs. lots of little bjts for small random stuff, fets for injector drivers and the igbt for ign driver, but they arent very robust (vb921) i prefer to use ignitors like these mitsi ones : http://s50.photobucket.com/albums/f323/ ... atsink.jpg as they are virtually bullet proof. the vb921s have dwell and current limitations, the mitsi ones just soldier on unless you fry them with no heatsink. also, proding around at a component thats mounted could be proding at some other connected component thats more sensitive. there are a few ics on there too. fred. Quote Link to comment Share on other sites More sharing options...
Zealot Posted November 4, 2007 Share Posted November 4, 2007 BJT = bipolar junction transistor, MOSFET = metal oxide semiconductor field effect transistor. They're just different types of transistors. Quote Link to comment Share on other sites More sharing options...
fishtailfred Posted November 4, 2007 Share Posted November 4, 2007 kinda, the way the work and behave is totally different though, and i well know what they stand for http://i50.photobucket.com/albums/f323/fredio54/amp.jpg Quote Link to comment Share on other sites More sharing options...
Zealot Posted November 4, 2007 Share Posted November 4, 2007 Given that you mentioned the words "gate", "collector" and "emitter" in one post I had assumed you did It was more for V8Pete's benefit. They're both pretty similar in behaviour though. Did you build that amp yourself? looks pretty huge Quote Link to comment Share on other sites More sharing options...
fishtailfred Posted November 4, 2007 Share Posted November 4, 2007 fair enough the difference is of course that a bjt has say 10ma in and a beta of 100 you get 1amp out, whereas a mosfet you send in 1mv and get 1volt out similar, but different. mosfets are fast, thats why they are used in switching circuits like that as they spend very little time between on and off dissipating power. not sure why igbts are used for ign. yeah, i built the amp myself. its good for about 250wrms standing by itself, but about 600rms with a pair of fans on the heatsink (or playing music which disipates less heat than a sinewave) and an external power supply plugged in. unfortunately the amp modules are very average indeed, but its modular so i can swap them out at some stage. i designed the boards for the protection circuits myself and layed it all out. its about 70mm high, 280 deep and 450 wide, so not huge physically, but very compact for whats inside. fred. Quote Link to comment Share on other sites More sharing options...
Zealot Posted November 4, 2007 Share Posted November 4, 2007 More importantly you've got the whole 0 gate current thing, meaning you can have one mosfet driving a much larger number of other mosfets. Fan-out, that's the word I was looking for. That amp sounds pretty awesome Quote Link to comment Share on other sites More sharing options...
fishtailfred Posted November 4, 2007 Share Posted November 4, 2007 dont forget gate capacitance and its effect on impedance at higher frequencies pretty much right though. its ok. i havent used it that much, but it has powered a few parties. kinda dropped the audio hobby in favour of the turbo ute one i'll get back to that at some point when i have too many fast cars at my fingertips. Quote Link to comment Share on other sites More sharing options...
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