Post by thread:stupid question about sink/source

Hey, really dum question... I have a small 5v, 75ma, 70ohm coil relay that i was fooling around with using a pic. now if i attatch one end to +5v and the other end to the pic PORTB with the pin driven low the relay clicks. If i attatch one end on the relay to ground and then the other end to a pin on PORTB driven high nuthing happens. Now i assume that this is because the 16f84 can sink 25ma while only source 20ma right?? a 75ma coil though is like 3 times as much so shouldnt the pin being Low and the other end of the relay connected to 5v still not be able to have enought current only sinking 25ma to switch the relay? YES I KNOW USE TRANSISTORS BUT IM JUST SAYING cause i know half of you are gonna reply relling me to use transistors to switch the relay ;) mike, again. Send someone a cool Dynamitemail flashcard greeting!! And get rewarded. GO AHEAD! http://cards.dynamitemail.com/index.php3?rid=fc-41

Mike M wrote: > > Hey, really dum question... I have a small 5v, 75ma, 70ohm coil relay that i was fooling around with using a pic. now if i attatch one end to +5v and the other end to the pic PORTB with the pin driven low the relay clicks. If i attatch one end on the relay to ground and then the other end to a pin on PORTB driven high nuthing happens. Now i assume that this is because the 16f84 can sink 25ma while only source 20ma right?? a 75ma coil though is like 3 times as much so shouldnt the pin being Low and the other end of the relay connected to 5v still not be able to have enought current only sinking 25ma to switch the relay? YES I KNOW USE TRANSISTORS BUT IM JUST SAYING cause i know half of you are gonna reply relling me to use transistors to switch the relay ;) Half? possibly about 125% of them Mike. However, I dare say sinking isn't completely killing the micro, as you have an external +5V supply, but sourcing is. Of course it will kill the micro, port, or port pin eventually. Don McKenzie spam_OUTdonTakeThisOuTdontronics.com http://www.dontronics.com World's Largest Range of Atmel/AVR and PICmicro Hardware and Software. Free Basic Compiler and Programmer http://www.dontronics.com/runavr.html

Mike M wrote: > > Hey, really dum question... I have a small 5v, 75ma, 70ohm coil relay that i was fooling around with using a pic. now if i attatch one end to +5v and the other end to the pic PORTB with the pin driven low the relay clicks. If i attatch one end on the relay to ground and then the other end to a pin on PORTB driven high nuthing happens. Now i assume that this is because the 16f84 can sink 25ma while only source 20ma right?? a 75ma coil though is like 3 times as much so shouldnt the pin being Low and the other end of the relay connected to 5v still not be able to have enought current only sinking 25ma to switch the relay? YES I KNOW USE TRANSISTORS BUT IM JUST SAYING cause i know half of you are gonna reply relling me to use transistors to switch the relay ;) > > mike, again. "Use HexFets, not bipolar transistors", Mike <G> I didn't hear anything about a protection diode across the relay, Mike; Even with a HexFet, it's still a good idea. Mark -- I re-ship for small US & overseas businesses, world-wide. (For private individuals at cost; ask.)

Mitchell - the two things I know about a hexfet that makes them better than a transistor is 1)there are versions that only require 5 v to turn them on. This makes them a snap to connect directly to a PIC to handle large power loads of any (reasonable) voltage. 2) A hexfet (or any FET) has a very low resistance when "turned on". I do not remember the exact figures, but I think a transistor may have an internal resistance in the range to 10's to 100's of ohms. A hexfet may have a resistance on only 1ohm down to .006 ohms. The more resistance present, the more energy that is converted into heat that must be dissipated, plus it is wasted energy. So the bottom line is, hexfet's are easy to interface and energy efficient. David V. Fansler Network Administrator TripPath Imaging, Inc. (formerly AutoCyte, Inc.) 336-222-9707 Ext. 261 dfanslerKILLspamautocyte.com Now Showing! http://www.mindspring.com\~dfansler\ <http://www.mindspring.com\~dfansler\> Updated December 16, 1999 Ann's Cancer & David's Observatory -----Original Message----- From: Mitchell D. Miller [.....mdmiller2KILLspam.....HOME.COM] Sent: Tuesday, December 28, 1999 4:23 PM To: EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU Subject: Re: stupid question about sink/source > "Use HexFets, not bipolar transistors" What's special about HexFets? Mitch Miller, Omaha, NE mdmiller2spam_OUThome.com

What is your question? Fansler, David wrote: {Quote hidden}> Mitchell - the two things I know about a hexfet that makes them better than > a transistor is 1)there are versions that only require 5 v to turn them on. > This makes them a snap to connect directly to a PIC to handle large power > loads of any (reasonable) voltage. 2) A hexfet (or any FET) has a very low > resistance when "turned on". I do not remember the exact figures, but I > think a transistor may have an internal resistance in the range to 10's to > 100's of ohms. A hexfet may have a resistance on only 1ohm down to .006 > ohms. The more resistance present, the more energy that is converted into > heat that must be dissipated, plus it is wasted energy. > > So the bottom line is, hexfet's are easy to interface and energy efficient. > > David V. Fansler > Network Administrator > TripPath Imaging, Inc. (formerly AutoCyte, Inc.) > 336-222-9707 Ext. 261 > @spam@dfanslerKILLspamautocyte.com > Now Showing! http://www.mindspring.com\~dfansler\ > <http://www.mindspring.com\~dfansler\> > Updated December 16, 1999 > Ann's Cancer & David's Observatory > > {Original Message removed}

In message <KILLspamF138C4434C8BD111B2120060082AEB7D45BC86KILLspamrias.com>, Fansler, David <RemoveMEDFANSLERTakeThisOuTAUTOCYTE.COM> writes >Mitchell - the two things I know about a hexfet that makes them better than >a transistor is 1)there are versions that only require 5 v to turn them on. >This makes them a snap to connect directly to a PIC to handle large power >loads of any (reasonable) voltage. 2) A hexfet (or any FET) has a very low >resistance when "turned on". I do not remember the exact figures, but I >think a transistor may have an internal resistance in the range to 10's to >100's of ohms. A hexfet may have a resistance on only 1ohm down to .006 >ohms. The more resistance present, the more energy that is converted into >heat that must be dissipated, plus it is wasted energy. A transistor doesn't have an 'internal resistance', when turned fully on it has a voltage drop across it, depending on the particular transistor this is around 0.1-0.2 volts. In many circumstances this makes them easier to use than an FET, and transistors only need 0.7 volts to turn on, but do need a fair amount of current depending on the spec of the device. -- Nigel. /--------------------------------------------------------------\ | Nigel Goodwin | Internet : spamBeGonenigelgspamBeGonelpilsley.demon.co.uk | | Lower Pilsley | Web Page : http://www.lpilsley.demon.co.uk | | Chesterfield | Official site for Shin Ki and New Spirit | | England | Ju Jitsu | \--------------------------------------------------------------/

Mitchell D. Miller wrote: > > > "Use HexFets, not bipolar transistors" > > What's special about HexFets? > > Mitch Miller, Omaha, NE > TakeThisOuTmdmiller2EraseMEspam_OUThome.com Several others have it right <G> Transistors are current amplifiers, a FET is more like a voltage-controlled switch or (non-inductive) relay. I like HexFETs, just a quirk maybe <G> The logic gated ones have really been convenient in some packages I've made in the past. With a FET instead of a bipolar, you do have to make sure you saturate the FET (the logic gated FETs handle this for you.) The advantage is that, when switching a 1A 12V load (for example), instead of dissipating base drive * base current + Vce * Ice i.e (0.7V*(1A/hFE)) + (0.3V*1A) or about 0.31 watts in that transistor, you dissipate (1A^2*0R01) or 0.01W in that FET, if you're running off a battery pack that can make a big difference. (The 1A is the real killer, of course, in this case that was just the startup surge, and you need to saturate the transistor/FET for THAT; the sensor's supply current then dropped down to about 30mA - no use dissipating 20mA into that transistor, to keep a 30mA load switched on, the HexFET made a big difference here. Net savings, about 35% of the power budget, for one project.) Mark -- I re-ship for small US & overseas businesses, world-wide. (For private individuals at cost; ask.)