On Friday, April 20, 2012 04:10:40 PM Viesturs Lācis did opine: > Hello, gentlemen! > > I would appreciate, if someone could advice, what is wrong with my diy > relay driver. > I built it according to this scheme: > http://www.cutting.lv/fileadmin/user_upload/bd140_scheme.gif > > Relay is PE014005: > http://www.te.com/catalog/pn/en/1393219-3 > > The transistor is BD140-16 > http://www.datasheetcatalog.org/datasheet/stmicroelectronics/4186.pdf > > The diode is 1n4007 > > R1 is 220 ohm. > > M is output pin on Mesa card, set for current sinking. > > Base current should be 22mA, minimum HFE should be slightly below 40, > so current on collector should be several hundreds of mA, while the > datasheet says that relay coil is rated at 200mW, so it needs 40 mA. > > Output pin on Mesa card is working - it swings from +5,07 VDC to 0,60 > VDC as I switch it on and off. > > But it seems that the transistor is not working - the voltage on > collector does not rise above 0,00 VDC regardless of the state of Mesa > output pin. > > I have tried: > 1) shortcircuit base of transistor to GND (disconnected from Mesa > card): relay not clicking; > 2) shortcircuit collector of transistor to +5V: relay is clicking; > > So I know that relay is ok, but there is something about the > transistor. I have exchanged 3 transistors already without any luck. > > What could I be missing here? > One, shorting the base of that transistor to ground will blow the base emitter junction instantly, if not sooner.
It may be safe to ground the left end of that 220 ohm resistor, but I'd be tempted to use a 1k resistor as additional current limiting. Test your transistors. Those you have pulled back out will likely not have a detectable emitter/base junction. Modern digital meters should have a diode function on their function switch, which applies about a volt across the junction, and then displays the voltage at which the junction diode has become forward biased enough to start conducting the quite low test current available from the meter, and should, on a good junction, show something in the range of (for an Si semiconductor which that one is) between .55 and .75 volts as displayed by the meter. Reverse the probe placement and the meter should show whatever it displays when the probes aren't touching anything, usually OD or OL depending on the meter. A reading both directions is a no-no. This circuit is a current load on the Mesa, activating the relay when the Mesa output pin goes to 4.2 volts or below. If the Mesa pin goes to below a volt, you will have close to 20-22 milliamps of base drive, which given a beta of 40 min, will give around .9 amps to drive the relay, which needs only .03 amps to close it, albeit that is likely the ragged edge of low. So I would raise that 220 ohm resistor by 2 to 4x, basing how high on the measured drop across the transistor when it is turned on. When you raise it until the voltage drop across the transistor has risen to .5 volts when the relay is energized, I would install for good, 30 to 50% of the value of whatever the test resistor in place of the 220 was then being used. IOW if 2200 gives that half volt, install a 1000 permanently. Make sure D1 is good, its purpose is to absorb the inductive spike that goes below ground when the transistor is suddenly turned off, thereby protecting the transistor from a Vceo breakdown which is said to be at -45 volts according to that .pdf data sheet. If the relay is noticeably slow at opening, a small ohmage resistor could be inserted in series with that diode in order to speed the power dissipation of the free wheeling current the diode allows. 100 ohms should be safe but I'd say 47 would be a SWAG value to use there. And although the resistor will never develop any great amount of heat, I would not use a resistor rated for less than 1/2 watt in that position because the smaller ones will not have enough internal breakdown under the surge conditions and will over time drift low, but will finally open, removing the diode from the circuit completely. It will not be discolored or have any other visible damage when that happens, but the transistor will likely be 'punched thru' and the relay then stuck on till the circuit is powered down and the transistor replaced. BTW, when checking the transistors, when you find the direction thge probes need to be to find the base-emitter junction, leave the probe on the base and move the other probe to the collector, you should see a similar reading there, and trading the probes should also be equ to no connection. As that is a Si transistor, I'd also say that readings below .45 volts make the devices health quite suspect. Ge transistors OTOH can be below .2 volts. HTH. Cheers, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> Knowledge is power. -- Francis Bacon ------------------------------------------------------------------------------ For Developers, A Lot Can Happen In A Second. Boundary is the first to Know...and Tell You. Monitor Your Applications in Ultra-Fine Resolution. Try it FREE! http://p.sf.net/sfu/Boundary-d2dvs2 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
