Post by thread:[PIC]:interfacing power transitor

Hi Just wondering if anyone has any information or know of any on-line for interfacing a power transistor. I'm trying to drive about a non-inductive 2 amp load. This circuit for lack of a better description would be the blinking led on steroids. The signal is 2 pulses (1/10th duration) at 1hz. Board voltage is 3.3vdc. I've tried a few various fets and such but haven't found one I can get to turn on cleanly. I also tried driving a mosfet with a 2n2222 but that didn't work out either (even though it should). I'm missing something here but I think I've been staring at it too long to figure out what so HEEEELLLLPPPPP!!!! Cheers Dave -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

> >I'm missing something here but I think I've been staring at it too long to >figure out what so >HEEEELLLLPPPPP!!!! What is the Vth of the fet? You have to get the gate above Vth, to turn it fully on. For very low voltage designs, you end up needing either some mechanism to create a gate voltage supply, or just use bipolars. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

At 03:31 PM 2/12/02 -0800, Dave King wrote: >Hi > >Just wondering if anyone has any information or know of any on-line for >interfacing a >power transistor. I'm trying to drive about a non-inductive 2 amp load. >This circuit for >lack of a better description would be the blinking led on steroids. The >signal is 2 pulses >(1/10th duration) at 1hz. Board voltage is 3.3vdc. Do you have a higher supply voltage available? Most FETs - even logic level FETs - will not fully enhance below 4V. You can use a Darlington bipolar transistor: one NPN part I use is TIP121. Feed the base with a 220R resistor (Ib ~ 10 mA if Vdd=3.3V). The collector will handle 2 Amps easily - but with a fairly nasty saturation voltage of about 1V. If you DO have a higher supply available, use it to turn on a MOSFET. For example, where is your 3.3V derived from? Can you use that? dwayne Dwayne Reid <spam_OUTdwaynerTakeThisOuTplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 18 years of Engineering Innovation (1984 - 2002) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Dave, For lack of information, I will explain a few ideas. First use an N-Channel FET and drive the circuit from the ground side. Use a logic level FET if using a microcontroller directly. Example: controller output pin to a 100 ohm resister to the gate of a logic level FET. If trying to high side switch the FET, you will require a gate driver that can produce about 15Vdc above the input voltage source. You pulse period is to long to boot strap the gate. Use a PNP transistor on the high side and drive it like a series pass regulator. Good Luck Gordon Varney http://www.iamnee.com {Quote hidden}> Hi > > Just wondering if anyone has any information or know of any on-line for > interfacing a > power transistor. I'm trying to drive about a non-inductive 2 amp load. > This circuit for > lack of a better description would be the blinking led on steroids. The > signal is 2 pulses > (1/10th duration) at 1hz. Board voltage is 3.3vdc. > > I've tried a few various fets and such but haven't found one I can get to > turn on cleanly. > I also tried driving a mosfet with a 2n2222 but that didn't work > out either > (even though it > should). > > I'm missing something here but I think I've been staring at it too long to > figure out what so > HEEEELLLLPPPPP!!!! > > > Cheers > > Dave > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > > -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

>Do you have a higher supply voltage available? Most FETs - even logic >level FETs - will not fully enhance below 4V. I actually compromised and went with 3.3V on the board. I thought that was a good thing as the per board cost dropped from about $47 to about $7. I didn't think I was going to give myself a stroke getting these #$% transistors to behave. >You can use a Darlington bipolar transistor: one NPN part I use is >TIP121. Feed the base with a 220R resistor (Ib ~ 10 mA if Vdd=3.3V). The >collector will handle 2 Amps easily - but with a fairly nasty saturation >voltage of about 1V. I'll see if that would work with this. I'm sure there is some part that will but I've not stumbled across it yet. >If you DO have a higher supply available, use it to turn on a MOSFET. For >example, where is your 3.3V derived from? Can you use that? I have a smart smps as I don't know exactly what my power is going to be out in the field. So all I can count on is the 3.3. I've been debating if I need to scrap the 3.3 and go back to 5vdc but then I need to start adding back in parts. TNX Dave -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

PIC pin via resistor to base of 2N2222 Emitter of 2N2222 to ground Collector of 2N2222 via resistor to base of PNP power transistor Emitter of PNP power transistor to +V Collector of PNP power transistor is your output Driving the PNP into full saturation your output voltage will be +V - Vcesat Select R's to ensure full saturation of both transistors You may also want a resistor between PNP base and +V to ensure it remains in cutoff when 2N2222 isn't driven. Bob Ammerman RAm Systems {Original Message removed}

--- David VanHorn <.....dvanhornKILLspam@spam@CEDAR.NET> wrote: > > > > What is the Vth of the fet? > > You have to get the gate above Vth, to turn it fully on. > For very low voltage designs, you end up needing either some > mechanism to > create a gate voltage supply, or just use bipolars. Another way to do it is to use a dedicated MOSFET driver which contains a charge pump to create the necessary high gate drive voltage. One such circuit is the MAX622 (see http://www.maxim-ic.com). I seen recommendations for it on some pages, but never used it as it's not available from any retailer here in Romania. ===== ing. Andrei Boros Centrul pt. Tehnologia Informatiei Societatea Romana de Radiodifuziune __________________________________________________ Do You Yahoo!? Send FREE Valentine eCards with Yahoo! Greetings! http://greetings.yahoo.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Tue, 12 Feb 2002, Andrei B. wrote: > Another way to do it is to use a dedicated MOSFET driver which contains > a charge pump to create the necessary high gate drive voltage. One such > circuit is the MAX622 (see http://www.maxim-ic.com). > > I seen recommendations for it on some pages, but never used it as it's > not available from any retailer here in Romania. > Andrei, maybe that's because romanians know to compute and design easily a step-up driving transformer for any MOSFET or better to use bipolar transistors, cheapest and easyest to drive ( but with problems at really high currents... ) regards, Vasile -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Hi Dave, Is your 2A load using the 3.3V supply or a higher voltage? ******************************************************************** This email and any attachments are confidential to the intended recipient and may also be privileged. If you are not the intended recipient please delete it from your system and notify the sender. You should not copy it or use it for any purpose nor disclose or distribute its contents to any other person. ******************************************************************** -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

--- Vasile Surducan <vasileKILLspamS3.ITIM-CJ.RO> wrote: > On Tue, 12 Feb 2002, Andrei B. wrote: > > Andrei, maybe that's because romanians know to compute and design > easily > a step-up driving transformer for any MOSFET or better to use bipolar > transistors, cheapest and easyest to drive ( but with problems at > really high currents... ) Eh, in cautarile mele despre punti H am dat si peste solutia asta. Oricum, pentru un montaj cat mai compact posibil, dar care tine la curent mare, parca l-as prefera. Iar la facut rost/confectionat traf-uri, intru in alta belea si mai tare... Cel putin cu materialele pe care le gasesc pe aici. ===== ing. Andrei Boros Centrul pt. Tehnologia Informatiei Societatea Romana de Radiodifuziune __________________________________________________ Do You Yahoo!? Send FREE Valentine eCards with Yahoo! Greetings! http://greetings.yahoo.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

>Select R's to ensure full saturation of both transistors > >You may also want a resistor between PNP base and +V to ensure it remains in >cutoff when 2N2222 isn't driven. > >Bob Ammerman >RAm Systems Thanks Bob! This was close to what I had tried with a few differences. I used a ZTX851 as one of the other fellows suggested as well as your suggestions and it worked (sort of). It's now functional but I need to clean up the signal a bit more to make it perfect. On the scope it would be best described as somewhat ugly and grungy. I've also lost my clean square wave and have a saw tooth now. Cheers Dave -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

----- Original Message ----- From: "Dave King" <.....kingdwsKILLspam.....SHAW.CA> To: <EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU> Sent: Wednesday, February 13, 2002 2:04 PM Subject: Re: [PIC]:interfacing power transitor > >Select R's to ensure full saturation of both transistors > > > >You may also want a resistor between PNP base and +V to ensure it remains in > >cutoff when 2N2222 isn't driven. > > > >Bob Ammerman > >RAm Systems > > Thanks Bob! > > This was close to what I had tried with a few differences. I used a ZTX851 > as one of the > other fellows suggested as well as your suggestions and it worked (sort of). > > It's now functional but I need to clean up the signal a bit more to make it > perfect. > On the scope it would be best described as somewhat ugly and grungy. I've also > lost my clean square wave and have a saw tooth now. > > Cheers > > Dave Sounds like one or more of the following: The source impedence on your signal is a bit high. You have some capacitance floating around that is slowing transitions. Your R's are too high and you are not driving the transistors fully into saturation. Q: Refresh my memory: 1: source of signal 2: frequency of signal 3: duty cycle of signal Bob Ammerman RAm Systems -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

>The source impedence on your signal is a bit high. K >You have some capacitance floating around that is slowing transitions. >Your R's are too high and you are not driving the transistors fully into >saturation. I have a feeling this is most of it. If you looked at the scope the signal would be one or two pulses fairly clean then it behaves like it never cleanly shuts off or over drives >Q: Refresh my memory: >1: source of signal 25ma source from a 74hc04 which is in turn driven by the pic. Everything is perfect going in and coming out of the 04. I'm actually pulling two identical signals from the 04. Each separately goes to a 2n2222 then to the ztx851 and then to the load. Each 851 has a separate line to the power source. >2: frequency of signal 2 pulses @ 1hz >3: duty cycle of signal total 20% - 2 x 100ms duration pulses with a 100ms low space between the pulses then 700ms of low. looks like (bad ascii art following....) 1010000000 or -_-_______ I know there is a stoooopid simple solution to this but I've been staring at it too long. ;-] Dave -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body