Post by thread:[EE]: Detecting low input voltage

part 1 981 bytes content-type:text/plain; (decoded 7bit) I responded to an earlier post asking how to detect low input voltage to a LM7805 regulator, and posted a schematic with a "fancy" solution. That was a cut and paste from another design, and I realized after hitting send that some of the constraints in the original design don't apply to the simple question that was asked. Attached is a simpler design that may be good enough for the purpose. It has a softer knee, but that may be fine depending on the application. I didn't reply to my other post because it seems to have vanished into never never land again. I probably forgot to restore the [EE]: tag that Outlook Express stripped off the reply. Argh! I know this isn't the list's fault, but Outlook Express is such a common email client. Is there anything that can be done about this? -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body part 2 3866 bytes content-type:image/gif; (decode) part 3 2 bytes -

Hi, I'd like to ask a couple of questions and I apologize if they are "dumb" but I'm pretty weak with analog electronics. Can you explain what is meant by a softer knee? and is there a reason you used the transistor you did or could I use a 2n3906 instead? Thanks, FJ {Quote hidden}>From: Olin Lathrop <.....olin_piclistKILLspam@spam@EMBEDINC.COM> >Reply-To: pic microcontroller discussion list <PICLISTKILLspamMITVMA.MIT.EDU> >To: .....PICLISTKILLspam.....MITVMA.MIT.EDU >Subject: [EE]: Detecting low input voltage >Date: Thu, 10 Apr 2003 09:12:52 -0400 > >I responded to an earlier post asking how to detect low input voltage to a >LM7805 regulator, and posted a schematic with a "fancy" solution. That >was a cut and paste from another design, and I realized after hitting send >that some of the constraints in the original design don't apply to the >simple question that was asked. > >Attached is a simpler design that may be good enough for the purpose. It >has a softer knee, but that may be fine depending on the application. > >I didn't reply to my other post because it seems to have vanished into >never never land again. I probably forgot to restore the [EE]: tag that >Outlook Express stripped off the reply. Argh! I know this isn't the >list's fault, but Outlook Express is such a common email client. Is there >anything that can be done about this? > >-- >http://www.piclist.com#nomail Going offline? Don't AutoReply us! >email EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body > > ><< x2.gif >> _________________________________________________________________ Tired of spam? Get advanced junk mail protection with MSN 8. http://join.msn.com/?page=features/junkmail -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

> I'd like to ask a couple of questions and I apologize if they are > "dumb" but I'm pretty weak with analog electronics. Can you explain > what is meant by a softer knee? The output will no instantly click from high to low as the input voltage drops. There will be a transition region around the input threshold where the output will be at intermediate levels. The size of the input voltage transition region is larger in this circuit than in the previous. > and is there a reason you used the > transistor you did or could I use a 2n3906 instead? Just about any small signal PNP transistor will do. The 2N4403 happens to be the PNP "jelly bean" transistor I keep around. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

Olin Lathrop <KILLspamPICLISTKILLspammitvma.mit.edu> wrote: > I didn't reply to my other post because it seems to have vanished > into never never land again. I probably forgot to restore the > [EE]: tag that Outlook Express stripped off the reply. Argh! I > know this isn't the list's fault, but Outlook Express is such a > common email client. Is there anything that can be done about > this? You mean, besides switching from that POS non-standard email client to a real one? Probably not. I use Pegasus Mail, by the way; it's absolutely free (not even shareware) and is available from http://www.pmail.com . -Andy === Andrew Warren -- RemoveMEaiwTakeThisOuTcypress.com === Principal Design Engineer === Cypress Semiconductor Corporation === === Opinions expressed above do not === necessarily represent those of === Cypress Semiconductor Corporation -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body

> -----Original Message----- > From: Olin Lathrop [SMTP:TakeThisOuTolin_piclistEraseMEspam_OUTEMBEDINC.COM] > Sent: Thursday, April 10, 2003 2:13 PM > To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU > Subject: [EE]: Detecting low input voltage > > I responded to an earlier post asking how to detect low input voltage to a > LM7805 regulator, and posted a schematic with a "fancy" solution. That > was a cut and paste from another design, and I realized after hitting send > that some of the constraints in the original design don't apply to the > simple question that was asked. > > Attached is a simpler design that may be good enough for the purpose. It > has a softer knee, but that may be fine depending on the application. > I don't understand how this circuit works. Assuming (dangerous!) that the regulator will always have at least it's dropout voltage between it's intput and output, (typicaly 2 volts for a generic 780x) the transistor will continue to be biased into conduction even the the regulators output is very low. The UNREG_OK signal will simply track the output. Please tell me where I'm going wrong in this analysis... Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. ======================================================================= Any questions about Bookham's E-Mail service should be directed to postmasterEraseME.....bookham.com. -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestmitvma.mit.edu>

At 08:09 AM 4/11/03 +0100, Michael Rigby-Jones wrote: > > >I don't understand how this circuit works. Assuming (dangerous!) that the >regulator will always have at least it's dropout voltage between it's intput >and output, (typicaly 2 volts for a generic 780x) the transistor will >continue to be biased into conduction even the the regulators output is very >low. The UNREG_OK signal will simply track the output. > >Please tell me where I'm going wrong in this analysis... Assume low drop-out regulator, then all should make sense. You have to add 1 extra resistor between E-B if you want to use a 7805 type regulator. dwayne -- Dwayne Reid <RemoveMEdwaynerEraseMEEraseMEplanet.eon.net> Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 19 years of Engineering Innovation (1984 - 2003) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' 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: To leave the PICList RemoveMEpiclist-unsubscribe-requestspam_OUTKILLspammitvma.mit.edu>

> I don't understand how this circuit works. Assuming (dangerous!) that > the > regulator will always have at least it's dropout voltage between it's > intput > and output, The point was to test whether the input voltage is too low. If the regulator always has enough voltage to work with, then there isn't a problem. This circuit will start raising UNREG-OK when the drop on the regulator gets to around 1.4V. UNREG-OK should be fully high when the drop is about 1.9V. You can add another diode in series with D1 if you want the thresholds to be a little higher 600mV to 700mV higher. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu>

> Assume low drop-out regulator, then all should make sense. You have to > add 1 extra resistor between E-B if you want to use a 7805 type > regulator. That would work. You could instead add another diode in series with the emitter. That would raise the threshold point to about 2V while maintaining a sharper response at a slightly higher cost. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestspamspamBeGonemitvma.mit.edu>

part 1 2005 bytes content-type:text/plain; format=flowedHi everyone, This is the circuit that I have (see the attachment). Since the turn-on of the transistor is based on its gain, the circuit still suffers from the transistor's gain variation (correct me if I am wrong!). The diode is there to compensate for the transistor Hfe changes due to temperature. This circuit has steeper turn on/off slope than the one Olin suggested. What do you think? Regards, Thomas {Quote hidden}>From: Olin Lathrop <RemoveMEolin_piclistKILLspamEMBEDINC.COM> >Reply-To: pic microcontroller discussion list <PICLISTSTOPspamspam_OUTMITVMA.MIT.EDU> >To: spamBeGonePICLISTSTOPspamEraseMEMITVMA.MIT.EDU >Subject: [EE]: Detecting low input voltage >Date: Thu, 10 Apr 2003 09:12:52 -0400 > >I responded to an earlier post asking how to detect low input voltage to a >LM7805 regulator, and posted a schematic with a "fancy" solution. That >was a cut and paste from another design, and I realized after hitting send >that some of the constraints in the original design don't apply to the >simple question that was asked. > >Attached is a simpler design that may be good enough for the purpose. It >has a softer knee, but that may be fine depending on the application. > >I didn't reply to my other post because it seems to have vanished into >never never land again. I probably forgot to restore the [EE]: tag that >Outlook Express stripped off the reply. Argh! I know this isn't the >list's fault, but Outlook Express is such a common email client. Is there >anything that can be done about this? > >-- >http://www.piclist.com#nomail Going offline? Don't AutoReply us! >email KILLspamlistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body > > ><< x2.gif >> _________________________________________________________________ MSN 8 helps eliminate e-mail viruses. Get 2 months FREE*. http://join.msn.com/?page=features/virus -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestEraseMEmitvma.mit.edu part 2 5552 bytes content-type:image/gif; name="circuit.GIF" (decode) part 3 2 bytes -

> This is the circuit that I have (see the attachment). Since the > turn-on of the transistor is based on its gain, the circuit still > suffers from the transistor's gain variation (correct me if I am > wrong!). The diode is there to compensate for the transistor Hfe > changes due to temperature. This circuit has steeper turn on/off slope > than the one Olin suggested. > > What do you think? You need to define what this circuit is supposed to do, else it's impossible to judge if it does it successfully. However, the diode most likely doesn't belong there. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> -----Original Message----- > From: Olin Lathrop [SMTP:@spam@olin_piclist@spam@spam_OUTEMBEDINC.COM] > Sent: Friday, April 11, 2003 8:20 PM > To: spamBeGonePICLISTKILLspamMITVMA.MIT.EDU > Subject: Re: [EE]: Detecting low input voltage > > > I don't understand how this circuit works. Assuming (dangerous!) that > > the > > regulator will always have at least it's dropout voltage between it's > > intput > > and output, > > The point was to test whether the input voltage is too low. If the > regulator always has enough voltage to work with, then there isn't a > problem. This circuit will start raising UNREG-OK when the drop on the > regulator gets to around 1.4V. UNREG-OK should be fully high when the > drop is about 1.9V. You can add another diode in series with D1 if you > want the thresholds to be a little higher 600mV to 700mV higher. > The point I was trying to make is that the drop accross the regulator is pretty constant once it goes out of regulation. For a 7805 this is over 2 volts so the circuit wouldn't work, but as already explained by someone, this application is for an LDO. Regards Mike ======================================================================= This e-mail is intended for the person it is addressed to only. The information contained in it may be confidential and/or protected by law. If you are not the intended recipient of this message, you must not make any use of this information, or copy or show it to any person. Please contact us immediately to tell us that you have received this e-mail, and return the original to us. Any use, forwarding, printing or copying of this message is strictly prohibited. No part of this message can be considered a request for goods or services. ======================================================================= Any questions about Bookham's E-Mail service should be directed to .....postmasterspam_OUTbookham.com. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.