Post by thread:[OT]: 7905 getting hot!

Hi all, I thought I'd ask this question here as my pool of friends here can't seem to offer any help. I have a circuit where I am feeding -24V into a 7905 linear regulator. It gets very hot, can't keep your finger on it. It is on a pretty hefty heatsink. I took the output leg loose and measured the current draw its supplying and it shows to be 280ma. I would think that it should not have any problem with this and shouldn't be getting hot. I thought maybe it was getting so hot because there was such a large voltage feeding it so I then took the circuit off of the 24v supply and fed it with 12V and again it still gets too hot to touch. I had a .1uf cap on the input and output. I looked at the data sheet and from that I tried changing them out to a 2.2uf on the input and a 1uf on the output. At the same time, I chaged out the regulator to one of a different brand. Still the same results. My questions would be should it be getting that hot? I thought that it shouldn't at only 280ma. If not, can you point me toward any problems I might be having that could cause this? Suggestions of things to look at would be greatly appreciated. Thanks, FJ _________________________________________________________________ The new MSN 8: smart spam protection and 2 months FREE* http://join.msn.com/?page=features/junkmail -- 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

-24 is nearing the -25 limit, which is probably why it's getting hot - but -12 is still probably a bit more than it needs. Can you drop it down to 7 for testing purposes? As long as the rest of your circuit is right, and your sure it's drawing 280mA... there doesn't seem to be an obvious reason why it would heat up. Quoting fred jones <spam_OUTboattowTakeThisOuTHOTMAIL.COM>: {Quote hidden}> Hi all, > I thought I'd ask this question here as my pool of friends here can't seem > to offer any help. I have a circuit where I am feeding -24V into a 7905 > linear regulator. It gets very hot, can't keep your finger on it. It is on > a pretty hefty heatsink. I took the output leg loose and measured the > current draw its supplying and it shows to be 280ma. I would think that it > should not have any problem with this and shouldn't be getting hot. I > thought maybe it was getting so hot because there was such a large voltage > feeding it so I then took the circuit off of the 24v supply and fed it with > 12V and again it still gets too hot to touch. I had a .1uf cap on the input > and output. I looked at the data sheet and from that I tried changing them > out to a 2.2uf on the input and a 1uf on the output. At the same time, I > chaged out the regulator to one of a different brand. Still the same > results. > > My questions would be should it be getting that hot? I thought that it > shouldn't at only 280ma. If not, can you point me toward any problems I > might be having that could cause this? Suggestions of things to look at > would be greatly appreciated. > Thanks, > FJ > > _________________________________________________________________ > The new MSN 8: smart spam protection and 2 months FREE* > http://join.msn.com/?page=features/junkmail > > -- > 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 > ---------------------------------------- This mail sent through http://www.mywaterloo.ca -- 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

Even at 12V, there is a 7V drop across the regulator, which is about 2W of power dissipation. A 7905 with no heatsink would definitely get hot at 2 W dissipation. You said that it was on a pretty hefty heatsink, but did you keep it attached to the heatsink for the 12V test? I think that at -24 V in and 280mA, you might well be outside the maximum power dissipation of the 7905. Also, is the output voltage correct? Are you sure you have the pinout correct? I'm not sure what the tab is connected to in the 7905 but are you sure that you aren't shorting the tab to something it is not supposed to be connected to by attaching it to the heatsink? In other words, the tab is ground on the 78xx series, but it might not be on the 79xx ones. Sean At 10:01 PM 3/9/2003 -0500, you wrote: {Quote hidden}>-24 is nearing the -25 limit, which is probably why it's getting hot - but -12 >is still probably a bit more than it needs. Can you drop it down to 7 for >testing purposes? As long as the rest of your circuit is right, and your sure >it's drawing 280mA... there doesn't seem to be an obvious reason why it would >heat up. > >Quoting fred jones <.....boattowKILLspam@spam@HOTMAIL.COM>: > > > Hi all, > > I thought I'd ask this question here as my pool of friends here can't seem > > to offer any help. I have a circuit where I am feeding -24V into a 7905 > > linear regulator. It gets very hot, can't keep your finger on it. It > is on > > a pretty hefty heatsink. I took the output leg loose and measured the > > current draw its supplying and it shows to be 280ma. I would think that it > > should not have any problem with this and shouldn't be getting hot. I > > thought maybe it was getting so hot because there was such a large voltage > > feeding it so I then took the circuit off of the 24v supply and fed it with > > 12V and again it still gets too hot to touch. I had a .1uf cap on the > input > > and output. I looked at the data sheet and from that I tried changing them > > out to a 2.2uf on the input and a 1uf on the output. At the same time, I > > chaged out the regulator to one of a different brand. Still the same > > results. > > > > My questions would be should it be getting that hot? I thought that it > > shouldn't at only 280ma. If not, can you point me toward any problems I > > might be having that could cause this? Suggestions of things to look at > > would be greatly appreciated. > > Thanks, > > FJ > > > > _________________________________________________________________ > > The new MSN 8: smart spam protection and 2 months FREE* > > http://join.msn.com/?page=features/junkmail > > > > -- > > 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 > > > > > > >---------------------------------------- >This mail sent through http://www.mywaterloo.ca > >-- >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

Hi Fred The temperature of the 7905 will be proportional to how much power it is dissipating. Power here is the voltage drop across it from input to output multiplied by the current it is supplying to the load. It's own quiescent current figures in too, about 10mA. So in your "It gets very hot, can't keep your finger on it" circuit, the poor little regulator sees (24in - 5out)* 28mA = 19V * .28A = 5.32 SMOKIN' WATTS! With 12 volts in, it sees 7V * .28A = 1.96 WATTS. With Iq (10mA) call it 2W. Yes, this will get hot. Solutions? 1. Lower the input voltage. 2. If you can't do that, put something in to drop it. A resistor between the 24V and the regulator can take the heat off the regulator. Of course, then the resistor will get real hot. 3. Use a switcher. Look up buck converters. 4. If your load is fixed, you can bypass the regulator with a resistor to supply some of the current that way and the regulator supplies the rest keeping things at 5volts. Don't do this one unless you *really* understand how it works. 5. Use two or more regulators in parallel with their outputs separated with "ballast" resistors to make them share the load. Good luck! Hope the blisters heal real soon. Tom M. -- 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

Thanks to all of you who answered my question. I now understand the problem. I am using a 7912 in the circuit as well. I guess i could feed the 7905 off of it and improve things. Should I do as Jinx suggests and put a resistor on the output of the 7912 to drop it and then feed the 7905? I believe the right answer here is to use a switching regulator but this circuit can't tolerate any extra noise it would generate. I knew you guys would be able to help. Thanks so much, FJ {Quote hidden}>From: fred jones <boattowKILLspamHOTMAIL.COM> >Reply-To: pic microcontroller discussion list <.....PICLISTKILLspam.....MITVMA.MIT.EDU> >To: EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU >Subject: [OT]: 7905 getting hot! >Date: Sun, 9 Mar 2003 20:43:20 -0600 > >Hi all, >I thought I'd ask this question here as my pool of friends here can't seem >to offer any help. I have a circuit where I am feeding -24V into a 7905 >linear regulator. It gets very hot, can't keep your finger on it. It is >on >a pretty hefty heatsink. I took the output leg loose and measured the >current draw its supplying and it shows to be 280ma. I would think that it >should not have any problem with this and shouldn't be getting hot. I >thought maybe it was getting so hot because there was such a large voltage >feeding it so I then took the circuit off of the 24v supply and fed it with >12V and again it still gets too hot to touch. I had a .1uf cap on the >input >and output. I looked at the data sheet and from that I tried changing them >out to a 2.2uf on the input and a 1uf on the output. At the same time, I >chaged out the regulator to one of a different brand. Still the same >results. > >My questions would be should it be getting that hot? I thought that it >shouldn't at only 280ma. If not, can you point me toward any problems I >might be having that could cause this? Suggestions of things to look at >would be greatly appreciated. >Thanks, >FJ > >_________________________________________________________________ >The new MSN 8: smart spam protection and 2 months FREE* >http://join.msn.com/?page=features/junkmail > >-- >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 _________________________________________________________________ Tired of spam? Get advanced junk mail protection with MSN 8. http://join.msn.com/?page=features/junkmail -- 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

You don't say what you are trying to do with power other than you would like it to be low noise. Does all of the -5 need to be low noise or just some? Some options: 1. You can use a buck regulator to drop -24 down to -7 or so. Then use your 7905 to drop to -5. 2. Low pass filter (L C) the output of the buck regulator before it goes to the the 7905. 3. Use an opamp such as a LM324 to give a very low noise -5 for your low noise section. 4. Use an opamp with a hefty transistor to do the linear regulation from -7 down to -5. This eliminates the 7905 all together. cheers, eric But, if you need a low noise -5. Use an OPAMP such as LM324 {Quote hidden}>Thanks to all of you who answered my question. I now understand the >problem. > >I am using a 7912 in the circuit as well. I guess i could feed the 7905 off >of it and improve things. Should I do as Jinx suggests and put a resistor >on the output of the 7912 to drop it and then feed the 7905? I believe the >right answer here is to use a switching regulator but this circuit can't >tolerate any extra noise it would generate. > >I knew you guys would be able to help. >Thanks so much, >FJ > > > > >>From: fred jones <boattowspam_OUTHOTMAIL.COM> >>Reply-To: pic microcontroller discussion list <@spam@PICLISTKILLspamMITVMA.MIT.EDU> >>To: KILLspamPICLISTKILLspamMITVMA.MIT.EDU >>Subject: [OT]: 7905 getting hot! >>Date: Sun, 9 Mar 2003 20:43:20 -0600 >> >>Hi all, >>I thought I'd ask this question here as my pool of friends here can't seem >>to offer any help. I have a circuit where I am feeding -24V into a 7905 >>linear regulator. It gets very hot, can't keep your finger on it. It is >>on >>a pretty hefty heatsink. I took the output leg loose and measured the >>current draw its supplying and it shows to be 280ma. I would think that it >>should not have any problem with this and shouldn't be getting hot. I >>thought maybe it was getting so hot because there was such a large voltage >>feeding it so I then took the circuit off of the 24v supply and fed it with >>12V and again it still gets too hot to touch. I had a .1uf cap on the >>input >>and output. I looked at the data sheet and from that I tried changing them >>out to a 2.2uf on the input and a 1uf on the output. At the same time, I >>chaged out the regulator to one of a different brand. Still the same >>results. >> >>My questions would be should it be getting that hot? I thought that it >>shouldn't at only 280ma. If not, can you point me toward any problems I >>might be having that could cause this? Suggestions of things to look at >>would be greatly appreciated. >>Thanks, >>FJ >> >>_________________________________________________________________ >>The new MSN 8: smart spam protection and 2 months FREE* >>http://join.msn.com/?page=features/junkmail >> >>-- >>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 > > >_________________________________________________________________ >Tired of spam? Get advanced junk mail protection with MSN 8. >http://join.msn.com/?page=features/junkmail > >-- >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 PICList is archived three different ways. See http://www.piclist.com/#archives for details.

re 7905 connections - > Do you have it correctly connected? The centre leg is NOT the ground leg. > Don't ask how I know - but it was learnt many years ago. :))))) Knowing WHY these things are so can be helpful: (Almost) all 3 terminal regulators have the most negative part of the circuit connected to the tag and the centre electrode. This is due to the fact that the most negative part of the die is the substrate and this is electrically and mechanically and thermally connected to the mounting frame. Therefore, on positive regulators the middle tag is ground as this is most negative in that case. AND on negative regulators the middle leg is Vin as this is the most negative potential in that case. (It is possible to build regulators differently but in most cases this is the easiest and cheapest and most effective way to do it so that's how it's usually done.) Russell McMahon -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Just a thought, it is connected up correctly isn't it? The negative regulators have a different pinout than the positive regulators. IGO for positive and GIO for negative. If connected up wrong, and damaged, can sometimes put out a strange voltage and gets hot. Keep the bypass caps at 0.1uf. Rick >From: fred jones <RemoveMEboattowTakeThisOuTHOTMAIL.COM> >Hi all, >I thought I'd ask this question here as my pool of friends here can't seem >to offer any help. I have a circuit where I am feeding -24V into a 7905 >linear regulator. It gets very hot, can't keep your finger on it. It is >on >a pretty hefty heatsink. I took the output leg loose and measured the >current draw its supplying and it shows to be 280ma. I would think that it >should not have any problem with this and shouldn't be getting hot. I >thought maybe it was getting so hot because there was such a large voltage >feeding it so I then took the circuit off of the 24v supply and fed it with >12V and again it still gets too hot to touch. I had a .1uf cap on the >input >and output. I looked at the data sheet and from that I tried changing them >out to a 2.2uf on the input and a 1uf on the output. At the same time, I >chaged out the regulator to one of a different brand. Still the same >results. > >My questions would be should it be getting that hot? I thought that it >shouldn't at only 280ma. If not, can you point me toward any problems I >might be having that could cause this? Suggestions of things to look at >would be greatly appreciated. >Thanks, >FJ -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Hi Alan, Yes I'm sure its connected correctly, it puts out a nice clean -5V. Its dropping 19V which equates to 5W so now that I understand this, I understand why I can cook dinner on it. Thanks, FJ {Quote hidden}>From: "Alan B. Pearce" <spamBeGoneA.B.PearcespamBeGoneRL.AC.UK> >Reply-To: pic microcontroller discussion list <TakeThisOuTPICLISTEraseMEspam_OUTMITVMA.MIT.EDU> >To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU >Subject: Re: [OT]: 7905 getting hot! >Date: Mon, 10 Mar 2003 10:08:01 -0000 > >Dop you have it correctly connected? The centre leg is NOT the ground leg. > >Don't ask how I know - but it was learnt many years ago. :))))) > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. _________________________________________________________________ Protect your PC - get McAfee.com VirusScan Online http://clinic.mcafee.com/clinic/ibuy/campaign.asp?cid=3963 -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Jai Dhar wrote: > -24 is nearing the -25 limit, which is probably why it's getting hot > - but -12 is still probably a bit more than it needs. Can you drop it > down to 7 for testing purposes? As long as the rest of your circuit > is right, and your sure it's drawing 280mA... there doesn't seem to > be an obvious reason why it would heat up. If you think that 280mA x 19V (more than 5W) is not hot enough to burn your finger, than I wonder your finger is dead already... :) I would suggest you to use some voltage drop device to reduce this 24V to something around 9 to 12V, so the 7905 heat dissipation would be less than 1 Watt, what would be nice, isn't it? Suggestions would go from power resistors to lamps in series with the input of the 7905. Lets see, a regular 50 ohms 5 Watts resistor would drop the input voltage to the 7905 to around 10V. A truck (24V x 10W) would reduce the voltage to around 9V. All of this if your circuit continuous to drain around 290mA. Of course, there are nice dc-dc conversions that will produce 5V with much less power dissipation loss. Wagner. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

>I am using a 7912 in the circuit as well. I guess i could feed the 7905 off >of it and improve things. Should I do as Jinx suggests and put a resistor >on the output of the 7912 to drop it and then feed the 7905? Taking 280 mA from 24v to 5v will generate 5 watts of heat no matter how you do it (linear, not switching). So think of it as a budget-- you can dissipate it in the 7095 or partly in the 7912 or even add a resistor and shed part of it there. It all adds up. Also not a bad idea to measure the current going INTO the regulator; you would catch any surprises such as oscillation, big leakage current, shorted tab, and all that. Barry -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestEraseME.....mitvma.mit.edu>

Hi, By this, what exactly do you mean. What I did was cut the output pin of the 7905 and put a current meter across this and measured 280ma. I am still trying to figure out how I want to handle this problem. I want to try using a negative buck regulator to go from -24 to -5 but haven't had much success at finding a suitable circuit. In the past when I've played with switchers I've had too much noise and this particular circuit can't tolerate more noise. My inexperience with these probably doesn't help :0) Thanks, FJ >Also not a bad idea to measure the current going INTO the regulator; >you would catch any surprises such as oscillation, big leakage >current, shorted tab, and all that. > >Barry _________________________________________________________________ The new MSN 8: smart spam protection and 2 months FREE* http://join.msn.com/?page=features/junkmail -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestmitvma.mit.edu>

Take a look at Roman Black's regulator - just use PNP transistors instread of NPN & reverse the diodes etc. Should work well for this application - except you may need to look carefully at the output noise - add more filtering etc. You'll get noise issues with any switcher regardless! One possibility might be to use the switcher to drop the voltage to 8V or so & then use the linear regulator (7905) from here. This would cut your dissapation to about a watt. and should help the noise problem. Richard P fred jones <boattow@HOTMA To: RemoveMEPICLISTEraseMEEraseMEMITVMA.MIT.EDU IL.COM> cc: Sent by: pic Subject: Re: [OT]: 7905 getting hot! microcontrolle r discussion list <PICLIST@MITVM A.MIT.EDU> 13/03/03 12:44 Please respond to pic microcontrolle r discussion list Hi, By this, what exactly do you mean. What I did was cut the output pin of the 7905 and put a current meter across this and measured 280ma. I am still trying to figure out how I want to handle this problem. I want to try using a negative buck regulator to go from -24 to -5 but haven't had much success at finding a suitable circuit. In the past when I've played with switchers I've had too much noise and this particular circuit can't tolerate more noise. My inexperience with these probably doesn't help :0) Thanks, FJ >Also not a bad idea to measure the current going INTO the regulator; >you would catch any surprises such as oscillation, big leakage >current, shorted tab, and all that. > >Barry _________________________________________________________________ The new MSN 8: smart spam protection and 2 months FREE* http://join.msn.com/?page=features/junkmail -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestspam_OUTKILLspammitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTspammitvma.mit.edu>