Post by thread:[PICLIST] [EE] power supply heat

I have a circuit with a 7805 regulator that was set-up with a 12.6V CT transformer and two diodes on the input side. This gave it an input voltage of 8.9V (or, 6.3V x 1.414 = 8.9). The 12.6V transformer output is rated with the primary at 230V. Now I am considering using it on 277VAC. This should bring the input to the 7805 up to about 120% of the 8.9V, 0r about 10.7V. At 8.9V no heatsink was required with a .5A load. How can I pre-figure how much power will be dissipated when on the 277V primary, so I can select a heatsink, or perhaps decide that the whole idea is either OK or NG??? I checked the National datasheet for the 7805, and still came up short. Is it as simple as saying that I will have 5.7V dropout at .5A, thus will have 2.85W to deal with? TIA, Chris -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

At 02:53 PM 6/19/02 -0400, you wrote: Power dissipation is just: P= (input voltage - output voltage) * (output current + regulator current) Regulator current is typically around 5mA for the 78xx05. You should work out heat sink size based on worst case input voltage, output voltage tolerance, and ambient temperature. Best regards, {Quote hidden}>I have a circuit with a 7805 regulator that was set-up with a 12.6V CT >transformer and two diodes on the input side. This gave it an input voltage >of 8.9V (or, 6.3V x 1.414 = 8.9). > >The 12.6V transformer output is rated with the primary at 230V. Now I am >considering using it on 277VAC. This should bring the input to the 7805 up >to about 120% of the 8.9V, 0r about 10.7V. > >At 8.9V no heatsink was required with a .5A load. How can I pre-figure how >much power will be dissipated when on the 277V primary, so I can select a >heatsink, or perhaps decide that the whole idea is either OK or NG??? > >I checked the National datasheet for the 7805, and still came up short. >Is it as simple as saying that I will have 5.7V dropout at .5A, thus will >have 2.85W to deal with? > >TIA, >Chris > >-- >http://www.piclist.com#nomail Going offline? Don't AutoReply us! >email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body Spehro Pefhany --"it's the network..." "The Journey is the reward" speffKILLspaminterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com 9/11 United we Stand -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

At 02:53 PM 6/19/02 -0400, Chris Loiacono wrote: >The 12.6V transformer output is rated with the primary at 230V. Now I am >considering using it on 277VAC. This should bring the input to the 7805 up >to about 120% of the 8.9V, 0r about 10.7V. Don't do that! I think that you will cook your transformer if you over-voltage the input that much. Core saturation. How much power do you need? We use several custom transformers for some of our products - one of those has 120V, 277V, 347V primary taps and supplies 20 Vac @ 4A. It mates with a little card that has a NS simple switcher that you could set to whatever voltage you wanted within the range of 5Vdc - 14Vdc. dwayne -- Dwayne Reid <EraseMEdwaynerspam_OUTTakeThisOuTplanet.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#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

> >The 12.6V transformer output is rated with the primary at 230V. Now I am > >considering using it on 277VAC. This should bring the input to the 7805 up > >to about 120% of the 8.9V, 0r about 10.7V. > > Don't do that! > > I think that you will cook your transformer if you over-voltage the input > that much. Core saturation. Doesn't core saturation depend on current, rather than voltage? I would think that you'd have less chance of saturating at 277V than 230V. Bob Ammerman RAm Systems -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body

{Quote hidden}> > >The 12.6V transformer output is rated with the primary at > 230V. Now I am > > >considering using it on 277VAC. This should bring the > input to the 7805 > up > > >to about 120% of the 8.9V, 0r about 10.7V. > > > > Don't do that! > > > > I think that you will cook your transformer if you > over-voltage the input > > that much. Core saturation. > > Doesn't core saturation depend on current, rather than > voltage? I would > think that you'd have less chance of saturating at 277V than 230V. > > Bob Ammerman > RAm Systems > But no-load current depends heavily on primary coil voltage. How about having 1A no-load current with 277V? (Core saturation). Mike. -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu

At 11:39 PM 6/19/02 -0400, Bob Ammerman wrote: > > > > I think that you will cook your transformer if you over-voltage the input > > that much. Core saturation. > >Doesn't core saturation depend on current, rather than voltage? I would >think that you'd have less chance of saturating at 277V than 230V. Try it and see! Do it outside so that the stink has doesn't get to you ;) I'm not sure how best to describe this. A transformer core can handle only a certain flux density before it saturates. This is directly proportional to applied voltage as follows: Peak AC Flux (Gauss) B(AC) = Vrms x 10^8 / 4.44 * N * f * A B gauss V volts N #turns f Hz A cm^2 One of many web pages that describe this: <http://www.coilws.com/magneticandhow.html> I had a couple of our power supplies returned after failing because the installer had inadvertently connected 347 Vac to the 277 Vac primary tap. They didn't smell nice :( dwayne -- Dwayne Reid <RemoveMEdwaynerTakeThisOuTplanet.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: To leave the PICList spamBeGonepiclist-unsubscribe-requestspamBeGonemitvma.mit.edu

>> I think that you will cook your transformer if you over-voltage the input >> that much. Core saturation. > >Doesn't core saturation depend on current, rather than voltage? I would >think that you'd have less chance of saturating at 277V than 230V. > >Bob Ammerman >RAm Systems Nope. The core must standoff the volt*second product. Thus, more volts needs more core. Likewise, lower frequency means more seconds and also means more core needed. This is why higher freq operation is often desirable. If a transformer is designed for one input voltage and you find you must increase the voltage, it helps to offset this by raising the operating frequency also if possible. The core in question *may* work at 277V but this can only be determined by testing or consulting the manufacturer. Since excess core usually means excess expense, the cores are usually made to just barely do the job and typically run warm at rated load. Raising the voltage will usually raise the temperature also. Determination of suitability is left as an exercise to the student. Tom M. -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu

This is why I contacted the manufacturer at the same time the original message was posted here. I don't expec to hear from them any time soon...Since changing the freq is out of the question, I am most curious about Dwayne's solution... CL {Quote hidden}> > The core in question *may* work at 277V but this can only be > determined by > testing or consulting the manufacturer. Since excess core > usually means > excess expense, the cores are usually made to just barely do > the job and > typically run warm at rated load. Raising the voltage will > usually raise > the temperature also. Determination of suitability is left as > an exercise > to the student. > > Tom M. > > -- > http://www.piclist.com hint: To leave the PICList > RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu > > > -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestEraseME.....mitvma.mit.edu

What sort of wattage are you after? RS Components have 50/100/200VA kits that comes with a pre wound 115/230 primary on it, and a piece of paper with tables for calculating a secondary. The unit is not potted, and so it should not be too hard to either remove the existing primary, and replace it, or else add a supplementary primary winding before putting a secondary on it. If you did the latter then make sure you put sufficient insulation in between the primary and secondary. The kit has a plastic bobbin with primary windings, and a stack of laminations to put in afterwards, along with the necessary metal pieces to clamp it all up afterwards. At rshttp://www.com/ look for catalogue # 182-9919 for 50VA, 182-9925 for 100VA and 182-9931 for 200VA -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestmitvma.mit.edu

On Wed, 19 Jun 2002, Chris Loiacono wrote: >I have a circuit with a 7805 regulator that was set-up with a 12.6V CT >transformer and two diodes on the input side. This gave it an input voltage >of 8.9V (or, 6.3V x 1.414 = 8.9). > >The 12.6V transformer output is rated with the primary at 230V. Now I am >considering using it on 277VAC. This should bring the input to the 7805 up >to about 120% of the 8.9V, 0r about 10.7V. > >At 8.9V no heatsink was required with a .5A load. How can I pre-figure how >much power will be dissipated when on the 277V primary, so I can select a >heatsink, or perhaps decide that the whole idea is either OK or NG??? > >I checked the National datasheet for the 7805, and still came up short. >Is it as simple as saying that I will have 5.7V dropout at .5A, thus will >have 2.85W to deal with? Maybe you can remove 1 diode and have more ripple. This will reduce the power on the regulator. Simulate your circuit with spice. I am amazed that you ran 9V into it (dissipating 2W) and did not need a heatsink. You will need a small heatsink for 3W but you can remove the diode as I said or you can add a resistor to spread the load. 3.3ohms/2W in series will go a long way I think. You can put it before the filter cap too but then you will need another value. Peter -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestEraseMEEraseMEmitvma.mit.edu

If you want to run off of 277VAC and you only have 230V transformers available, get a 230V to 48V transformer and wire it as a stepdown autotransformer. 230V + 48V = 277V aprox. so you can input 277 to both windings in series (correct phasing) and get 230 out, everything will be in spec. Ideally you would get a transformer with both 48V and 12.6V windings so you don't need two chunks of iron. To test phasing try putting a light bulb in series with the primary on your first test. If the phasing is wrong the core will saturate and the light will light. If the lamp stays dark you have the phasing right. This test is done with no load on the output. Doug Butler Sherpa Engineering > {Original Message removed}

On Fri, 21 Jun 2002 12:41:21 -0500, you wrote: >If you want to run off of 277VAC and you only have 230V transformers >available, get a 230V to 48V transformer and wire it as a stepdown >autotransformer. 230V + 48V = 277V aprox. so you can input 277 to both >windings in series (correct phasing) and get 230 out, everything will be in >spec. Ideally you would get a transformer with both 48V and 12.6V windings >so you don't need two chunks of iron. ,,and you need to have the full mains spec isolation between the 48 and 12.6v windings, which will often not be the case for standard transformers, which only have full isolation between primary and secondary, not between secondaries.. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads