Post by thread:[EE]: PS configuration

Hello All, I am building a +/-15 volt power supply, using a dual sec transformer, full bridge, filter caps and LM7x15 regulators. I thought of configuring it as 2 independent supplies,using a 7815 on each secondary, then tying them in series to get my +/- 15. (for op-amp use) Is there any advantage the above config as opposed to tying the secondaries to make a center tap and using a 7815 and a 7915? Regards, Robert Francisco -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> > I am building a +/-15 volt power supply, using a dual sec transformer, full > bridge, filter caps and LM7x15 regulators. > I thought of configuring it as 2 independent supplies,using a 7815 on each > secondary, then tying them in series to get my +/- 15. > (for op-amp use) > Is there any advantage the above config as opposed to tying the secondaries > to make a center tap and using a 7815 and a 7915? > Hi Robert, The short answer is Cost. With a 78xx + 79xx you can use a centre tapped secondary with a bridge rectifier. For a 2 x 78xx you require 2 x secondary windings + 2 x Bridge rectifiers. Although ........ Without checking, I have a niggle that there is a difference in the performance of the 78xx and 79xx regulators. You will need to look at the data sheets for that and beware there is a difference between manufacturers, it may be subtle, and not even revealed on the datasheets. Regards Chris -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Robert Francisco wrote: > > Hello All, > > I am building a +/-15 volt power supply, using a dual sec transformer, full > bridge, filter caps and LM7x15 regulators. > I thought of configuring it as 2 independent supplies,using a 7815 on each > secondary, then tying them in series to get my +/- 15. > (for op-amp use) > Is there any advantage the above config as opposed to tying the secondaries > to make a center tap and using a 7815 and a 7915? Yes, the advantage is that you could have two completely independant 15v supplies. They could be +15/-15, +30, +15x2A or +15 and +15 etc. I see this as being a big advantage. Just be careful not to earth the tabs of either 7815. :o) -Roman -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

Thanks for all who had contributed. I have decided to use 2x7815's mainly for the availability of parts even at my local RS. And while I'm at it, I added a 7805 on one secondary and an LM317 variable output on the other. BTW- I read on the specs that the maximum input-output differential is 37 volts and there is a way to increase the regulator output voltage. Has anyone tried using these devices at higher voltages at long as the in/out diff is less than 37? I dont know if it's allowed to post another question on a reply or should have I posted a new topic? Regards, Robert Francisco {Original Message removed}

Robert Francisco wrote: > > Thanks for all who had contributed. > > I have decided to use 2x7815's mainly for the availability of parts even at > my local RS. And while I'm at it, I added a 7805 on one secondary and an > LM317 variable output on the other. > > BTW- I read on the specs that the maximum input-output differential is 37 > volts and there is a way to increase the regulator output voltage. Has > anyone tried using these devices at higher voltages at long as the in/out > diff is less than 37? Hi Robert, the answer is yes and no. What you really need to decide is the max volts in,out,gnd, and the max current that will be drawn and then check if you can do it within the max power dissipation recommended in the specs. These regulators have an inbuilt thermal shutdown and the greatest cause of failure I have seen is from running too hot for too long, causes dry solder joints which arc and the arc blows the regulator "brain". -Roman -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

> > BTW- I read on the specs that the maximum input-output differential is 37 > > volts and there is a way to increase the regulator output voltage. Has > > anyone tried using these devices at higher voltages at long as the in/out > > diff is less than 37? Yes, I have an application where the maximum input voltage of the LM317 is 47VDC while the output voltage is 24VDC. But, just like Roman said, beware of the dissipation in the LM317 with large regulator drop voltages. Daniel... -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads