On Monday, May 26, 2014 1:54:51 PM UTC-7, John Fields wrote:> The real problem lies in not letting the magic smoke out of the > rectifiers during turn-on, and that's easily side-stepped by sizing > (overrating) the rectifiers properly.It doesn't help that rectifier ratings are by average current passed through a whole cycle. A nominal '1A' rectifier (1N4003) will be OK with 10A (nonrepetitive) during startup, and 2A output current in a fullwave bridge (because it has only 50% duty cycle) and that amounts to 1A average, but is also 2.8 A peak, and 1.4A RMS. Then there's the problem of overrated components (low ESR capacitors and low series resistance rectifiers and oversize copper windings) causing excessive startup currents. You might need to add NTC or other resistive elements if your capacitors, windings, and rectifiers have been overrated improperly. I shudder to recollect some of the attempts of golden-eared audiophiles to redo power supply components according to vague ideas like 'properly overrating'.
Benchtop Power Supply Options
Started by ●May 7, 2014
Reply by ●May 26, 20142014-05-26
Reply by ●May 26, 20142014-05-26
On Sun, 25 May 2014 18:33:16 +0100, "Ian Field" <gangprobing.alien@ntlworld.com> wrote:> > >"John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message >news:e794o9tj2im9tflvlbisagg29g1crl7468@4ax.com... >> On Sun, 25 May 2014 17:53:03 +0100, "Ian Field" >> <gangprobing.alien@ntlworld.com> wrote: >> >>> >>> >>>"Jurd" <guitardorkspamspameggsandham74@gmail.com> wrote in message >>>news:llrkq5$hr4$1@news.albasani.net... >>>> On 5/24/2014 8:48 PM, John Larkin wrote: >>>>> On Sat, 24 May 2014 20:06:45 -0500, Jurd >>>> >>>>> >>>>> That bridge configuration will in theory charge the caps to 1.41 times >>>>> the RMS voltage of the transformer secondary, because a sine wave has >>>>> a peak voltage 1.41x its RMS. >>>>> >>>>> In real life you'd typically get more DC than that at light loads and >>>>> less at heavy loads. And the "DC" will have ripple, which makes the >>>>> voltage dip at 120 Hz (100 Hz in the hinterlands). >>>>> >>>>> >>>> >>>> Ah thanks. Good to know about the ripple, as that's certainly something >>>> I'd like to avoid. Back to the Googling board! >>> >>>Search under "active ripple cancelling" - pretty much just an emitter >>>follower with some bias and a not quite as huge electrolytic as you'd need >>>on its own. >> >> That doesn't help when you're building a power supply. You may as well >> just connect the rectifier caps to the main linear regulator. That's >> better, actually; a ripple canceler ahead of the regulator makes >> things worse. >> >> The issue is energy storage. 120 times a second, the transformer >> output goes to zero volts. If you want to keep powering the load then, >> the energy has to come from somewhere, and in this case it's the >> filter caps. > >I never said don't use reservoir caps - an unregulated emitter follower with >a heavily decoupled base does its best to follow the insignificant ripple on >its base.Problem is, it will also follow the significant ripple on its collector.
Reply by ●May 26, 20142014-05-26
On 5/26/2014 9:33 PM, John Larkin wrote:> On Sun, 25 May 2014 18:33:16 +0100, "Ian Field" > <gangprobing.alien@ntlworld.com> wrote: > >> >> >> "John Larkin" <jjlarkin@highNOTlandTHIStechnologyPART.com> wrote in message >> news:e794o9tj2im9tflvlbisagg29g1crl7468@4ax.com... >>> On Sun, 25 May 2014 17:53:03 +0100, "Ian Field" >>> <gangprobing.alien@ntlworld.com> wrote: >>> >>>> >>>> >>>> "Jurd" <guitardorkspamspameggsandham74@gmail.com> wrote in message >>>> news:llrkq5$hr4$1@news.albasani.net... >>>>> On 5/24/2014 8:48 PM, John Larkin wrote: >>>>>> On Sat, 24 May 2014 20:06:45 -0500, Jurd >>>>> >>>>>> >>>>>> That bridge configuration will in theory charge the caps to 1.41 times >>>>>> the RMS voltage of the transformer secondary, because a sine wave has >>>>>> a peak voltage 1.41x its RMS. >>>>>> >>>>>> In real life you'd typically get more DC than that at light loads and >>>>>> less at heavy loads. And the "DC" will have ripple, which makes the >>>>>> voltage dip at 120 Hz (100 Hz in the hinterlands). >>>>>> >>>>>> >>>>> >>>>> Ah thanks. Good to know about the ripple, as that's certainly something >>>>> I'd like to avoid. Back to the Googling board! >>>> >>>> Search under "active ripple cancelling" - pretty much just an emitter >>>> follower with some bias and a not quite as huge electrolytic as you'd need >>>> on its own. >>> >>> That doesn't help when you're building a power supply. You may as well >>> just connect the rectifier caps to the main linear regulator. That's >>> better, actually; a ripple canceler ahead of the regulator makes >>> things worse. >>> >>> The issue is energy storage. 120 times a second, the transformer >>> output goes to zero volts. If you want to keep powering the load then, >>> the energy has to come from somewhere, and in this case it's the >>> filter caps. >> >> I never said don't use reservoir caps - an unregulated emitter follower with >> a heavily decoupled base does its best to follow the insignificant ripple on >> its base. > > Problem is, it will also follow the significant ripple on its > collector. > >Nah, that's a capacitance multiplier. As long as you don't let it saturate, it's the bomb for supply rejection. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●May 26, 20142014-05-26
In article <hp97o9hv1tvf0g01h68huuh6scoh01moba@4ax.com>, jfields@austininstruments.com says...> > There is no amount of caps you can add that will remove the ripple 100% > >and expect to be able to get full use of the supply.. > > --- > Sure there is. > > All you have to do is make sure that the ripple valleys are high > enough to give the regulator the headroom it needs to provide a > ripple-free output at the supply's rated output current. > ---That still does not remove the ripple from the caps, what the F is wrong with you? I understand Phil better now. Jamie
Reply by ●May 27, 20142014-05-27
Jurd <guitardorkspamspameggsandham74@gmail.com> wrote:> On 5/24/2014 8:48 PM, John Larkin wrote: >> On Sat, 24 May 2014 20:06:45 -0500, Jurd >> >> In real life you'd typically get more DC than that at light loads and >> less at heavy loads. And the "DC" will have ripple, which makes the >> voltage dip at 120 Hz (100 Hz in the hinterlands). > > Ah thanks. Good to know about the ripple, as that's certainly > something I'd like to avoid.Quick version: I am almost certain John is talking about the DC *on C1 and C2*, not the DC at the output of the supply. Long version... There will *always* be some ripple "before" the LM317 - across C1 and C2 in the diagram you posted.* If you had a transformer with a 24 V AC secondary, and you were drawing 1 A (DC) from the output of the power supply, and you put an oscilloscope probe across C2, you'd see a DC voltage varying between about 30 V and 32 V DC. The variation would be at 120 Hz. You can reduce the amount of this ripple by making C1 and C2 bigger, but you can never get it to go away completely. (* From "Getting Started in Electronics" by Forrest Mims, I think.) There will be *much less* ripple "after" the LM317 - across C3 - as long as you have R1 adjusted to give an output voltage less than about 27 V (in this example). If you put an oscilloscope probe across C3, you'd see a DC voltage varying between (say) 27.000 V and 27.005 V. You can reduce this ripple a little by putting another capacitor across R1 - see the LM317 data sheet. A lot of things you would run from this power supply won't care about this. Audio stuff might care a little (you might hear a 120 Hz hum in the audio), depending on the audio voltage levels you are using. The main reason you have to care about the ripple "before" the LM317 is that it effectively sets the highest output voltage you can get from the supply. There will always be a couple of volts of drop "through" the LM317 - in other words, even if you set the ADJ pin for maximum output, the OUT pin will always be a couple of volts less than the IN pin. If the voltage on C1 and C2 is rippling between 30 V and 32 V, and you set R1 for 27 V output or less, then there is always at least 3 V available to lose in the LM317, and the output will be stable at 27 V. If you tried to turn up R1 to get 29 V on the output, there would only be between 1 and 3 V available to lose in the LM317. 1 V isn't enough for the LM317, so part of the time, you wouldn't get the full 29 V on the output. There is an article by Don Lancaster on how to pick the filter capacitor size on PDF page 104 of http://www.tinaja.com/glib/hackar1.pdf . In that article, Figure 1-B is you, the capacitor is the combination of C1 and C2 in your schematic, and the resistor represents your LM317 and everything "after" it. Matt Roberds
Reply by ●May 27, 20142014-05-27
On Mon, 26 May 2014 16:16:45 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:>On Monday, May 26, 2014 1:54:51 PM UTC-7, John Fields wrote: > >> The real problem lies in not letting the magic smoke out of the >> rectifiers during turn-on, and that's easily side-stepped by sizing >> (overrating) the rectifiers properly. > >It doesn't help that rectifier ratings are by average current passed >through a whole cycle. A nominal '1A' rectifier (1N4003) will be OK >with 10A (nonrepetitive) during startup, and 2A output current in >a fullwave bridge (because it has only 50% duty cycle) and that >amounts to 1A average, but is also 2.8 A peak, and 1.4A RMS. > >Then there's the problem of overrated components (low ESR capacitors >and low series resistance rectifiers and oversize copper windings) causing >excessive startup currents. You might need to add NTC or other resistive >elements if your capacitors, windings, and rectifiers have been overrated >improperly. I shudder to recollect some of the attempts of >golden-eared audiophiles to redo power supply components according >to vague ideas like 'properly overrating'.--- Indeed. But then there's always active soft-start... John Fields
Reply by ●May 27, 20142014-05-27
On Mon, 26 May 2014 22:50:37 -0400, "Maynard A. Philbrook Jr." <jamie_ka1lpa@charter.net> wrote:>In article <hp97o9hv1tvf0g01h68huuh6scoh01moba@4ax.com>, >jfields@austininstruments.com says... >> > There is no amount of caps you can add that will remove the ripple 100% >> >and expect to be able to get full use of the supply.. >> >> --- >> Sure there is. >> >> All you have to do is make sure that the ripple valleys are high >> enough to give the regulator the headroom it needs to provide a >> ripple-free output at the supply's rated output current. >> --- > > That still does not remove the ripple from the caps,--- From the caps??? Of course not, silly boy; I thought you were talking about the supply's regulated output. --->what the F is wrong with you?--- I guess I'm just not hung up enough to not write "fuck". ---> I understand Phil better now.--- Rising out of your torpor, are you? John Fields
Reply by ●May 27, 20142014-05-27
"whit3rd" <whit3rd@gmail.com> wrote in message news:30ec191b-ff90-47fd-954d-b95bc3c0ef04@googlegroups.com...> On Monday, May 26, 2014 1:54:51 PM UTC-7, John Fields wrote: > >> The real problem lies in not letting the magic smoke out of the >> rectifiers during turn-on, and that's easily side-stepped by sizing >> (overrating) the rectifiers properly. > > It doesn't help that rectifier ratings are by average current passed > through a whole cycle. A nominal '1A' rectifier (1N4003) will be OK > with 10A (nonrepetitive) during startup, and 2A output current in > a fullwave bridge (because it has only 50% duty cycle) and that > amounts to 1A average, but is also 2.8 A peak, and 1.4A RMS. > > Then there's the problem of overrated components (low ESR capacitors > and low series resistance rectifiers and oversize copper windings) causing > excessive startup currents. You might need to add NTC or other resistive > elements if your capacitors, windings, and rectifiers have been overrated > improperly. I shudder to recollect some of the attempts of > golden-eared audiophiles to redo power supply components according > to vague ideas like 'properly overrating'.So far I don't recall ever having seen a PTC in front of a transformer PSU. Over the past year or so my web trawling has dredged up all manner of weird and wonderful arrangements - most often a relay to switch a resistor in/out of the primary feed, the variety is in the circuits controlling the relay.
Reply by ●May 27, 20142014-05-27
On Tue, 27 May 2014 16:46:28 +0100, "Ian Field" <gangprobing.alien@ntlworld.com> wrote:> > >"whit3rd" <whit3rd@gmail.com> wrote in message >news:30ec191b-ff90-47fd-954d-b95bc3c0ef04@googlegroups.com... >> On Monday, May 26, 2014 1:54:51 PM UTC-7, John Fields wrote: >> >>> The real problem lies in not letting the magic smoke out of the >>> rectifiers during turn-on, and that's easily side-stepped by sizing >>> (overrating) the rectifiers properly. >> >> It doesn't help that rectifier ratings are by average current passed >> through a whole cycle. A nominal '1A' rectifier (1N4003) will be OK >> with 10A (nonrepetitive) during startup, and 2A output current in >> a fullwave bridge (because it has only 50% duty cycle) and that >> amounts to 1A average, but is also 2.8 A peak, and 1.4A RMS. >> >> Then there's the problem of overrated components (low ESR capacitors >> and low series resistance rectifiers and oversize copper windings) causing >> excessive startup currents. You might need to add NTC or other resistive >> elements if your capacitors, windings, and rectifiers have been overrated >> improperly. I shudder to recollect some of the attempts of >> golden-eared audiophiles to redo power supply components according >> to vague ideas like 'properly overrating'. > >So far I don't recall ever having seen a PTC in front of a transformer PSU.--- With good reason; a PTC's resistance increases when it heats up. John Fields
Reply by ●May 27, 20142014-05-27
"Ian Field" <gangprobing.alien@ntlworld.com> wrote in message news:lr2hv.389759$Sr2.34074@fx08.am4...> > > "whit3rd" <whit3rd@gmail.com> wrote in message > news:30ec191b-ff90-47fd-954d-b95bc3c0ef04@googlegroups.com... >> On Monday, May 26, 2014 1:54:51 PM UTC-7, John Fields wrote: >> >>> The real problem lies in not letting the magic smoke out of the >>> rectifiers during turn-on, and that's easily side-stepped by sizing >>> (overrating) the rectifiers properly. >> >> It doesn't help that rectifier ratings are by average current passed >> through a whole cycle. A nominal '1A' rectifier (1N4003) will be OK >> with 10A (nonrepetitive) during startup, and 2A output current in >> a fullwave bridge (because it has only 50% duty cycle) and that >> amounts to 1A average, but is also 2.8 A peak, and 1.4A RMS. >> >> Then there's the problem of overrated components (low ESR capacitors >> and low series resistance rectifiers and oversize copper windings) >> causing >> excessive startup currents. You might need to add NTC or other resistive >> elements if your capacitors, windings, and rectifiers have been overrated >> improperly. I shudder to recollect some of the attempts of >> golden-eared audiophiles to redo power supply components according >> to vague ideas like 'properly overrating'. > > So far I don't recall ever having seen a PTC in front of a transformer > PSU.OOPS! - typo - that should read NTC.
