Hi Unless there is something wrong with the linear supply, it will produce heat proportional to the output load. They all have very low “parasitic” current in the control circuits.
A unit with a damaged transformer or bad caps *can* indeed heat up with no load. In that case, it needs to be repaired. Bob > On Feb 12, 2020, at 10:34 PM, Taka Kamiya via time-nuts > <[email protected]> wrote: > > Yes, I did consider use of a second linear supply for 12V. But decided > against it. This particular project was to use ONE 24V supply only. This is > because of my previous project. Also the issue with another 12V linear is > that these open frame supplies are awfully inefficient. They generate > enormous amount of heat for little current. Since this is a rubidium box, > heat management was important. > > On previous project, I used 24V linear supply, and 12V switcher supply. AND > 5V DC/DC off 24V, 5V 3 terminal off 12v, and 3.3V internal to one device. > Each was routed all over the place. The end result was a good working > system. But ground situation became very complicated. I was very concerned > because EFC can easily be affected. > > This is still a work in progress. There is already a plan for 3rd iteration > with GPDSO, and 4th iteration with OCXO. I'd expect similar problems. I'm > still debating a good design as OCXO requires +/-5V supply that has to be > very clean and stable. > > At one point, I plan to remake the first project. Besides the fact it works, > I didn't practice good engineering process. Lack of foresight and slapping > on STUFF as needed led to this awful configuration. There got to be a much > better way, and I got to get there for my own satisfaction. > --------------------------------------- > (Mr.) Taka Kamiya > KB4EMF / ex JF2DKG > > > On Wednesday, February 12, 2020, 9:49:42 PM EST, Charles Steinmetz > <[email protected]> wrote: > > Taka Kamiya wrote: > >> I started with 24V linear power supply (open frame type). This is the only >> power supply in the box. From there, it branches to Rb, a small dist amp, >> and a simple interface circuit. Dist amp is 12V rated and interface is 5V. >> I modified the dist amp by including a 3 terminal regulator. * * * I >> crated a 5V source right at Rb's interface board to be used as EFC power. >> This is the only purpose of this branch. I also created a separate 5V for >> the simple interface circuit. >> >> * * * EFC circuit was fine but digital interface board killed the >> regulator. Put a heat sink on it and issue is resolved. A bit surprising >> but it shouldn't be.... The circuit only uses 25mA or so but voltage drop >> is huge. * * * I will have to consider use of small smp board for large >> voltage drops. > > Good idea to use two, 5 V supplies in this situation. > > The EFC circuit presumably draws no more than 0.5 mA, so regulator power > dissipation is probably at most 19 V x 0.5 mA, or <10 mW. As you found, > no problem. > > At ~ 25 mA, the interface board dissipates ~ 19 V x 25 mA, or ~ 500 mW. > As you found, nothing a moderate heatsink can't handle. But you don't > have to dissipate all 500 mW in the regulator. For example, you could > feed the 5 V regulator from the 24 V supply through a 400 ohm, 1 W > resistor (putting a capacitor of, say, 470 uF to ground at the input of > the 5 V reg). This would share the ~500 mW dissipation equally between > the 5 V regulator and the resistor. (A 10v, 1 W zener diode would also > work.) > > Another option would be to use two, mains-powered linear power supplies > -- one 24v supply, and one 12v supply. Then sub-regulate the two 5v > supplies from the 12v supply rather than from 24v. > >> My only remaining concern is if regulator fails short, then what happens?? >> It will kill the particular device, which I'm fine with, but what else? >> Perhaps simple fuse is in order for every branch. > > A fuse by itself will not necessarily open in this situation. If you > are worried about failing short, use a simple zener diode crowbar with > the protective fuse. Use a shunt zener with a breakdown voltage about > 2v greater than the regulated power supply voltage across the load, fed > through the protective fuse. The fuse should be rated at about 200% of > the load current. The zener must be sufficiently robust to withstand > the fault current until the fuse opens. > > With a zener rated 2v greater than the operating voltage, there is a > good chance the downstream load will survive without damage. > > Best regards, > > Charles > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
