Power Supply for Home lab?

This supply by JSPAYNE seems to be pretty capable without getting too complex:

Variable Voltage, Variable Current Power Supply

The main challenges would be to get an appropriate transformer and big enough heat sinks.

I would add a couple of low cost modules to display current and voltage.

Just look for "3 wire voltmeter" on your favourite shopping source.

Doug

On more careful examination there are some issues with this design - it will probably work but isn't sensibly capable of 30V and 5A.

The TIP142 and TIP2955 can cope with maximum currents of 3A with 30V across them (SOA curves from data sheets) so at 2V output or short circuit with >3A current limit destruction of Q1 or Q2 is pretty much guaranteed. Even at normal working at, for example, 15V out and 5A, Q1 needs to dissipate at least 75W, the Multicomp data sheet on E14 gives the junction to case thermal resistance at 1.39 C/W, add in 0.5 for case to heatsink and you get a temperature rise of 141.75C, so the maximum allowed heatsink temperature is 8.25C - or to put it another way Q1 is going to cook.

24V ac isn't enough - only 33.9V peak which will be more like 32V after the bridge rectifier, with 10,000uF cap the ripple at 5A load will be about 5/0.01*0.007 = 3.5 v, so the raw supply will dip to less than 28.5, allow at least each for Q1 and Q2 and you'll be lucky to see 24.5V max output at 5A.

There's more - it suggests that C1 should be rated at Vac * 1.5 - this is far too low: if the AC comes from a transformer you need to allow :

Vac

* 1.414 for peak to rms ratio

* 1.1 for mains quality

* 1.1 for transformer regulation (may be worse or better 10% is common but it gets better for really big transformers -check the spec)

* 1.05 because you should have some additional margin for specmanship in components -real mains etc.

= Vac * 1.8

All this suggests that the design hasn't been properly analyzed or tested - so anyone building it take care and don't expect more than 25V and 2A max out of it !

MK

On more careful examination there are some issues with this design - it will probably work but isn't sensibly capable of 30V and 5A.

The TIP142 and TIP2955 can cope with maximum currents of 3A with 30V across them (SOA curves from data sheets) so at 2V output or short circuit with >3A current limit destruction of Q1 or Q2 is pretty much guaranteed. Even at normal working at, for example, 15V out and 5A, Q1 needs to dissipate at least 75W, the Multicomp data sheet on E14 gives the junction to case thermal resistance at 1.39 C/W, add in 0.5 for case to heatsink and you get a temperature rise of 141.75C, so the maximum allowed heatsink temperature is 8.25C - or to put it another way Q1 is going to cook.

24V ac isn't enough - only 33.9V peak which will be more like 32V after the bridge rectifier, with 10,000uF cap the ripple at 5A load will be about 5/0.01*0.007 = 3.5 v, so the raw supply will dip to less than 28.5, allow at least each for Q1 and Q2 and you'll be lucky to see 24.5V max output at 5A.

There's more - it suggests that C1 should be rated at Vac * 1.5 - this is far too low: if the AC comes from a transformer you need to allow :

Vac

* 1.414 for peak to rms ratio

* 1.1 for mains quality

* 1.1 for transformer regulation (may be worse or better 10% is common but it gets better for really big transformers -check the spec)

* 1.05 because you should have some additional margin for specmanship in components -real mains etc.

= Vac * 1.8

All this suggests that the design hasn't been properly analyzed or tested - so anyone building it take care and don't expect more than 25V and 2A max out of it !

MK

Forgive my ignorance, but where is the schematic (in discussion)? I did see it (the schematic) once, but cannot find it now. AAR with all this discussion of Power Dissipation b/c of the Voltage Drops across "Series-Pass" Linear Designs, what considerations would lead one to consider Switching Power Supply Designs? I've been waiting for the MOOC site www.coursera.org to run their Course on Switching Pwr Supplies again. Then there was a Reply in one of these element14 Blogs that gave me an URL to a Commercial Switching Pwr Supply Manufacturer whose Site promised a PDF "book". I never got sent one. Anybody received that .pdf Book on switching pwr supplies? BTW Thx to MK for your remarks. Your feelings toward the Patent Systems (Both US and abroad) are Not uncommon among small inventors who see Art in what they "invent." To them it is the inventing that is the Buzz. I quite agree Privately. But Business is Business.

There is a link to the schematic in question in Douglas Wong's post of 9 th Jan:

I've copied it here:

"

This supply by JSPAYNE seems to be pretty capable without getting too complex:

Variable Voltage, Variable Current Power Supply

The main challenges would be to get an appropriate transformer and big enough heat sinks."

I don't have time to go into all the pros and cons re. switching v linear power supply regulation, suffice to say that for lab use at low to medium powers (up to about 150W) a linear supply usually offers lower noise, better transient response and more load protection than a switcher and with simpler designs. For home build without much test gear it is much easier to get  a linear regulator working correctly.

MK

To: Mike (MK). Thx again for being gracious in both your sharing of technical and Real-World Knowledge. I'm pretty sure everybody feels the same. Thx Again. Not being a Circuit Designer, I respect those that are. Reading a schematic along-side a Circuit Description is Like Google Mapping a Location and looking at the "Street View(s)." It transforms you to a different Place and Time. Ack your comments about "Linear" vs. "Switching". I suspected as much, but confirmation by you is greatly appreciated. Context is Everything. For this Particular Discussion Thread, Linear is the Way to Go.

I can contribute a Grilling Recipe though. "Flank Steak -LGA" 1. Flank Steak (or w/ Skirt Steak). Marinated (One- few Hours) in 2. "L"-Lemon juice from one Lemon; 3 "G"-Garlic, minced, as much as you like, 4. "A"-Anchovies- maybe 2-3 small/medium Pieces on both sides. Marinate in Plastic Bag. Grill.  BTW I live about two miles South of LGA (the airport).

ELY (Edward L. Yee) aka: elyee000, aka: uscdadnyc