Trying To Cheat The Laws Of Physics . . . Again. Multi Voltage Power Supply

[Rory Starkweather]

I'm trying to build a project that would be much easier if I had several different voltages available locally. (+18, +12, +5, +3.3) I don't want to use any wall warts.

So I found a 120 VAC to 24 VAC, 7 Amp transformer. Rectification and filtering are all designed.

The question that remains, is what is the most efficient way to produce the 4 voltages.

My first was a voltage divider, but I understand that would be very inefficient and produce a lot of heat.

Next thought was too feed the 78xxs directly off the <24 volt feed. Again, doesn't seen very efficient and would produce heat where I least need.

So, option 3. Suppose I hook the 18 volt regulator directly to 24 volts, then the +12 to the output of the +18, then a +9 to the +12, then the +5 to the +9 and the +3.3 to either +5 or +9?

I could even stick in a few variable regulators to cut the size of the steps.

I think that 7 amps is definitely over-kill for the needs, so I'm not worried about that.

What kind of problems would I run into? I know I'm not the first person to ever think of this, but I've never seen it done. There must be a reason. What am I missing?

 

[(*steve*)]

If you use linear regulators you will always suffer the same amount of losses due to heat.

I would recommend you look into a variable output switchmode regulator.

 

[hevans1944]

Your "totem pole" scheme will work, but as pointed out by @(*steve*) it won't be very efficient. How much current do you need from each of your outputs?

 

[KrisBlueNZ]

I would recommend you look into a variable output switchmode regulator.

Or multiple switching regulators, assuming you need all of those voltages simultaneously.

Switching regulators aren't the best for some applications - audio, in particular. Tell us more and we can advise more

 

[Rory Starkweather]

This would be for a QRP transmitter (Ramsey 30 Meter kit) with some add ons, mainly for experimentation. I think that, in the end, there will be at least one sub circuit that would be happiest with each one of the four voltages.

I plan on doing a lot of active filtering, adding an RF pre-amp and a modified audio stage. 7 Amperes is probably more than I will ever need, but it was the cheapest transformer I could find with enough voltage as I think I will need.

After I posted last night I thought about just using voltage dividers in the same configuration, but I haven't done the math yet, so don't know if I would be gaining anything.

For some reason I have never done any research on Switching regulators. The one I googled turned out to be an OnSemi LM2575. Excellent data sheet. Really explains the concept and the circuit design well. I have to admit I didn't absorb it all. What makes them more efficient?

 

[KrisBlueNZ]

What makes them more efficient?

The magic of inductance

https://www.google.com/?q=efficiency+of+switching+vs.+linear+power+supplies

 

[davenn]

This would be for a QRP transmitter (Ramsey 30 Meter kit) with some add ons, mainly for experimentation. I think that, in the end, there will be at least one sub circuit that would be happiest with each one of the four voltages.

In this case I would then use standard linear regulators
Switching reg's may prove too noisy and difficult to filter for this sort of application

cheers
Dave

 

[CDRIVE]

Sarge, you didn't say what your current requirements are but next time check out this supplier.
http://www.mpja.com/Transformers-Power/products/45/

BTW, is that a 1st Cav patch on your sleeve?

Chris

 

[Rory Starkweather]

Sarge, you didn't say what your current requirements are but next time check out this supplier.
http://www.mpja.com/Transformers-Power/products/45/

BTW, is that a 1st Cav patch on your sleeve?

Chris

That's one of the problems. I haven't gone far enough with the design to nail down the current drains from all the modules, and the current requirement will change as I add the modules, so I thought it best to start high.


Yes. I was in B Troop, 1/9 from 1974-76.

 

[hevans1944]

You wouldn't happen to have a radio amateur license would you, what with messin' around on the 30 M amateur radio band? If so, please tell us what class license and your call sign. BTW: I love HF QRP operation and your Facebook page.

73 de AC8NS
Hop

 

[CDRIVE]

That's one of the problems. I haven't gone far enough with the design to nail down the current drains from all the modules, and the current requirement will change as I add the modules, so I thought it best to start high.


Yes. I was in B Troop, 1/9 from 1974-76.

Maybe things have changed but back in the day QRP was only 1W. Not much current demand there. Back then multitap xfmrs were easy to find.

The 1st Cav is a great old outfit. I served some brief TDY with them in1966 while participating in the S.E. Asia War Games..

Chris

 

[hevans1944]

Maybe things have changed but back in the day QRP was only 1W. Not much current demand there. Back then multitap xfmrs were easy to find.

Chris

I dunno about early definitions of QRP, but today the ARRL defines it as 10 W or less input to the final or less than 5 W measured output (to the antenna I presume). My KX3 puts out a maximum of 12 W on a warm day with freshly charged Li-ion cells, and fer shure that much with an external power supply. I think most hams run the KX3 at 5 W or less so the output transistors don't get too hot and automagically cut the power back to that level, unbeknownst to the operator. If I ever get a real antenna up, I have a little KXPA-100 with built-in antenna tuner, still unused, to spice things up a bit. I just read nice article in QST January 2105 issue for a solid-state gallon (well, 1.25 kW) that can be driven directly from the KX3. Does HF from 160 m and up and also 2 m. I haven't added the 2 m module to my KX3 and probably won't. My cheapo Baofeng UV-5R+Plus reaches all the repeaters I need with an extended whip to replace the rubber ducky that came with the HT.

 

[Rory Starkweather]

You wouldn't happen to have a radio amateur license would you, what with messin' around on the 30 M amateur radio band? If so, please tell us what class license and your call sign. BTW: I love HF QRP operation and your Facebook page.

73 de AC8NS
Hop

I'm working on a Technician's license now. For right now I just sit and evesdrop.

 

[CDRIVE]

I dunno about early definitions of QRP, but today the ARRL defines it as 10 W or less input to the final or less than 5 W measured output (to the antenna I presume).

Ha, it very well could be that my memory has failed me again. It could be that those ARRL specs were the same in the 60's. My wife tells me that the only part of our 41 year marriage that I remember correctly was the day we met!

Chris
WA2ERQ

 

[hevans1944]

I'm working on a Technician's license now. For right now I just sit and evesdrop.

FB! There is an on-line course I signed up for in preparation for my Technician, General, and Extra Class exams. I highly recommend HamTestOnlne. There is also plenty of free study stuff online too, but you probably have looked into that. I would also recommend that you study for all three exams since you can test at the same sitting if you pass each one. You need at least a General Class License to operate on 30 meters. Eavesdropping okay of course. There are some interesting data modes propagating on 30 meters that you can play with. RTTY and data only are allowed on that band; no voice modulation (AM, SSB) allowed; maximum 200 W PEP.

I was a SWL (shortwave listener) as a kid, but didn't get my Novice license until I was 22 while serving a tour in the Air Force. After my enlistment was up in 1967 I left the Air Force and let my Novice license expire without upgrading to Technician or General. Back then it was upgrade or out. Novice license was good for one year and not renewable. There was also a Morse Code test for Technician (5 WPM) and General (13 WPM) that I could have easily passed because as a Novice I worked CW on 80 m. I regret being out of ham radio from 1967 to 2013. I missed a lot of the changes in the hobby. Now not only is there no code test, the questions and answers for the exams are published! A person with a good memory (not me anymore) can pass the exams by memorizing all the questions and answers without knowing a thing about electronics or amateur radio.

 

[CDRIVE]

I was a SWL (shortwave listener) as a kid, but didn't get my Novice license until I was 22 while serving a tour in the Air Force. After my enlistment was up in 1967 I left the Air Force and let my Novice license expire without upgrading to Technician or General.

Every now and then I read about guys who had had hobbies while serving their hitch. It's a real head scratcher for me because free time in my world was little to none. The only hobbies that I and any of my buddies had was drinking and working hard to qualify as tour guides of red light districts. If the Army had merit awards for it I think I would have qualified expert!

 

[Rory Starkweather]

I've wanted to get a license since I was a teenager, but just didn't do it.

I've been taking practice exams, but, at the moment, my scores are trending down, so I thought I'd cool off a little.

Most of the mistakes I make are in frequency allocation and operational rules.

Oh well. I have time.

TY for the suggestion on doing General at the same time.

 

[davenn]

go for broke .... amateur radio is an awesome hobby
I have been licenced for over 30 years .... WOW almost 35 years

I have never regretted it, learning RF electronics has been rewarding
I have never been a fan of HF bands, in fact ask any of my ham friends and they will tell you ...

" ohhh Dave, yeah, anything below 1 GHz is DC for him" haha

I used to be a very active 6m band operator, but most of the time these days
its 70cm and 23cm bands from home and those and up into the microwave bands from hilltops etc

firing 10GHz up and down the coast, pretty much a full sea path. 75km south to one station 150 km north to the other station from my location

 

[CDRIVE]

go for broke .... amateur radio is an awesome hobby
I have been licenced for over 30 years .... WOW almost 35 years

I have never regretted it, learning RF electronics has been rewarding
I have never been a fan of HF bands, in fact ask any of my ham friends and they will tell you ...

" ohhh Dave, yeah, anything below 1 GHz is DC for him" haha

I used to be a very active 6m band operator, but most of the time these days
its 70cm and 23cm bands from home and those and up into the microwave bands from hilltops etc

firing 10GHz up and down the coast, pretty much a full sea path. 75km south to one station 150 km north to the other station from my location

View attachment 17676

Dave, you may have posted photos of yourself before that I've missed but it's good to put a face on you. BTW, is it an optical illusion or is that dish pointing at the ground? Err... ground wave?

Chris

 

[davenn]

BTW, is it an optical illusion or is that dish pointing at the ground? Err... ground wave?

Yes it does point to the ground like that, its an offset fed dish as used for a lot if satellite TV etc> And because it is offset fed, it changes the angle compared to a normal parabolic dish ( which would point straight to the other station) ... If I pointed an offset dish straight to the other station, the signal would be actually aiming up into the sky and going nowhere .... OK for moonbounce I suppose hahaha

Dave