30v & +-15vdc psu suggestions

Thread Starter

Plummer

Joined Nov 18, 2013
13
New here and not an electronics professional , so please bear with me if you don't mind. I came here, as it seems the best forum for a safe reliable project.

My project requires 3 voltages, filtered clean and 2amps each. 30,+15 & -15vdc. I have successfully built the 15's using 78 & 79's with a step down transformer. The transformer has 3 2ndaries. A 5vac, c-tap 16vac, and a 5 tap 19vac & 42vac.
I used a LM317 for the 30vdc. Currently I am current limited....haha and I don't know if I can use the one transformer for both regulated voltage boards I built and share a common ground. I'm thinking I cannot use just one secondary for both voltages properly. The transformer is 8-10amps.

Is there a better way?, devices, or suggestions. I have been searching for weeks, and have found two very similar psu's but the to did not have any more current than what the 78 & 79 & 317 provides.
I will post some pics of the modulation board and plans of what I have done so far. Thanks for all the great reading so far.
 

#12

Joined Nov 30, 2010
18,224
It's the 5 tap transformer that needs a drawing here. There are several ways to do this. Show me a drawing of the secondaries and I'll guide you to an answer.

Apparently you want +30V compared to the center of the +/- 15V supply. Entirely doable.
 

#12

Joined Nov 30, 2010
18,224
And while we're on the subject of, "you can't get there from here", this is a drawing of the 30 volt supply added to the split 15's with only one transformer...as in, "not isolated".
 

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Thread Starter

Plummer

Joined Nov 18, 2013
13
Yes there are 2 transformers, the larger one also has 3 secondaries 5vac=2 taps, 14vac=3 taps and 48vac=5 taps. On the AC side there are 3 taps, but nothing was connected to the loop or third tap. I get slightly less output when connected to the loop....couple volts.
Many thanks for the quick replies. Ask away, I'm standing by
 

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#12

Joined Nov 30, 2010
18,224
Dreadful math results. The 19 volt taps will only achieve about 25 volts after they are rectified and the 42 volt taps will achieve about 58 volts, which means a lot of waste heat. Stacking the 5 volt winding on the 19 volt winding will achieve 32.5 volts under no load conditions, but it won't hold up under load. Stacking a 16V on a 19V gives about 48 volts instead of 58 volts, but it could only be half wave rectification.
Your second transformer sucks for this application.

Maybe the answer is a variation of the drawing I already posted, or are you sharp enough to do a fast switching design?
 
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Thread Starter

Plummer

Joined Nov 18, 2013
13
Not sure on the fans witch, but if I have a schem I can build it No problems. Building the drawing you posted is no problem, if that means anything. I'm not stuck on the transistors I listed either. I have two NTE suppliers local, and a shop full of junk electronics to salvage from. Right now I am looking for a quick and dirty for getting my audio set up. Then building a permanent supply. Plans change depending on what I have on hand.

This is the board I'm feeding, the 24vdc is actually the 30v tie. Some are running as low as 22v-28v. Same with the 15's, it works with 13.8. It's just easier to get a clean asymmetrical wave with 15 & 30
 

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Thread Starter

Plummer

Joined Nov 18, 2013
13
Ok, thanks, that's real easy. I assume I will still be limited to 1amp each on the 15's and 1.5a on the 317 30v supply. This might be enough current. I have mje3055t's & SD188 but every plan I've seen calls for the 2955. We will see.
Your help is much appreciated 12.
 

#12

Joined Nov 30, 2010
18,224
Your current limits are determined by the regulators and the filter capacitors. You can buy regulators that provide more than 1 amp. The basic difference is the package they are in so they can get rid of the heat.
 

#12

Joined Nov 30, 2010
18,224
My project requires 3 voltages, filtered clean and 2amps each. 30,+15 & -15vdc. The transformer is 8-10amps.
LM338T or LM338K has a 5 amp limit, but you will never get there without a complete study of heat sinks.

LM350T is a 3 amp chip

L78S75CV by ST Microelectronics = 2A

That's the best on the first of 5 pages. Go to a vendor site and do a parametric search for voltage regulators to find over a hundred that can do 2 to 5 amps.

http://www.mouser.com/Semiconductor...0w8p8Z1z0vlw7Z1yzryzwZ1z0wd5eZ1z0wbywZ1z0wbdj
 

Thread Starter

Plummer

Joined Nov 18, 2013
13
Thanks, I have plenty and many types of heat sinks. I use a sink on each transistor anyways. I also have plenty of fans, AC and dc.
 

GopherT

Joined Nov 23, 2012
8,009
Connecting the 42 volt to the closest 19 volt lug will net 23vac. After rectification you are about 31.3 volts. If the OP can tolerate a bit under 30VDC after regulation it becomes an option (depending on the drop out of the regulator.
 

#12

Joined Nov 30, 2010
18,224
I shied away from that because the measurements are unloaded and you can bet the peak voltage will fall 5% or more under load.
 

Thread Starter

Plummer

Joined Nov 18, 2013
13
Can't thank you gents enough for your input. Really made my project easy. Now I just need enough uninterrupted time to complete.

My next project will be making this work in the mobile off 12v or inverter.
 

#12

Joined Nov 30, 2010
18,224
Arrrgh! The only inverter I bought had a 2 step square wave pretending to be an AC voltage and it will not work for this circuit because the peak voltage of a sine wave is not provided.

It looked like this:
 

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