Bench Power Supply

chopnhack · 2016-04-01 1:39 am

Grin and bear with me, novice with bench psu questions.

I am interested in building the psu from this issue of Nuts and Volts.

I am having trouble sourcing transformers. Does anyone have a good source of 120 to 9vac trafos? Ideally one with four secondary bobbins rated at 1A each. If not, then two with two secondaries? I am space limited in the case I intend to use.

Thanks in advance!

p.s. - If not a source specifically, can anyone help me with better search terms to use? I have been looking on google with 9vac, 10va transformer, etc.

powerbob · 2016-04-01 2:49 am

T3 and T4 are not 1A, the panel meters require almost a un-measurable amount of power. The article lists them as .065A and even this is overkill.

List of transformers.

Power Transformers | Transformers | DigiKey

Least expensive transformers that will work for T3, T4. 9V not available must use 10V.

Power Transformers | Transformers | DigiKey

chopnhack · 2016-04-01 3:04 am

powerbob said:
T3 and T4 are not 1A, the panel meters require almost a un-measurable amount of power. The article lists them as .065A and even this is overkill.

Least expensive transformers that will work for T3, T4. 9V not available must use 10V.

Power Transformers | Transformers | DigiKey

You are awesome Bob!
Yes, agreed, the two panel meters I have in mind state that they only use less than 20mA. Thank you for the second link to digikey - your search was a lot better than mine!! I only came up with this at 3 times the price. But at least I was moving in the right direction.

I read on some of Triad's (manufacturer of some trafos) datasheets that the secondaries must be wired in series or parallel, this particular unit states that the secondaries can be operated independently (with a 300v difference). Was that a search parameter you selected?

powerbob · 2016-04-01 4:05 am

All of the secondary's can be used independently. The manufacture is just giving the specs for the most common use. The 300v isolation is not relevant to your usage.

If you want to use one secondary only or both secondary's that is up to you. In your case you want two secondary's that are separate from each other, so no center tap. A center tap output would have 3 leads and you need 4 leads.

I selected "parallel 10v, series 20v", This can only bring up transformers with two separate secondary's.

chopnhack · 2016-04-01 4:12 am

powerbob said:
All of the secondary's can be used independently.

Thanks again Bob. I see how you narrowed the results.

Take a look at this Triad made trafo.
It has the specs that I am looking at for making two independent low voltage ouputs, but the manufacturer says at the bottom that the secondaries need to be in series or parallel, not independent?

Could I use this for two separate supplies? I intend to make one 5v and the other 3V at no more than 1A. The 3V v. regulator may have some heat issues...

powerbob · 2016-04-01 4:16 am

The simplest and cheapest transformer will have only a single primary of 120vac. If you select 115v under the heading of "primary voltage" you will have only four choices left.

powerbob · 2016-04-01 4:18 am

Again these transformers we are selecting are only to power the meter, they are .065A not 1A. Give me a minuet to look at the Triad.

powerbob · 2016-04-01 4:20 am

"Primary and secondary windings are designed to be connected in series or parallel. Winding are not intended to be used independently."

WOW, i have not seen that before, good thing you read the document. You are right, do not use that transformer.

chopnhack · 2016-04-01 4:22 am

Oh, no worries.

Yes, I selected a trafo for the meters! Thank you for that

Sorry for the confusion! I have moved on to the next trafo needed, which will be to power one 5v output and another for a 3v output. These are solely to be dedicated to powering mcu projects and the associated gizmos that might accompany them. That is why I want no more than 1A.

LOL, I have been tutored on reading the datasheet ;-)

powerbob · 2016-04-01 4:23 am

The cheapest one looks like a good match for you.

"The F20-055-C2 is a single primary and dual secondary, split bobbin design which
operates with an input of 115V. The secondaries are 10V @ 0.055A each. They
can be used independently (up to 300V difference between them) or in series for
double the voltage or in parallel for double the current."

powerbob · 2016-04-01 4:32 am

OK, moving right along... 5v 1A may be a little light. If we stick with the article he wisely recommends a ratio of 1.5 for current rating. So if you want 1A out you need a 1.5A transformer. Did you mean 1A out?

chopnhack · 2016-04-01 4:36 am

Well spotted. Forgot about headroom. Yes, 1A out would be grand! So, a 1.5A unit is needed. Perhaps something like this Triad unit?

powerbob · 2016-04-01 4:45 am

That looks like a good choice. Let me search a little though.

chopnhack · 2016-04-01 4:50 am

Of course and thank you!

Incidentally, I was able to pickup two NOS Hammond trafos from e-bay today for $14 shipped!! I will be using them for my two regulated outputs 0-~18vdc. Hopefully that will be all I ever need in a power supply

powerbob · 2016-04-01 4:56 am

This search i looked for single secondary and wider voltage range, because you want to do 3.3vdc also. With a LDO low dropout regulator you can use a much lower voltage transformer. These transformers all came up as chassis mount.

Power Transformers | Transformers | DigiKey

powerbob · 2016-04-01 4:58 am

Those Hammond transformers look good.

chopnhack · 2016-04-01 4:59 am

powerbob said:
Those Hammond transformers look good.

Thanks! I think I got lucky on those!

chopnhack · 2016-04-01 5:02 am

powerbob said:
This search i looked for single secondary and wider voltage range, because you want to do 3.3vdc also. With a LDO low dropout regulator you can use a much lower voltage transformer. These transformers all came up as chassis mount.

Power Transformers | Transformers | DigiKey

Maybe I was looking at this problem in the wrong way - isolated like the meters required. Are you hinting at, that I can run both v. regs off of the same secondary output so long as there is enough headroom? If that is the case then I should look for a single secondary with about ~3v difference from each required output, add it up, add up the required I plus 50% and search

Rubbish...

I see what you are saying, the required headroom for the LDO is much much less... as in mV! Brilliant, apologies. I now know why you are recommending 6.3v trafos. LOL

powerbob · 2016-04-01 5:14 am

Lots of possibility's here. The article stressed all isolated outputs so you can put them in series if you want. For that you need separate transformers for 5v and 3.3v. But if you want to share a transformer for the 3.3 and 5v out, that would work. But the 3.3v regulator would have to work a little harder dropping more voltage than it has to.

chopnhack · 2016-04-01 5:25 am

powerbob said:
Lots of possibility's here. The article stressed all isolated outputs so you can put them in series if you want. For that you need separate transformers for 5v and 3.3v. But if you want to share a transformer for the 3.3 and 5v out, that would work. But the 3.3v regulator would have to work a little harder dropping more voltage than it has to.

Thank you Bob, let me ponder on this some and come back a little more focused with more constrained questions. I can see where I can drift into different designs quite easily from here!

Agreed, though, the 3.3 LDO would have to drop ~2W at 1A (LD1086 in TO-220 package) which would be 10 deg C above room temp, not terrible..

As for isolated, the 3v and 5v outputs would never be series'd in my mind, just don't see the need to when there is two good outputs that can cover that.

I appreciate your time and timely conversation, but its off to bed for me! Thanks again and I will certainly come back to bounce some ideas!

Good night!

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