PSU Transformer ???s

[phishman13]

Now, I have perused the Meta's for PSUs and searched for every term I could think of regarding this question, and I just couldn't find what I am looking for.  Maybe I just don't have enough patience to go to the very end on every thread, but I think that putting this info in an easy to reach place, would be helpful not to just me.

So, I am trying to finish off a pair of S800's that I have and all i need is the PSU and a case.  I have an extra PSU that came with my Mnats 1176 A set.  I want to use that PSU to power the 2 channels of S800 eqs.  SO, I can easily pick out all of the rest of the parts, but one thing I cannot find is information on how to choose a proper power transformer.  I have seen discussion of choosing one for current requirements, but not about proper voltage requirements.  Say I want +/- 15v.  Do I just buy a 15-0-15 transformer?  Or since the mnats little psu is technically an adjustable one, would I buy one that gives a higher voltage to allow it to be used for higher supplies?

Heres the deal however.  I would prefer to NOT be given a part # or just a quick answer.  Because when I have to go find a transformer for my next project that may or may not have different type of power supply or voltage requirements, I don't want to have to bother everyone with this same question, but slightly different. 

I just want to know how to choose em.  Take the SSL Comp for example.  It uses the 7815 and 7915 regulators, and in some of the boms that I have seen they say a 15-0-15v transformer.  Is that fine to make sure it provides a very regulated +/- 15V?  Or do you need to feed it a bit over 15v?  And then is the method for choosing one different for the LM317/337 regulators?  Do you need to overshoot the target voltage to allow whatever voltage drops may or may not occur?  If so, by how much?  My guess is that that will determine the maximum supply voltages that you can provide.  I just want a general rule of thumb to follow for various projects of varying voltage requirements. 

Any help is appreciated!

Thanks!

 

[Ptownkid]

This link helped me a lot when starting out.

http://www.kpsec.freeuk.com/powersup.htm

 

[Ptownkid]

You are confusing AC with DC I think...basically your 15V-0-15V xfmr can give you about 21Vdc when using a full wave rectifier. V(secondary in AC)*1.414V = V(output in DC)

Beyond that your concerns are then having enough amps, and feeding your regulators a volt or two MORE than you need out of them. So if you need 15V out of your regulator, 21V is golden. As for the amps, IE the VA rating, it comes down the the current draw of what you are powering. The VA rating is simply V(secondary)*Amps, so if your 15V-0-15V xfmr was rated 15VA, you have 1 amp, enough to power 4 things drawing >250mA. if it's rated at 30VA, you have 2 Amps (15VAC*2A=30VA)

Hope that helps a bit.

 

[phishman13]

So from what you're saying, the rating of output voltage of 15-0-15 is lower than what I would measure with an AC voltmeter?  Or is it 15vac and when converted to DC it ends up being 15v?  Is this saying that they kinda build in easy to the specs?  So if I want the mnats psu to be adjustable up to +/- 24v, I should get a  24-0-24 transformer?  And if I wanted +/-15v on my SSL comp onboard psu, then the 15-0-15 transformer is right on?  That would make sense, I just didn't understand exactly what that spec means.

Let me know if this is on the right path.  And thanks!

 

[Ptownkid]

So from what you're saying, the rating of output voltage of 15-0-15 is lower than what I would measure with an AC voltmeter?

No, it would measure 15Vac.

Or is it 15vac and when converted to DC it ends up being 15v?

No, when converted to DC it would be approx 21Vdc

So if I want the mnats psu to be adjustable up to +/- 24v, I should get a  24-0-24 transformer?

You could...but 18-0-18 or 22-0-22 would be cheaper and have to dissipate less heat.

if I wanted +/-15v on my SSL comp onboard psu, then the 15-0-15 transformer is right on? I just didn't understand exactly what that spec means.

In that case you probably would use the 15-0-15, but you are still confusing AC with DC. 15-0-15 means you have three wires coming from your transformer. One that is the center tap (used as ground or 0V) and two separate 15Vac (with reference to the center tap.) What you need for your SSL, is +/- 15Vdc not ac. So the power supply is going to take that 15Vac and turn it into approx 21Vdc, feed it to the regulators, and give you 15Vdc.

 

[phishman13]

No, I knew that the output of the transformer needs to be converted still.  Thats where the rectification and filtering come in.  I DID however confuse ac and dc by not knowing how it converted from one to the other.  I mean, I guess I knew, but my electronics knowledge was a minor in college, college was a mild blur... etc. etc.  You get the idea.  I'm just now digging up those memories and RElearning them.  So basically the output of a 15-0-15 is 15vac rms each leg w/ reference to the center tap/ground.  the peak value is about 21vac.  Therefore, once rectified and filtered, ends up somewhere around 21vDC?  Then into the regulator and outputs 15vDC.  forgot about that whole rms/peak business.

Am I getting closer?

What you said definitely cleared up a lot, so thanks again.

 

[Ptownkid]

closer, but still off... ;D

Re-read what I wrote and hit up that link about psu's. The little light will come on soon.

 

[CJ]

i use 1.3 times ac voltage to get APPROX dc volts.

caps un loaded charge to root 2, = 1.414, so derate for dcr of winding, eddy currents, blah blah, and you usually get 1.3.


but how big?

unless running weird choke input, the xfmr will be a certain size for a certain heat.
triad in the middle, hammond in the basement, chicago transformer at the top.

use VA.

add  up all your currents times volts:

6.3 times 1 amp = 6.3 va.
2 amps = 12.6 va.

then B+.

100 ma at 300 volts = .1 times 300 = 30 va.

so 12.6 va plus 30 va = 42 va, double it for safety, = 80 va transformer to run 24/7 and cool.


api/1176, same deal, just use 30 volts times whatever current, you can even get a battlefied estimate by multyplying the fuse by 120, eg 120 times 3/4 amp fuse =90 va.