[sdiy] A transformer question.

Bob Weigel sounddoctorin at imt.net
Tue Nov 4 04:16:40 CET 2008

Yeah things will run a little hotter with the higher voltages of course 
unless you're using switching power supplies where there's very little 
heat difference.  But with an analog regulator, you are imposing a 
variable load in the line with the regulator.  So it has to dissipate 
the current times the potential drop across it of course.   The voltage 
ratings of the regulators will impose no problem..what are they 36V or 
so I seem to recall limit?  Anyway but depending how much current you 
are drawing you could run into a problem with heat dissipation going 
from 18 to 24V .  But remember if you are talking about an 18VAC winding 
on the secondary, then through full wave rectification it will give more 
headroom than that filtered.  You've read up on front end power supply 
design I'll assume, but just wanted to mention that since you didn't 
say.   If you meant untapped 18VAC with fw rectifier (eg on this page 
http://my.integritynet.com.au/purdic/power1.html ) then you'll really be 
having plenty of headroom and I wouldn't go to 24V by any means.  It'll 
just make heat I think.  Let's see 1.4 x 18 = 25.2VDC on the filter with 
no load.  In japan you'd still have 21V or something like that no load 
which should be ample headroom unless you are sucking lots of current.  
But if you just build it with a transformer that has plenty of headroom 
current wise, and overrated bridge diodes and plenty of filter capacitor 
(do the calculation for change in voltage.  Here you use delta Q (Change 
in charge) = C * delta V (Change in voltage)  .  And then apply the 
amount of current draw you are expecting out of the system to the TIME 
between charging pulses. (With FW rectifier 1/120th of a second or like 
that).  So say you wanted 1A of current.  That's the same as 1 
coulumb/sec for 1/120th of a section.= .01C or a little less.  So now 
ask yourself "How much change in voltage is acceptable? (ie. max 
ripple).  Say a little over 1V.   So just insert that into the change 
equation above and you get C= delta Q/ Delta V = .01F or 10,000uF.  So 
that's the way to quick calculate to make sure you have better than what 
you need since really it's not 1/120th second of total current void :-)  
You could really get away with 6800uF to get that spec probably.  Anyway 
hope that helps.   -Bob

Tom Bugs wrote:

> Thanks Bob - that makes a lot of sense and, I think, gives me the 
> direction to figure things for the most part.
> I'd already been planning to use 18v secondary outputs to feed v-regs 
> for 15volts (LM317/337)- I'll do some calculations to check whether 
> it'd be worth heading up to 24v secondaries to cover the potential 
> 100v situation. Making sure that the V-regs will receive happy 
> operating voltages..
> - that'd be right, right?!
> Cheers,
> Tom
> Bob Weigel wrote:
>> Tom,
>> Normally the variation in 20V around 230V divides to a variation of a 
>> coule volts in the stepped down side... and in most circuitry that's 
>> a tolerable variation.   And same percentage variation in the 110 to 
>> 120...usually ok.  The 100V in Japan can cause some 'brown out' 
>> simulation, but some things will also work just fine with it.  For 
>> example a Siel DK80 I'm pretty sure wouldn't like the 100V without a 
>> step up transformer because the headroom is too limited on the 
>> regulators as I recall.  It'll buzz as the ripple drops below the 
>> regulator's headroom requirement.
>> You can get step up transformers for 100V to 120V.   For running 
>> Russian gear here I had a good sized 2X transformer so I just wired 
>> it up in a plastic box and made a simulation of the funny round 
>> pinned outlets on the top of the plastic box :-).  Anytime you have 
>> to step DOWN there are real cheap options out there.  Step up 
>> requires a more expensive solution.  -Bob
>> Tom Bugs wrote:
>>> Hi,
>>> A quick question about twin primary transformers for which I've not 
>>> been able to find answers...
>>> If there are twin 115v primaries.. (ie series for 230v and parallel 
>>> for 115v)
>>> - should this be able to cover Euro vs UK ranges of c.220v to 240v?
>>> - and would it cover US 120vAC and 110vAC in places like Cuba?
>>> - how can one deal with the 100v in Japan (other than replacing the 
>>> trafo with a different spec one?)
>>> I'd guess there's a bit more to it too - dependent on current draw, 
>>> trafo type & spec, etc..
>>> Any help greatly appreciated!
>>> Ta,
>>> Tom
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