# Transformers and regulators

Discussion in 'Electronic Basics' started by Abstract Dissonance, Feb 12, 2006.

1. ### Abstract DissonanceGuest

Since I could not get the dual split power supply voltages I needed from one
of my transformers I decided to use two "smaller" ones but with higher
voltage in series to get a larger voltage to work with.

Basicaly I had only [email protected] and switched to using two [email protected] in
series to give me +-40 volts to work with. Basicaly I combined them so that
I have like one large transformer that gives me [email protected] I figure for now
I don't need that much power but I do need atleast +-30 volts to play around
with(as most of my projects will be small circuits). Later on I can worry
about making a better PSU but for now I just want to get something done.

Anyways, after hooking up the transformers in "series" on the secondary side
and "parallel" on the primary and putting them through a bridge rectifier
and through two caps I end up measuring about +-60V at about
115Vrms(Actually about 56V).

Now I ended up with a new issue ;/ The voltage regulators I plan on using
all are rated at max 40V input cts.(they will be variable regulators too,
not that it matters much).

So I have ~20V's(depending on the mains voltage of around 105-125Vrms) to do
something with. I don't think that I can put those extra volts across the
regulators? ;/ Ofcourse with a load the voltage will drop but can the
regulators handle it?(I only have about 5 regulators of that type so I can't
really blow them up to find out). If the max voltage is to prevent some
type of heading issue then I think it might be ok since I have adaquate
cooling(although maybe not).

Although I guess I could reconfigure the secondary transformers in series to
lose that 20V

------ VoA

--- CTA

----- |
|
|--GND
----- |

--- CTB

------ VoB

Thats my configuratio now with with CT's not connected...

but I guess I could do

------ VoA

--- CTA

----- |
|
|--GND
----- |

--- VoB

------ CTB

which should drop some of the voltage(~20V)?

This will make it unsymmetric though and I'll have something like -40 +60 ;/

I tried to figure out how to connect the transformers in a way so that I
have +-40V but I can't seem to get it to work with a common ground. i.e., if
I act like I'm building two seperate power supplies I can easily get +40 on
one and -40 on the other but then when I try to hook up the grounds I end up
getting shorts. If I use a full wave rectifier then that cuts my voltage
down to 20V or so which isn't acceptable(I need ~40V).

So my main question is if its acceptable to put a higher voltage into the
regulator than it says on the datasheet when there is no load? Cause last
time I posted about something like this you guys said that the transformer
gives more output voltage than normal at no load conditions cause it will
drop anyways. So can I expect a all those "extra" volts to drop enough not
to cause problems?

Also, I'm a little confused on how to figure this stuff out. If, say, my
transformer has a turns ratio of N and an input voltage in rms of VPrms then
the output voltage would be VPrms/N but when I go through the bridge
rectification process I get sqrt(2)*VPrms/N for my peak output. When I
filter that I get ~ sqrt(2)*VPrms/N.

So say for VPrms = 115VAC with N = 2.875 which corresponds do a 115VAC to
40VAC step down transformer(?) I have a DC voltage on my filters of about
57VDC. So if I really wanted 40VDC comming out of my filters to use in my
regulators I should have gotten a "smaller" transformer? Or get a regulator
that can handle it but I have several such as the LM317/LM337, 78XX/79XX,
LM2931 and they all have max about 40VDC input. (which means that my 40VAC
transformer is to much for them?)

(so my 25.2VAC transformer will give me about 35VDC which is atleast under
what I need(so I can regulate to betwee 1.25n and 32VDC which is better than
burning up my regulator... but unfortunately I only have 1 of those
transformers so I can't get - polarity without having to cut those values in
1/2... which is not acceptable).

Ultimately I guess what you guys are going to say is get the right
components. Unfortunately at the moment I can't and need to work with what
I've got. I know I have to compromise but I think I have to have +-40VDC and
atleast 100mA(which should keep me busy for a while until I can get the
right parts for the job).

Thanks for any help,
Jon

2. ### Abstract DissonanceGuest

I guess what I'm saying is it seems a little misleading when you see the
transformers secondary voltage as something like V VCT since it represents
VACrms and if you use in DC then your output voltage after your bridge and
filter won't be V but V*sqrt(2)(assuming basic full wave bridge rectifier
and stuff).

So in reality for 40VDC I need ~ 28VAC on the secondary? (So next time I
order some parts I can look for that).

(and what I mean by misleading is not if you know but if you don't know you
might get confused that you would get that on the output and wind up with
something larger than you expect. Not that its really misleading so don't
get on my case about "Well, it not misleading to me." i.e., when I brought
the transformers and I saw 40VCT I didn't think of it has AC but DC for some
stupid reason(probably since this is my first excursion into electronics and
I don't have much experience... probably not a good enough excuse for some
of you guys though).. sure, my fault, I know.).

Jon

3. ### ChrisGuest

Hi, Jon. First off, here are a couple of resources that could probably
help you better understand the whole transformer-rectifier-filter
thing:

http://www.play-hookey.com/ac_theory/ps_elements.html
http://synergy.sager.com/ProductPDFs/PowerTransformer_FilterRatings.pdf

For the first link, also read the following sections to see types of
transformer-rectifier-filter configurations. The second link is a
4-page appnote from Signal transformer on these issues. I would guess
you might want to print that out and keep it in your reference binder
-- it's that good. These will answer your questions on this post.

A power supply is one of those essentials you have to have on the
bench. It might be better to just cobble something together that will
work for you, and spend your time learning about electronics rather
than just getting ready to learn about electronics.This is starting to
look like a chicken-and-egg problem.

If I were in your shoes, I'd look at Surplus Sales of Nebraska

http://www.surplussales.com/

and just get a couple of open frame linear power supplies, bolt them to
the bottom of your bench, and be done with it.

Linear open frames are almost always based on the venerable LM723, and
have excellent line and load regulation, and remarkably low ripple.
They're pretty much bulletproof as long as you don't connect their
output to line voltage or connect two differing output voltages
together.

Here's the plan. Buy

(PS) 10656X Linear open frame triple power supply [email protected], +/-12V to
[email protected] \$35
(PS) IHB28-1 Linear open frame power supply [email protected] \$25

set the current limit on the 5V supply to 3A, the current limit on the
+/-12V supplies to 1A, and the current limit on the 28VDC supply to
1/3A. Now bolt them to the bottom of your workbench. Hook them up
like this (view in fixed font or M\$ Notepad)

|
| .---------.
| | |
| | +senseo-------------.
| | | A1 ___ B1 | +5V
| SW1 | +out o--o-|_R_|-o--o---------------------------o
| L1 _/ | +5V |
| o-o/ o--. | |
| | | | -out o-------------o---------------------------o
| 120VAC| | | | | COM1
| | o | -senseo-------------'
| | ( | |
| | ) | ------|
| | o | |
| | | | +senseo-------------.
| | | | | A2 ___ B2 | +12V
| | | | +out o--o-|_R_|-o--o---------------------------o
| | '--oL1 |
| | | +12V |
| | | -out o-------------o---------------------------o
| | | | | COM2
| | | -senseo-------------o
| o--o--)-----oL2 | |
| L2 | | | ------| |
| | | | | |
| | | | +senseo-------------o
| | | | | |
| | | | +out o-------------'
| | | | -12V |
| | | | | B3 ___ A3 -12V
| | o | -out o--o-|_R_|-o--o---------------------------o
| | ( | | | _____
| | ) | -senseo-------------' | | 1.2-24V
| | o | | .---o-|LM317|--o---o------o
| | | '---------' | +| |_____| | +|
| | | | --- | .-. ---
| | | .---------. | --- | 120| | ---
| | | | | | | | | | |
| | | | +senseo-------------o-' === | '-' ===
| | | | | A4 ___ B4 | COM3 | | COM3
| | | | +out o--o-|_R_|-o--' '-----o
| | '-----oL1 +28V | |
| | | | .-. .---o
| | | -out o-------------. .--->| |2.5K |COM3
| | | | | | | | |
| '--------oL2 -senseo-------------o----. | '-' |
| | | | | | |
| '---------' '-----o-----o------'
| COM3
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

using lamp zip cord to connect to a terminal box on your bench
(2-conductor 18AWG, one for out and one for sense). Wire it up inside
the box, with four 0.1 ohm 1% resistors
(RWF) RW67-.1-1, 5 watts, \$1.25 ea. and a set of 5-way binding posts.
Now you can hook up the A/Bs for each resistor to a 4PDT
break-before-make rotary switch, and have the output go to a pair of
binding posts where you can connect your voltmeter to measure current
(1mV = 10mA). This will be accurate to within 1% plus the accuracy of
your meter, up to the resolution of your meter. The cool thing about
these open frame linears is that they have remote sense, which means
you can put a current-sense resistor in series (up to about 1/2V
burden) without worrying about voltage drop.

The LM317CK (use the metal TO-3 package here) can also be mounted in
this terminal box, with the caps and the 2.5K pot. This will give you
a 1.2-to-24VDC variable supply, which is current-limited by the open
frame linear to 1/3A. Make sure to use a heat sink that can dissipate
10 watts.

Use a motor-rated AC switch to turn things on and off, and don't forget
the fuses. 3AG 4A for the big one, 3AG 1A for the small one.

There. You're done, and thanks to the miracle of surplus, you can
learn about electronics with an acceptable bench power supply without
having paid an arm and a leg. You've got good LM723-based [email protected],
+/-12V or +/-15V (your choice) at 1A, and a variable 1.2-to-24V at 3A,
for less than \$100.

Good luck
Chris

4. ### Abstract DissonanceGuest

Thanks for taking the time out to write this. I'll look into it and see what
I can do. At the momement though I'm more interested in building a simple
linear power suppy to use and to learn in the process of building it.
Ofcourse my main issue is not having the proper components but sometimes you
don't always have what you need and have to fudge it. When I go to buy a
good PS I'll keep what you said in mind.

Thanks,
Jon

5. ### Peter BennettGuest

The Canadian transformer manufacturer Hammond has a Transformer
Selection Guide at http://www.hammondmfg.com/pdf/5c007.pdf that you
may find useful.

It details the relation between expected DC voltage and current
outputs and transformer rating for various rectifier configurations.

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

6. ### Peter BennettGuest

I think you're confusing things somewhat by talking about the primary
voltage and turns ratio here. I do know that the secondary voltage is
determined by the turns ratio, but power transformers are always
specified by the input and output voltage - I've never seen them
specified by turns ratio. AC voltages are always specified as RMS
Volts (unless otherwise stated).
If you want 40 VDC peak, then the AC peak voltage must be about 40.7
volts for a full-wave rectifier, or 41.4 volts for a bridge (allowing
0.7 volts drop in each diode). The required AC RMS voltage will then
be 0.7 x 40.7 = 28.5

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

7. ### Peter BennettGuest

Oops - with a full-wave rectifier, you are only using half the
secondary at a time, so you'll need 57 Vrms, Center-tapped. If you
want to get +/- 40 VDC from a single transformer, you'll also need 57
volts rms, CT, since you are effectively using two full-wave
rectifiers (although they are often drawn as a bridge.)

--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

8. ### Guest

The reason that you have such a great loss in voltage using two
seperate transformers is possibbly due to the AC waves are not in sync.
If you are combining two wave forms you possibly have positve and
negative ac voltage which are canceling each other out. In my opinion
you should try to rectify each transformer seperatly, then connect them
in series.

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