Power supply question

Not clear exactly what you're doing, but the devil is in the details. Depending on the line voltage and the design of the transformer, but the unloaded voltage of the transformer may result significantly higher than 30V.

The word "24V tap" begs the question of how the transformer is configured to get the 30VDC.

You really need to characterize the 12V board. It doesn't take a "whole bunch" of discretes to make a 723 supply. There may be other things going on. You don't know what input min/max it can tolerate. You're gonna have a wide range in input voltage depending on line voltage and load current. And a couple of volts of ripple. Once you figure out how much headroom you have, insert a zener in the power input. You can make one from a small zener and a power transistor. Doesn't have to be all that accurate.

And there are a lot of potential gotchas using an unknown board. For example, I build supplies that sense output current in the negative lead. They run from floating transformer windings. If I were to try to use the AC for some other purpose, stuffing current into my floating winding relative to ground would make a real mess of things.

Making a dual-purpose power supply where both purposes are known is much easier than building an dual-output supply where none of the "purposes" are known in advance.

The best aproach would be to use a transformer with both 12V and 24V windings. Feeding 24VAC into a bridge rectifier will give you about

32 - 36 volts, depending on load. Using a rough approximation, your regulator would be dissipating 15 watts. I'd also boost the value of the main filter cap by at least a value of 10.

An alternative approach may be to use a 7812 series regulator as an adjustable regulator. By using a voltage divider between the output, the reference (ground) pin, and true ground you could have an intermediate 18V regulated output to feed to your regulator. Check the 7812 data sheets for voltage readings and how to do this.

If you have a variable power supply, or a variable transformer it may be possible to evaluate your regulator. Use a 100 ohm resistor between the power supply and the input pin of the regulator. Measure the voltage across the resistor while slowly increasing the voltage into the regulator. If a slight increase in input voltage results in a signiificant increase in voltage across the 100 ohm resistor you are above the maximum input voltage for the regulator.

PlainBill

It can handle up to 40volts on the input..

There is PDF with examples at the bottom for its use..

Jamie

With a full-wave bridge rectifier, if you use an 8300 uF capacitor, the volts of ripple will equal the amps of load current. (Don Lancaster taught me this.) The proposed 1000 uF capacitor will have about 8 volts of ripple at a 1 amp load current. This is still above what the regulator probably needs as a minimum input, but you might want to consider a bigger capacitor.

I have a USB power supply that plugs in to the 12 V in my car that does this. The 12 V DC goes through a power resistor (around 5 W rating) and then into the input of a 7805 regulator in a TO-220 case. I'm not sure if they did this to move some of the heat dissipation from the regulator to the resistor, or just wanted to provide some current limiting for some failure modes of the regulator, or what. I've never put a meter on it but it seems to work OK. (The USB socket provides 5 V DC at 0.5 A, or 2.5 W.)

If you don't have any better specs on what the parts will do, I'd try it and see. Install the transformer, bridge rectifier, and capacitor, but leave out the regulator. Load the 24 VAC output to 4 A. (If you don't have some power resistors sitting around, car tail light bulbs in series are good for this and you probably already own some.) Load the output of the bridge rectifier to 1 A. Plug the transformer in to the lowest line voltage you expect it to work at (use a Variac if you have one). Measure the DC at the output of the rectifier - that is the lowest DC input to the regulator you can expect. If you want, you can then disconnect the load on the 24 VAC output and on the rectifier, plug into the highest line voltage you expect (Variac again), and measure the DC again, to get the highest DC input to the regulator that you can expect.

Then, you need to know the minimum DC input you can have to your regulator board for it to still regulate. If you already have a variable DC power supply, this is easy to figure out. If you don't, you have to guess; if it was a 7812 the standard answer is that you need about 13.5 to 14 V DC minimum for 12 V DC out. Your board may be designed for more than this, though.

Once you know the lowest DC voltage you can expect from the rectifier, and the lowest DC voltage you can supply to the regulator, you can figure out how many volts maximum you have to drop in the resistor.

Just on paper, if I guesstimate a minimum DC input to the regulator of

14 V, and a 1 A load, I have...

Peak secondary voltage 24 V * 1.414 = 33.9 V Minus two diode drops 33.9 V - 2 V = 31.9 V Ripple with 1000 uF @ 1 A 31.9 V - 8 V = 23.9 V Minus regulator minimum 23.9 V - 14 V = 9.9 V Resistor needed 9.9 V / 1 A = 9.9 ohms 9.9 V * 1 A = 9.9 watts

I would probably then look for something like an 8 ohm, 15 watt or better resistor. In perfect conditions, this would provide a little more DC to the input of the regulator, and in the real world, it would also account for getting less than 24 V DC from the transformer, slightly more diode drop, slightly worse ripple, etc.

The series resistor *is* a voltage divider. The regulator is the bottom resistor in the divider.

Matt Roberds

"klem kedidelhopper"

** Likely about 35VDC unloaded

** A string of say 20 x 3A diodes is the cheapest solution.

Wired in series with the "+" terminal of the bridge, all mounted on a tag strip.

Drops the DC level by about 12 to 15 volts, depending on the load.

.... Phil

** The correct value is 6300uF.

Such a cap will drop 1 volt in 6.3mS, the typical discharge time in a full wave rectifier at 60Hz.

For 50Hz full wave supplies, the value is 7500uF for 1V ripple at 1 amp.

.... Phil

l

This is some great advice. Thank you everyone for all your input. I especially like the diode and Zener ideas. They/re cheap and should work well. I've got lots of diodes around here and I'm going to experiment with them. However after all this I remembered that the transformer is center tapped! At the time, I simply wire nutted the tap, tucked it down under the transformer and forgot about it. How this simple fact and the possibilities it presents slipped my mind is anyone's guess. I've been building this thing in the evenings after my usual work and perhaps I've been tired. The transformer measures 25.2V at 120V line unloaded. Loading the transformer in this fashion will probably unbalance it somewhat and drop the 24V a bit as well but I don't see it as a real problem. I should be able to use one side of the secondary and the tap now get at least 15 -17 VDC out of the bridge and filter, and that will provide a healthier input to my regulator. Lenny

he

r

full

p.

e

But won't that still give me 36VDC into my regulator? Lenny

ain

er

ed

to

n d e

is

bout

the

FW

k e

ld,

ing

nd

to

ng

er

wer

I'm grateful for everyone's input, and although this power supply as well as the equipment that it will power is to be used in a family business, never the less I'm trying to build this as "technically correct" as possible. I found some 6.8V 10W stud mount zeners in the junk box last night. I'm thinking that they may be a bit light for this but I might try stringing two or possibly three of them together on a heat sink as Arfa suggested or perhaps try the series diode trick, (from Phil), both as previously mentioned on the output of the filter and see what happens. Even though given the load it may work fine it would be nice as Arfa said to not have to "unbalance the transformer. Lenny

,

ster

a

he

s

my

2V

the

ain this properly. I have a 24VCT transformer. The transformer needs to pow er 24VAC equipment as well as 12VDC equipment. For the DC circuit I first t ried using a bridge directly off the 24V winding. As soon as I connected up the filter cap the DC output went to 35V, which is probably too high to fe ed my 12V regulator.

n realized that I had a center tap that was not being used. So I connected my bridge across the center tap and one side of the secondary. This time th e 13VAC when FW rectified using the bridge went to about 19VDC, which is  a safe input to the regulator. Although this worked, I wasn?t happy ab out unbalancing the transformer this way so I posted my results.

the full secondary output with two diodes, cathodes tied together, (typical FW rectifier), and use the center tap as my negative return. I didn't thin k that would make any difference in the output voltage from using a bridge without the center tap however I tried it anyway. As I suspected it would, the DC output again went to 35V when I connected the filter up.

atching up with me. I guess I just don't understand the explanation of how to get a lower voltage to my DC regulator by what I thought sounded like ju st configuring the transformer properly. So at the risk of sounding like a complete idiot here can someone please explain this further? Thanks, Lenny

ode

     o       |       | to your 24V AC device       |       |       |       |       |       |       |       |           |    18DC       |           +------++       |     ===  ===  |           |     /-\  GND  |           |      |    ===                           GND

,

ster

a

he

s

my

2V

the

ain this properly. I have a 24VCT transformer. The transformer needs to pow er 24VAC equipment as well as 12VDC equipment. For the DC circuit I first t ried using a bridge directly off the 24V winding. As soon as I connected up the filter cap the DC output went to 35V, which is probably too high to fe ed my 12V regulator.

n realized that I had a center tap that was not being used. So I connected my bridge across the center tap and one side of the secondary. This time th e 13VAC when FW rectified using the bridge went to about 19VDC, which is  a safe input to the regulator. Although this worked, I wasn?t happy ab out unbalancing the transformer this way so I posted my results.

the full secondary output with two diodes, cathodes tied together, (typical FW rectifier), and use the center tap as my negative return. I didn't thin k that would make any difference in the output voltage from using a bridge without the center tap however I tried it anyway. As I suspected it would, the DC output again went to 35V when I connected the filter up.

atching up with me. I guess I just don't understand the explanation of how to get a lower voltage to my DC regulator by what I thought sounded like ju st configuring the transformer properly. So at the risk of sounding like a complete idiot here can someone please explain this further? Thanks, Lenny

ode

     o       |       | to your 24V AC device       |       |       |       |       |       |       |       |           |    18DC       |           +------++       |     ===  ===  |           |     /-\  GND  |           |      |    ===                           GND

I know it's difficult when we're only using text but I just can't understand, (read that is) your schematic. From your description though it sounds like it might be the FW arrangement I've already tried and got 35V out of. Is there any way to make that appear any more readable? Lenny

,

ster

a

he

s

my

2V

the

ain this properly. I have a 24VCT transformer. The transformer needs to pow er 24VAC equipment as well as 12VDC equipment. For the DC circuit I first t ried using a bridge directly off the 24V winding. As soon as I connected up the filter cap the DC output went to 35V, which is probably too high to fe ed my 12V regulator.

n realized that I had a center tap that was not being used. So I connected my bridge across the center tap and one side of the secondary. This time th e 13VAC when FW rectified using the bridge went to about 19VDC, which is  a safe input to the regulator. Although this worked, I wasn?t happy ab out unbalancing the transformer this way so I posted my results.

the full secondary output with two diodes, cathodes tied together, (typical FW rectifier), and use the center tap as my negative return. I didn't thin k that would make any difference in the output voltage from using a bridge without the center tap however I tried it anyway. As I suspected it would, the DC output again went to 35V when I connected the filter up.

atching up with me. I guess I just don't understand the explanation of how to get a lower voltage to my DC regulator by what I thought sounded like ju st configuring the transformer properly. So at the risk of sounding like a complete idiot here can someone please explain this further? Thanks, Lenny

ode

     o       |       | to your 24V AC device       |       |       |       |       |       |       |       |           |    18DC       |           +------++       |     ===  ===  |           |     /-\  GND  |           |      |    ===                           GND

I'm not sure if my last reply worked so I'll try it again. I know that it's very difficult when all we can use here is text but I'm afraid that I just can't understand, (read) your schematic. From you description however it appears to be the FW arrangement that I've already tried and got 35 V out of after I connected up the filter. Is there any way to make your diagram any more readable in this medium? Thanks, Lenny

this properly. I have a 24VCT transformer. The transformer needs to power 24VAC equipment as well as 12VDC equipment. For the DC circuit I first tried using a bridge directly off the 24V winding. As soon as I connected up the filter cap the DC output went to 35V, which is probably too high to feed my 12V regulator.

realized that I had a center tap that was not being used. So I connected my bridge across the center tap and one side of the secondary. This time the 13VAC when FW rectified using the bridge went to about 19VDC, which is a safe input to the regulator. Although this worked, I wasn?t happy about unbalancing the transformer this way so I posted my results.

full secondary output with two diodes, cathodes tied together, (typical FW rectifier), and use the center tap as my negative return. I didn't think that would make any difference in the output voltage from using a bridge without the center tap however I tried it anyway. As I suspected it would, the DC output again went to 35V when I connected the filter up.

catching up with me. I guess I just don't understand the explanation of how to get a lower voltage to my DC regulator by what I thought sounded like just configuring the transformer properly. So at the risk of sounding like a complete idiot here can someone please explain this further? Thanks, Lenny

Copy it and paste it into Wordpad, with the Courier New font. Google Groups uses a proportional font which screws up drawings.

properly. I have a 24VCT transformer. The transformer needs to power 24VAC equipment as well as 12VDC equipment. For the DC circuit I first tried using a bridge directly off the 24V winding. As soon as I connected up the filter cap the DC output went to 35V, which is probably too high to feed my 12V regulator.

realized that I had a center tap that was not being used. So I connected my bridge across the center tap and one side of the secondary. This time the 13VAC when FW rectified using the bridge went to about 19VDC, which is a safe input to the regulator. Although this worked, I wasn?t happy about unbalancing the transformer this way so I posted my results.

full secondary output with two diodes, cathodes tied together, (typical FW rectifier), and use the center tap as my negative return. I didn't think that would make any difference in the output voltage from using a bridge without the center tap however I tried it anyway. As I suspected it would, the DC output again went to 35V when I connected the filter up.

catching up with me. I guess I just don't understand the explanation of how to get a lower voltage to my DC regulator by what I thought sounded like just configuring the transformer properly. So at the risk of sounding like a complete idiot here can someone please explain this further? Thanks, Lenny

THe FW you tried was a 4 diode bridge, this is a 2 diode full wave config. THe difference being is, the CT (Center tap) is your common, the negative terminal and where the Cathode of the 2 diodes come together are the (+) terminal which will be around 18VDC with Cap attached

If you truly do have a 24 volt transformer with CT, this means you'll get 12 Volts from each outer leg with respect to the CT. Because they are out of phase with each other (180), this is a split phase configuration, like seen in residential pole pigs in the USA.

If you have done this and gotten 36 volts, then you actually have a

48V transformer with CT>

Jamie

..

or,

caster

in a

1 I d

the

is

r my

5.2V l I f

the

lain this properly. I have a 24VCT transformer. The transformer needs to po wer 24VAC equipment as well as 12VDC equipment. For the DC circuit I first tried using a bridge directly off the 24V winding. As soon as I connected u p the filter cap the DC output went to 35V, which is probably too high to f eed my 12V regulator.

en realized that I had a center tap that was not being used. So I connected my bridge across the center tap and one side of the secondary. This time t he 13VAC when FW rectified using the bridge went to about 19VDC, which is  a safe input to the regulator. Although this worked, I wasn?t happy ab out unbalancing the transformer this way so I posted my results.

the full secondary output with two diodes, cathodes tied together, (typica l FW rectifier), and use the center tap as my negative return. I didn't thi nk that would make any difference in the output voltage from using a bridge without the center tap however I tried it anyway. As I suspected it would, the DC output again went to 35V when I connected the filter up.

catching up with me. I guess I just don't understand the explanation of how to get a lower voltage to my DC regulator by what I thought sounded like j ust configuring the transformer properly. So at the risk of sounding like a complete idiot here can someone please explain this further? Thanks, Lenny

ode

u

rd

       o         |         | to your 24V AC device         |         |         |         |         |         |       |       |           |    18DC       |           +------++       |     ===  ===  |           |     /-\  GND  |           |      |

•    ===

                           GND

nfig.

What you're referring to is a 12VCT transformer. Connecting either a bridge and not using a center tap or just two diodes across the full secondary with cathodes tied together using CT as common connected to an output filter will yield 18VDC. A 24 V transformer under the same circumstances will produce 36VDC. I've tried it and it does. Lenny.

this properly. I have a 24VCT transformer. The transformer needs to power 24VAC equipment as well as 12VDC equipment. For the DC circuit I first tried using a bridge directly off the 24V winding. As soon as I connected up the filter cap the DC output went to 35V, which is probably too high to feed my 12V regulator.

realized that I had a center tap that was not being used. So I connected my bridge across the center tap and one side of the secondary. This time the 13VAC when FW rectified using the bridge went to about 19VDC, which is a safe input to the regulator. Although this worked, I wasn?t happy about unbalancing the transformer this way so I posted my results.

full secondary output with two diodes, cathodes tied together, (typical FW rectifier), and use the center tap as my negative return. I didn't think that would make any difference in the output voltage from using a bridge without the center tap however I tried it anyway. As I suspected it would, the DC output again went to 35V when I connected the filter up.

catching up with me. I guess I just don't understand the explanation of how to get a lower voltage to my DC regulator by what I thought sounded like just configuring the transformer properly. So at the risk of sounding like a complete idiot here can someone please explain this further? Thanks, Lenny

absolutely not, you are greatly mistaken or you don't know what you have for a transformer.

THe transformer is spec'ed out via its outer legs, the highest voltage. The CT is the voltage divider. when you use the CT as your common, the supply will yield only half of the rated voltage of that transformer and is when you use only 2 diodes from the outer legs to form a full wave into a single node where this becomes the (+) terminal.

With a real 24V Transformer with CT and using the CT as your common (-) terminal, you will get 12*1.414 = 16.968-0.650 = 16.318VDC at the peak of a marginal ripple.

You have your logic mixed up

Jamie

e

m...

itor,

ancaster

e in a

at 1

. I

uld

he

ed the

ow

nd is

ter my

25.2V

ill

t I

of

the

xplain this properly. I have a 24VCT transformer. The transformer needs to power 24VAC equipment as well as 12VDC equipment. For the DC circuit I firs t tried using a bridge directly off the 24V winding. As soon as I connected up the filter cap the DC output went to 35V, which is probably too high to feed my 12V regulator.

then realized that I had a center tap that was not being used. So I connect ed my bridge across the center tap and one side of the secondary. This time the 13VAC when FW rectified using the bridge went to about 19VDC, which is  a safe input to the regulator. Although this worked, I wasn?t happy a bout unbalancing the transformer this way so I posted my results.

ff the full secondary output with two diodes, cathodes tied together, (typi cal FW rectifier), and use the center tap as my negative return. I didn't t hink that would make any difference in the output voltage from using a brid ge without the center tap however I tried it anyway. As I suspected it woul d, the DC output again went to 35V when I connected the filter up.

s catching up with me. I guess I just don't understand the explanation of h ow to get a lower voltage to my DC regulator by what I thought sounded like just configuring the transformer properly. So at the risk of sounding like a complete idiot here can someone please explain this further? Thanks, Len ny

g.

iode

you

d

or

       o         |         | to your 24V AC device         |         |         |         |         |         |       | +       |           |    18DC       |           +------++         |     ===  ===  |           |     /-\  GND  |           |      |    ===                           GND

config.

ll

o n

What I decided to do is use the full secondary and employ the two 6.8V zeners in series with the cap. That arrangement gives me 21V out of my power supply to feed the 12V regulator. As far as I can tell the other methods would be using the center tap and one side of the transformer, a method which I didn't want to use. I tried this full secondary arrangement with a 1.2A load and the 21V across the zeners holds pretty steady. I have a nice heat sink for the two diodes, (they will be insulated) that will be going into the box as well. Thanks everyone who advised and helped me on this project. Lenny

"klem kedidelhopper"

What I decided to do is use the full secondary and employ the two 6.8V zeners in series with the cap. That arrangement gives me 21V out of my power supply to feed the 12V regulator.

** He sure is one stubborn SOB - ain't he?

.... Phil

"Arfa Daily" >

** All the fool had to do was LOOK at his wiring and spot the mistake.

The CT wire was swapped with one of the ends.

FFS, what a annoying moron.