# How do you calculate the REQUIRED voltage from a transformer secondary

Started by April 2, 2017
```How do you calculate the REQUIRED voltage from a transformer secondary?

Ok, here's the plan.
A friend of mine installed a bunch of LED fixtures to run off a solar
panel and a 12V marine battery. It's drawing about 15amps at 12VDC.

It works fine when the sun is shining, but drains the battery when there
is no sun. He said he gets tires of having to hook a battery charger to
that battery on cloudy days. (I would think that is also hard on the
battery too).

I suggested that he build a 12VDC power supply to connect to a regular
120V outlet, and put a switch, so he can switch off the battery and
switch on this power supply on cloudy days.

Now we need to build this power supply. It does NOT need to be regulated
for this use. Just a basic transformer, bridge rectifier, and a
capacitor. Capable of outputting 12 to 14 VDC at 15 amps or greater.

He ordered a large bridge rectifier from ebay, (with heat sink), rated
at 50V 20A. for around \$2 from China. (more than enough power).

But now comes the question. I know that when using a bridge rectifier,
the output voltage is "close" to the output of the transformer. But
there is still some voltage drop, from the rectifier.

The ideal voltage desired is 13.8 VDC (same as a battery). But anything
from 12V to 14 V would be fine. How does someone determine the secondary
voltage of the transformer that is needed to accomplish this?
(I'm guessing 16V would work, but thats only a guess).

One other thing, he plans to put a capacitor across the output. I know
this cap needs to be rated at 25 volts or higher, but what capacity
should be used? From looking at some schematics which use bridge
rectifiers, it appears they use fairly large caps, such as 1000uf or

For this use, the power supply does not need to be precision, or
filtered to the extreme, but a fairly constant voltage with filtering to
avoid flicker is desired.

Suggestions?

Thanks

```
```On Sunday, April 2, 2017 at 4:08:25 PM UTC-4, olds...@tubes.com wrote:
> How do you calculate the REQUIRED voltage from a transformer secondary?
>
> Ok, here's the plan.
> A friend of mine installed a bunch of LED fixtures to run off a solar
> panel and a 12V marine battery. It's drawing about 15amps at 12VDC.
>
> It works fine when the sun is shining, but drains the battery when there
> is no sun. He said he gets tires of having to hook a battery charger to
> that battery on cloudy days. (I would think that is also hard on the
> battery too).
>
> I suggested that he build a 12VDC power supply to connect to a regular
> 120V outlet, and put a switch, so he can switch off the battery and
> switch on this power supply on cloudy days.
>
> Now we need to build this power supply. It does NOT need to be regulated
> for this use. Just a basic transformer, bridge rectifier, and a
> capacitor. Capable of outputting 12 to 14 VDC at 15 amps or greater.
>
> He ordered a large bridge rectifier from ebay, (with heat sink), rated
> at 50V 20A. for around \$2 from China. (more than enough power).
>
> But now comes the question. I know that when using a bridge rectifier,
> the output voltage is "close" to the output of the transformer. But
> there is still some voltage drop, from the rectifier.
>
> The ideal voltage desired is 13.8 VDC (same as a battery). But anything
> from 12V to 14 V would be fine. How does someone determine the secondary
> voltage of the transformer that is needed to accomplish this?
> (I'm guessing 16V would work, but thats only a guess).
>
> One other thing, he plans to put a capacitor across the output. I know
> this cap needs to be rated at 25 volts or higher, but what capacity
> should be used? From looking at some schematics which use bridge
> rectifiers, it appears they use fairly large caps, such as 1000uf or
>
> For this use, the power supply does not need to be precision, or
> filtered to the extreme, but a fairly constant voltage with filtering to
> avoid flicker is desired.
>
> Suggestions?
>
> Thanks

This one runs \$18 at 15A, looks protected. Doesn't get any cheaper. http://tinyurl.com/oyremzv
```
```On 4/2/2017 3:07 PM, oldschool@tubes.com wrote:
> How do you calculate the REQUIRED voltage from a transformer secondary?
>
> Ok, here's the plan.
> A friend of mine installed a bunch of LED fixtures to run off a solar
> panel and a 12V marine battery. It's drawing about 15amps at 12VDC.
>
> It works fine when the sun is shining, but drains the battery when there
> is no sun. He said he gets tires of having to hook a battery charger to
> that battery on cloudy days. (I would think that is also hard on the
> battery too).
>
> I suggested that he build a 12VDC power supply to connect to a regular
> 120V outlet, and put a switch, so he can switch off the battery and
> switch on this power supply on cloudy days.
>
> Now we need to build this power supply. It does NOT need to be regulated
> for this use. Just a basic transformer, bridge rectifier, and a
> capacitor. Capable of outputting 12 to 14 VDC at 15 amps or greater.
>
> He ordered a large bridge rectifier from ebay, (with heat sink), rated
> at 50V 20A. for around \$2 from China. (more than enough power).
>
> But now comes the question. I know that when using a bridge rectifier,
> the output voltage is "close" to the output of the transformer. But
> there is still some voltage drop, from the rectifier.
>
> The ideal voltage desired is 13.8 VDC (same as a battery). But anything
> from 12V to 14 V would be fine. How does someone determine the secondary
> voltage of the transformer that is needed to accomplish this?
> (I'm guessing 16V would work, but thats only a guess).
>
> One other thing, he plans to put a capacitor across the output. I know
> this cap needs to be rated at 25 volts or higher, but what capacity
> should be used? From looking at some schematics which use bridge
> rectifiers, it appears they use fairly large caps, such as 1000uf or
>
> For this use, the power supply does not need to be precision, or
> filtered to the extreme, but a fairly constant voltage with filtering to
> avoid flicker is desired.

There will be voltage drop in the transformer coil, so this also needs
to be factored in.  The other issue is that the transformer is likely
rated at voltage RMS which is not directly a useful number in this case.
Multiply by 1.4 to get the peak voltage.

Fuse         Diode
o----o==o---+  +--+--|>|-----+----+----o
|  |  |          |    |
C||C  +--|<|--+  |    |
AC         C||C          |  |   === Cap
Line        C||C  +--|>|--|--+    |
|  |  |       |       |
o----o==o---+  +--+--|<|--+-------+----o

So you want 13.8 volts output.  We'll make that the peak voltage on the
output cap.  The diodes give 1.4 volts drop total (assuming silicon,
Schottky diodes drop less).  So the peak voltage from the transformer
under load needs to be 15.2 volts.  That would be about 10.8 volts RMS.
Then you need to factor in the IR drop in the transformer.  When all is
said and done I would shoot for a 12 volt transformer.  Open circuit it
will give 15.4 volts.  With a coil resistance of 0.1 ohm it will drop
1.5 volts in the transformer at 15 amps giving 13.9 volts peak on the cap.

The cap is sized by the voltage droop acceptable during the
non-conducting portions of the power cycle.  For an estimate assume all
the power comes from the line in one instant at the peak voltage, 8.3 ms
apart.  If 1 volt of droop is tolerable, that will give...

C = I / (dV / dt) = I * dt / dV = 15A * 0.0083s / 1V = 0.124 Farad...

Did I miss a decimal place somewhere?

Why aren't you buying this power supply?  Better yet, can't you just
hang a charger on the battery?

--

Rick C
```
```On Sunday, 2 April 2017 23:11:05 UTC+1, rickman  wrote:
> On 4/2/2017 3:07 PM, oldschool@tubes.com wrote:
> > How do you calculate the REQUIRED voltage from a transformer secondary?
> >
> > Ok, here's the plan.
> > A friend of mine installed a bunch of LED fixtures to run off a solar
> > panel and a 12V marine battery. It's drawing about 15amps at 12VDC.
> >
> > It works fine when the sun is shining, but drains the battery when there
> > is no sun. He said he gets tires of having to hook a battery charger to
> > that battery on cloudy days. (I would think that is also hard on the
> > battery too).
> >
> > I suggested that he build a 12VDC power supply to connect to a regular
> > 120V outlet, and put a switch, so he can switch off the battery and
> > switch on this power supply on cloudy days.
> >
> > Now we need to build this power supply. It does NOT need to be regulated
> > for this use. Just a basic transformer, bridge rectifier, and a
> > capacitor. Capable of outputting 12 to 14 VDC at 15 amps or greater.
> >
> > He ordered a large bridge rectifier from ebay, (with heat sink), rated
> > at 50V 20A. for around \$2 from China. (more than enough power).
> >
> > But now comes the question. I know that when using a bridge rectifier,
> > the output voltage is "close" to the output of the transformer. But
> > there is still some voltage drop, from the rectifier.
> >
> > The ideal voltage desired is 13.8 VDC (same as a battery). But anything
> > from 12V to 14 V would be fine. How does someone determine the secondary
> > voltage of the transformer that is needed to accomplish this?
> > (I'm guessing 16V would work, but thats only a guess).
> >
> > One other thing, he plans to put a capacitor across the output. I know
> > this cap needs to be rated at 25 volts or higher, but what capacity
> > should be used? From looking at some schematics which use bridge
> > rectifiers, it appears they use fairly large caps, such as 1000uf or
> >
> > For this use, the power supply does not need to be precision, or
> > filtered to the extreme, but a fairly constant voltage with filtering to
> > avoid flicker is desired.
>
> There will be voltage drop in the transformer coil, so this also needs
> to be factored in.  The other issue is that the transformer is likely
> rated at voltage RMS which is not directly a useful number in this case.
>   Multiply by 1.4 to get the peak voltage.
>
>       Fuse         Diode
> o----o==o---+  +--+--|>|-----+----+----o
>              |  |  |          |    |
>              C||C  +--|<|--+  |    |
>   AC         C||C          |  |   === Cap
> Line        C||C  +--|>|--|--+    |
>              |  |  |       |       |
> o----o==o---+  +--+--|<|--+-------+----o
>
> So you want 13.8 volts output.  We'll make that the peak voltage on the
> output cap.

The circuit you describe is unstabilised. V_out varies due to load & mains voltage. Not really good for connecting to the battery long term, whih would be the sensible option here.

> The diodes give 1.4 volts drop total (assuming silicon,
> Schottky diodes drop less).

That is such a common mistake. Diodes do not have 0.65v or 0.7v drop when you hit them with their rated current. 1v is closer, though >2v drop diodes are also to be found.

>  So the peak voltage from the transformer
> under load needs to be 15.2 volts.  That would be about 10.8 volts RMS.
> Then you need to factor in the IR drop in the transformer.  When all is
> said and done I would shoot for a 12 volt transformer.  Open circuit it
> will give 15.4 volts.  With a coil resistance of 0.1 ohm it will drop
> 1.5 volts in the transformer at 15 amps giving 13.9 volts peak on the cap.

Transformer V_outs are for full load. At no load V_out rises due to copper loss no longer happening (much). So 12v ac with 10% regulation would deliver 12x1.414x1.10 = 18.66v off load. Diodes at low current will steal 1.3v giving you 17.35v.

> The cap is sized by the voltage droop acceptable during the
> non-conducting portions of the power cycle.  For an estimate assume all
> the power comes from the line in one instant at the peak voltage, 8.3 ms
> apart.  If 1 volt of droop is tolerable, that will give...
>
> C = I / (dV / dt) = I * dt / dV = 15A * 0.0083s / 1V = 0.124 Farad...
>
> Did I miss a decimal place somewhere?

15A 1v drop 50/60Hz is a whole lot of uFs. Regulated supplies can permit greater droop and thus smaller reservoirs.

> Why aren't you buying this power supply?  Better yet, can't you just
> hang a charger on the battery?

quite!

NT
```
```On 3/04/2017 3:07 AM, oldschool@tubes.com wrote:
> How do you calculate the REQUIRED voltage from a transformer secondary?
>
> Ok, here's the plan.
> A friend of mine installed a bunch of LED fixtures to run off a solar
> panel and a 12V marine battery. It's drawing about 15amps at 12VDC.
>
> It works fine when the sun is shining, but drains the battery when there
> is no sun. He said he gets tires of having to hook a battery charger to
> that battery on cloudy days. (I would think that is also hard on the
> battery too).
>
> I suggested that he build a 12VDC power supply to connect to a regular
> 120V outlet, and put a switch, so he can switch off the battery and
> switch on this power supply on cloudy days.
>
> Now we need to build this power supply. It does NOT need to be regulated
> for this use. Just a basic transformer, bridge rectifier, and a
> capacitor. Capable of outputting 12 to 14 VDC at 15 amps or greater.
>
> He ordered a large bridge rectifier from ebay, (with heat sink), rated
> at 50V 20A. for around \$2 from China. (more than enough power).
>
> But now comes the question. I know that when using a bridge rectifier,
> the output voltage is "close" to the output of the transformer. But
> there is still some voltage drop, from the rectifier.
>
> The ideal voltage desired is 13.8 VDC (same as a battery). But anything
> from 12V to 14 V would be fine. How does someone determine the secondary
> voltage of the transformer that is needed to accomplish this?
> (I'm guessing 16V would work, but thats only a guess).
>
> One other thing, he plans to put a capacitor across the output. I know
> this cap needs to be rated at 25 volts or higher, but what capacity
> should be used? From looking at some schematics which use bridge
> rectifiers, it appears they use fairly large caps, such as 1000uf or
>
> For this use, the power supply does not need to be precision, or
> filtered to the extreme, but a fairly constant voltage with filtering to
> avoid flicker is desired.
>
> Suggestions?
>
> Thanks
>
>
>
>
>
>
MEH !! why mess around with a tranny, as said before you can get SWP
supplies for peanuts and with no great chunk of hardware to put
somewhere. Also for what difference it makes the output is adjustable on
the switch mode jobs.
```
```olds...@tubes.com wrote:

** The boring old troll has another hair brained ide he wants us to waste time on.

> How do you calculate the REQUIRED voltage from a transformer secondary?
>
> Ok, here's the plan.
> A friend of mine installed a bunch of LED fixtures to run off a solar
> panel and a 12V marine battery. It's drawing about 15amps at 12VDC.
>
> It works fine when the sun is shining, but drains the battery when there
> is no sun. He said he gets tires of having to hook a battery charger to
> that battery on cloudy days. (I would think that is also hard on the
> battery too).
>
> I suggested that he build a 12VDC power supply to connect to a regular
> 120V outlet, and put a switch, so he can switch off the battery and
> switch on this power supply on cloudy days.

** Power supplies are not battery chargers.

>
> Now we need to build this power supply. It does NOT need to be regulated
> for this use.

** It very much does, actually.

> Just a basic transformer, bridge rectifier, and a
> capacitor. Capable of outputting 12 to 14 VDC at 15 amps or greater.
>

** The capacitor is no use at all, batteries act like giant caps already.

Such a crude unit will over charge the battery and ruin it

> But now comes the question. I know that when using a bridge rectifier,
> the output voltage is "close" to the output of the transformer. But
> there is still some voltage drop, from the rectifier.
>

** FFS - the AC supply often voltage varies by over 10% during any 24 hour period.

The max voltage must be limited by some means to a safe value.

> One other thing, he plans to put a capacitor across the output.

** Must be an ever bigger idiot than you.

>
> For this use, the power supply does not need to be precision, or
> filtered to the extreme, but a fairly constant voltage with filtering to
> avoid flicker is desired.
>
> Suggestions?
>

** Don't tempt me.....

.....   Phil

```
```On Sun, 02 Apr 2017 18:10:58 -0400, rickman wrote:

> On 4/2/2017 3:07 PM, oldschool@tubes.com wrote:
>> How do you calculate the REQUIRED voltage from a transformer secondary?
>>
>> Ok, here's the plan.
>> A friend of mine installed a bunch of LED fixtures to run off a solar
>> panel and a 12V marine battery. It's drawing about 15amps at 12VDC.
>>
>> It works fine when the sun is shining, but drains the battery when
>> there is no sun. He said he gets tires of having to hook a battery
>> charger to that battery on cloudy days. (I would think that is also
>> hard on the battery too).
>>
>> I suggested that he build a 12VDC power supply to connect to a regular
>> 120V outlet, and put a switch, so he can switch off the battery and
>> switch on this power supply on cloudy days.

<snip>

> Why aren't you buying this power supply?  Better yet, can't you just
> hang a charger on the battery?

Yup.  These days, buying a surplus 12V supply is probably cheaper than
buying a suitable transformer, much less all the other bits.  Just get
one that's rated for 15A or more.

I'd try www.mpja.com first, although eBay may be a good source.
www.allelectronics.com and www.herbach.com come to mind, too -- there's a
bunch of decent surplus Internet sources out there.  I'd trust MPJA more
than some anonymous eBay seller -- you're less likely to get nameless
Chinese junk (although I've gotten nameless Chinese junk from MPJA --
buying cheap is a crap shoot).

--
Tim Wescott
Control systems, embedded software and circuit design
I'm looking for work!  See my website if you're interested
http://www.wescottdesign.com
```
```On 2017-04-02, oldschool@tubes.com <oldschool@tubes.com> wrote:

> A friend of mine installed a bunch of LED fixtures to run off a solar
> panel and a 12V marine battery. It's drawing about 15amps at 12VDC.

> Now we need to build this power supply. It does NOT need to be regulated
> for this use.

well since you know everything, why are you asking us?

--
This email has not been checked by half-arsed antivirus software
```
```On 3 Apr 2017 08:28:28 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

>On 2017-04-02, oldschool@tubes.com <oldschool@tubes.com> wrote:
>
>> A friend of mine installed a bunch of LED fixtures to run off a solar
>> panel and a 12V marine battery. It's drawing about 15amps at 12VDC.
>
>> Now we need to build this power supply. It does NOT need to be regulated
>> for this use.
>
>well since you know everything, why are you asking us?

PLONK

```
```On Mon, 03 Apr 2017 00:04:43 -0500, Tim Wescott <tim@seemywebsite.com>
wrote:

>On Sun, 02 Apr 2017 18:10:58 -0400, rickman wrote:
>
>> On 4/2/2017 3:07 PM, oldschool@tubes.com wrote:
>>> How do you calculate the REQUIRED voltage from a transformer secondary?
>>>
>>> Ok, here's the plan.
>>> A friend of mine installed a bunch of LED fixtures to run off a solar
>>> panel and a 12V marine battery. It's drawing about 15amps at 12VDC.
>>>
>>> It works fine when the sun is shining, but drains the battery when
>>> there is no sun. He said he gets tires of having to hook a battery
>>> charger to that battery on cloudy days. (I would think that is also
>>> hard on the battery too).
>>>
>>> I suggested that he build a 12VDC power supply to connect to a regular
>>> 120V outlet, and put a switch, so he can switch off the battery and
>>> switch on this power supply on cloudy days.
>
><snip>
>
>> Why aren't you buying this power supply?  Better yet, can't you just
>> hang a charger on the battery?
>
>Yup.  These days, buying a surplus 12V supply is probably cheaper than
>buying a suitable transformer, much less all the other bits.  Just get
>one that's rated for 15A or more.
>
>I'd try www.mpja.com first, although eBay may be a good source.
>www.allelectronics.com and www.herbach.com come to mind, too -- there's a
>bunch of decent surplus Internet sources out there.  I'd trust MPJA more
>than some anonymous eBay seller -- you're less likely to get nameless
>Chinese junk (although I've gotten nameless Chinese junk from MPJA --
>buying cheap is a crap shoot).

That MPJA site sure has a lot of stuff. I never knew that existed...