12vdc out of transformer?

[338_MtRushmore]

I have a transformer that is putting out 14.6vac. I assume I need a bridge rectifier and that I would get something less than 14.6vdc out of it. What would be required to drop the voltage if it isn't close enoughto 12vdc? It is a 2 amp transformer.

I am struggling to find relevant info on the web to learn, but its probably due to bad search terms.

 

[sp5it]

Measure it's voltage under load.
Mike

 

[nomadradio]

So just what is the desired outcome here?

A power supply to run a radio meant for "12 Volts"? Most radios meant to use in a vehicle expect 13.8 Volts DC, more or less.

If you want a power supply that is not regulated, a bridge rectifier connected to a transformer that provides about 10 Volts DC will do. The bridge rectifier will charge up your filter capacitors to 1.4 times the AC voltage on the winding.

The transformer you have will deliver about 20 Volts DC if you feed it into a bridge rectifier and filter capacitors. Makes it sound as if this transformer was meant to use with a voltage-regulator circuit that drops that down to 12 or 13.8 or that ballpark. Most regulator circuits use a so-called "pedestal" between the unregulated vltage and regulated side between 6 and 10 Volts. Your 20-Volt unregulated supply would work with any number of regulator circuits.

Sounds to me as if what you really need most to begin with is a plan.

73

 

[AudioShockwav]

As Nomad has said the voltage will go up, after you run it thru a full wave bridge and into the caps.
You need to regulate this down to 12 or well 13.8 volts to use it.

Here is a very simple power supply page to get you started understanding how a power supply works.
This is a very small supply, not enough to run a radio, but I think it might help you see how it goes together.

http://www.learningaboutelectronics.com/Articles/How-to-build-a-DC-power-supply.php

Read through that and take a gander at the pictures.

This will give you a idea how it works.

To build a supply that delivers more amps, as needed to run a radio, you will need a little more complicated regulated set up with pass transistors.
If you need less than 5 amps
NEC used to have a single 5 amp regulator ....but I don't think it is available any more.
I will go look after I post this
Any way.
Take a look at this and then post back with questions and we can go from there.


73
Jeff

 

[338_MtRushmore]

Obviously I didn't think this through at all. Maybe I can just find a regulator board on ebay.

 

[AudioShockwav]

Here is aother resource page that you might find useful to read, and help you understand what is going on with building a power supply.

https://www.homemade-circuits.com/12v-5-amp-fixed-voltage-regulator-ic/

73
Jeff

 

[338_MtRushmore]

I read the first link and it demystified the circuit. I really liked that it explained tha AC power cord, it made me feel smart because I already understood that.

I was planning to build 2 supplies. One for my router/modem, and another for powering meters and such in the shack. The router says 12v, and the supply puts out 12.06 open, so I am thinking actual 12v would be best for that.

How much current is the transformer capable of supporting if its rated at 12v 2a?

 

[nomadradio]

Sounds like 24 Volt-Amps, er, Watts transformer power rating. This is your limiting factor.

It will be delivering 20 Volts DC unregulated. 24 Watts divided by 20 Volts says 1.2 Amps if you use an analog series regulator. The 12-Volt output times 1.2 Amps gets you 14.4 Watts of regulated output The other 9.6 Watts get thrown overboard as heat in the regulator circuit.

A switchmode regulator is more efficient. Would get you more 12-Volt current by losing less power in the regulator. But not a full 2 Amps. That would require a regulator that's 100% efficient. A switchmode regulator that's 90% efficient would still only deliver 1.8 Amps.

Wall warts have evolved to use switchmode technology. They're cheap on the surplus market. Marlin P Jones or All Electronics would usually have this sort of item. Here's one rated at 3 Amps for $6.95 https://www.allelectronics.com/item/ps-1272/12-vdc-3-amp-power-supply-special/1.html

Doesn't get much cheaper than that.

73

 

[338_MtRushmore]

Here's one rated at 3 Amps for $6.95 https://www.allelectronics.com/item/ps-1272/12-vdc-3-amp-power-supply-special/1.html

Doesn't get much cheaper than that.

73

How do you ensure that it is quiet? The rfi from my existing wall wart is insane, and I failed miserably in the past to quiet one down.

 

[nomadradio]

Clearly a potential drawback to the cheap solution. Depends what you want to power with the thing.

73

 

[338_MtRushmore]

I found a different supply for the router that was quieted down with 7 turns on a ferrite. Since these transformers are too small to be useful I will just toss them in the junk box.

I'm still shocked that a 20 oz. bare transformer is only capable of putting out the same power as an entire 12 oz. wall wart transformer. Then again, there must be a reason that a 12v 2a wall wart transformer is impossible to find.

 

[nomadradio]

The wart contains a switchmode circuit. The transformer inside operates at 30 or 40 kHz, most likely. The key here is the core of the transformer. The 60-Hz transformer transfers energy from the primary winding, through the core in the form of a magnetic field, and then into the secondary winding. The core must store that energy for a 60th of second at a time. The more Watts, the bigger the core needed.

The switchmode just rectifies the 60 Hz AC without a transformer. An oscillator circuit drives a transformer at a frequency that only requires the core to store energy for a 30-thousandth of a second more or less. That shorter storage interval allows the transformer to have a core that's radically smaller per Watt than the 60 Hz version.

And lighter weight, to boot.

73

 

[Handy Andy]

Since you mentioned the 12V - the simple option is to use a DC power-pack for a CB radio.

IF you have a spare "coaxial plug" connector you know that fits the Modem, just clip it off the older wall-wart and run that cord thru that type of power supply (Linear) - Polarity is usually stated on the original device or on its' wall-wart you're replacing this with - and wire accordingly.

​

• - else the Switchmode or even the capacitor coupled "transformerless" types (less power rating capacity but heavy on the switching and capacitor for filters) although work for small load applications - their noise generated is horrible - buzzy and consistent and unless you can locate the radio you want to use somewhere else in the house, you're at the mercy of these things feeding back into your home wiring and generating problems with other equipment you have.

The problem with the "wall-wart" is their Switchmode, which you'll need the ferrites - chokes - coat hangers and various other materials including home made Leyden jars to try and tame the noise...

Might as well use a Zinc-steel and Copper foil - wining scheme and Lemons

It's not cheap but it's entertainment...​

 

[338_MtRushmore]

Is a full switchmode power supply a transformer? If so, what is the correct way to differentiate between a coil transformer and a full switchmode?

Can I use both of these transformers in parallel into a regulator? At this point I'm looking for a way out as to not have to admit total defeat.

 

[Handy Andy]

There are ways to approach the "switchmode" - look back at the "Schematic", you'll see a MOSFET that pulses the line voltage appearing on the Transformer.

It uses an ordinary transformer, but it sends power thru it differently

If you notice, Europeans use 50Hz, while US uses 60Hz - but did you know that you can increase the effective power A transformer can pass by increasing the frequency? 80Hz or even 120Hz? You can lessen the power thru the primary because the Secondary can couple to the Primary more efficiently at higher frequencies - this principle works, but is a bane to tame it's harmonics.

If you take a typical transformer - if you use a really high-impedance meter, you'll see that both the PRIMARY and SECONDARY will try to rise and MATCH each other due to the nature of the coil coupling. It's when you place a load across the Secondary, does the power present in it's winds drop. They can use this proximity effect - as a means to determine the load - noting the Primary appearance and the Secondary appearance, once the Secondary starts to drop in voltage (power) the load sense circuit then increases the length or duration of pules into the PRIMARY - they it determines the amount of the sine wave that goes thru the primary wind, or the SECONDARY if they equip it as such...they can take some current from a capacitor placed as a tap, along with a resistor - that drains off current stored on the plates and can be "Captured" to power a load sense. To utilize this they put it thru a bridge rectifier, to either power the load sense circuit that looks forward into the output to sense a drop in voltage across the output terminals or provide a secondary power supply to power the switchmode circuit and the MOSFET which then the Switchmode sends pulses to the GATE of the MOSFET - for a length of time - as the demand increases - the pulses turn into more of a rapid on to off - then nearly fully on as a duration of the duty cycle becomes more towards 100% on - read this as the Primary or Secondary is sending power thru it's winds to capture and use - as the load or level of work rises.
.
So I showed you a means to make this work thru a battery, the Schematic shows it as Vt - but the concept of a pulse train at a higher and higher frequency of pulses - RAISES the coupling effect and hence the level of power transfer into the Secondary from the Primary in the transformer - it increases it's efficiency.

Maybe this is more than you need to know but it' helps explain the noise they produce.