transformer with multiple outputs

[whiz115]

I have a small transformer which haves multiple outputs
raging from 1,5V to 12V/350mA. The transformer has many windings
and you can choose the output (I don't know how do you call these transformers)

i've noticed that when i measure it with my multimeter the output
is higher than it should...and from my school i learned that i should use
a "load resistor" paralleled with the output so my transformer can have a small
dissipation.

so...by myself i tried to figure out what i need and i ended up to this: 12x0.35=4.2W
12V because it's the maximum output in volts and 0.35A because its the maximum output
in amperes.

do i really really need 4.2W resistor????
and how am i going to calculate how many ohm of resistor i need?

also can someone explain to me why the output is higher than it should?

Thank you guys!

 

[audioguru]

The windings in the transformer have resistance that causes the output voltage to drop when it is loaded. It is made to give the correct voltage at full load.

The 12V/350mA resistor is calculated with Ohm's Law to be 34.3 ohms. Use a 33 ohm 5W resistor in series with a 1.2 ohm 1/4W resistor.

 

[stevez]

If we were to take a look at just the 12 volt winding:

1. As Audioguru pointed out - the impedance of the transformer secondary winding will result in a small but noticeable voltage drop as the load on the secondary is increased from zero to rated load - in this case you said 350 ma. The voltage on the secondary goes down as you increase the load. Quite often the transformer symbol includes a series resistor to represent the transformer impedance.

2. You can expect the secondary voltage to be proportional to the input voltage. As you make measurements carefully check the input voltage at the same time you measure the secondary. If the primary (input) is high - the secondary will also be high. As an example - if your transformer secondary is 12v@350 ma with an input of 220 volts - and you apply 240 volts - expect to see somewhat more than 12v@350 ma.

 

[Nigel Goodwin]

also can someone explain to me why the output is higher than it should?

Simple, it's a crap unregulated power supply, you should buy a better quality regulated one.

 

[whiz115]

The windings in the transformer have resistance that causes the output voltage to drop when it is loaded. It is made to give the correct voltage at full load.

that's why it shows higher output to my multimeter when no load exists?
i was afraid that i was right with the resistor...what am i going to do?
5W resistor is to big to fit inside the transformer's little box.

@Nigel Goodwin yes it is crappy...but if i don't like it any more i can dissemble it and make my own regulated 1,5-12V/350mA regulated power supply with zeners diodes! i think
somewhere i have read that zener diodes can be used for regulation!

 

[stevez]

Somehow I think there is a bit of a misunderstanding. The 5W resistor that you are worried about represents the maximum load that can be placed on the transformer secondary. I am not sure I see the point in connecting a resistor - except to see how the transformer behaves.

If your goal is to get the secondary voltage down there might be better ways to do it. It might help if you explained more about what you are trying to do.

 

[whiz115]

Somehow I think there is a bit of a misunderstanding. The 5W resistor that you are worried about represents the maximum load that can be placed on the transformer secondary. I am not sure I see the point in connecting a resistor - except to see how the transformer behaves.

at first i want to experiment a little...i was told that if i leave it like this it is
possible that i can burn a device that has strict tolerance in peak voltage
that's because the capacitor of the transformer charges to that voltage...
let's say we want 12V and the transformer momentarily outputs 13,5V

probably you could say that the same thing can happen if the voltage at the primary is raised....well..i don't know.....



P.S do you think that the regulation with zener diodes is a good idea to try it
some time?

 

[audioguru]

The resistor is connected as a load on the transformer only while you measure the fully loaded voltage to see if it is correct.

A zener diode is a very old method to regulate a voltage. It wastes a lot of power. If you need the voltage to be regulated then you should use a modern inexpensive voltage regulator IC. The LM317 has an adjustable regulated output voltage from 1.25V up to about 2.5V less than the input voltage. It doesn't waste power.

 

[whiz115]

A zener diode is a very old method to regulate a voltage. It wastes a lot of power. If you need the voltage to be regulated then you should use a modern inexpensive voltage regulator IC. The LM317 has an adjustable regulated output voltage from 1.25V up to about 2.5V less than the input voltage. It doesn't waste power.

you mean i can disable all the windings and keep only the 12V and with the
LM317 i can use a trimmer and adjust the voltage where i want it to be?
and i always have for output exactly what i expect to have?

do you agree with my last post? is it a reason not to use the transformer as is without a load resistor?

 

[audioguru]

The only disadvantage of using the LM317 adjustable voltage regulator is that it gets hot and needs a heatsink if its voltage is set low and its load draws the max current. If the input is 12V and the output is 1.5v then it has 10.5V across it. With a load current of 350mA then it dissipates 3.7W so it needs a little heatsink.

The transformer doesn't need a load resistor.

You talk about a 12V transformer. A transformer has an AC output. A voltage regulator IC needs to have a smoothed DC input. So the transformer needs to have a bridge rectifier and big filter capacitor. If the transformer is 12VAC then the unregulated DC voltage is 15.6V. The extra voltage will create more heat in the regulator.

 

[whiz115]

You talk about a 12V transformer. A transformer has an AC output. A voltage regulator IC needs to have a smoothed DC input. So the transformer needs to have a bridge rectifier and big filter capacitor. If the transformer is 12VAC then the unregulated DC voltage is 15.6V. The extra voltage will create more heat in the regulator.

it's rectified with i think 1N4004 diodes...and a smoothing capacitor 470uF/25V

 

[audioguru]

The capacitor is small so the power supply will have ripple voltage. If the lowest voltage of the ripple is 12V then the max output from the LM317 is +9.5VDC.

 

[jess]

I just skimmed the above posts. Often a circuit will work OK if the voltage is within +/- 10% or maybe even 20%. Think about circuits which run on batteries, say 9V. Some circuits will operate OK down to 7V, perhaps lower before battery replacement or recharging is needed.

Components are selected with maximum rated currents and voltages well above the design rated voltages and currents so it doesn't matter if the supply voltages are exact. If a circuit is designed for 12 volts dc, the electrolytic capacitors should probably be rated at 20 to 25V (12 x 1.4 = 16.8). The peak voltage applied to an electrolytic filter capacitor will be about 1.4 x the transformer input voltage. Diode rectifiers should be rated 2 to 4 times the appled ac voltage for a simple full wave rectifier. Transistors used in a 12V circuit are probably rated for at least 20V, probably more.

Some circuits in critical applications need a highly regulated voltage and this is where the 317 or other voltage regulators come into play. Some regulators are fixed at a certain voltage. Examples are the LM7805, +5 volt regulator. The LM317 can be used as an adjustable regulator with proper circuit design.

Transformers typically supply higher voltages than the rated output. As someone mentioned the transformer output voltage will decrease as the load current increases due to resistances in the transformer primary and secondary windings.

And by all means DO EXPERIMENT safely.

If I have missed the point here please excuse me.

Jess

 

[whiz115]

no you haven't missed the point...you have absolutely right!
most of the components in a circuit have tolerances that can withstand
those small voltage changes...i know it because i have noticed it many times!
especially with capacitors that rated far way than the actual power supply.

the problem is that i am newbie...and when i hear something from someone
who knows more that i know...i usually take them seriously!

 

[cadstarsucks]

I have a small transformer which haves multiple outputs
raging from 1,5V to 12V/350mA. The transformer has many windings
and you can choose the output (I don't know how do you call these transformers)

i've noticed that when i measure it with my multimeter the output
is higher than it should...and from my school i learned that i should use
a "load resistor" paralleled with the output so my transformer can have a small
dissipation.

so...by myself i tried to figure out what i need and i ended up to this: 12x0.35=4.2W
12V because it's the maximum output in volts and 0.35A because its the maximum output
in amperes.

do i really really need 4.2W resistor????
and how am i going to calculate how many ohm of resistor i need?

also can someone explain to me why the output is higher than it should?

Thank you guys!

Audio is correct.

Typically transformers will have a % regulation spec. which is 100 - (full load/no load). typical values are from 10-30%. This number represents the sum of all the transformer losses and can be lumped into line with one winding or the other of an ideal transformer for analysis.

What you have in your hand is a simple multitap secondary transformer.


D.

 

[Hero999]

Correct, transformers do have a regulation specification, generally smaller stransfromers are worse than larger transfromers. A small 3VA transformer might have a 40% while a huge 1MVA power company transormer will be <0.1%.