transformer - can someone explain

[wildbill]

Hi to everyone, can someone explain the VA calculation in a transformer ?
is 12v@ 500ma = 6VA, 12v@ 1A = 12 VA ?
What about 12 -0- 12 @ 1A, is that 24VA ?
Thanks in advance.

 

 

[BeanieBots]

Without getting too complex, when the load is a simple resistive one, the VA equates to the power (W) that the transformer can deliver. So fundamentally, your assumption is correct and yes a 12-0-12 @ 1A would be 24VA.
(It is written VA rather than W to emphasise that it is V*A without taking the phase difference into account which is required to give true watts.)

 

[wildbill]

Thanks for that, i intend building a new variable bench supply, using the LM317T reg, ( about 1.5 to 35vdc )which should be fairly straightforward and once i get a bit more experience, i can incorporate an 'axe to give me a digital readout. Thank you for the rapid response to my query.
Wildbill.

 

 

[BeanieBots]

Just a quick word of warning with such a supply. The power dissipated by the LM317 will be the volt drop across it multiplied by the current drawn. This can be very high when using a low output voltage with high current. Make sure you do not exceed the device's maximum power spec /temperature as well as not exceeding it's current spec. Just a few watts will make it get very hot unless you have very good heatsinking.

Edited by - beaniebots on 1/17/2006 12:21:25 PM

 

[wildbill]

Thanks for that, heatsink will be bolted to metal chassis, i may also put in a small fan. I dont know off hand if there is a negative version of this reg, as it would be good to make a dual supply !

 

 

[ylp88]

There is a negative voltage version, LM337 from memory.

Bolting to the chassis may not be sufficient if you do not have a large enough heatsink. Some power supplies change the voltage tap on the transformer to reduce the input voltage to the regulation circuit so that the voltage drop across the regulator (or similar pass device) is greatly reduced.

eg.
<code><pre><font size=2>
_____ _____&gt; This tap at high O/P voltages
)(
)(
)(_____&gt; This tap at low O/P voltages
)(
)(
_____)(________________&gt; common tap

transformer
</font></pre></code>

<b><i>ylp88 </b> </i>

 

[wildbill]

thanks ylp, if i decide on a dual supply, the volt regs will first be bolted to a suitable heatsink and then the heatsink bolted to the chassis, all with thermal paste, this plus a small fan that i have should be adequate.
thank you again for your input.
wildbill

 

 

[ylp88]

Out of interest, what is our typical load current (max.) and maximum and minimum voltage output?

<b><i>ylp88 </b> </i>

 

[wildbill]

Hi ylp, as this is going to be a bench supply, it will be used mainly for testing and experimentation, having 1.5 to 35 vdc means it can power most projects like dual rail op-amps etc, it will also be powerful enough to operate a medium output stereo power amp although i need to be careful not to exceed 1.5A. i've still to decide on the final spec for this project that i have been meaning to build for many years now. i cant remember how many times i've had to stick taped bare wires into the 3.5 plugs on the ends of a couple of power adapters to get a + &amp; - 12vdc. time for a proper one.

 

 

[ylp88]

The regulators have a 4'C/W thermal resistance (j-c). Assuming that you do use a centre tap for low output voltages, say, 18V in to 9V out, you will have a 9V drop. Drawing 0.5A will produce a power dissipation of 4.5W. A typical heatsink and fan solution may have a thhermal resistanece of 1'C/W (c-a). Thus there is a j-a resistance of 5'C/W. At 4.5W, this is a 22.5'C temperature rise above the heatsink's ambient temperature.

Ultimately, please be careful when using moderate to high currents at relatively low voltages. Remember, this scenario is assuming you have a low tap to supply the lower voltage. If you don't, the power dissipation will be even higher (more than twice) and I havn't considered case to heatsink (c-h) resistance.

Furthermore, please be even more careful with your mains wiring.

All the best!

<b><i>ylp88 </b> </i>