How to size Transformer

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bsparky
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How to size Transformer

Post by bsparky » Mon Sep 12, 2005 7:53 am

How do you size a transformer. Most trans. are rated in KVA right? So say I have a 250 watt, 24 VDC heligan bulb and want to build a power supply for this, I know 250 watts = 1.2 amps. I have tried Googling for my answer, will try more probably not use the right key words. Thanks for any help.
Single phase transformer KVA is V*Amp but is this for input or out put?<p>[ September 12, 2005: Message edited by: bsparky ]</p>

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Chris Smith
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Re: How to size Transformer

Post by Chris Smith » Mon Sep 12, 2005 8:16 am

In and out is the same value, but for a safety margin add in a little more watts. 25% more is a good start.

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MrAl
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Re: How to size Transformer

Post by MrAl » Mon Sep 12, 2005 10:01 am

Hi there,<p>I'd like to add the following...<p>Since you have a 250 watt bulb that runs at
24vdc it will draw about 10.4 amps, so you need
a transformer with a secondary that can handle
at least that much current at 24vac. As Chris
says, you should go a bit higher just to have
some safety margin.<p>Since you're also going to have to rectify
this, you'll probably use a bridge rectifier
or some diodes which should also be rated for
the current.
This means you have to consider the effect of
the waveform which wont be smooth dc unless
you add filter caps. You also wont get the
required output level without adding some caps
either (24vac transformer output).
Since you want 24vdc out, if you start with a
24vac transformer you'll need about 2000uf for
the filter caps. If you use more than this you'll
get a higher output dc level, which will burn up
the bulb too quickly.
You probably want to go with four 500uf caps in
parallel, depending on their ripple current rating.
Once you get it up and running for an hour or
so, feel the caps with your hand to see if they
get hot. If they get hot you'll have to use
different caps with a higher ripple current rating
or else go with more in parallel, like 20 x 100uf.<p>The voltage rating of the caps should be at
least 50vdc.<p>Take care,
Al
LEDs vs Bulbs, LEDs are winning.

bsparky
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Re: How to size Transformer

Post by bsparky » Mon Sep 12, 2005 11:02 am

Wow thanks Al I was way off on my calculation for amps don't know were I got 1.2 amps from. At first when I read your reply I thought were did he get that from so I did it agian and sure enough your right. This is on a microscope here at work and they need pretty smooth DC because they see the ripple through the scope they said. But still not sure spec. on the transformer. Transformer being the most costly item, the heavyest, and will use the most room, it would be best to size it properly then to over kill like I've done in the past.

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Re: How to size Transformer

Post by Robert Reed » Mon Sep 12, 2005 4:50 pm

Bsparky
First a tidbit-KVA Does not neccesarily imply wattage. Wattage is true power (V-I in phase) and KVA is apparrent power (V-I not nessesarily in phase). This has to be looked at during initial design, but in your case you should be OK.
2000 MF capacitance will still allow too much ripple, since you say you need smooth DC. Better to go with a higher secondary (30V) and regulate it down to 24VDC. This way regulator input capacitance can be on the stingy side and still produce smooth regulated DC out. This project will get quite beefy and expensive with a fair amount of heat dissapation. A better way to go might be to use a couple of 12V tractor batterys in series with a low current charger. Again I am basing my assumptions on your need for "smooth DC"

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sofaspud
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Re: How to size Transformer

Post by sofaspud » Mon Sep 12, 2005 5:07 pm

What about a switched mode power supply? A lot less bulk and heat, and one could probably be found at a comparable price for a transformer supply.
I have some 100 watt 12 volt halogens (think golfball-sized Sun) but don't really have a use for them right now because of their power requirements.

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Re: How to size Transformer

Post by jwax » Mon Sep 12, 2005 6:19 pm

250 watt microscope lamp? Wow! What kind of microscope is this? If it is like most, you'll want a variable voltage supply, so you can adjust the intensity of the light. 24 volt is simply the max the bulb is rated for. You want an adjustable 24v volt supply, at 12 amps or so.
DC? Not needed on an optical microscope. Many use a lamp dimmer (SCR control)- nobody will see 60 Hz ripple.

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Chris Smith
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Re: How to size Transformer

Post by Chris Smith » Mon Sep 12, 2005 6:52 pm

The input to the transformer will draw around 2 amps, possibly where you got your 1.2 amps from? <p>2.08 amps IN at 120 volts is 250 watts. [draw, not deliver]<p>10.4 amps OUT at 24 volts is also 250 watts.[delivery]<p>Add in a buffer of at least 25% extra for good measure because there is always a loss. Also 24VAC recitified and filtered will be a lot more than 24VDC. Closer to 30?

bsparky
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Re: How to size Transformer

Post by bsparky » Mon Sep 12, 2005 8:48 pm

Jwax you are right it is for an adjustable 24v. You realy don't think they would see the 60hz ripple. From what I interperat from the Chinese tech there were a lot of capaciters in there but they removed the guts and put something else in place that is not working for them, they can't dim the bulb enough. So I quessed that it was DC with alot of filtering. Wish they had left what was in there, might have been able to fix or at least seen what was in there.

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Chris Smith
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Re: How to size Transformer

Post by Chris Smith » Mon Sep 12, 2005 9:18 pm

250 watts for a microscope? <p>I use lasers on mine, much brighter, better focus, and only a few milli watts. <p>For white light I use a few white LEDS, and other color LEDs for different approaches to the different problem. <p>250watts?

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Re: How to size Transformer

Post by Enzo » Mon Sep 12, 2005 9:29 pm

A 250 watt bulb under a microscope slide ought to pretty well cook whatever is on it. And it ought to make the scope too hot to touch. Are you sure about those numbers? That sounds insanely large.

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Re: How to size Transformer

Post by terri » Tue Sep 13, 2005 1:07 am

The transformer ought to have about 2-3/4 square inches of core to handle 250 watts without transformer saturation, according to the ARRL Handbook. This is about 1-5/8 inches on a side for the central part of the transformer core.<p>250 Watts does seem like a lot, but on the other hand, there's a lot of light loss through all those lenses. When you say they can see the ripple, is this maybe because of core saturation on the current peaks in the original setup? Still, seeing a 60Hz ripple seems a little strange for an incandescent lamp.<p>But then again, I'm no microscopist. *<p>Maybe you ought to use a car battery charger or its transformer? :) <p>Can you use a variable aperture to vary the brightness?<p>Is there a cooling tank between the lamp and the object?<p>Hm.<p>( * "Microscopist" seems so awkward to say. Make that "Microscope Looker-Througher." )<p>[ September 13, 2005: Message edited by: terri ]</p>
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Re: How to size Transformer

Post by bsparky » Tue Sep 13, 2005 4:45 am

I think I will go out there agian and investigate the scope for myself. Will get back to you all then . Thanks for all the replys, you gave me alot of food for thought. I think I may have not been understanding their problem right.

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jwax
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Re: How to size Transformer

Post by jwax » Tue Sep 13, 2005 4:46 am

bsparky, you still haven't said what kind of microscope this is. Some differential interference contrast (DIC) scopes use both lamp intensity power control and a variable iris to throttle light into the sample. Varying the lamp power allows them to change the color temperature of the bulb, hence its' output color, allowing better viewing of certain samples. The iris controls intensity without changing the color temp of the light.
I've yet to see the point in going to a DC supply. A pair of 10 amp battery chargers in series, with a lamp dimmer on their primaries would work fine.

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Re: How to size Transformer

Post by terri » Tue Sep 13, 2005 6:16 am

Someone's sure to challenge you on the use of lamp dimmers on the supposedly "inductive" transformer primaries.<p>This actually works pretty good, within limits, and the actual load is the resistive load of the lamps "reflected" through the transformers.
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