24VAC rectified to ?

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perfectbite
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24VAC rectified to ?

Post by perfectbite » Sat Jul 24, 2004 10:16 pm

I'm going to try to get this question in before the forum evaporates (and you thought my BTWs were way off topic). I was going to get the values of the 24VAC txs I have (I think they're 40VA) before posting but they are under piles of other handy junque and unavailable at this time.<p>If I rectify a 24VAC/40VA tx output to a DC voltage what voltage and amperage output could I reasonably expect?<p>Rough percentages with amperage draw limits would do fine. It isn't for NASA or a dialysis machine.

bodgy
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Re: 24VAC rectified to ?

Post by bodgy » Sun Jul 25, 2004 2:27 am

Tx is this a transmitter or a transformer?<p>Anyhow the absolute answer will depend on the type of rectification you are going to use, the efficiency of the transformer on load and the resistance of the load.<p>Now to the general formula.<p>With capacitor filter <p>half wave rectification Vdc = Vac * 1.414 Idc = Iac * 0.28<p>full wave rect Vdc = Vac * 0.71 Idc = Iac * 1
full wave bridge rect = Vac * 1.414 Idc = Iac *0.62<p>With choke (inductor) filter<p>
FW Vdc = Vac * 0.45 Idc = Iac *1.54<p>FWB Vdc = Vac * 0.90 Idc = Iac * 0.94<p>Resistive load no filter<p>HW Vdc = Vac * 0.45 Idc = Iac * 0.64
FWB Vdc = Vac * 0.90 Idc = Iac * 0.90 <p>The rectifying diodes will lower the voltage by one or two diode drops.<p>Mosfet active rectifiers would not have a voltage drop, but would only be recommended for an experienced electronics/electrics person.<p>Colin
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cato
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Re: 24VAC rectified to ?

Post by cato » Sun Jul 25, 2004 7:25 am

Assuming the input line voltage is nominal and assumeing you use a cap to filter the rectified output, the cap will charge to the peak of the secondary output (24 *1.414) or 33.94V. That voltage will drop as you draw current during the rest of the ac line cycle... how much depends on how much current you draw. I'm not sure how to do the current calculation...but 1.5 amps feels about right.... the more you draw, the hotter the transformer will get....

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Externet
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Re: 24VAC rectified to ?

Post by Externet » Sun Jul 25, 2004 11:27 am

Hi.
Try to look at a ham radio handbook, the power supply chapter shows nice set of curves for each condition of filtering and loads for half or full wave rectification.
Miguel
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sofaspud
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Re: 24VAC rectified to ?

Post by sofaspud » Sun Jul 25, 2004 11:55 am

I just recently ran across a rough rule-of-thumb guideline for full-wave rectification. Was it in the newest N&V? I'm not certain, but I remember it saying to figure a 30% increase in voltage and 40% decrease in amperage for the xformer rating vs. dc output.

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sofaspud
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Re: 24VAC rectified to ?

Post by sofaspud » Mon Jul 26, 2004 9:55 am

This is a different but related question. I can guesstimate the rating of an unmarked transformer by its size and mass, or from the equipment it was removed from if that is known. Is there a way to obtain a better knowledge of the rating, say anywhere between 1% to 20% of the actual? From a practical standpoint, I'm thinking of transformers with 110VAC primaries here. A resistance measurement/Ohm's Law calculation??
Just trying to elaborate a little... Maybe energize the primary with 12 or 24 VAC 60Hz with a dummy load on the secondary, which would provide volts/amps/resistance that could be used to determine the rating.
Not worth it if it takes a page of calculations, but I was wondering if there might be some trick I'm unaware of.<p>[ July 26, 2004: Message edited by: Morgen ]<p>[ July 26, 2004: Message edited by: Morgen ]</p>

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Edd
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Re: 24VAC rectified to ?

Post by Edd » Mon Jul 26, 2004 11:27 am

My quick roughing /guesstimation of the power rating of transformers after testing 120V line rated transformers of many manufacturers thru many years was to just take the raw ac output voltage of the secondary and then use an adjustable power rheostat resistor adjusted to bring the voltage down 10 % from the unloaded voltage. Then one computes the current/voltage/wattage specs from that information. All of the popular Stancor/Thordarson /Meissner/Merit transformers patterned to this load spec. Unfortunately the current, readily available Radio Shack series of transformers are not nearly as conservatively rated , especially in their resulting operating temperature.<p>73's de Edd
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Re: 24VAC rectified to ?

Post by toejam » Mon Jul 26, 2004 12:21 pm

60 hz. transformers from my experience with them go at around 12 watts per pound for a good reliable one with a 100 percent duty cycle. A watt per ounce would probabally work for general use, but oem specs must always be considered.

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Re: 24VAC rectified to ?

Post by dyarker » Mon Jul 26, 2004 11:30 pm

50-60Hz transformers may have a larger (heavier)core.<p>Wire gauge of the secondary winding(s) gives some indication of current capability.<p>Edd - I think your 10% voltage drop test would give a more realistic continous rating for Radio Shack transformers too! The RS transformers are okay, but the sales dept. prob redid the specs to show peak instead of continous load. Not cheating because they don't say, one way or the other ;)
Dale Y

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