suggest a circuit to convert 24vac to 12vdc

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eoinitb
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suggest a circuit to convert 24vac to 12vdc

Post by eoinitb » Thu Apr 07, 2005 8:20 am

maybe its been asked before but could someone suggest a circuit for stepping down and converting 24vac to 12vdc

Sterling Martin
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Re: suggest a circuit to convert 24vac to 12vdc

Post by Sterling Martin » Thu Apr 07, 2005 9:19 am

School project?

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Chris Smith
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Re: suggest a circuit to convert 24vac to 12vdc

Post by Chris Smith » Thu Apr 07, 2005 12:14 pm

A bridge rectifier and caps to convert the AC to DC, and a LM 7812 voltage regulator to step it down to 12 volts.
Keep in mind There are amperage limitations to consider.

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Re: suggest a circuit to convert 24vac to 12vdc

Post by Mike » Thu Apr 07, 2005 12:58 pm

Yes, like Chris said. Depending on your application, the 1.5A from a 7812 may or may not be enough.<p>If it isn't, there are adjustable regulators that can handle more power (5A from a lm338). If 1.5A is enough and you want varable, I have a schematic for a lm317 1.5A adjustable regulator. See http://www.electronet.dydns.org/electro ... /lm317.htm<p>
78xx series regulators are very easy to use. Just transformer, rectifier, capacitor, regulator, capacitor, load.

Intimidator#3
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Re: suggest a circuit to convert 24vac to 12vdc

Post by Intimidator#3 » Thu Apr 07, 2005 4:47 pm

the lt1084 can go up to 7.5 amps if you need more than 5a<p>[ April 07, 2005: Message edited by: Intimidator#3 ]</p>

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Re: suggest a circuit to convert 24vac to 12vdc

Post by Michael J » Sat Apr 09, 2005 3:32 am

Hi all I'm new to this forum.<p>Stepping 24v AC down to 12v DC can be tricky and
not as easy as you might think.
A fully rectified 24v AC becomes around 36/37 volts DC no load which is very close to the recomended 40v max for most 3 terminal regulators. Depends on the transformer used.
But that is usually not the main problem,, heat
dissipation is.
Depending on the 12v load current you require a lot of heat can be created in the process.<p>I have used 2 x 27 Ohm 5W wire wound resistors up to 2 x 33 Ohm 5W w/w resistors to give the 7812 regulator an easier life. This spreads the heat between the 2 resistors and the regulator and still allowing the nessesary 2 volts or so input above the regulated output for the regulator to function correctly,,, @ about half an amp output.<p>Hope this helps.

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Re: suggest a circuit to convert 24vac to 12vdc

Post by Michael J » Sat Apr 09, 2005 4:06 am

P.S. Forgot to mention I put the resistors in
series with the 24v AC transformer output, before the Bridge Rectifier, filter cap and 7812 regulator,,, ~;o)

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Re: suggest a circuit to convert 24vac to 12vdc

Post by dyarker » Sat Apr 09, 2005 9:17 am

Adding to what Michael J said; if your 24VAC source is center tapped, use two diodes for a full wave center tap configuration. That results in less voltage to drop and is more efficient.<p>It would help if you said what you're going to use the 12VDC for, and how much current you need.<p>Cheers,
Dale Y

eoinitb
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Re: suggest a circuit to convert 24vac to 12vdc

Post by eoinitb » Mon Apr 11, 2005 12:08 pm

im only going to need less than half an amp. im using it to power some ics which require anywhere from 10 -14v dc.<p>after following everyones advice i think im nearly there. the only problem im having is that the voltage fluctuates from around 9volts back up to around 14volts i think it might have something to do with my capacitor valuee.could someone suggest what values i should use

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sofaspud
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Re: suggest a circuit to convert 24vac to 12vdc

Post by sofaspud » Mon Apr 11, 2005 1:33 pm

I would aim for about a 3300µF 50V electrolytic filter capacitor. A slightly lower or higher value should be fine too. It really depends on how much ripple is allowed by your circuit.

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Re: suggest a circuit to convert 24vac to 12vdc

Post by Mike » Mon Apr 11, 2005 1:47 pm

Are you using the 7812 regulator or an adjustable one?

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Re: suggest a circuit to convert 24vac to 12vdc

Post by Michael J » Tue Apr 12, 2005 2:18 am

A capacitor in the range of 1000 to 4700 µF
should work in most cases, 63v.<p>The input of the regulator has to be 2 volts or
so higher than the regulated output, while supplying the rated current load you require for it to function correctly. Otherwise they become
a non regulator. Low dropout types are available
only needing half a volt or so higher to operate.<p>If your voltage is going high,, you may have a faulty regulator or incorrectly wired. Unusual.

eoinitb
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Re: suggest a circuit to convert 24vac to 12vdc

Post by eoinitb » Tue Apr 12, 2005 9:50 am

the problem with it going to high is that the input voltage which i tough was 24v is actually around 27 volts i presume this would explain why the voltage is going up to around 14v

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sofaspud
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Re: suggest a circuit to convert 24vac to 12vdc

Post by sofaspud » Tue Apr 12, 2005 12:28 pm

Are you using a 7812 regulator? If so, the output voltage should remain virtually constant in spite of any 3V difference in the input voltage, as long as the input to the regulator stays between 15V and 35V.

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