Transforming 24vac to ~5vac for +5vdc - Mitch

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Discussion Overview

The discussion revolves around the conversion of 24VAC to approximately 5VAC for subsequent rectification and regulation to +5VDC. Participants explore transformer options, current requirements, and efficiency considerations in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Mitch inquires about using a transformer rated for 115VAC input to step down 24VAC to 5VAC, questioning if this would yield the expected output.
  • One participant suggests finding a transformer with the appropriate step-down ratio and inquires about the current requirements for the LM7805 regulator.
  • Mitch specifies a current budget of up to 250mA and expresses concern about the LM7805's maximum input voltage being close to the expected rectified voltage.
  • Another participant explains that feeding a transformer with lower voltage may lead to voltage sag due to the transformer's resistance, particularly under varying load conditions.
  • Mitch asks about determining transformer current requirements at lower input voltages and seeks a formula for this calculation.
  • A participant provides a resource link for further reading on transformer specifications.

Areas of Agreement / Disagreement

Participants express various viewpoints on the feasibility of using a transformer rated for a different input voltage and the implications of load variations on transformer performance. No consensus is reached on the best approach or specific calculations.

Contextual Notes

Participants mention limitations regarding the maximum input voltage for the LM7805 and the effects of transformer resistance on output voltage under load. The discussion includes unresolved questions about current calculations for transformers at different input voltages.

Who May Find This Useful

This discussion may be useful for individuals interested in power supply design, transformer applications, and voltage regulation in electronic circuits.

mitchellch
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Hey,

[Newbie question]

I need to convert 24vac to ~5vac for ultimate rectification/regulation to +5vdc. I'm looking at transformers to step down to 5vac from 24vac. Most of what I'm finding take 115vac input. Would I be foolish to use a transformer rated at 115vac input to 24vac output, but input my 24vac instead of the 115vac and expect ~5vac out (i.e. 24v*(24v/115v) = ~5.00v)?

My goal is to rectify the ~5vac (resulting in ~7vdc) and then regulate that to +5vdc using an LM7805.

TIA.
-Mitch

PS I tried a solid state approach using a TL783 to go from rectified 24vac (34 vdc) directly to 5vdc, but producing 50-70ma the TL783 heated a very hefty heat sink to the point that it was too hot to touch in ~15 minutes.
 
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If you can find a "regular" transformer that has the step down ratio you need (I think around 120->35v) you should be good to go.

However, you don't mention how much current you are interested in? Looking at the specs, the 7805 would do ok with circa 34vdc as input if you don't need the full output current. Or, if your load is fairly constant and power efficiency is not an issue you could just put a power resistor and filter cap in line to the regulator to drop the input voltage to something reasonable. Ohms law will help in figuring out the resistance you need -- think of it as a voltage divider that operates at your needed 5v current.
 
Thanks for the reply.

The 'current budget' for my board is <=250ma. My most recent measurement was ~195ma but I do expect peak current consumption to hit (and possibly on occasionally briefly exceed) 250ma.

Power efficiency is not an issue for this project as it is supplied by a 24 vac, 1 amp wal-wart.

However, the LM7805 has a maximum VI of 35v, so I'm worried about 34vdc being too close to the max to allow for spikes, etc.

One of the questions I have is how to determine the current requirements of the transformer I choose at lower input voltages. For example, given a 120/35v transformer rated at 200ma, how much current can I expect with a 24v input and 7v out? I'm sure there is a simple formula for this, but I can't seem to find it (or figure it out myself).

I like your "power resistor" suggestion. However, while power efficiency is not an issue, my load does vary (i am driving 8 5vdc/12ma relay coils that come on/off in an unpredictable fashion). Does the variable nature of my load eliminate this as an option?

Thanks again for your help.
-Mitch
 
feeding the transformer with lower voltage will sort of work.
what you'll find is the transformer's resistance will make the voltage sag .

it's built to have some nominal voltage drop at full current, let's just say 5%.

well, 5% of 24 volts is what % of 5 volts?

when you lower the applied voltage you don't lower the resistance of the windings.
and it gets worse - since you only draw current in big gulps near the sinewave peaks, the effect is exaggerated.

you should run that experiment it won't take long and you'll get that gut feel for the problem. someday you'll use it to advantage so get it into your bag of tricks now.

I'd look in thrift shops. There's always a basket full of "wall warts", those little transformers for video games and cellphones.
If you can find a charger for 7.2 volt nicad battery packs...now there's a transformer !
 
One of the questions I have is how to determine the current requirements of the transformer I choose at lower input voltages. For example, given a 120/35v transformer rated at 200ma, how much current can I expect with a 24v input and 7v out? I'm sure there is a simple formula for this, but I can't seem to find it (or figure it out myself).

here - these guys literally wrote the book...


http://www.hammondmfg.com/pdf/5c007.pdf
 

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