ac/dc conversion and regulation

[reb]

what is the least expensive and/or best way to convert voltage? my application is to convert 24v ac to 5v dc.

[bodgy]

The least expensive (but dangerous for the unwary) is to use a capacitive dropper (if low current 50mA or so) or a resistive dropper and a zener.<p>None of the above would be isolated from the mains.<p>For safety bite the bullet and spend the money on a transformer.<p>Never tried, but maybe an active rectifier might chop the voltage<p>Colin

[bodgy]

oops read an extra zero on the 24 there.<p>A series resistor and zener diode would be the cheapest, with an extra diode for a half wave formation.<p>Not the most efficient, but the cheapest - what current are you needing?<p>colin

[Chris Smith]

Bridge rectifier to make it DC, and a 7805 voltage regulator, plus ripple caps across the output and input of the reg.

[IRONMAN]

I agree with chris, the I.C. regulators are usually the cheapest most reliable way to go, however if you're talking about 24 VAC then be careful becuase when you run that through a bridge rectifier your out-put is about maximum for the 7805's input voltage(about 35 volts peak.), the higher the voltage across the device the higher the power it dissapates.

[rshayes]

A bridge rectifier will work if your 24 VAC is an ungrounded winding. Output voltage could be over 40 volts if the line voltage is high, the transformer regulation is poor (ie. a cheap transformer), and the load is light. Probably too high for the 7805.<p>National makes an LM317HV which can withstand a 60 volt input. This needs a pair of resistors to set the output voltage.<p>Dropping 40 volts down to 5 volts is very inefficient, and will heat up the regulator. If this is a problem, put a resistor in series with the input of the regulator. Figure it for about a 25 volt drop at your maximum load current.<p>If your 24 volt winding has a center tap, use a full wave rectifier. This will put the maximum voltage at about 20 volts, and halve the power needed and the heat dissipation. In this case, figure the resistor for about a 12 volt drop. In this case, you could also use the 7805, since the input would be well below 35 volts.

[reb]

Let me give more detail. First, I'm a programmer with a good understanding of low voltage circuits but new to IC's. My project is probably simple. I have 24vac input which ranges between 23 to 28vac from a 240v to 24v transformer. I need to reduce the voltage to about 5 vac to operate a few transistors for switching purposes. It seems like the bridge rectifier and the 7805 regulator is the way to go and is what I had thought would be. What size caps are required for the reg and I think a high value cap should be used for smoothing the supply to the transistors?

[russlk]

The regulator would like .01uF on the output for stability. You can calculate the filter cap from: C = I*dT/dE. In your case, dT is the time between voltage peaks = 20mS and dE is the ripple voltage that you will tolerate. I is the load current.

[IRONMAN]

Rebel:
You really should use a different transformer, as stephen pointed out dropping 35-40 volts down to five is extremely inefficient, Think about it lets say your circuit uses 1/2 an amp(not unreasonable.), @ a forty volt input thats 17.5 watts of power wasted across the regulator!
Give in and buy a lower out-put transformer, otherwise ANY scheme you can come up with will either be wasting a lot of power or be complicated and expensive(which you're trying to avoid in the first place.).
Good luck.

[reb]

The transformer is not supplied by me. The transformer is part of the equipment that I am attempting to draw power from. The equipment is 240vac and has a 240vac to 24vac transformer that is used for its control circuits. My circuits that I'm adding need to use the equipment power to operate a few transistors(mine) to switch the 24vac control circuits that is allready present in the equiment. As I said in a previous reply the voltage range is around 23 to 28vac not more.

[IRONMAN]

I understand that the supply is no more than 28 VAC, however when you rectify that 28 VAC with a bridge circuit and add a filter cap. you'll end up with very close to 40 VDC. Thats just about to much for a little 7805 to deal with.
If you need any other voltages higher than 5 volts in the circuit, you could use a higher voltage reg. for them and then use a 7805 off of that lower voltage.
Other than that you'll have to look at data sheets to see which I.C. regulator will work with that level of input(probably an adjustable.), or you could build your own with a zener for reference and a series pass transistor. Thats what I would do in this case.
Of course if the 5 volts is not very critical, simply use a voltge divider after the filter cap.<p>Good luck.<p>[ July 14, 2003: Message edited by: IRONMAN ]</p>

[rshayes]

You may need an additional transformer. The 24 VAC from the equipment may be tied to a definite ground at some point. If you use a bridge rectifier with this winding, the negative output (which you would consider ground for your added circuits) might be close to ground (one diode drop) for half the AC cycle and follow the other half cycle up to a peak value close to 40 volts. This can be awkward if you are trying to add external control signals.<p>A small transformer can be used to convert the 24 VAC to a lower voltage, probably about 10 VAC. The secondary of this transformer will be isolated from the primary, and can be rectified and filtered, and regulated to provide your 5 VDC power supply. Since this transformer is not connected to the AC line, it will not need high voltage insulation between the primary and secondary. Since the voltages are low it might be practical to hand wind a suitable transformer.<p>A 120 volt to 50 volt transformer could be used at the lower voltages, but this is not an easy part to find. A 110 volt to 25.2 volt transformer could be modified by removing the primary and rewinding it for 10 volts. The secondary would then be used as the primary. The 10 volt winding would not have very many turns, and the wire size would be reasonable. This would be especially easy if the transformer used a bobbin split into two sections, as many of them are.

[bruinbear714]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by IRONMAN:
I agree with chris, the I.C. regulators are usually the cheapest most reliable way to go, however if you're talking about 24 VAC then be careful becuase when you run that through a bridge rectifier your out-put is about maximum for the 7805's input voltage(about 35 volts peak.), the higher the voltage across the device the higher the power it dissapates.<hr></blockquote><p>Also don't forget that the amount of power the 7805 dissapates also depends on how much power is drawn from it.<p>I personally would put that 24VAC through a bridge, add some supply caps, and use a step-down converter IC such as ones available from Maxim-IC or Linear. They both have 1 chip solution for low current draw, or use of external FET for higher powered applications.<p>[ July 15, 2003: Message edited by: Lien ]</p>