# AC DC Power Supply

Discussion in 'The Projects Forum' started by yoamocuy, Jan 5, 2011.

1. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
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I'm working on a simple AC DC power supply, but am running into a bit of a problem in regards to the transformer. I know that I am going to have a 1.4V drop across the diodes in a bridge rectifier so I should need a secondary voltage of this plus my desired output voltage all divided by 1.414. I've realized that getting the right output voltage is not as simple as getting the right turns ratio according to this formula and then using a capacitor resistor combination to smooth out my output into a output with a small ripple. In saying that, my problem is that I know my load resistor is going to affect this secondary voltage but I'm not exactly sure how the two relate. After doing some research it seems that it will be different for each transformer based on the load regulation. How do I know what load resistance I need in order to get the desired output voltage if it appears to be different for each type of transformer?

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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1,125
That is why voltage regulation was one of the first subjects covered in electronics school. You need one everywhere you have AC/DC transformer rectifiers circuits.

3. ### wayneh Expert

Sep 9, 2010
15,266
5,586
Power supply modeling can be a challenge. For instance even the diode drop isn't fixed, but will vary with current and temperature. RMS voltage after the cap will depend on conduction phase, coil inductance and impedance, and so on.

But perhaps you can share what you're trying to do? That may be much easier to answer.

4. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
I'm just trying to build a 15V power supply with an output of 1A and only a small amount of ripple. I don't think getting the output current will be a problem once I'm able to get the voltage output, and the ripple will be easy to reduce. Yea, I have realized that a regulator would work but can't you do it without a regulator as well?

5. ### wayneh Expert

Sep 9, 2010
15,266
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Big cap = small ripple

The cap needs to supply 1A for the non-conducting phase of the rectifier, about 1/120 seconds. The voltage drop - ripple - is estimated from first order discharge of the cap.

6. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
1,125
NO not if you want the 'ripple' to be 'easy' to reduce.

the current from the circuit MUST be regulated in some way or form or fashion. If not, the voltage will have whatever AC signal is produced by the load impressed on it.

Something as simple as a high wattage voltage divider which draws more current than the circuit can serve to regulate the output voltage level.

SOMETHING has to have a constant, steady, smoothing effect on the supply OTHER than the load for it to remain a smooth DC level.

If you just want to run LED lighting then you are golden without the regulator, not wise, but good to go.

If the load in not a steady DC amp load, and instead varies its current demands, then the DC output level of the transformer will also vary.

Careful selection of resistor values and transformer specs can reduce that variation to fairly low levels, but it you want DC that does not make a multimeter display flutter in the last digit, then you NEED a regulator.

Dec 26, 2010
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Whether or not this is practicable depends on how much ripple (and average voltage variation) you are willing to tolerate. Obtaining a small ripple voltage is particularly difficult without using a regulator. For example, with a reservoir capacitor of 10,000uF and a load current of 1A, with full-wave rectification one would expect perhaps 600mV pk-pk of ripple, depending on the mains frequency and the rectifier conduction angle.

Clearly further filtering can be added but the result is likely to be bulky and uneconomic. Better to use a regulator. Nowadays, a switched-mode power supply may be preferred.

8. ### #12 Expert

Nov 30, 2010
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9,683
Another problem is the voltage deliver to you by the power company. I am in Florida and I get 125 RMS at my house. That is so high that the heating element in my 1970's clothes dryer burns out in only a few hours! (I replaced it with a Calrod. Now it survives.) What is the voltage at your house? Will it ever change? It will affect the voltage you get from any transformer.

9. ### yoamocuy Thread Starter Active Member

Oct 7, 2009
84
0
Hm ok thanks for the insights. I get 120V AC in my house. I guess I'll look at some spec sheets for 15 V regulators and then find a transformer that will step it down enough for my regulator. Thanks again.

10. ### wayneh Expert

Sep 9, 2010
15,266
5,586
You didn't specify what you mean by "small" residual ripple, and it clearly means different things to different people. It would also help to elaborate on what you're trying to power. The solution may be easier than you think but no one can really help without more detail.