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High power "lumpy DC" remover pumping out +4V

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Zappyguy111

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dc transformer 4v

Hi,

I am building a chiller for my marine tank and have run into a problem with the rectifyer. I have been intending is to run the circuit on 12V of power, what's been happening is that the rectifyer is generating 16V. I have checked the transformer, it is putting out 12V on both of it's outputs. They both read 12V prior to the rectifyer, but as soon as the power goes through the rectifyer, 4V is added to the power. I have bought a rectifyer from my local shop. The capacitor is a 1600uf (could be a lot bigger, I can't remember) at 25V electrolytic.

Thank you in advance for your assistance.

Zappyguy
 

When you measure the output of your transformer, you see the RMS voltage, that is equivalent to a peak voltage equal to Vp=1.41*Vrms. (sqr(2)=1.41)

12Vrms * 1.41 = 16.9Vp

16.9V minus the direct voltage of your rectifier will be near 16V.

Your circuit is working OK.

If you need 12Vdc, you can changue your transformer to a (12Vdc+0.7V)/1.41=9Vac if you use a single rectifier, or (12Vdc+1.4V)/1.41=9.5Vac if you use a full bridge rectifier.

Another solution is to use a 12V linear regulator like the 7812 attached to your actual circuit.
 

The above caculations are valid for a high (infinite) capacitor value respectively zero or low load only. The loaded voltage can be expected lower in a real design. Also load dependant transformer output voltage has to be considered in real life.

Generally, there's no easy stable and low ripple AC/DC supply design. If the load can tolerate the ripple, you simply can omit the reservoir capacitor.
 

Why do they use the RMS voltage instead of the peak voltage in AC?
 

Average power is more meaningful. Suggest you connect load, then measure voltage across it. You will probably find it drops a fair bit. Sounds like what you are doing is not going to be that voltage critical anyway. If you need exact 12v You could always buy a plug in switched mode supply, they are pretty cheap these days
 

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