psu problem.

[guestn6600]

Hi.
i made a psu for my friend's small fridge. the circuit is attatched.
the problem is when i checked the output with dmm, it shows 10 volts without load. and it showed 18 volts when i connect to the fridge.
the fridge didnt run cos protected by a overvoltage protection ckt.
another problem is when i try to power a peltier module for about 30 minutes both of the diode bridge getting warm. but the diode bridge is rated at 10 amps each and the peltier only draws 2.5 amps. any explainations?
the reason i use two transformer is 2 small transformer is cheaper than 1 big transformer.
any explaination would be appretiated. thanks

 

[akg]

drawing 2.5A can cause the bridge to get warm , but the no load and loaded voltage u specified is confusing??, what is the current rating of the Tfr??

 

[Glyph]

your circuit has no capacitors!

this should explain the strange meter readings:

http://www.kpsec.freeuk.com/powersup.htm

your fridge probably has a large input capacitor which is giving the appearance of higher voltage, really its just showing you the peak voltage.

 

[audioguru]

Your transformers are 12VAC with a full load. The rectifier bridge rectifies the peak voltage of 17V at full load and maybe 19.6V without a load and the diodes drop the peak voltage to 18V.
You don't have a filter cap so the DMM reads the average of the fluctuating voltage, about 10V without a load.
The fridge probably has a filter cap that charges to the 18V peak voltage from the rectifiers.

Your circuit has three problems:
1) Cheap transformers have poor voltage regulation, even if you parallel a million of them. A single powerful transformer has pretty good voltage regulation because its windings have a much lower resistance.
2) A 12VAC transformer produces about 15VDC at full load when using a bridge rectifier and a filter cap. To get 12VDC the transformer must be only 9.9VAC at full load.
3) The bridge rectifiers have 1.25A of current and about 2V across them when powering the peltier. That's 2.5W of heat for each. A 10A bridge is rated with its case cooled to 25 degrees C, sometimes cooled to 70 degrees C. At its rated current, its internal temp is so high it is almost melting. How huge is your heatsink for them?

 

[eblc1388]

2) A 12VAC transformer produces about 15VDC at full load when using a bridge rectifier and a filter cap. To get 12VDC the transformer must be only 9.9VAC at full load.

Only true at light load. At full load, one is lucky to get 12V DC.

Why do people make this same assumption of increased DC voltage at rated current again and again?

A 12VA, 12V rated transformer can provide 15V for 0.1A perhaps, it just cannot provide 15V at 1A full load. The power doesn't add up.

But I guess this is the norm in audio trade. :lol:

 

[audioguru]

Only true at light load. At full load, one is lucky to get 12V DC.

The peak is 17V but is squashed a little due to the massive momentary rectifier current. The average current is only 1.25A per transformer, so any half-decent rectifier bridge will have about a 1.8V drop. The result with a properly calculated filter capacitor is about 15VDC.
Maybe at 20A then a 12VAC transformer produces 12VDC. The transformer's output will resemble a square-wave and the rectifier bridge will be cooking with nearly 4V across it. Even the mains will resemble a square-wave.

Why do people make this same assumption of increased DC voltage at rated current again and again?

A 12VA, 12V rated transformer can provide 15V for 0.1A perhaps, it just cannot provide 15V at 1A full load. The power doesn't add up.

People forget to account for the power lost by heating the rectifier bridge.
A 12VAC/12VA transformer cannot produce 15VDC at 1A, it can produce 12/17= 705mA at 15VDC and the remaining power heats the rectifier bridge.

But I guess this is the norm in audio trade. :lol:

Been there, done that. At work, I was asked to test a well-known competitor's new pro-audio amplifier at full output. It smoked after only two minutes. Our amps could operate at full power all day long!
Our amps were over-built for reliability. Audio amps aren't operated at full power continuously, the rated power occurs only during the peaks of music or speech. Most of the time an amplifier averages at 10% of its rated power when it is barely clipping the peaks. Think about the thermal stress of its parts heating and cooling again and again. :lol:

 

[guestn6600]

Thanks alot for the reply..
i try putting two 4700uF and the fridge works!!!
and for the peltier i didnt put any heatsink in the diode because i think the diode can handle it easyly...(1 diode bridge is rated at 10 amps)
the transformer is 12 volts 5 amps each.
this forum really helpfull and fasttt...