why is my full wave bridge overheating?

In the arrangement shown assuming DC current of 70 mA, the RMS current upstream of the filter capacitor is a lot more. Although I would decrease the value of the filter capacitor to 470 uF (to nearly half the RMS input current to make things easier on the transformer).

Let me go a bit further as best as I know...

70 mA over 1/120 of a second will discharge a 470 uFcapacitor by only about 1.24 volt, and a 100 uF one by only about 5.83 volts.

Assuming no resistance nor other reactance in the way to stretch out the input current pulses to the filter capacitor, *and* taking a first order approximation assuming that the filter capacitor has output load with no input for 1/120 of a second every half-cycle that is!

Continuing with same assumptions, a 470 uF capacitor sucks in current from 34-peak VAC (24V RMS) about 15.5 degrees out of 180, if I worked this out right! (ratio to 1 of cosine of 15.5 degrees is about [34-1.244]/34!) This means that assuming within-conduction-cycle-portion RMS-average ratio close to the 1.11 of a sinewave and RMS-average ratio throughout the cycle being the within-conduction-cycle-fragment times the square root of duty cycle of conduction at all... 34V peak 470 uF 120 Hz 70mADC means 262 mA RMS through the transformer secondary winding.

Repeat the calculation for 100 uF and 5.8 volt "ripple" - .07 amp over

1/120 second discharges a 100 uF cap by 5.8 volts over 34 degrees of a 120 Hz cycle. Correcting as I would do pulling straws from a hat for now) for time needed to recharge the filter cap from 180 degrees to 150 degrees 70 mA over 1/144 second (5/6 of 1/120) discharges a 100 uF cap from 34V by 4.86 volts - so I would expect a 34V peak rectifier output 120 Hz sinewave to a 100 uF cap with a 70 mA load to have peak-to-peak ripple of close to 5 volts! And with conduction 34 degrees out of 180, I would assume that the conducted current waveform during the conduction time fragments is close enough to half a sinewave with the half chopped at the peak from a sinusoidal half-cycle. During that conduction fragment, my assumptions predict RMS/average ratio of the usual 1.11. To account for conduction duty cycle within a 1/120 second half-cycle (assuming 60 Hz AC), divide this by the square root of the duty factor of the rectifier passing current from the transformer secondary. With 30 degrees, RMS current through a fullwave rectifier would then be about 2.7 timesthe DC load current. (Just be thankful that rectifier heating is determined more by the average current!) Transformer winding resistance will stretch out the curent conduction portion of each AC half-cycle and reduce the RMS current somewhat...

This RMS current value through transformer secondary winding will:

• Roughly, by first order approximation, vary directly with square root of load current from load current of above-assumed 70 mA. Doubling the load current from 70 to 140 mA will only increase the 189 mA RMS transformer secondary winding RMS current (assuming 100 uF main filter capacitor value) to 267 mA. I would say real-world guessing from a hat more like increase from 150-155 to about 230 mA with doubling of load curerent from 70 to 140 mA from 24V transformer. And that use of a 470 uF capacitor rather than a 100 uF one will close to or at least almost double (and assuming no resistance will worsen more rather than the real-world-usual-less compared to multiply by square root of 4.7) these RMS values, which I figure for now being indicating 70 mA from 470 uF to mean transformer secondary current even real-world likely exceeding .3 amp and theoretically maybe .38 amp or so.

• Roughly, by first order of approximation, vary nearly enough inversely with the square root of value of the filter capacitor.

• Have peak value close to 1.4-1.45 times the actual RMS value, while I have been predicting RMS current values easily enough to be seen as a bit on the alarmist side.

- Don Klipstein ( snipped-for-privacy@misty.com)

Who calculated what RMS currents? I calculated a lot more witheven 100 uF filter capacitor, maybe exceeding 300 mA with a 470 uF one, and worse still with the original 2200 uF one!

- Don Klipstein ( snipped-for-privacy@misty.com)

In message , dated Sun, 13 Aug 2006, slow_mac snipped-for-privacy@yahoo.com writes

0.001 uF +/-10% (coded as 'k')

10 0000 pF +80/-20% (coded as 'z')

10 00 pF +/-10% (coded as 'K')

10 000 pF +/-20% (coded as 'M'), made by KCK

10 0000 pF +/-10% (coded as 'k')

33 0 pF no tolerance given.

--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

The best poison for pigeons is to wrap small chunks of metallic sodium in fresh thin cut white bread - if they swallow it whole they burst shortly after takeoff!

That works with calcium carbide as well#

Two pieces of bread, with a couple of yards of fishing line, tied between them, thrown off the back of a ferry, will annoy two seaguls

martin

Your electrolytic is toast. And your regulator will be toast too if you dont fit it onto a heatsink. With the kind of current you are drawing the rectifier should not heat up at all , the regulator, however, will. As suggested by several people you should connect a 0.1u ceramic cap to the regulator between the "in" terminal and ground, keeping the leads as short as possible. You do need to scale down the input voltage quite a bit as it is too high for a 7805. A 12V primary should be more than adequate on the transformer.

I'm assuming you aren't using heatsink. The thermal resistance junction to case is 65 DegC/Watt. If you want to keep the junction temp below 150 Deg C, then the max power you can dissipate is (150-25)/65 = 1.92W (Assuming the local ambient temperature is 25 Deg C). Assuming the cap charges up to the peak, the filtered DC voltage will be approx 33V (give or take). The voltage across the regulator will be 33-5 = 28V. I = P/V, so the maximum load must be lass than

1.92/28 = 0.067A. Anything greater than this current will cause the junction temperature to exceed 150 Deg C. Regards, Jon

[...]

(newserver died on me again, so missed this).

I've just checked with the Spice. It's 115ma rms and +66ma average through the secondary, (with the OPs 2200uF). john.