basic bridge rectifier question.. ac on output

[wicked1]

I built a bridge w/ 4 MUR diodes, and am reading a lot of AC on the output.. Input is 18/0 ouput is 25v DC and 52v AC.
This is w/ no load.

Is this ok? Maybe things will change when its got a load? My scope is in storage (remodeling basement) so all ive got right now is my multi meter.

Thanks!

-also, I know this is a very basic question, but having no ability to search for 2 letters is really crippling! so much of what we do uses 2 letter acronyms!!!

[richie00boy]

Without a cap for smoothing you will just get very bumpy DC...

[wicked1]

I did go through caps.. CRC 2x 22000uf caps. Get the same readings with and w/out caps. But, i don't know how small of a ripple will show up as AC on my meter, and dont have my scope to see it.

Hmm.. just checked it again, and i only read AC w/ the probes hooked up red to red, black to black... so its not really AC. Reverse the probes, and 0....
ripple it is.

[gain-wire]

hi,
this might seem silly, but what is the multimeter you are using? I have seen some weird reactions like this before. I canT' remember exactly what it was, but something similar.

Without seeing it on the oscilloscope it's kind of hard to tell.

also, I think you should put a load on there, especially with that much capacitance. Put a 2,2kOhm resistor 5W or maybe a 22kOhm 2w, or just make something up but defintiely use some load, I think that might be the source of the problem. Your circuit has nowhere to discharge itself but in the meter. Which as you know, its input is very high impedance.

O.

[TechGuy]

Originally Posted by wicked1

I did go through caps.. CRC 2x 22000uf caps. Get the same readings with and w/out caps. But, i don't know how small of a ripple will show up as AC on my meter, and dont have my scope to see it.

Hmm.. just checked it again, and i only read AC w/ the probes hooked up red to red, black to black... so its not really AC. Reverse the probes, and 0....
ripple it is.

Try using an output inductor between the bridge and the inductor. An Output inductor is required to smooth the AC rectified ripple.

if you have really high ripple, I suspect that you may also have some aging caps that are performing below there spec. As electrolytic caps age the ESR (Equivent Series Resistance) increases. If they been sitting for a while the also need to be regenerated as the oxide layer degenerates over time without use. A good set of filter caps should show a voltage output of 1.414 times the (AC voltage reading without the caps attached.) If the output voltage is significantly lower, they you make have a problem with your caps. Also measure the capacitance value using your multimeter. if the value is 10% below the value marked on the caps, they are probably suspect.

Also note that very big caps with a very high voltage rating usually have poor ESR, and significant amount inductance that degrades performance. FWIW, bigger caps doesn't necessarily means it better. You may be able rectify the problem by connected a smaller cap (less than 10uf) that has a voltage rating close to the output voltage. Small electrolytic or even using ceramic MLCCs can help get rid of ripple. If the output voltage is low (less than 35V),tantalum caps are a good option since they have very low ESR. Caps with low ESR and low inductance reduce ripple the best.

Assuming your using polar caps with a big black marking pointing to the Negative terminal, you did connect the neg cap terminal to, the negative side of the bridge correct? (sorry if this is condensending, but it happens!)

[megajocke]

Originally Posted by wicked1

Input is 18/0 ouput is 25v DC and 52v AC.

Those cheap multimeters show nonsense if connected to a non sinusoidal signal on the AC range.

They just half-wave rectify the voltage and show the average of this multiplied by a factor to compensate.

1V AC half wave rectified has an average of 1*sqrt(2)/pi V making the multiplicator pi / sqrt(2)

25V * pi / sqrt(2) = 56V, but second-order-effects like diode drop was disregarded so this is where your 52V display comes from.

[ChrisA]

Your big filter cap has ESR. Your meter might by reading the peak to peak switching transient and then getting fooled to thing the uSecond long tranient is an AC sin wave.

This might searve as a reminder to place some small caps in parallel with the big one if not already done, Also a bleader resistor to act as a load and drain the big cap when the power is off.