I am currently in the process of building a project that requires a 30 volt DC power supply. I just purchased a toroidal power transformer that is rated at 24-26 VAC secondary voltage. I found several bridge rectifiers in my parts bin marked Motorola MDA970-2. So I connected the output of my transformer into this bridge rectifier with the following result:
Primary Voltage: 124 VAC
Secondary Voltage: 27 VAC
DC output: 24.2 VDC
This made no sense to me, so I replicated the same circuit with the other bridge rectifiers I had on hand and got the same result.
Next, I built up a little bridge rectifier using 1N5408 diodes and repeated the experiment. This time I got 32 VDC output from the bridge circuit. Yes, this gives me what I need because I will be feeding this into a LM317 regulator, but I expected a much higher DC output - more like 37-38 VDC.
Can anyone help me figure out what is going on here?
P.S. I checked the voltages with two different DMMs and they were within 1% of each other.
Extech EX330
Amprobe AM220

 

A copy of your diagram would be helpful. And you said nothing about using a smoothing capacitor. AC gives you an RMS voltage, which it sounds like you are aware of. But when you change it to pulsating DC (no filtering capacitor) it's still going to give you somewhere around the RMS voltage of the secondary.

Try putting a filtering (or smoothing) capacitor on the output of the bridge and see if the voltage doesn't go up.

 

Did you have a filter capacitor at the bridge output?
That's needed to get the peak voltage of the sinewave you expected.
Without the cap you will measure the average value of the sinewave at the output of the bridge.

Edit: Tony beat me by a minute.

 

I am currently in the process of building a project that requires a 30 volt DC power supply. I just purchased a toroidal power transformer that is rated at 24-26 VAC secondary voltage. I found several bridge rectifiers in my parts bin marked Motorola MDA970-2. So I connected the output of my transformer into this bridge rectifier with the following result:
Primary Voltage: 124 VAC
Secondary Voltage: 27 VAC
DC output: 24.2 VDC
This made no sense to me, so I replicated the same circuit with the other bridge rectifiers I had on hand and got the same result.
Next, I built up a little bridge rectifier using 1N5408 diodes and repeated the experiment. This time I got 32 VDC output from the bridge circuit. Yes, this gives me what I need because I will be feeding this into a LM317 regulator, but I expected a much higher DC output - more like 37-38 VDC.
Can anyone help me figure out what is going on here?
P.S. I checked the voltages with two different DMMs and they were within 1% of each other.
Extech EX330
Amprobe AM220

First, you need to check the connections. It seems like you have a bad/incorrect connection when using the bridge module.

Secondly: the voltage directly out of a bridge will not be the peak voltage (37V) unless you have a filter capacitor.
Lastly, just because a filter cap shows an output voltage (no load) of 37v, doesn't mean your output voltage will be the same with a load attached. It will likely drop down to the RMS voltage coming out of the transformer if you are approaching the rated current of the transformer.

Edit: my wife interrupt me - two people beat me.

 

The bridge is not connected correctly. I'd point it out on your schematic...

 

Depending on your BR (bridge rectifier) you're going to have a loss of about 1.4 volts (you're going through 2 diodes at all times). Using 24 volts RMS, multiplying by 1.414 (the square root of 2) you should see 33.94 volts peak. 26 volts X 1.414 = 36.76 volts. Take away 1.4 volts from the BR and your 33.94 becomes 32.5 volts. So if your secondary is giving you 24 volts and your goal is a steady 30 volts, as long as you're not draining current faster than the torrid transformer can put it back in - going through a LM317 regulator, you're coming pretty close to your 30 volts.

Now, I'm no expert on regulators, but I think they require a fair bit more voltage going into them than they have coming out of them. You'll have to consult the data sheet to see how much more input voltage you need to get your output voltage to be stable. All things to be considered.

Just saying.

Oh, and a picture of your circuit would help us understand the problem you're facing better, and thus, give us a better chance of helping you find a solution.

 

Hello,

The average DC output from a transformer with bridge rectifier with no filter capacitor is close to this:
Vdc=0.9*(Vrms-1)

so for 27vrms we get Vdc=23.4 volts.

The average DC output with a capacitor and no load however would be more like this:
Vdc=1.4*(Vrms-1)

Now the ratio of these two is about 1.5, so it is possible that you used a capacitor with one measurement and not with the others.
Of course an incorrect connection for one circuit or the other could cause a discrepancy as well.

 

Forgive my oversight; you said you had an unloaded secondary output of 27 volts AC. After rectification I'd expect to see (accounting for losses in the BR) 36.78 volts. That should be enough to drive your LM317.

On the internet there are many different LM317's out there. Do you have an LM317 you're planning on using? If so, there should be more nomenclature beyond the LM317. For instance: LM317LZX, which is a TO-92 package capable of 100 mA. I'm guessing you probably want more current than that. Some packages are in SMT as well. So if you can share with us (again) your circuit, your project, your goal and what you have on hand I'm sure someone here can help you find a plausible (if not good) answer to assist in your endeavor.

[edit] Mr Al, I'm not sure I follow your math. Can you explain how you come to your conclusions? Thanks - appreciated.

 

I am currently in the process of building a project that requires a 30 volt DC power supply. I just purchased a toroidal power transformer that is rated at 24-26 VAC secondary voltage. I found several bridge rectifiers in my parts bin marked Motorola MDA970-2. So I connected the output of my transformer into this bridge rectifier with the following result:
Primary Voltage: 124 VAC
Secondary Voltage: 27 VAC
DC output: 24.2 VDC
This made no sense to me, so I replicated the same circuit with the other bridge rectifiers I had on hand and got the same result.
Next, I built up a little bridge rectifier using 1N5408 diodes and repeated the experiment. This time I got 32 VDC output from the bridge circuit. Yes, this gives me what I need because I will be feeding this into a LM317 regulator, but I expected a much higher DC output - more like 37-38 VDC.
Can anyone help me figure out what is going on here?
P.S. I checked the voltages with two different DMMs and they were within 1% of each other.
Extech EX330
Amprobe AM220

Thanks to all who replied. Yes, in my haste to check out the new transformer I omitted the smoothing capacitor from the circuit. After adding a 6800 uF smoothing capacitor here are the results:
With the MDA970-2 bridge - 38.5 VDC
With the 1N5408 bridge - 37.6 VDC
Not sure why the 1 volt discrepancy but I am a very happy camper!

 

Edit: my wife interrupt me - two people beat me.

Better the the vice versa, I suppose?

 

With the MDA970-2 bridge - 38.5 VDC
With the 1N5408 bridge - 37.6 VDC
Not sure why the 1 volt discrepancy

It's likely the MDA970-2 BR is a better rectifier, less loss through the PN Junctions.