Question 1

I have a 14 vac input, but i'm getting 38 vdc output (no load). can anyone help me understand why?

Accepted Answer

That is a surprise, I have not run into that with mine. I think mine is used in a power supply I built using a 20 amp, 120 VARIAC (Auto Transformer) and I have both the AC and DC output and each have their own voltmeters and they track very close and accurately.  Is the Bridge fully connected into the circuit or do you just have the 14VAC connected to the AC input and then nothing on the DC output? Is the transformer primary single voltage or tappable for either 120 or 240 wiring configuration, and then are you using 120 or 240 input voltage. I assume you have checked everything, but assuming transformer primary is 120v and then are you center tapped 14 VAC out put or just one 14VAC output? If your Primary was 120 and you plug into 240, it I am sure will run hotter, but it being 2x of the 120, you would get 28 volts out. If the transformer is primary 120 volts, and you have a center tapped secondary coil, meaning you have 3 wires on the low voltage coil output, you might have one of two things 28 volts across the entire 2ndary coil and then 14 volts from center tap to either side of the other 2 secondary windings.   Just asking about the Bridge rectifier sadly does not help since we do not know what or how your transformer is wired or the type of circuit being powered after the bridge.  You can make voltage double/ tripler and x voltage multipliers out of just diodes and capacitors. Confirm your primary input voltage to the transformer assuming the primary is rated for 120V, make sure you have 120v going to it. Then not connected to the bridge rectifier measure the output of your transformer. if you only have 2 wires on it's secondary, confirm you are getting 14v from it, or what ever 2 wires of the secondary you are connecting to the bridge rectifier.  SO make sure the transformer by its self is providing the expected voltage.  Then again making sure you connect the transformer output to the ~ marked terminals connect them and then measure the AC on them terminals to make sure they are still the expected voltage. now without the circuit or filter capacitors, measure the DC from it. Are you still 14 or should be slightly less due to voltage drop across the diodes.  Now connect up your filter circuit,  capacitors / inductors and etc and measure the dc output before powering up  what ever circuit it will be used to power. With out an actual schematic of your circuit and very specific details on the transformer, hard to say how you are getting 38 volts DC out of a transformer that is suppose to output 14VAC.  All diodes do is block current flow in one direction and it passes it in the other. I don't know what your electrical or electronics skills are or education in that field. I have an associates in electronics, and build a great deal of power supplies, charging systems and things and never experienced that unless the transformer was not correctly marked or it has issues in the windings or wrong voltage applied to the primary. In simple terms a transformer works by electromagnetic induction and everything is based on coil winding rations between primary coil and secondary coil. There are alot of great electronic videos on Youtube, so if you are just new to D.Y.I. and working with electronics, check some of the videos out. Sorry I could not be of further help, but maybe helped you in testing the transformer, your issue could actually be the transformer with wrong markings or secondary coil wired wrong. Best of luck.

Question 2

Will this work converting my (post terminal) low voltage ac train transformer to dc?

Accepted Answer

this is a "bridge rectifier" and will change the AC sine wave from to full wave DC

Question 3

what is the forward voltage drop of each leg of the rectifier?  Is there a  link to a data sheet with specs?  thanks

Accepted Answer

I tried to google the MFG or for a spec sheet and no luck, see if the seller can provide one. else for the price and if it meets or exceeds  your ratings, buy it. It can be returned. I used it in a DC power supply that what voltage drop the diodes did have did not make any difference.  Googling the question of forward voltage drops across silicon geranium diodes: A Silicon Diode will have a voltage drop of approximately 0.7V across its leads.   Germanium Diode has a voltage drop of approximately 0.3V across its leads.  1amp or 100 amp diode I don't think the voltage drop will drift  that much.  I assume they are still using silicon & Germanium. More amps bigger chunk of silicon or Germanium is used.  typically building a DC power supply the AC input voltage will be just a littler higher that what you might want for the DC output and then after rectification and any slight voltage drop loss it will be closer to what you want for a DC voltage and or you then would have your voltage regulator circuit after the rectified to give you your regulated output.

Question 4

is this a full wave rectifier 120vac to 100vdc @what amp ac input to get 100adc output?

Accepted Answer

Yes.
I would recommend A current derating 85 AMPS up to 90 AMPS to be safe on the output DC load. You must provide a very good heat sinking for this rectifier bridge. You will lose 1.2 volts on the output. 100 Amps through a 1.2 Volts rectifier loss equals 120 WATTS in heat with this module under A full 100 AMPS load.

Manufacturer	Baomain Electric Co.,Ltd
Part number	MDQ-100A
Item Weight	159 g
Package Dimensions	10.9 x 8.1 x 3.8 cm; 159 g
Item model number	MDQ-100A
Item Package Quantity	1
Batteries included?	No
Batteries Required?	No

Baomain Bridge Rectifier Mdq-100A 100A 1600v Single Phase Full Wave Diode Module

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