Issues with a Full wave bridge rectifier

Thread Starter

ConstructionK88

Joined Jul 25, 2018
257
Ok. For safety sake i have experience with hv. I built a full wave bridge rectifier to run off of a 1:1 transformer using high value resistors i was able to bring the voltage down to a manageable 32v ac and on output of the diode bridge i get a perfectly wonderful 27.5vdc. however. when connecting anything to the output, nothing happens, no light lights up. no motor twirls. multimeter clearly displays 27.5vdc and 35v ac, but it doesnt seem to actually power anything. where am i going wrong?
 
The high value resistors are probably limiting your ability to supply power to a load.

The transformer turns ratio is usually used to change the voltage and still maintain a high power output capability.

Add a schematic of the supply if you want more input...
 

Thread Starter

ConstructionK88

Joined Jul 25, 2018
257
the diode bridge follows most common diagrams. and i was told the resistors were the cause. unfortunately i dont have easy access to a transformer to provide the voltage needed. 25v is ideal but i dont have one in pocket so to speak. is there any other way to reduce what is essentially 120vdc down to about that 25v?
 

dendad

Joined Feb 20, 2016
3,611
You need a lower voltage transformer.
If you feel up to it, you can rewind it to suit.
Pull your transformer apart, without damaging the laminations if you can.
Unwind the secondary, counting the turns. Then divide the number of turns by the voltage, and that will tell you how many turns per volt you need to rewind a new secondary.
If you cannot find heavier wire, use the old secondary wire, but folded over 4 or 5 times, and wind on the new secondary.
Reassemble the transformer and you should have working low voltage transformer.
Here is a video I found. I've not watched it all.
But it is a lot eaier to get another transformer.
 

vu2nan

Joined Sep 11, 2014
165
the diode bridge follows most common diagrams. and i was told the resistors were the cause. unfortunately i dont have easy access to a transformer to provide the voltage needed. 25v is ideal but i dont have one in pocket so to speak. is there any other way to reduce what is essentially 120vdc down to about that 25v?
What would you want to power with the DC supply? What would be the required DC voltage and current ?

- Nandu.
 
Last edited:

crutschow

Joined Mar 14, 2008
25,633
is there any other way to reduce what is essentially 120vdc down to about that 25v?
Actually a rectifier-capacitor will charge to near the peak value of the 120Vrms so will have an open-circuit voltage of about 1.4 * 120Vrms or 168Vdc, which is a lot of voltage to drop to get to 25Vdc.
A linear regulator would dissipate too much power for any but very small loads (its dissipation is (168V-25V) * Iload), so a switching regulator is a better choice.
But finding one that goes from 168Vdc to 25Vdc may be difficult.
 

Thread Starter

ConstructionK88

Joined Jul 25, 2018
257
Thanks guys! I didnt have the math on hand, i was simply using a hypothetical guestimate. I certainly do have and know how to rewind the transformer. it just the ones i have are rather heavy. from metal halide lamps. they weight about 10lbs. soso heavy. i was just looking for a more compact way to do it. Its to power a coleman air mattress fan that runs on a standard 12v car plug. it works just fine on a 12v adapter but not much power. a direct line to a 12v car battery has to much power. i was looking for a nice middle ground that can use my lm317s to adjust so about 25 to 30v would have been nice. i tried using CAPs to knock off some of the voltage. worked great to dim a nightlight. did nothing when rectified to dc. im guessing the caps couldnt keep up with the drain? I suspect ill just have to carry around a heavy A-word power supply :p
 

dendad

Joined Feb 20, 2016
3,611
I made a reasonable power supply from a battery charger. It had enough room between the windings and the core to add another layer of windings to increase the ACV.
You should be able to pick up a good power supply from second hand or junk stores.
A good one to use is an old 19V laptop power brick, with an external switch mode buck converter.
 

Thread Starter

ConstructionK88

Joined Jul 25, 2018
257
I suspect ill just rewind one of my cores and suck it up for now. I did have several laptop supplies and a few 48v printer supplies but they seem to just pulse the output. Odd but moot. But to be sure, is a full bridge rectifier the best way to conver the ac for my needs or is some other way recommended? I have quite a few HV high watt diodes so im not terribly concerned about what i may thow at them. I choose to build my own power supply simply because prebuilt supplies over heat in this environment, burn out, ect. Ive burned out over 5 already
 

Thread Starter

ConstructionK88

Joined Jul 25, 2018
257
thank you sir. i have reverse powered a transformer i had on hand and outputs various voltages. oddly the lowest voltage rectified powers the fan much more aggressively. output is 25, 35, 41, and 47. (there are 4 inputs that are now outputs)
 

RPLaJeunesse

Joined Jul 29, 2018
123
You state "a direct line to a 12v car battery has to much power." That tells me the problem is not voltage, it is that everything except the car battery doesn't have enough current capability. It would not surprise me if the motor took 6 to 8 amps from the battery, or roughly 100W. Measure the motor current while on the car battery, measure the battery voltage, and figure out the wattage from those two values. Any supply you build should be capable of that wattage number, plus 10-15% for safety, and at NO MORE than the measured battery voltage or you risk seeing some magic smoke. One thought: find a dead microwave oven, extract the transformer, lift the end of the secondary that is connected to the chassis (thus isolating the secondary). Feed your line voltage into the secondary and measure what comes out the primary. It might be close enough to what you need. Add a full-wave bridge rectifier and see what power you can get.
 

Thread Starter

ConstructionK88

Joined Jul 25, 2018
257
Ive rewired a halide lamp transformer yesterday and it does exactly as i needed it to. 11.7v rectified @ 6.8amps. but i used solid core speaker wire and i have 2 outputs. trying to firgure out what i can do with the other and how to use my lm317s to control the amperage hahaha. gonna take some research.
 
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