Issues with a full wave bridge rectifier.

Ok. For safety sake i have experience with hv. I built a full wave bridge rectifier to run directly off of a 110v ac main. 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 but it doesnt seem to actually power anything. where am i going wrong?

The voltage dropped by a resistor depends on the current flowing through the resistor:

V = I × R

If you measure with a meter, current is very small. The resistor that drops 100 volts at a light load will drop more voltage as the load increases.

You need a transformer to drop the voltage down to the level you want (much explanation left out). A FAR safer method would be to use a used laptop supply (usually 19.5 volts) or to buy a power supply of the desired output voltage.

thanks for the reply. So to some extent im guessings its not really possible to even produce a usable dc directly from a 110v main?

It's absolutely possible. The picture here shows a basic regulated power supply.

The transformer drops the voltage down the the range you want. The bridge rectifier changes the AC waveform to pulsating DC. The filter capacitor smooths the pulsating DC to (approximately) a steady voltage.

On the right side of the diagram, a voltage regulator provides a regulated DC voltage.

that i understand easily. my issue is obtaining a suitable transformer. realistically any transformer i use is simply adding extra components that can fail. I FULLY EXPECT THIS TO EVENTUALLY FAIL. it will be in a fairly harsh environment. hence a transformerless supply. since any component that fails can easily be replaced. no so much a transformer for on the spot repairs.

Transformerless supplies are dangerous and can be deadly. Perhaps replacing a transformer is easier than replacing you??

Transformerless are suitable only for applications that provide a constant load AND ALL PARTS OF THE CIRCUIT are isolated from contact with living beings.

it would indeed be fully isolated from me. its to run a small air pump but a regular 12v supply is to little and its intended 12v car battery is to strong. so i was looking for a 25 to 30v dc to use with lm317s to give me a nice middle ground to work with. I have transformers but they are heavy and require rewraping.

How much current does the pump draw?

Constructionk88 said:

its to run a small air pump but a regular 12v supply is to little

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Transformerless supplies are sometimes used in things that have no external "user" connections, like plug-in remote controlled power sockets.

They are normally used for very low power devices.

For one to work, the impedance of the "voltage dropping" components must be matched to the maximum load current, and there must be a load to always take that current, or the output voltage will rise to the same as the input voltage.

With a typical tiny bit of electronics, the permanent load can just be a small zener diode, to take the excess current.

Your 12V air compressor probably takes 5 - 10A.

That means the stabilising "load" would have to take more that 10A to prevent the voltage rising above the 12V range.
And the series impedance has to be such that 10A is passed through the rectifier to the load all the time.

The 120W shunt regulator will cost at least $10 and run very hot, needing a lot of cooling.

The total series impedance to pass 10A from 110V will be around 10 ohms. Doing that with a resistor would dissipate another 1000 Watts as heat; basically an 1KW electric heater element.

Using a capacitor in place of a resistor, you would need a 270uF 250V AC rated one.
Something like this:

As you would be using a bridge rec, you should actually use four of those to keep it a balanced circuit, so two in parallel connected from each AC input to the bridge rec.


You would end up with something massive, weighing possibly around 10 - 20lb, costing at least a couple of hundred dollars, that wastes large amounts of power and heat, & is also lethally dangerous.

Or you just buy a proper transformer, or pay about $20 for a complete, ready built, 12V DC power unit.

In other words, "transformerless" power supplies are totally impractical for anything other than trivial loads.

Constructionk88 said:

that i understand easily. my issue is obtaining a suitable transformer. realistically any transformer i use is simply adding extra components that can fail. I FULLY EXPECT THIS TO EVENTUALLY FAIL. it will be in a fairly harsh environment. hence a transformerless supply. since any component that fails can easily be replaced. no so much a transformer for on the spot repairs.

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Transformerless power supplies are crap, unreliable, will almost certainly fail far sooner and more often than a proper transformer, and limited to only very low current.

It's a BAD idea, abandon it! - do the job properly.

Thanks for the input guys. I have several okd cores i can rewind for an isolation/reducing transformer. They are just simply quite heavy. 15lbs at least. I had only hoped a transformerless would have been much lighter but after 10+hours of tinkering, yall are correct. Its useless. As for the drain, im not sure. Its not so much a pump as a fan. For blowing up air mattresses and pools. Its the perfect size for my needs and i have plenty of dc motors to replace the once inside should it inevitably burn out. So i propose this question: should i go with a full wave bridge rectifier or is there another preferred way to do it?

Constructionk88 said:

Thanks for the input guys. I have several okd cores i can rewind for an isolation/reducing transformer. They are just simply quite heavy. 15lbs at least. I had only hoped a transformerless would have been much lighter but after 10+hours of tinkering, yall are correct. Its useless. As for the drain, im not sure. Its not so much a pump as a fan. For blowing up air mattresses and pools. Its the perfect size for my needs and i have plenty of dc motors to replace the once inside should it inevitably burn out. So i propose this question: should i go with a full wave bridge rectifier or is there another preferred way to do it?

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You seem a bit confused?, a 'bridge rectifier' and a 'full wave rectifier' aren't the same thing. A bridge is ONE TYPE of full wave rectifier, and is just called a 'bridge rectifier' - a 'full wave rectifier' would normally refer to a twin diode one, along with a dual winding secondary.

Your question is also fairly meaningless, yes you should go for a full wave rectifier, of either type depending on what transformer you use (either single or dual winding) - or if you use a switch-mode design, you just need a single diode, as full wave isn't required (but that's a MUCH more complicated design).

Your earlier comment makes no sense either.

a regular 12v supply is to little and its intended 12v car battery is to strong

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A correctly specified 'regular' 12V supply would be fine, as would a 12V car battery - why would you imagine it would be 'too strong'?.

First though you need to find out what you actually need, and a good way to do that would be to use a car battery to power it, and monitor the current it takes - assuming it's less than 10A you should be OK with most multimeters, as they normally have at least a 10A range.

rjenkinsgb said:

Using a capacitor in place of a resistor, you would need a 270uF 250V AC rated one.
Something like this:

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Are you sure about that? I've always been told a start capacitor was an electrolytic and should never be used on AC, other than the very short time to start the motor by phase shifting the start winding.
https://en.wikipedia.org/wiki/Motor_capacitor A run cap may work though.

shortbus= said:

Are you sure about that? I've always been told a start capacitor was an electrolytic and should never be used on AC, other than the very short time to start the motor by phase shifting the start winding.
https://en.wikipedia.org/wiki/Motor_capacitor A run cap may work though.

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Regardless, it's a very poor and unsafe way of trying to do it

My apologies for the confusing questions. Been a bit busy with to much and was responding in a rush. I did try using caps with resistors to build a capacitive potential divider? It worked just fine to dim a bulb or run an ac motor slowly but did nothing when i attemped to rectify to dc. Multimeter showed dc output but didnt run even a small dc motor. Dangers aside i dont plan to try doing it this way so ill rewind a transform. But curious me wants to know if a core can be built or is it best to just rewrap an existing core?

Constructionk88 said:

My apologies for the confusing questions. Been a bit busy with to much and was responding in a rush. I did try using caps with resistors to build a capacitive potential divider? It worked just fine to dim a bulb or run an ac motor slowly but did nothing when i attemped to rectify to dc. Multimeter showed dc output but didnt run even a small dc motor. Dangers aside i dont plan to try doing it this way so ill rewind a transform. But curious me wants to know if a core can be built or is it best to just rewrap an existing core?

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Why not just buy a transformer? - you talk about reliability, and then plan reducing it by rewinding an old transformer.

But I refer you to post #12, you need to find out what power you require for it first before you either rewind, or buy one.

Nothing is written on the casing of the fan or serial information other than the sn of the model but has no results online other than typical use and price. Granted its designed to run directly off of a 12v x*-amp car battery but theres no solid information on the motor other than dont run longer than 10 minutes. Ive used a: 12v 1.5 and 2 amp adapter and it runs meh. Sure itll blow air but nothing usable. Ive also used a 19.5v 1amp adapter and ran slightly better. Along with a 6v 1amp and it was the same as the 12s. A car battery will run it to the point of a leaf blower but as i stated thats just to strong. I can likely be wrong but it seems a higher voltage and a modest 1 to 2 amps seems great. Just the need for the greater voltage has put me outside of what im able to buy at the moment.

Nigel Goodwin said:

But I refer you to post #12, you need to find out what power you require for it first before you either rewind, or buy one.

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Better yet for reliability, buy a commercial power supply or find a nice 12 volt wall-wart switcher at a thrift store for a couple bucks.

Constructionk88 said:

higher voltage and a modest 1 to 2 amps seems great.

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What is actually happening is that the motor is overloading the low-current supply and pulling the voltage down well below 12V, so the motor runs slower.

The motor speed at any given load is directly proportional to the voltage across it; lower speed = lower voltage.

Constructionk88 said:

Nothing is written on the casing of the fan or serial information other than the sn of the model but has no results online other than typical use and price. Granted its designed to run directly off of a 12v x*-amp car battery but theres no solid information on the motor other than dont run longer than 10 minutes. Ive used a: 12v 1.5 and 2 amp adapter and it runs meh. Sure itll blow air but nothing usable. Ive also used a 19.5v 1amp adapter and ran slightly better. Along with a 6v 1amp and it was the same as the 12s. A car battery will run it to the point of a leaf blower but as i stated thats just to strong. I can likely be wrong but it seems a higher voltage and a modest 1 to 2 amps seems great. Just the need for the greater voltage has put me outside of what im able to buy at the moment.

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No - you don't want greater voltage - if a 12V car battery makes it too powerful you need LESS than 12V.

Don't you have a multimeter?, you need to measure the current it takes from a 12V battery - then lowering the voltage will lower the current as well (ohms law).

The ONLY reason you thought it was 'better' off the higher voltage supply was that it was seriously overloaded (which will make it VERY unreliable) and the voltage will have been dropping down to considerably less than 12V.

As common with many newcomers, you probably don't understand the relationship between volts and amps on a power supply (battery or anything else).

You mention 12V 1.5A and 2A adaptors, and that it ran 'meh' - that means the motor needs MORE then 2A at 12V.

Essentially the current rating isn't of consequence, the load (the pump in this case) will take what it needs - if it needs 5A, and you have a 10A supply, then the motor will draw it's 5A and the rest is spare unused capacity, this means the power supply runs cooler, is under less strain, and will be far more reliable.

Likewise, if you only have a 2A supply, the motor will still try and take 5A - the supply can't do this, so it's seriously overloaded, the voltage will drop massively as a result, and the power supply will soon die.

Think of it as a car - if you've got the correct power supply it's like been sat on the motorway tootling along at 70 miles an hour in 5th gear, everything is smooth and happy. If you've got too small a power supply then it's as if you're only in second gear, thrashing along at 70 on the motorway - how long do you think the engine will last?.