Why is this so hard to find help with? Can I reduce 120 ac to about 8 volts ac then to dc with a bridge rectifier that has maybe something like 1N5624 diodes and a series resistor in each branch of bridge to create the voltage drop and produce about 8 volts dc out before I use a switch mode dc-dc buck converter and potentiometer for the variable (0-5) volt DC out that I need. Please help. I do not want to use a bulky transformer. Thank you very much if someone is able to help.

 

When you have to step down a very high voltage through resistors all the energy is wasted as heat. The resistor will get very hot. Your solution is to use a switched mode power supply or a step-down transformer.

 

Why is this so hard to find help with?

maybe because its a lot safer to have a step down transformer. thats the way i'd be doing it, saves the possibility of being electrocuted.

 

OK so you want a full wave bridge and each diode in the bridge will have a series resistor. So each diode will have a voltage drop on alternate half cycles of what? Maybe around .7 volts maybe? What will the voltage drop be on each leg if I use 100 Ohm resistors? You tell me and how about 200 Ohm resistors? Tell me what the voltage drops will be because I don't know? The diodes you mention have a max forward current of 3 Amps so what wattage should my resistors be? When you get that worked out you will understand why you can't do what you are suggesting. What is your planned load with your "about 8 volts"?

Ron

 

Can I reduce 120 ac to about 8 volts ac then to dc with a bridge rectifier ......produce about 8 volts dc out before I use a switch mode dc-dc buck converter and potentiometer for the variable (0-5) volt DC out that I need. .

You will produce ~11vdc with a 8vac secondary using bridge and smoothing cap.
For 8vdc you would need ~6vac sec.
Max.

 

I do not want to use a bulky transformer.

Then use a switch mode power supply, SMPS, such as a laptop power brick or an AC-to-USB power adapter. The latter is almost free these days and you can easily get 5V at 2A. Should be plenty.

 

using resistors will be very dificult, the voltage drop will vary with load, low loads will have less drop, and high curent loads will have high voltage drops. use a transformer to drop the voltage.

 

OK so you want a full wave bridge and each diode in the bridge will have a series resistor. So each diode will have a voltage drop on alternate half cycles of what? Maybe around .7 volts maybe? What will the voltage drop be on each leg if I use 100 Ohm resistors? You tell me and how about 200 Ohm resistors? Tell me what the voltage drops will be because I don't know? The diodes you mention have a max forward current of 3 Amps so what wattage should my resistors be? When you get that worked out you will understand why you can't do what you are suggesting. What is your planned load with your "about 8 volts"?

OK so you want a full wave bridge and each diode in the bridge will have a series resistor. So each diode will have a voltage drop on alternate half cycles of what? Maybe around .7 volts maybe? What will the voltage drop be on each leg if I use 100 Ohm resistors? You tell me and how about 200 Ohm resistors? Tell me what the voltage drops will be because I don't know? The diodes you mention have a max forward current of 3 Amps so what wattage should my resistors be? When you get that worked out you will understand why you can't do what you are suggesting. What is your planned load with your "about 8 volts"?

Ron

Ron

Thanks for you initial comments, what I said in my post was that I want a bridge rectifier to convert to dc obviously but also drop the AC in from 120 to 8 - I was hoping to do this with a high wattage resistor (in each branch of the bridge if necessary) because I cannot fit a bulky transformer into the mix - my downstream load will be maximum 3 amps and variable load by way of potentimeter 0-5 volts dc. Hope you can help, and thanks again.

 

Then use a switch mode power supply, SMPS, such as a laptop power brick or an AC-to-USB power adapter. The latter is almost free these days and you can easily get 5V at 2A. Should be plenty.

That's great and already in the plan, however, I believe that the switch mode power supply cannot take 120 volts input which is what I must have because as I said, I cannot fit in a bulky transformer. Help please and thanks again.

 

I cannot fit a bulky transformer into the mix.

Can you fit 336 watts of cooling in?
You would need to buy resistors rated for about 700 watts. Are they bulkier than a 3 amp transformer?
Yes.

Why do you think a switch mode power supply can't use 120 volts?
Personally, I have 4 of them that can make 5 volts out of 120 VAC.

 

using resistors will be very dificult, the voltage drop will vary with load, low loads will have less drop, and high curent loads will have high voltage drops. use a transformer to drop the voltage.

I understand that but I CANNOT use a bulky transformer, I was hoping that since my required dc out is 1.5 - 5 volts, that there was a way to use high wattage resistors in the relevant branches of the bridge to get the voltage drop that I need and convert to dc at the same time - or can I get a solid state ac drop first to get to 8 - 10 volts then use the bridge and switch mode power supply circuit downstream before my poteniometer to get the required 1.5 - 5 volts dc out. Thanks for your interest and a solution hopefully.

 

Not on this website. Powering circuits directly from the power line without an isolation transformer is a forbidden topic.

 

Can you fit 336 watts of cooling in?
You would need to buy resistors rated for about 700 watts. Are they bulkier than a 3 amp transformer?
Yes.

Why do you think a switch mode power supply can't use 120 volts?
Personally, I have 4 of them that can make 5 volts out of 120 VAC.

That' interesting, are you saying that I can use a switch mode power supply with 120 volts ac in and drop that to 8-10 volts then convert to dc with a poteniometer as the load?

 

Not on this website. Powering circuits directly from the power line without an isolation transformer is a forbidden topic.

Not sure what you are saying, I intend to plug this thing into a 120 ac wall outlet but I have very small space restrictions, therego no room for a bulky step-down transformer

 

Yes. You only need to find one.
Get specific about the method for adjusting the final output voltage and then you will know exactly what voltage to look for. A linear regulator will need a couple of extra volts. I don't know what the headroom is on a switch mode regulator.

 

You will produce ~11vdc with a 8vac secondary using bridge and smoothing cap.
For 8vdc you would need ~6vac sec.
Max.

Please elaborate

 

Not sure what you are saying, I intend to plug this thing into a 120 ac wall outlet but I have very small space restrictions, therego no room for a bulky step-down transformer

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