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# smooth AC -> DC without transformer.

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#### Berserk87

##### New Member
I need a way to take 120VAC to under 80V DC, or alternating DC (going between 10V-70V)

i dont have much experience with AC stuff so just to double check...120VAC is 60V and -60V alternating in a sine wave.

im going over multiple things in my head and i just ended up confusing myself.

IF it is 60V alternating, i just need to know what cap to use to keep it from dropping below 10V at 180'degrees into the cycle (normally 0V) and ill just use a full wave bridge rectifier.

No, 120V is 120V RMS, each peak (positive and negative) is 1.414 times that figure, so +/-170V

Alternating DC?

There's no such thing, alternating DC, is AC.

bah!

kinda figured that, but was hoping i was wrong and got lucky
im a little 'off' right now since i didnt get any sleep last night.

Alternating DC?

There's no such thing, alternating DC, is AC.

you know what i mean !

the positive stays positive, and the negative stays negative, and the voltage is switching between two voltages like a square wave or saw tooth or triangle and all that.

You really need a transformer.

For 80VDC you need a 58V transformer plus a bridge rectifier and smoothing capacitor.

Use a 60V transformer or 30V transformer and a voltage doubler to get 80V.

how bout a bridge rectifier and a horribly wasteful regulator?

the 120V would be coming for a couple seconds a couple times a day.
im trying to figure out a kind of "surge protection".
maybe ill just go with a relay and a micro.

It depends on whether it needs to be regulated or not.

If no regulation is required then you can skip the horribly wasteful regulator.

It depends on whether it needs to be regulated or not.

If no regulation is required then you can skip the horribly wasteful regulator.

i would prefer to use a regulator after the rectifier, that way my device will continue functioning during the 'spike'.

without the regulator i would need a secondary power supply in order to turn the relay on/off for a few seconds.
im also concerned with the relay method, i would have to detect the spike and disconnect the switch before it damages the rest of the circuit, where as relays are slow.

edit:
another option.
a reset-able fuse that would function under normal conditions then "blow" with the 120VAC spike, then have a micro check the fuse every couple seconds, and if its 'blow' reset it.

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So what are you powering that needs 80 volts DC or so?

Voltage spikes are short transients and are normally attenuated by the RFI filter and filter capacitor.

How much current do you need?

Voltage spikes are short transients and are normally attenuated by the RFI filter and filter capacitor.

How much current do you need?

by the end of the circuit ill be using somewhere around 1A.

What's it powering?

Hi. Try a dimmer, diode and filter

120 VAC Phase~ -----dimmer ------------------|>|-------------A--------------- + output

120VAC Neutral -----------------------------------------------------B--------------- common

Connect an electrolytic capacitor > 250V between A and B
---|>|--- = 3A 400 PIV diode

The capacitor could be canibalized from a discarded PC power supply, and the diode perhaps is also in there waiting for you

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What's it powering?

im building a multi-mode device for charging various things and some led lighting if necessary.

but the 120VAC is not whats powering it.

also, i do have a couple old psu's i can 'cannibalize' for parts if i need to.

Hi. Try a dimmer, diode and filter

120 VAC Phase~ -----dimmer ------------------|>|-------------A--------------- + output

120VAC Neutral -----------------------------------------------------B--------------- common

for the dimmer, do you just mean a normal dimmer knob, so i can adjust it until i find the perfect spot?

I wouldn't recommend using a phase controller for controlling the voltage on the DC side of a rectifier.

The surge current will destroy the TRIAC pretty quickly and the non-sinusoidal current drawn is bound to 'confuse it' causing undesirable operation.

ive been searching for a voltage regulator that can handle somewhere around 120VAC but havnt been able to find one.

such a thing exist?

With out knowing exactly what your getting your power from since you stated the 120 volt Ac is not whats powering it and you have not given a specific regulated voltage range or current capacity we are still a bit confused as to what to recommend.

I need something to make my assorted stuff go. Got any suggestions?

With out knowing exactly what your getting your power from since you stated the 120 volt Ac is not whats powering it and you have not given a specific regulated voltage range or current capacity we are still a bit confused as to what to recommend.

I need something to make my assorted stuff go. Got any suggestions?

if i continue down the regulator approach, the 120VAC WILL be powering it for that couple of seconds, other than that the regular input will be 60V and somewhere around 100mA.

the current output for the regulator has to be lower than 70V.

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