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How to convert 120VAC to 90VDC @ .5amps? Answered

Hello, I'm looking for a circuit the will convert 120VAC to about 90 VDC at .5 amps. If anyone could suggest a schematic or point me in the right direction of one that would much appreciated!


Jack A Lopez

22 days ago

Well, I think the cheap-and-dirty way to do it would be start with a lamp dimmer, then feed the output from that to a simple rectifier circuit, e.g. bridge rectifier plus a big filter capacitor.

However, this cheap implementation has some disadvantages, including lack of isolation (i.e. some parts of your load might be at voltages close to the mains voltage, so you would not want to touch it). Also I*T/C ripple noise,


where I*T is the amount of charge moving in and out of the filter capacitor, each half cycle.

Like the formula suggests, this kind of noise is big when I, output current, is big, and the only way to make it smaller is to make C, in the denominator, bigger.

A better way, would probably be to just buy a professionally made DC power supply, whose output range includes 90 volts DC. These are somewhat rare, but they do exist. In fact I found a number of them on eBay using the search text,

"dc 0-120 adjustable power supply"

I am not sure why that particular range, i.e 0 to 120 volts DC, is popular. I also tried looking for the range 0-100 volts DC, but this search seemed to return fewer, and more expensive, results.

"dc 0-100 adjustable power supply"

Most of these seem to be "lab bench" style power supplies, which is good I suppose, since these are versatile. Typically with adjustable voltage AND current limits. (The actual mode it works in, either constant voltage or constant current, depends on the character of the load.) You know, you could potentially use it for other different projects... provided you don't break it.

By the way, I did not mention this at the beginning, but 90 volts DC, or adjustable output with a range that includes 90 VDC, is kind of rare, not so easy to find. You know, the DC voltages most people want for their gizmos, are typically much lower, like usually lower than 30 VDC.

Like for example, if you wanted 19 VDC, or 12 VDC, at comparable power levels (e.g. 50 watts) you could almost find handfuls of those in any thrift store, or dumpster, just for the trouble of reaching down to pick them up.

bigjohn412Jack A Lopez

Reply 22 days ago

The good thing about this circuit is I have a good +/-10v wiggle room to work with. And yeah I know it's an uncommon voltage in this era, where as landline phones for example use this high of voltage to ring to phone.

The reason I ask for a circuit instead of a bench supply is it's going into an invention of mine and I am trying to get it as small as possible (Even though it's going to be big with transformers anyway). I could look into a lamp dimmer and see what results that'll give me (never even thought of that).

Jack A Lopezbigjohn412

Reply 21 days ago

Well, if your load does not mind large ripple voltage, and by this I mean voltage that varies up and down every half cycle, then I think a lamp dimmer driving a full-wave rectifier, plus filer capacitor, will work.

Did I mention Vripple = I*T/C?

Well, supposing C = 470 uF, I=0.5 A, T=(1/120) s,

then Vripple = (0.5)*(1/120)/(470e-6) = 8.8652 V

So the size of the filter capacitor needed is not crazy huge, at 0.5 A, the amount of current you wanted.

Regarding circuit diagrams, the kind of power supplies that just do some rectifying, plus a filter capacitor, these are easy. Also easy are the kind that rely on a linear voltage regulator IC.

However the ones that use switching, also called switched mode power supply, or SMPS, those get complicated fast.

I was searching the web, trying to find someone publishing SMPS circuits, and also explaining them, but I have not found someone who does a good job of both. Although I found this person, at 320volt.com, who has published a large number of example SMPS circuits, here:


As I was saying before, I think it is preferable to buy SMPS circuits, rather than build them, provided you can find them cheap, and the usual place I look for cheap power converters, is the Chinese eBay sellers.

I did some more looking there, on eBay, and I might have found something thereI had not seen before, described as,

"DC/AC Motor Speed Driver Controller PWM Spindle Governor 15-160V/12-110V"


Also here,


22 days ago

A cheap alternative would be to use an adjustable transformer.
The chinese sell them on ebay as "voltage regulators" LOL
All you need as to add a rectifier, filtering capacitor and set the transformer so the output is what you need.
I got a 500W model for under 50 bucks on Fleabuy....


Reply 22 days ago

Issue with that is I need it as small as possible to fit into a box, that's the reason I asked for a circuit. This is the trouble I'm having trying to find a circuit that handles that high of voltage that I can build


Reply 22 days ago

There is always the chinese way of doing things...
These guys use a dead simple circuit for LED lights on mains power.
Deadly being the main part here ;)
You need a "resistor" to limit the power draw and voltage.
In China they use a capacitor and a standard resistor to do this.
On AV the capacitor can provide a high resistance and voltage drop - this can be calculated.
The resistor limits the actual current possible, like the 500mA you want.
Only problem with this is that your circuit hangs directly on mains power with no security or safety at all.
China gets away with it as all their stuff is exported and most of this to privates like on Ebay.
It works, it is simple but can be deadly.
And it only works with a load that does not change, like a fixed set of LED's.

Standard wall warts provide anything from 3 to 24V but that's about it.
90V at 0.5A still makes a whopping 45W.
So no matter what you try the power supply won't be tiny.