45v out of a bridge rectifier, what AV voltage goes in?

(45 - 2 diodedrops)/sqrt2 and then a but more to compensate for losses.

so (45 - 1.6)/1.141256 + 2= 33Volt more or less.

Thank you.

-- "He who will not reason is a bigot; he who cannot is a fool; he who dares not is a slave." - Sir William Drummond

(2^^1.5/pi*)45

All of that is well and good.

However, is that going to be peak, average, rms or just what that the origional question wanted. Is there going to be a capacitor to smooth out the voltage, or is the full wave rectified DC going to be used ?

Lots of unasked questions in the origional question.

** The original Q is a pile of utterly ambiguous drivel.

If Q posters would simply reveal the PURPOSE of their question, ambiguity would disappear and useful answers become possible.

But no, they want to play at being smart and be in control the answers by NOT revealing any such damn thing.

.... Phil

And depending on how the transformer is spec'd (assuming you are running off the mains) you should probably go with a current of 3 amps.

I was fresh out of navy boot camp assigned to an electronics shop and had the place to myself after hours. I wanted some music and found a junked turntable and FM radio front end. I modified our junked cannibalized donor transceiver into a stereo amp. One night listening to music the transformer vented (sealed steel case potted in tar, military grade thing) it was impressive, lot of noise and smoke literally shot out of it. Turns out I didn't allow for the true current the transformer could supply. The nameplate gave the milliamps it could supply as an AC rated part, but when you convert to DC the amps has to drop proportionately. Many transformers are rated in Volt-Amperes for that reason.

Oh dear.

I am looking at Fig 3.6 on p 42 of Marston's /110 operational amplifier projects for the home constructor/. The circuit depicted is that of a power supply delivering 3-30V at 0-1A. It is to be supplied with "+40 to 45V (unregulated)". The text has nothing to say about where that comes from. I have decided to use a bridge rectifier attached to the secondary of a transformer with 240(ish)V primary; and I wish to know what secondary I need.

The "2amps" in my OP was a guess of mine that if the PS delivers 1amp, then 2amps in would be more than enough.

--
"He who will not reason is a bigot; 
he who cannot is a fool; 
he who dares not is a slave." 
  - Sir William Drummond

** Shit, somebody actually took notice of my complaint - never happened before. **But it is easy to infer that the author means a transformer isolated supply of rectified AC with adequate smoothing for one or more electro caps - with 1 amp DC capacity and 43 volts average output under load and nominal house supply voltage.

** The transformer will need to have 2 amps AC capacity and a rated secondary voltage of 32 volts AC. This will give about 47V DC off load dropping to 42 under full(1A)load. About 4700uF(63V) filter capacitance should be OK.

** But there was no way previously for us to know what the heck you were up to.

.... Phil

ty would disappear and useful answers become possible.

by NOT revealing any such damn thing.

A bit of an old post, but one that never got a fair answer. To get 40v out, you need 40x 0.707 ac input = 28.3v rms plus the diode dr op of the bridge which under load will be 2x 1-2v. If we guess 1.5v diode d rops under load then you'd need 31.3v in. That much has mostly been covered already, though a lot of EEs mistakenly think diodes drop 0.65v all the ti me.

However there are 2 other factors not so far addressed. First, the reservoir cap will charge to its peak v but then decay until rec harged. You need to supply a bit more V to counter the decay for 100th of a second. Second, there is mains voltage variation to consider. If you want the psu t o be reliable & meet mains voltage specs it needs to get enough V to the re gulator when V_mains is at the lowest it is permitted to go, which is a fai r bit under nominal voltage.

Finally you should only need a 1A supply, parasitic drain is trivial in any normal 1A V reg.

And really finally, check the reg can dissipate the power at full load when mains V is max, I_out is max & V_out is min. And ensure you have sound sho rt/overload protection.

NT

Thank you. Much obliged!

--------------------------

** I beg to difefr...

** In my post, I allowed 2V for that.

The formula I = C dv/dt covers it,

where I =1, C =4700exp-6 and dt = 0.008

** Designing a mains transformer, linear, regulated DC supply is a non-trivial exercise if done entirely on paper. Coming up with an accurate spec for the tranny being the hardest thing.

Secondary AC too low and the PSU will hum under low mains.

Secondary VAC too high and the regulator IC or BJT will cook with high mains.

Tranny VA too small and it will cook.

A few clever designers installed a pre-regulator using a Triac and feedback in front to ease the requirements on the poor tranny and heatsinking.

..... Phil

just means a bigger pass tr & heatsink are needed.

There are others ways to do it too, but for someone needing a 'how to' it's easier to stick to dissing the heat in the pass tran.

NT

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** A f****it's worthless solution.

** A f****it's worthless comment.

..... Phil

it's the standard solution for a linear reg.

that comment certainly is of no worth.