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Chris Cristini

Sun Mar 28 2010, 07:13PM

Registered Member #1749 Joined: Fri Oct 10 2008, 02:04AM
Location: Claremont New Hampshire
Posts: 497

How do I get smooth DC like a computer PSU from a 60 HZ transformer?

dmg

Sun Mar 28 2010, 08:07PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

A fullwave rectifier with a large smoothening capacitor feeding a voltage regulator with another capacitor afterwards?

I do a bit overkill, but I use capacitors for smoothening that are in the hundred thousand microfarad range,
but maybe smoothening capacitors (capacitance depends on the transformer's output) before and after a voltage regulator will yield the desired smooth DC,
also, how about putting a choke between the first smoothening capacitor and the voltage regulator, and then the final capacitor will take care of any residual ripple.
although, this method is rather inefficient in compareson to a Switch mode power supply like in a computer,
As SMPS is almost twice more efficient then this method.
hope that helps.

Mattski

Mon Mar 29 2010, 08:37PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Wikipedia talks a bit about the ripple when you have the simple full wave rectifier and smoothing capacitor. As you make the capacitor bigger the ripple decreases, but ripple increases as you increase load current.

If you add a voltage regulator after the smoothing capacitor, and the minimum voltage after the smoothing capacitor (determined by the ripple) is at least about 2V above the output voltage (less for low dropout regulators), then you can get very low ripple. It "burns off" the excess voltage which wastes power but gets you a stable voltage. Look at datasheets or application notes for LM341, LM7805, and similar regulators.

But these days it's more common to use switching regulators since they are more efficient.

Myke

Mon Mar 29 2010, 10:02PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

In some old tube circuits, they use a filter cap to ground, then put another to ground in series with a resistor (looks like pi). Some of the filter networks use a fairly large inductor.
When building a tube circuit, I couldn't get the ripple on the plate to disappear so I looked at what other people used for filtering. I added another resistor and cap and the ripple disappeared. You might want to try it out?

Mattski

Mon Mar 29 2010, 11:23PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Putting a cap to ground, then a small resistor in series with the power supply, then another cap to ground which is near the device power input is a technique that can be used to increase isolation between circuits. Adding the resistor provides a low-pass filter to prevent high frequency noise from entering or leaving the circuit. It can be useful when amplifying a very small signal and later amplifier stages couple back into the input. Or anytime when you have high speed and high gain.

I've also seen a network which involved a cap to ground in parallel with a series RC combo to ground. This is used when you need a particularly uniform wide bandwidth decoupling.

This appnote mentions the latter at the end.

But these things are more relevant to device bypassing once you have a DC power supply.

Steve Conner

Tue Mar 30 2010, 09:21AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

How smooth do you want? Maybe just a rectifier followed by a large filter capacitor will do.

Chris Cristini

Tue Mar 30 2010, 02:59PM

Registered Member #1749 Joined: Fri Oct 10 2008, 02:04AM
Location: Claremont New Hampshire
Posts: 497

Clean DC for a new ZVS driver and a few I.C.s that need clean DC I have simple switchers for the I.C.s but they are only 3A max so they would be no good for high demands of a Flyback Driver. We all know that A computer SMPS uses rectified mains to get around 180V from 120V can I build one that uses a lower voltage say 36VAC in?

Oh and I need it for my 12V to 66V boost converter can i achieve high power from such a low voltage in say around 5A?

Mattski

Tue Mar 30 2010, 05:59PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

A flyback driver usually draws quite a bit of power, meaning you'll need a big 60Hz transformer. If you're okay with 12V input to the ZVS I'd recommend just using a computer power supply, you can pick those up for cheap or free usually and they'll typically give you at least 10A at 12V.

But if you do use a 60Hz transformer then you just pick a capacitor that gives you maybe 5-10% voltage ripple at your expected current draw. See the wikipedia link I gave for that.

With 12V 5A that gives you 60W for your boost converter. P=V*I.

HV Enthusiast

Wed Mar 31 2010, 12:49AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Chris Cramer wrote ...

How do I get smooth DC like a computer PSU from a 60 HZ transformer?

Well, as others have said, a simple rectifier/capacitor is the simplest method.

However, its not "like a computer PSU." Computer PSUs are switching power supplies - a completely different beast then a simple rectifier and filter circuit. Tons of info though. Just search google or wiki for switching power supply.

cjk2

Wed Mar 31 2010, 04:06AM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

I would not run signal electronics and IC's from the same power supply as a ZVS driver is connected. If you insist on doing this, make sure you filter the power supply for the sensitive electronics with something like an LC (lowpass) filter first.

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