Rectifier output?

About 119.

John

"Gloria West"

If you connect a 120 volt lamp to the DC output, it will be the same brightness as usual.

If you add a filter capacitor, the rms voltage rises up to a maximum of 170 volts DC.

..... Phil

Homework?

No, smart guy.

Relays run with this rectified supply are buzzing. These are in a 45-year-old printing press.

I want to replace them with something more modern (the contacts are also intermittent).

I found some that are rated for 110 dc:

Will these buzz also? Is there a simple way to keep them quiet?

"Gloria West"

Add a 4.7uF, 250 volt electro across the output of the bridge and put a 4700 ohm, 1 watt resistor in series with the 110 volt relay.

This should give you 160 volts DC average on the cap and 110 volts on the relay.

I used ohms law and the formula: I = Cdv/dt to get the values.

...... Phil

And who is to say the contact rating will be correct? Why don't you fix the problem (whatever it is ) rather than trying to redesign the machine from a position of not enough knowledge?

John G.

Thank you Phil.

Relays coils are inductive enough that they don't follow the ripple voltage. So the effective (average) DC voltage becomes Vpeak/(2/pi) or about 108 volts. The 110 volt DC relays should be happy and probably won't buzz. Try one maybe.

If you add a filter capacitor, you'll get about 170 DC, which won't buzz but would fry the 110 DC coils. You could add a series resistor to fix that.

But why not use some nice octal-socket P&B AC-coil relays?

John

Actually there is no way to answer her question because she never gave enough data. And it has long been proven that the best way to repair a device is to return it to its design standard rather than redsign it without enough data or design knowkedge.

John G.

Oops, it's Vp * 2/pi. Still 108 volts.

John

So even if it was designed improperly it's still the best to fix it exactly the way it was? That sounds very intelligent. In any case, that is not the problem. The problem is that you are assuming she is a moron. That may or may not be the case but why do you do so? She asked a question and you gave her a statement. Not just any statement of course but a condescending and arrogant statement. A common trait among engineers and liberals.

Just because someone does not give enough information does not mean you get to be arrogant. Either assume what you can or ask for more information but personal attacks have nothing to do with engineering or helping someone.

~0.7*Peak. 120V is RMS. The rectified RMS is exactly the same. Think of it as power. Rectifying it will heat up a resistor jut as much cause the same amount of power is dumped into it. Mathematically, squaring a the abs of a function is the same as squaring it.

So |f|^2 = f^2 (assuming f is real)

So it does not effect the result.

(Note that 0.7*Peak is only valid for pure sinusoidal waveforms)

There are two main types of rectifiers, AC and DC. AC rectifiers work directly from AC without the need for rectification(They sorta have it built in). DC needs dc or you will get "chatter". The chatter is from the contact in the relay getting turned off and on repeatedly. This is due to the AC reversing the magnetic field that normally would pull(or push) the contact closed. What you get then is the ac pushing(or pulling) and then pulling(or pushing) the contact.

Even if you rectify the AC so it is "DC" the DC relay still will chatter because the rectified voltage is too low at some points in time. You can look up on the net exactly how relays work for it to be clear.

So, if they are DC relays, which most likely they are because of the rectification, then you have to use a filter cap, a cap that "smooths" the ac to dc like. Basically when the AC goes low, the cap will act like a battery and supply current to the relay until the AC can get back up. The cap has to be sized so that it is large enough to handle the current needed.

Generally electrolytic caps are used. It does not hurt too much to use too large of a cap but using too small will still create chatter. Electrolytic caps are polar which means you gott hook the + to the + side of the rectification.

ly

It's not 'fix' that's been described, it's 'redesign'. Changing a branch circuit from AC to DC might be OK, or (depending on the situation) might be disastrous. Switch contacts and fuses that work fine on AC are unsafe with DC, for instance.

An acquaintance once had to dismount a 1/4" steel plate to access a switching SCR, and wasn't keen to lift the plate back into position when he was done. He hunted up the original designer, who recalled that the circuit that SCR was on had a quarter-million joules of stored energy in a magnetic field. The steel plate went back up that afternoon.

I hope you meant to type "relays", rather than "rectifiers" in the first two lines above.

It's not 'fix' that's been described, it's 'redesign'. Changing a branch circuit from AC to DC might be OK, or (depending on the situation) might be disastrous. Switch contacts and fuses that work fine on AC are unsafe with DC, for instance.

An acquaintance once had to dismount a 1/4" steel plate to access a switching SCR, and wasn't keen to lift the plate back into position when he was done. He hunted up the original designer, who recalled that the circuit that SCR was on had a quarter-million joules of stored energy in a magnetic field. The steel plate went back up that afternoon.

----

I haven't seen anything mentioned about a redesign. She wants to replace components that have failed. Replacing components is not considered a redesign in my book. There have been many things I've "fixed" by replacing components that have failed yet it would not even have come close to what I would call being redesigned.

Obviously one has to have some idea about the circuit involved along with it's intentions. Replacing relays cannot ever cause any serious harm unless the design was flawed from the begining. Obviously there is the slight chance of a catastrophic failure but that can happen even with the original design. No design is perfect and hence has a redesign that can make it more perfect(theoretically).

Almost all designs, in the universe of designs, are implemented in a cost effective way. As you know, this directly competes with a perfect design. While the best option is usually to replace broken components it is not the only option. We've all experienced poorly designed systems that could be redesigned. It's neither her nor there because the origina question had nothing to do with a redesign. Simply replacing components is not redesigning the system.

When you replace a battery in your remote did you redesign it? Even if it's not the same battery? (Maybe if your a pedant)

In any case, it's not up to me, you, or anyone else to decide if anyone has the right to redesign something unless someone's safety is at issue. But we shouldn't take this too far since every person has the right to put himself at danger and even someone other people. Most devices that are in wide use have the potential to kill people but we don't not complain about such things and ban them. This is the cost of progress and convienence.

i.e., don't get overboard on such a simple thing. Replacing relays isn't going to cause a nuclear meldown and kill millions of people. Probably not even 1. If the wrong relays are used either:

1. They will burn up and need to be replaced again.
2. They will burn up something else.
3. They will not work as intented.
4. The device could work even better. (specially if the device no longer works)
5. Much progress can be made if the device did not work before or cause major problems because of the relays.

The probability of Gloria killing herself is pretty low. Probably lower than the probablity of you dieing from a car wreck tomorrow. (yet you don't hesitate to drive?) The probability of her killing or injuring someone else is even much lower. The relays could potentially malfunction at the most inoppurtune time but the likelyhood of this happening is probably rather low(mainly because relays don't generally fail in such a way).

This is a printing press and not a nuclear reactor. We see many people doing stupid things on a local scale and don't complain about it so no need to do it here. If we woried about what *might* happen then we wouldn't get much done.

If Gloria wants to redesign it then I'm all for it. Who knows... maybe she will make the thing work much better. If she kills herself in the process then at least she tried to do something instead of being paralyzed by fear. Progress has never been easy and never will be. (well, except if Obama gets his way)

Of course what you guys are really saying is "All you stupid people gotta leave it up to the "experts"! Just give me a call and we'll discuss how much it will cost you for me to redesign it for you."

Yes ;/ Guess I'm starting to get senile.

:120vac input to full-wave rectifier bridge. Without any filtering components :what is the output RMS voltage? : :This does not use a center-tapped transformer or such. : :Thanks.

The relays, presumably, did not "buzz" when the system was first designed. Unless the spring loading on the armature has been increased then the relay should not buzz when driven from a non-filtered bridge rectifier output. Since the mains input period is 16.7ms there is only a very brief period (somewhere in the region of 3-5ms) between the 8.35ms half cycles when the voltage will be reduced below the holding voltage of the relay and the normal magnetic inertia of the magnet system should be sufficient to keep the armature from chattering.

The new relays in your link have a nominal release time of 10ms and the 110Vdc coil has a resistance of 10Kohms - which indicates a large number of turns with significant inductance. When you replace the relays, and if buzzing does occur, try connecting a 10Kohm 3 to 5W resistor directly across the relay coil. This will allow the back emf to "slug" the release time and stop the buzzing.

Surely very slightly less, e.g. 118.5 due to 2 x Vf of the diodes in the bridge but without If being specified and the specific diodes, hard to be exact ?

Graham