Mains Power Supply Questions... 220VAC to 220VDC

[stewieg]

Hi all, im fixing up an old (1940's) hydraulic press/bending machine for my dad, He has a small machine shop with some quite old equipment... Ive repaired alot of the bits and pieces down there over the years but this one has me questioning myself and everytime i read and google more on the subject i get more and more confused! lol...

Now i did do a module on dc power supplys at TAFE/Tech about 13 years ago... but i havent had much to do with power supplies since... and the power supplys i dealt with then were with transformers to step down the voltage first and then diodes, caps and coils to rectify and filter the ac into dc..

The Machine im working on has a 2 way hydraulic valve/solenoid... with 220 volt DC coils! I tried to get my dad to go 24volt DC to make things a bit more safe, but they didnt have any stock of the 24VDC coils so now im stuck with 220VDC for everything anyhows ill explain the circuit

there is a push button that is momentary, in its normally closed position it causes one of the solenoid coils to energize and pull the solenoid back to the "return position" so all the rams retract...

Then when the push button is pushed.... Mains power goes through a bridge recitifer (i added this when we added new coils, full wave) through the push button switch which has now been depressed, and then into the alternate coil (cutting off the return coil in the process) which then makes the solenoid open and hydraulic fluid push's the rams down....

The moment you take your finger off the button the solenoid goes back to the return position and the rams retract... pretty simple!

We replaced the coils and plugs for the coils... and in the end of the plugs on each coil is a little circuit board with 4 diodes and a small circular red component similar to a ceramic capacitor with only "220" on the side of it.... i assume this is a capacitor? .22uf?  so i replaced these boards with the bridge rectifier... now ill probably need some kind of filtering capacitor to replace the one on that little board... what size should i be replacing it with? .22uf? Jaycar have a .22uf 630vdc greencap... will these do? they are the only capacitor i could find with high enough voltage ratings (enough for mains) and 0.22uf I would prefer someone who has some idea about power supplies to give me the go ahead before i hook it up and try it... The coils are worth about $300 each or something i was told... definatly dont want to go frying them the coils have 35Watts at 220VDC written on the side of them if you need to know that... Ive been reading up on this and got confused with ripple voltages etc etc... I dont own a scope yet and was scared with all the talk of ripple voltages and waveforms that i might have to finally go out and spend the cash on one

Anyhow if someone can point me in the right direction or if ive given enough info for someone who knows to say "yep you need this sized capacitor" that would be nice The forumla's etc that were starting to mess with my head are here :

http://www.tpub.com/neets/book7/27e.htm

http://answers.yahoo.com/question/index?qid=20070618213017AASsgCF

I know i was on the right path... but yea..

[RayJones]

If your solenoids are only rated for 220V, then you are actually safer to only run them from a bridge rectifier.

The RMS voltage of the rectified mains would be similar to the 220V DC.

As soon as you add a capacitor, the DC voltage will tend to rise and hold somewhere closer to the peak voltage of the rectified waveform, which is 1.4x higher, and your 220V capacitor will definitely be operating beyond its rating.

The whole thing about an RMS value is has the equivalent long term power of a steady DC voltage.

Hope this has not confused you further?

[stewieg]

Hi Ray! i actually understand that! while i was searching for an answer to my problem on google i came across a circuit that used capacitors and diodes in a ladder type configuration to "multiply" voltage but at the cost of current... i certainly dont want to do that! http://en.wikipedia.org/wiki/Voltage_multiplier <--- that one but on a different site..

so the little bit of ripple i would be getting through the coil from the rectified but unfiltered 220VDC wont hurt the coil will it? i was thinking along the lines of speaker coils... how it hurts them to feed DC into them for extended periods of time... but these coils are actually specified DC and i read that somewhere for this application coils that use AC directly arent strong enough and are very expensive due to a different design (constantly collapsing and expanding magnetic field through a coil = massive back emf) not to mention stress's on the core...

[RayJones]

Hi Stewie, no, as I said before, the RMS value of the rectified mains has the same heating effect as the equivalent steady DC value.

You may experience some buzzing from the solenoids without filter caps, but given the application it is probably not really an issue.

Your red round device may actually be a transzorb (surge arrestor), designed to absorb back EMF spikes over the nominal voltage....

[stewieg]

sweet ill give it a shot in the morning... physics wise i would hope it has the same heating effect otherwise einstein was wrong with this : http://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence energy doesnt magically go anywhere and cant be created magically without "work" or some sort of chemical effort... exactly why those water/hydrogen electrolysis injection into automobile engine gimmicks are a load of rubbish

[Zero999]

That's got nothing to do with it.

You don't understand the difference between RMS and peak voltage.

The peak mains voltage of 220VRMS is 311V.

The capacitor doesn't multiply the voltage, it just smooths the voltage so you get nearer the peak voltage all the time instead of just every half cycle.

The reason why AC voltages are measured in RMS for power distribution is that it gives the same heating effect as DC. Because of the non-linear relationship between voltage and power, P = V²/R the RMS voltage for a perfect sinewave is 1/?2 of the peak voltage or ?2×VRMS

[stewieg]

i know what root mean square is.... and i know what peak is too... my comment was in reference to ray's comment about heat, 220VAC and 220VDC is the same thing overall power wise... you get a slight loss throught the rectifier... but electrons dont just pop out of thin air! its just the waveform inversed for one half (for a full wave rectifier)... electrons are energy... which are governed by the general laws of physics...with the capacitor the wave form is averaged out even more... so you get closer to its peak... i understand this too... you missunderstood my comment about the voltage multiplier... im only adding one capacitor, so im not going to be building anything close to that... my whole post was about what capacitor value was suitable... i was saying i didnt want to end up doing something "silly like this".... sooo... anyways thanks for your help... ray answered the question for me.

Hero999, just a question... what prompted you to choose that username? its quite fitting *hacks eevblogs sql database and changes Hero999's username to Hero000* in another life...