Thread: Building an electrical control box

Ok this is my last post to this thread so let me explain the very basics here.
You have an inductor feeding an inductive load. The bridge diode setup will not stop harmonics of heavens knows what frequency and you could have an antenna in the mains lead or your solenoid.
These are the unknowns you are playing with. It may or may not be so. But let us assume your setup transmits on the aircraft band, Do you realise from what height you are going to be jumped on. To put a capacitor in to smooth the mains will not stop this as its resistance is too high. Anyway the capacitor suggested without a resistor is a waste of time. You have a 240V supply, it works the lights, There is a simple 1A supply on Ebay for less than $10 stick that somewhere out the way and run a 24V power line to your machine

So what is the difference between a bridge diode with capacitor and resistor to smooth the ripple and a power pack?

Originally Posted by rrobor

Ok this is my last post to this thread so let me explain the very basics here.
You have an inductor feeding an inductive load. The bridge diode setup will not stop harmonics of heavens knows what frequency and you could have an antenna in the mains lead or your solenoid.
These are the unknowns you are playing with. It may or may not be so. But let us assume your setup transmits on the aircraft band, Do you realise from what height you are going to be jumped on. To put a capacitor in to smooth the mains will not stop this as its resistance is too high. Anyway the capacitor suggested without a resistor is a waste of time.

Now that is an over-the-top post!

The load is inductive - it will smooth the current and I'd doubt that you'd get harmonics in to the RF band.

Also, we are talking power diodes here - they'll have enough junction capacitance to inhibit the high order harmonics.

Over the top or not, since it has been mentioned and I personally do not know any better, I have to research the situation some more before proceeding...

Broke my word didnt I. The bridge is a rectifier. If you pull any power supply apart you will find each diode will have a capacitor across it. A diode is non linear So its the diodes that will be the issue.
These capacitors will also act as speed ups to shut the diode off at the end of itc cycle, by not using them you tend to get leakage bofore the diode shuts off and it will get hot. So their value has to match the diode. Now when you go into high frequency, open up any old TV tuner and you will find strips of metal a few centimeters long, they are tuning coils and the capacitors are what is known as stray capacitance, between the print etc. So he has 4 non linear devices and a heap of stuck together stuff. He may get away with it, but is this really worth while. The coils in the load would be nothing to RF that I agree with, but the junk out of the bridge will be random and not damped by anything. What it is is anyones guess.

Originally Posted by .RC.

Over the top or not, since it has been mentioned and I personally do not know any better, I have to research the situation some more before proceeding...

RC,

If you are concerned, just try the AM radio trick. Wire it up and place an AM radio near it. If you can hear hash, then the item is emitting radiation.

And try running an electric drill near an AM radio for comparison too!

People who fit antennas will tell you there is a strange thing that can happen when fitting a lead.
You can tune the line. All channels are OK 1 is dead. Chop a few cm off the line and all is well.
Creating harmonics is like music scales, the second harmonic is the next scale up.
Square wave is sine wave with the addition of multiple harmonics. So once you create non linearity, you have no idea without proper test gear exactly what you are creating and where that harmonic will hit. That is why I suggest he uses a supply built and tested not to create problems.
Apple records years ago added a high pitched whistle to their records as they found the kids were taping them. This was above human hearing. Tape has a bias frequency on it so when the two beat together all the tape produced was a loud howl.

Someone has recommended that I get a 0.1 uF capacitor of at least 100V or so, and hook it across the 24V AC side of the rectifier bridge to remove any chance of any EMI.. Edit: Doing some research this is what I believe is called a snubber circuit..

I have discovered I have got a neutral wire to all the sockets (they used 5 core wire), but it is blanked off....And I have found a 5 pin plug here so all I need to get 240V to the machine is a 5 pin socket and some 5 core cable.. But then that renders the 24V transformer I have purchased worthless...

Here is a close up of my pendant control... It is ready for wiring...

Originally Posted by .RC.

So what is the difference between a bridge diode with capacitor and resistor to smooth the ripple and a power pack?

Nothing. except you don't need a resistor. Not sure how that resistor crept into the discussion...

The pendant came up looking well,

I would stick with the 24v DC control circuits, I have long been an advocate of ELV control circuits, apart from the ease of interfacing to control systems, it's much safer than running 240v around the various E-Stops and interlocks..

Regards
Ray

Nice work .RC.
Do you think you might need a jog button? or is it easy enough to turn over by hand if needed?

Going a little OT.
RayG Do you have any idea whats different between 24VDC and 24VAC relays(or contactors).
Go easy on me lol

Stuart

Originally Posted by Stustoys

Nice work .RC.
Do you think you might need a jog button? or is it easy enough to turn over by hand if needed?

Going a little OT.
RayG Do you have any idea whats different between 24VDC and 24VAC relays(or contactors).
Go easy on me lol

Stuart

Hi Stuart,

Apologies to .RC for the slight thread hijack..

DC coils are more expensive!

The main difference is in the magnetics, DC coils have more turns, and AC coils are wound differently.. I can't recall exactly what is different about the way they are wound except that they sometimes have shading loops. I have a mate who used to work for us, that's now working for a coil winding business in Melbourne, I'll ask him if he knows what the exact differences are.

I've mostly used DC coils over the years, contactors, pneumatic solenoids, hydraulic solenoids etc.. you can switch DC coils much faster than AC. But the main reason for using DC coils, is you can drive them with fets, which are faster, cheaper and more reliable (in the long term) than relays.

Regards
Ray

Originally Posted by Stustoys

Nice work .RC.
Do you think you might need a jog button? or is it easy enough to turn over by hand if needed?

Yes I do need a jog button and no I did not even think of it...

Lucky then I put the buttons near the bottom to leave room for other items should I need them... Off to ebay I go to buy another button...

Originally Posted by RayG

Nothing. except you don't need a resistor. Not sure how that resistor crept into the discussion...

The pendant came up looking well,

I would stick with the 24v DC control circuits, I have long been an advocate of ELV control circuits, apart from the ease of interfacing to control systems, it's much safer than running 240v around the various E-Stops and interlocks..

Regards
Ray

The resistor was to try to lower the voltage increase caused by the capacitor....Did some experimenting this afternoon and with a I think it was a 570uF capacitor I stole out of an obsolete circuit board, installed in parallel it rose the 24V to 34.9V..

When I was looking at contactors on ebay I could not find any 24V AC ones... All the low voltage contactors I found were all DC... I got two new tellecomme???? (however the hell you spell it) for $20..

Thanks Ray
I've wound the odd coil, but nothing to do with shaded poles. I'll have to do some reading.

I like DC suits my simple mind.

Would it make .RC.'s life much easier if he bought a 24VAC contactor?

Stuart

p.s. wow I type slow. "easier if he could find one"

Originally Posted by .RC.

The resistor was to try to lower the voltage increase caused by the capacitor....Did some experimenting this afternoon and with a I think it was a 570uF capacitor I stole out of an obsolete circuit board, installed in parallel it rose the 24V to 34.9V..

When I was looking at contactors on ebay I could not find any 24V AC ones... All the low voltage contactors I found were all DC... I got two new tellecomme???? (however the hell you spell it) for $20..

Telemecanique... is the word I think you are looking for.. which is schneider, You should aim to get the voltage under load to no more than 26volts or so.. (check the specifications for the coil, usually they are rated as 80% to 110% of the rated voltage, and drop out at 10-20%.
If you use a smaller filter capacitor the voltage drop under load will be greater. But the ripple will be higher.

You could choose a resistor that would give you the correct voltage under load, but the problem with that, is during the turn on period the in-rush current can be up to 10x the holding current, so you could be dropping too much voltage during that brief turn-on period.

Regards
Ray

PS.. Changing the contactor or transformer don't know.... might be hard to beat that $20 price...