PWM cap and power supply questions

[torker]

Oh boy...I'm lost now. Still working (when I get time) on my quill feed drive. Almost have all the parts made for the mechanical end. I'm using a GM wiper 12V wiper motor with a gear drive that runs a chain drive giving 7-1 reduction. I have a PWM controller and have settled on using a battery charger for a power supply. I read somewhere about another guy doing the same thing but he couldn't use his because it was only a half wave charger. How do you know if the charger is half wave and what is wrong with that if it is? Next question...the PWM controller says that if the power supply is a transformer and rectifying bridge type OR if the input lines are over one foot long then a capacitor must be used. I've read all I can find on this and understand that the capacitor is used to stop voltage spikes and other such magic. I'd assume that the cap would be wired into the cicuit before the PWM?? (correct?) Positive on one pole and negative on the other (correct?). Now what in the heck do I use for a capacitor?
There is no mention of type, size etc. in the wiring instructions for the PWM. Any help would sure be appreciated. Thanks!
Russ

[Forrest Addy]

Find a motorcycle battery that still has a little life in it and use the battery charger set on trickle to maintain the battery. Run your motor control from the battery; this way you get pretty good DC. If the battery is free you save the screwing around and some money.

[torker]

Forrest...you got me!!! Why a motorcycle battery? Would this do away with the need for a capacitor? Ha...if that's all it takes I'll buy a new battery...those I can find up here in the frozen north...capacitors, well that's another story. We do have a Radio Shack up here and a couple of industrial supply houses that won't sell me anything because I'm not an electrician.
Russ

[Evan]

Forest has good advice, but it may not suit in the long term. The windshield wiper motor really will not care if it runs on half wave or full wave DC. But, the controller may work better on full wave and well filtered DC. The rule of thumb is to use at least 1000 mfd per volt delivered to the filtered load. So, in your case the power supply should have at least a 12,000 mfd capacitor in parallel with the output. The voltage rating of the capacitor should be at least 15 volts.

Half wave DC is when the AC is passed through a single diode to the load. Full wave DC is when the AC is passed through either a full wave bridge rectifier or in the case of a center tapped transformer then through a pair of diodes. If you only have a single diode then it is half wave DC. The difference is that with half wave you only get power on the every other cycle of the AC, 60 times per second. With full wave rectification you have power 120 cycles per second. It is easier to filter and requires less capacitance to do so. I won't go into choke filters as they are only applicable to high voltage supplies.

What you need is a simple linear power supply. A battery charger with a filter capacitor will do just fine. The half wave/full wave bit doesn't matter. The thing about the wiring length is simply that you should put at least a 450mfd cap rated at least 50 volts at the point of entry of the power to the controller. This goes across the hot (+) and ground (-). It will smooth the power to the controller.

[Forrest Addy]

mispost

[This message has been edited by Forrest Addy (edited 01-03-2005).]

[zl1byz]

I agree with Forrest about the battery, doesn't have to be motorcycle could be a car 12v battery. This will get you up and running while you get your head around a power supply. Basicly you need a 12v DC power supply capable of delivering enough amps for the controler & wiper motor. You can probably pick up a ready built properly designed supply for less than the trouble of trying to coble one together if you don't already have the knowledge and parts.

John.

[rsr911]

Couldn't you just use a computer power supply? They have 12VDC outputs that could be ganged together, are readily available up to relatively high voltages etc. All you need is a switch on two of the wires in the connector that normally goes to the motherboard.

------------------
-Christian D. Sokolowski

[zl1byz]

Russ! Yes thats correct if you use a battery you don't need a capacitor. If you have a battery charger you could float the battery across that to keep it charged, still wont need a capacitor, unless the battery is totally stuffed.

You should be able to pick up a 12v DC supply from somewhere if and when you want to go that way. You also need to know what current you need, thats how many amps your wiper motor & the PWM controler needs to get the correct size supply.

John.

[zl1byz]

<font face="Verdana, Arial" size="2">Originally posted by rsr911:
Couldn't you just use a computer power supply? They have 12VDC outputs that could be ganged together, are readily available up to relatively high voltages etc. All you need is a switch on two of the wires in the connector that normally goes to the motherboard.

</font>

Yea I did think about computer supplies but I dont know how much current they can supply @ 12v. Also don't know how much current the PWM controller & wiper motor uses. Also don't know how much knowledge Russ has with electronics. A servicable battery & a charger is just least problematic & he probably has those lying around.

By the way Russ if you use a battery you might consider putting a fuse in the line from the battery to the gear. It may still not protect the gear if you do something wrong, but it may save you from getting a nasty burn if things get hot. Batteries can deliver huge amounts of current.

John

[torker]

Guys...Thanks so much! This helps a lot. Damn I envy you electrical guys! "Wrapping my head around it"...well said! I don't have the foggiest about electrics and to be honest am struggling with it. I entered "building 12 volt power supply" into Google every way I could imagine to find something I could understand. Found some good stuff...if I was an electrician! I just wish I had the time to sit down and really hash this out. Evan mentioned the computer supply route and even took the time to answer emails about it. I now have 4 computer supplies that I've robbed out of old computers but none have enough power. Guess I need to find a newer doner. I've looked locally for power supplies but to no avail. Ha...I spent too much time in the past learning how to build Holley carbs and tuning Enderle injection units! The bright spot is...I know more now about electrics now than I did a couple months ago . So I'm going to cobble this together and keep reading what I can. Thanks again!
Russ
Ooops...Almost forgot. How to mount the PWM control unit? It's just a board and only has two holes in one end. I was going to screw it into a plastic electrical box but it didn't seem right to clamp all the soldered connections down on the bottom. Should this be lifted up or mounted in silicon? Thanks.

[This message has been edited by torker (edited 01-03-2005).]

[J Tiers]

Reasons for the capacitor note on your PWM....

1) smoother DC, as mentioned above

2) connected close by the PWM because the PWM draws high frequency pulse currents, and long wires cause a voltage drop at higher frequencies. That drop reduces power, and also causes larger "spike" voltages that can damage electronics.

3) The PWM can dump current BACK INTO the supply, under certain conditions. That means the supply has to be able to absorb that current without letting the voltage get too high.

A diode bridge can't accept ANY current coming back, so you must add a capacitor. There should be some info with the PWM on how large a value in microfarads is required to allow the overvoltage shutdown to work in time. Bigger is better, in general, since bigger accepts more current at same voltage rise.

A battery will take any amount of "dumped" current, without complaint and without a significant rise in voltage.

[zl1byz]

<font face="Verdana, Arial" size="2">Originally posted by torker:
Guys...Thanks so much! This helps a lot. Damn I envy you electrical guys! "Wrapping my head around it"...well said! I don't have the foggiest about electrics and to be honest am struggling with it. I entered "building 12 volt power supply" into Google every way I could imagine to find something I could understand. Found some good stuff...if I was an electrician! I just wish I had the time to sit down and really hash this out. Evan mentioned the computer supply route and even took the time to answer emails about it. I now have 4 computer supplies that I've robbed out of old computers but none have enough power. Guess I need to find a newer doner. I've looked locally for power supplies but to no avail. Ha...I spent too much time in the past learning how to build Holley carbs and tuning Enderle injection units! The bright spot is...I know more now about electrics now than I did a couple months ago . So I'm going to cobble this together and keep reading what I can. Thanks again!
Russ
Ooops...Almost forgot. How to mount the PWM control unit? It's just a board and only has two holes in one end. I was going to screw it into a plastic electrical box but it didn't seem right to clamp all the soldered connections down on the bottom. Should this be lifted up or mounted in silicon? Thanks.

[This message has been edited by torker (edited 01-03-2005).]</font>

Hi again Russ. To answer your last question, you need something to use as stand offs. Basically something as a spacer under the mounting screws between the PWM board and the box. Just make sure what ever you use if it is capable of conducting doesnt connect to the electrical circuit.

I am no electrical guru. My knowledge comes from being a radio amateur. We often use high current 12v power supplies. I have built several that have been is service for over 20 years. I also have seen more problems caused by home brew power supplies than any other peice of equipment. There seems to be the thinking that power suplies are simple and can be cobled together out of any old junk lying around, well if all your running is a light bulb that may be true. But if any electonic equipment is to be hung on it then much more care in design needs to be taken. The least that can happen is the equipment doesnt work to spec, there by rendering the whole project useless or the supply is so bad that it lets the smoke out of the expencive gear you hung on it. Once the smoke is out things never work the same again.
A battery may not be a perfect long term solution. But it will get your project up and running with minimum fuss. Give you time to work it through properly. Those electrical supply outlets should be able to help with power supplies.
Good luck with your project. Some pic's of what you have done would be good if you can.

John.

[Paul Alciatore]

A few observations from the electronics world.

1. If you want to learn more about building DC power supplies, Radio Shack has/had a 80+ page paperback called "Building Power Supplise" by Dabvid Lines, cat # 276-5025. It may be an obsolete item now, I don't know. It is a good practical work.

2. Contrary to what J said, batteries can not take "any amount of 'dumped' current". Many NiCads have been ruined by overcharging. This is perhaps "true" when you consider only automotive, lead-acid batteries and the common commercial chargers as they are both fairly standard items and supposedly somewhat matched to each other. Even there you may get into trouble with excessive charging currents. Just try leaving an auto battery on full charge for a month.

3. Filter capacitors are not calculated in terms of uF per output volt. Instead, they are related to the load current. One generally accepted equation for determining the size of a filter capacitor in a full wave rectifier circuit is (Don't jump on me,I know there are other ways to do this but they are all related to the current.):

C = 2.4 I / Vrms

C is the capacitance in uF
I is the maximum load current
Vrms is the largest allowable ripple voltage

The ripple voltage refers to the AC or ripple component of the output voltage and represents how much the output voltage varies above and below the average DC voltage. Kind of like surface roughness above and below the desired dimension. The actual DC output voltage does not enter into the equation at all.

Another thing to consider when choosing a filter capacitor is it's voltage rating. It must have a voltage rating that is larger than the average DC output voltage PLUS the ripple voltage (this is an approximation).

One more consideration for switching or pulse mode supplies is that it is a good idea to add a 0.1 uF +1000/-90% ceramic or other non-electrolytic style capacitor in parallel with the above filter capacitor to surpress high frequency spikes. This is because the electrolytic capacitors are not as good at high frequencies.

[J Tiers]

<font face="Verdana, Arial" size="2">Originally posted by Paul Alciatore:
2. Contrary to what J said, batteries can not take "any amount of 'dumped' current". Many NiCads have been ruined by overcharging. This is perhaps "true" when you consider only automotive, lead-acid batteries </font>

NOPE, considering that automotive style batteries WERE what is being discussed.....you don't need to worry.....

AND, even MORE important, PWMs can only dump BACK what they RECEIVED, less losses........no there is clearly NO REASON to worry in the situation described above. The dump-back can never harm the battery from over-charging.

It is NOT like a charging-from-line situation, in which of course, too much charging is possible and is usually bad.

The battery under discussion, motorcycle or auto, can indeed take "any amount" of charge that the PWM can dump back in operation. ANY amount, as in...."not to worry".

Contrariwise, too small a capacitor in a rectifier/filter capacitor type supply can allow dumped charge to build up to a high voltage and potentially cause a shutdown.

I think IBEW discussed running into this back in the "Gecko threads". In PWM circles it is known as "pump-up", and can occur from several causes, including braking.

The braking resistor in a VFD drains charge and prevents dynamic braking from causing an over-voltage. But the VFD has a rectifier and capacitor power supply, with only a finite small "sink" capability.

[snowman]

Why dont you just look for a supply on Ebay?

13.8 Volt power supplies aren't that expensive, and are quite common. The CB people love them.

You'll need a pretty big one to run that motor though.

It seems like mcmaster sold an adjustable power supply for a reasonable price (around 230 dollars). If I remember right, the output on that was 40 amps, 20 continuous.

It might have been newark though...I really can't remember.

-Jacob