ac to dc

[cheech]

i have a couple of accessories that run on dc only. how would i use a rectifier to get them to work. is there a part number? how does the wiring work for the electrically challenged.

thanks
ralph

[Anonymous]

Go to Radio Shack or an on line electrical supply. Buy a bridge rectifier 6 amp or more,50 volts or more. Hook the transformer to the 2 leads marked ~. Hook the accessory to the + and - lead on the bridge to the accessory. Pay attention to polarity if the accessory is polarity sensitive. You may have to solder wires to the bridge or some take push on connectors. The Bridge will yield a pulsed DC. depending on the requirements of the device you may have to install an electrolytic capacitor across the + and - leads of the device. The - lead of the capacitor is marked.

Dale Hz

[cheech]

thanks dale

[johnandjulie13]

Hey Dale:

I know this is scary, but I understood everything in your post. As a matter of fact, when I first read Ralph's question, I knew a bridge rectifier was the way to go! Thanks to you and others I have learned a great deal in a very short time.

Thanks again for your feedback and contributions.

Regards,

John

[Anonymous]

Cheech and John

Thanks for the kind words. Keep in mind if you choose to add a capacitor to the bridge, the DC current is increased 40% over the AC input. So 10 volts AC in would give you 14 volts DC out. Dont know what you are powering but a 2200 or 4700 uf 35 volt or more capacitor should do the trick. Pay attention to polarity a backwards capacitor is a firecracker!

Dale Hz

[johnandjulie13]

Hello Dale:

What is the purpose of the capicitor in this case? Does it stop the "pulsing" nature of the rectifier output? If you have a DC device, how would you know whether a capicitor is necessary or not? Trial and error?

Regards,

John

[Jamesh1083]

Guys,

Couldn't you just use an old HO transformer to power anything needing DC voltage? I've done that in the past with some lights, when I was living in an apartment and forced to play with the HO trains instead of my 3-rail stuff. I guess I always assumed the HO transformers were putting out a DC voltage.

[thor]

No expert at electronics either, never graduated from discrete components to I.Cs but why buy a bridge rectifier instead of making one, its a simple enough item and the last time I did it, which was a ways back, 4 diodes were cheaper than buying one ready made. You can use chocolate block (terminal strip) no need for an expensive breadboard or etching a ciruit board. Do people still do this? I know I'm out of touch. Jamesh 1083's suggestion was probably the best.

[Anonymous]

John

It is hard to answer the question without knowing what is being powered. If for example you had some toy and you were trying to substitute a transformer for a battery,there may be some chips in the toy that do not take kindly to pulsed current. LEDs or light bulbs do not mind pulsed current but a voltage regulator run off batteries would be zapped for sure if it did not contain a filter capacitor. Most instuments take + 5 or +12 regulated DC current as a power supply. If it was battery operated it may not contain a filter capacitor. Without seeing the circuit it is difficult. A DC relay coil will work on pulsed current but will operate a bit smoother with a small capacitor on it. Cant really generalize but any circuit with chips would most likely use capacitors. Capacitors also smooth out voltage spikes and variations,filter out noise ect. Usually a capacitor wont hurt a DC device unless you exceed the voltage.

Dale Hz

[cheech]

Hi Dale. My education continues and my words cannot fully express my appreciation for your help here and in previous posts. i should have mentioned i am trying to power cornerstone oil pumps - 4 of them that need DC 10-12 volts. It has red and black wires for attaching power, so my assumption is that +/- is important to its operation.

i went to radio shack this am to shop and continue my education. i picked up a full wave rectifier [8 amps 400 volts] as well as a capacitor [10uf 50volts, 2kHz] . I observe the connection markings for the rectifier as you described perfectly, but can't find any markings on the capacitor to indicate polarity. I may have gotten the wrong one and will try again tomorrow when someone more knowledgeable than I is in the store to help me see the minus sign.

with all respect to Thor, the rectifier was 2.25 and the capacitor was .99. Since my goal is operating the layout, I can't imagine building one from scratch, but if i did, it would cost more than 3.25. I thought about the transformer but i nixed that idea--- i am at outlet capacity mentally-- i am plugging in 10 PoHos and 3 post war ZWs with the 14th spot reserved for a light to see under the benchwork.

I am at the deep end of the pool for my first layout in 40 years, but each day another accessory and 3 switches go on line...and each day a new connectivity opportunity presents itself... i will let you know how I progress on my oil pumps.

ralph

[Anonymous]

Hi Ralph

Sounds like you bought a non polarized capacitor! I am not familiar with those oil pumps so I dont know the requirements. My guess is you do not need a capacitor. Cornerstone makes mostly HO stuff and is most likely designed to run on an HO transformers. Most HO transformers have an AC tap for accessories and a DC tap variable to run the trains. I dont know if These have pulsed sine waves or not. It is probably just a DC motor inside the pump. May run a little smoother with a 220 uf 35v electrolytic across it,should not hurt anything.. Take the non polarized back,that is for audio and such. If you use the capacitor cut your voltage down until the unit just runs smoothly.

Dale Hz

[johnandjulie13]

Hello Dale:

Thank you for the clarification. If I understand correctly, it sounds like a capicitor (of the appropriate voltage) is a good precaution for these instances.

In my limited experience, I have only used AC accessories (LGB and Lionel). I am also used to having an AC tap on the back of my LGB power pack to handle switches, lights, etc.

Regards,

John

[lionelsoni]

I think this is the non-polarized capacitor that Ralph bought:
http://www.radioshack.com/product/index.jsp?productId=2102481&cp=2032058.2032230.2032267&pg=6&parentPage=family
That small a capacitor would be effective only for a load less than about 10 milliamperes. If the load is juat a DC motor, I agree that a filter capacitor is probably not necessary.

Ralph, what voltage do you have available for this project?

[cheech]

hi bob

i plan to run my accessories ---including these oil pumps-- off of 2 post war zw's. i have run a bus for these accessories---- which is separate from the power to the tracks around the layout ----and will power the bus with the zws. i will have around 50-55 operating accessories --not lights etc and not operating cars on the layout. i have a third zw for the switches and yard lights etc.

i am going to control all of them usng Acc Sw Controllers and Acc Motor controllers using tmcc assuming i don't run out of numbers.

i was going to give one separate tap of one of the zw's ---if that is the right term --- for these crazy ones and some lights, like the beacon etc...as i understand it, that is about 1/4 of the zw output of about 200Watts. these oil items and the beacons etc tend to want 10-12 volts

hope that i explained it well enough bob

i found the capacitor that dale suggested at all electronics and at allied electronics...my local radio shack didn't have it.

any additonal advice would be appreciated

ralph

[lionelsoni]

You might find that you can operate the oil-pump motors with half-wave DC from a 21-volt output from the ZW. My experience is that small DC motors, like those in a Tortoise, for example, tolerate this quite well. The DC component from half-wave rectification would be about 9 volts, which might be enough. Such a motor behaves somewiat like a capactor, with respect to the back voltage that it puts out when it is turning, so the effective voltage might well be higher than 9 volts.

You can use the rectifier that you bought for this. Just connect the ~ (AC) terminals together and to the transformer output terminal. Then run two of the four pumps from the + terminal and two from the - terminal. Connect the + terminal to red wires and the - terminal to black wires, with the other wire in each case connected to ground.

[cheech]

thanks bob
i'll try it over the next day or so and let you know how i make out
ralph