# AC to DC power question



## jimtyp (Jan 2, 2008)

I have a Malibu Lighting system outdoors to power building lights. The Malibu puts out 12v AC. I bought a set of crossing signals and a bell from South Bend Signals. The flashers can use 12v AC, but the bell needs 12v DC to work properly. If I get a Full Wave Bridge Rectifier from Radio Shack will this convert 12v AC to 12v DC? Part: http://www.radioshack.com/full-wave-bridge-rectifiers-for-power-supplies/2761146.html#.VHKY2YvF8nU

If I'm off the mark here, got any suggestions on how to convert 12v AC to 12v DC?

Thanks,
Jim


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## Treeman (Jan 6, 2008)

That is a small one, don't know the current rating.


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## armorsmith (Jun 1, 2008)

Jim - The full wave bridge rectifier should have markings to identify the AC input and the plus/minus output terminals. However keep in mind that 12VAC in will be somewhere in the neighborhood of 7.5 VDC out. Be sure that the device will operate on less than 12 VDC. For a bit more information see: http://www.electronics-tutorials.ws/diode/diode_6.html


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## ewarhol (Mar 3, 2014)

Jim- 

Don't know the answer about the rectifier, but be careful with the milliamps. Circuits can sometimes run fine on less voltage, it's too much current that kills them. What's the current output of the Malibu system, and what's the max current for the signals?


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## toddalin (Jan 4, 2008)

armorsmith said:


> Jim - The full wave bridge rectifier should have markings to identify the AC input and the plus/minus output terminals. However keep in mind that 12VAC in will be somewhere in the neighborhood of 7.5 VDC out. Be sure that the device will operate on less than 12 VDC. For a bit more information see: http://www.electronics-tutorials.ws/diode/diode_6.html


 

However, the addition of a capacitor in parallel with the output will raise the dc to 15.6 volts. (Watch the polarity.) The capacitor should be rated for at least 25 volts with 1,000 mfd per amp (i.e., 1 amp uses 1,000 mfd and 3 amps uses 3,000 mfd).

That may be more voltage than the bell can tolerate in which case you would want to add a 12 volt regulator to provide a continuous 12 volts dc. One amp is probably enough, but 3 amps, or more (for the rectifier and regulator), should be sufficient.


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## Dan Pierce (Jan 2, 2008)

THe bridge rectifier in the first posting was 4 amps, it is at the bottom of the spec sheet. Many Malibu transformers are 40 va and that is less than 4 amps at 12 volts.

12.0 volts rectified with a full wave bridge rectifier will give 10.6 volts average DC, not 7.5.


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## jimtyp (Jan 2, 2008)

Thanks for your help! I tested the flashers and bell with a 9v battery and they work, so it looks like 10.6 volts would work well. But I have a meter I can test with before actually attaching it to the flashers and bell. 

I calculate the 45 watt Malibu system I have at 3.75 amps, just as mentioned above. 45 watt /12volts = 3.75amps. So that 4amp rectifier should work. I'm not sure how much the flashers and bell use but hopefully less than that.

It was mentioned about a smoothing capacitor. I'll test the flashers and bell without one first as it looks like, and was mentioned above, if I add one it may raise the voltage over 12? The flasher and bell manual don't mention max voltage but it does warn in several places not to use track power as it will damage the circuits.


-Jim


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## ewarhol (Mar 3, 2014)

I can't imagine the circuit drawing more than 1 or 1.5 amps, if even. 4 amps should be way more than enough.


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## wigginsn (Jan 9, 2008)

Ok, I'm confused..

I learned that 12 VAC fully rectified supply will produce a DC voltage of approx 1.4 times the AC voltage. ie 12 VAC will give around 17 V DC (peak). 17 V will then become the RMS value if a smoothing capacitor is connected across the output.

I'm not sure how you can get less than 12 VDC out if the input is 12 VAC.

Can someone explain?

Cheers
Neil


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## Greg Elmassian (Jan 3, 2008)

I was reading this thread and, well, giggling.

OK, if you have a 12 volt ac signal, meaning it goes from plus 12 to minus 12, then a full wave bridge will change it to a pulsating signal that maxes at 12, and the RMS (like an average) value on a "normal" DC voltmeter would be .7 times 12 or as Dan stated.

You can add a filtering capacitor, and if you are not running a heavy load, can get pretty much 12 volts DC... but you cannot be pulling anywhere the limit of the transformer, as the capacitor needs to source the voltage when it goes to zero.

Start by looking at this picture:










Where you get the 12v ac to 15v dc is because your AC measurement is not really 12 volts, but 12 volts rms (there's a reason for the RMS measurement being pretty much the standard).

Now if you are getting 12ac RMS, then that is different, then the peak voltage is higher as stated.

Here's a graphic:










So it depends on whether your "12 volts ac" is rms or not... I suspect it is RMS, which should make Todd right.

Greg


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## wigginsn (Jan 9, 2008)

So, the q I see that needs answering is whether the Malibu output voltage is RMS or other? Can someone measure/answer definitively?

That determines if Jim will need a 12V regulator or not to avoid frying the bell circuitry..


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## jimtyp (Jan 2, 2008)

I have a true RMS meter (got it when I was curious as to what voltage my DCC system was putting out to the track), with no load it shows 12.74 volts AC.


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## Dan Pierce (Jan 2, 2008)

When rectifing AC to DC, the current draw through a bridge rectifier will give a diode drop and there are 2 diodes in series from the AC to the DC output.
Older diodes would give .7 volts each, newer ones are .3 volts.
I tried to show this as I do not know of any diode that has a 0 volt drop.

I use this feature often to create offset voltages, the LM7805 with a diode in the ground leg can give me 5.7 volts and I use this for 5 volt lights and smoke. (Greg, LGB uses there '5 volt' lights and smoke at as high as 6.8 volts)


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## jimtyp (Jan 2, 2008)

UPDATE:

I got a message from Jim at South Bend Signals. He is willing to modify the bell to work with what I have. And for the price of parts and return shipping. Now that's great customer service! So, I'm going to take him up on his offer.

I appreciate all the help here and most of all the great info I can use going forward. Once again I've learned a lot from you folks on MLS. Things I can use outside of trains even! Now I've got enough knowledge to be dangerous ;-)

-Jim


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## Greg Elmassian (Jan 3, 2008)

Dan, look into the "on" resistance of Schottky diodes, and also FET transistors.

Then look at the components on your decoders and a QSI and the Aristo Revolution. (hint, look at the "full wave bridge" part).

You will see a lot of lower resistance "diodes" in use in our hobby.

Regards, Greg


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## dbodnar (Jan 2, 2008)

Greg Elmassian said:


> Dan, look into the "on" resistance of Schottky diodes, and also FET transistors.
> 
> Then look at the components on your decoders and a QSI and the Aristo Revolution. (hint, look at the "full wave bridge" part).
> 
> ...


Actually, Greg, the Revolution doesn't use diodes to deal with track polarity - it uses a 2nd set of MOSFETS that act as an H-bridge that switches one way or the other based on which polarity is on the track - the add-on boards for DCC or AC on the track use diodes but not the Revo itself. The other set of MOSFETs controls speed and direction.

The new HO unit has diodes but not the G-scale one.

I have some more info and photos on my web page here:
http://www.trainelectronics.com/Revolution_Receiver_Repair/index.html 

dave


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## Greg Elmassian (Jan 3, 2008)

Dave, re-read the first sentence in my post... I used the word FET (of course I know they are mosfets)...

Notice that the word "diodes" is in quotes, since the FET is not a diode, but a transistor, but used as a diode in this instance.

You seem a bit overeager to correct me here, carefully re-reading my post will reveal no errors I believe. 

Greg


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## dbodnar (Jan 2, 2008)

Greg- I'm not interested in correcting anyone -- I just wanted to bring a rather interesting and somewhat unusual way of dealing with track polarity to everyone's attention.
Happy Thanksgiving!
dave


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## Dan Pierce (Jan 2, 2008)

Greg, in the first post there was a bridge rectifier link and I believe it is a diode bridge not a mosfet bridge thus I added in the diode drop for this unit of .3 or .7 volts dependent on diode type.

And many transformers were set to lead acid battery ratings of 6.3 and 12.6 volts as this was the standard for automobile lights and even vacuum tubes!!
People tend to round off these numbers.


Data sheet for the bridge rectifier in the first post, silicon diodes
http://www.datasheets360.com/part/d...d=paidsearch&gclid=COGXkLbYmsICFeJF7Aod4wQAkQ


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## jimtyp (Jan 2, 2008)

Reviving this thread as South Bend Signals wasn't able to fix the issue with the Bell after all, and they have given up on a solution.

The flasher and bell claim to require 12v DC, but I can even run them on a 9v DC battery. You could say well just use the 9v battery and be done with it, but I don't know how long a 9v transistor battery will last and they are not cheap. It seems if this full wave bridge rectifier really works then it is a cheap solution at $3.

So, I bought the full wave bridge rectifier, link in original post, and I measured the voltage as DC 11.23 volts with my True RMS meter. Thus, do you think I'm good to go? Or am I just being naive and more needs to be done? 

Thanks,
Jim


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## krs (Feb 29, 2008)

A couple of comments on this thread.

AC voltage is always specified as rms, never as peak or peak-to-peak unless that is specifically stated.
So the Malibu output is 12 volts AC rms and Jim confirmed that heasuring just over 12 volts rms.

Now, if one just adds the full wave rectifier at the AC output one gets the full wave rectified waveform that Greg posted - the bottom diagram in his post.
The meter measures 11.23 DC, that's close enough to 12 VDC to drive a "non-electronic" device like a relay for instance, but if you look at the waveform that Greg posted you will see that the voltage peaks will be at a higher voltage - 1.414 times the measured AC rms voltage actually or 12.74 x 1.414 or just over 18 volts.

I have no clue if the South Bend Signals bell etc includes any electronic circuitry and if that circuitry can handle just over 18 volts pulsed at 120 Hz which is what the signal would be with just the bridge rectifier.
That is something you would need to check with South Bend Signals.

If not, you would need to add a smoothing capacitor at the output of the bridge rectifier and possibly a small voltage regulator if 18 volts is too high a voltage.

In fact, South Bend Signals should be able to tell you exactly what components you need to convert the 12 VAC to a voltage suitable to drive their bell and flasher.

Knut


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## jimtyp (Jan 2, 2008)

Thanks Knut! On the South Bend Signal page and in the manual it says:

Power sources: 12 volt DC from battery or external power source .

I doubt that the flashers and bell can handle 18v as in multiple places in the manual it says track power will blow the electronics and not to use track power. 

So even though I read ~ 12v dc with my meter it's really putting out closer to 18v DC at peak? 
I have a DC regulator that is adjustable from 5v to 20v. In your opinion would I be safe in setting the regulator to 9V DC?
9 * 1.414 => 12.726
And not using a smoothing capacitor?
And does the 1.414 apply if I use a regulator? That is if the regulator is set to 9v that it does not go higher, like 12v peak?

Thanks,
Jim


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## wigginsn (Jan 9, 2008)

Hi Jim,

The voltage output will peak at 17 V (12.74 rms x 1.414 less 1V forward drop in the rectifier). I sourced the rectifier drop from generic datasheets for a KBU4D device you linked in the first post.

The bell will see the 17 V whether you smooth it or not - so unless SBS says 17 V is ok you will need to fit a regulator. 

Setting your regulator to 9V gives 9 VDC, no 1.4 multiplier - this is for AC voltage only. The possible problem with setting to 9V is overheating of the regulator. This is worked out from the voltage you want to drop (17-9=8V) times the current draw of the bell circuit, and the ability of the regulator to get rid of heat. We know the current is likely to be small as you can run it from a 9V battery.

You could set the regulator to 9V and see if it gets hot.. 

It's not hard to fit the extra bits, we can guide you through.

Cheers
Neil


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## krs (Feb 29, 2008)

Jim,

No point setting the regulator at 9 volts if the bell is designed for 12 VDC.

Just set the regulator at 12 volts, that would minimize the power dissipation (ie heating) of the regulator that Neil mentioned.
I would still add a small electrolytic capacitor at the output of the rectifier so that the bell and especially the flasher "sees" something close to a DC voltage.

I still think the best is to ask SBS what component values they suggest - they know their product the best.
Specifically what value of capacitor (that depends on the current draw of the SBS items powered by this little 12 VDC supply).
My guess would be that a 500 uf capacitor is adequate if you can run both the bell and flasher from a 9-volt battery.(which can't supply very much current)

BTW - which regulator do you want to use?

Knut


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## jimtyp (Jan 2, 2008)

Neil/Knut, okay gave it a go. The Full Wave Bridge Rectifier (link in original post) is connected to the Malibu 12 V AC system. I have the adjustable regulator attached to the rectifier. The one I'm using is in this link (LM2596)

http://www.ebay.com/itm/5PCS-LM2596...685?pt=LH_DefaultDomain_0&hash=item54041f18c5

Says it comes with a 2amp max, but can be modified to handle 3amps, I have not modified it, so 2 amp max.

I adjusted the output voltage until my meter said exactly 9 V DC.

Tested it with the South Bend Signal and Bell. IT WORKS!!!

With rectifier ($3) and regulator ($7/5 => $1.40) the cost is $4.40 

I'm thinking heat will not be an issue as the flashers and bell are only on while the train is in the crossing. However, do you think I should up the output voltage closer to 12 volts to be safe?

THANKS FOR YOUR HELP!
-Jim


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## wigginsn (Jan 9, 2008)

Great to hear. 

Look like the module already has some input smoothing, from what I see. Heat shouldn't be a problem, I was thinking a simple 3 pin device when I mentioned it..

I'd set it to 12 V. That's what SBS specify.

Cheers
Neil


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## krs (Feb 29, 2008)

I agree with Neil.

Set the voltsge to 12 volts since that's what SBS specifies.
Not that this makes it any "safer", but 12 volts may make the bell a bit louder and it may change the flash rate a bit.
However, if you're happy the way it works at 9 volts - that's fine too.

I was also thinking of a basic three-terminal regulator - the switching regulator you have has much better efficiency - so much less heat is generated than with a conventional 3-terminal regulator.


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## Dan Pierce (Jan 2, 2008)

When using a 12 volt A/C source and a rectifier to make DC, no one mentioned that the unit tied to the 12 volts DC could have a filter capacitor on the input and then get the 18 volts anyway!!

12 volts DC rating to me is just pure/filtered 12 volts DC, not rectified A/C with the 18 volt peaks.

FYI in Automobiles it is 12.6 from a battery when not running and up to 14 volts running.


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## krs (Feb 29, 2008)

Dan Pierce said:


> When using a 12 volt A/C source and a rectifier to make DC, no one mentioned that the unit tied to the 12 volts DC could have a filter capacitor on the input and then get the 18 volts anyway!!
> 
> 12 volts DC rating to me is just pure/filtered 12 volts DC, not rectified A/C with the 18 volt peaks.


Dan,

You are absolutely right!

However, I really doubt there would be an input capacitor in the SBS unit, commercial products, asnd even more so "Hobby" electronics, try to keep the manufacturing costs at an absolute minimum; an input capacitor wouldn't add to the functionality or value of the product.

In any case, with the regulator in place, it doesn't matter if there is an input capacitor or not - the voltage will be whatever the rgulator is set at.

Knut

PS: I wish companies would publish schematics of their products (at least the input and output circuitry) so we wouldn't be constantly guessing if a situation like the one in this thread comes up. It's not that these are highly classified designs.


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