AC vs DC Transformer

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  #1  
Old 04-22-14, 11:42 AM
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AC vs DC Transformer

Hi,

I recently set up a 24-volt gate operating system. I bought the system used on eBay and it works great. The only thing is, the control box has a 24-volt battery (well, actually 2 12-volt batteries in series) and when I hooked everything up the batteries were dead even though I had been keeping them charged, with the transformer that came with the system, until installation. I didn't use the transformer in the installation since I set the system up to run on a 24-volt solar panel. I bought 2 "lawnmower" batteries to replace the dead ones in the control box.
My question: I just noticed that the transformer that came with the system was 120 volt input, but 24 volts AC output. Can a transformer with an output in VAC charge a (DC) battery? Or did the seller give me the wrong transformer?
 
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  #2  
Old 04-22-14, 12:20 PM
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Can a transformer with an output in VAC charge a (DC) battery?
No,but it is possible the opener has a built in charger that needs 24 volts AC (unlikely though from my limited experience).Give us make and model or an online link to the manual so we can better answer.
 
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Old 04-22-14, 12:30 PM
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Low voltage AC transmits much more efficiently than DC does over distances. The logic with the gate opener is that if there is no existing source of 120V near the gate for the transformer to connect to, it is much cheaper and easier to run the low voltage cable back to the house than to bring a 120V circuit to the gate (shallower trench, no conduit, no electrician/permit required) - and that could be hundreds of feet or more. The voltage drop over that distance would be far greater with DC than with AC.

It is doing the conversion to DC at the gate controller.
 
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Old 04-22-14, 12:55 PM
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Since you nudged me to pull up the manual (found here: https://www.elitegates.net/files/PDF...1%20Manual.pdf )
I see that the manual shows (page 20) transformer output in VAC.
I set up solar per page 40.

I guess the control board converts to DC, huh?
 
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Old 04-22-14, 01:12 PM
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Yes, control board converts to DC.
 
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Old 04-22-14, 02:06 PM
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... Low voltage AC transmits much more efficiently than DC does over distances. ...
Are you sure? From what I read, for AC the number of volts lost in the wires (voltage drop) equals the amperes drawn at that moment times the impedance of the wires. For straight wires the impedance equals the resistance.

If AC transmits more efficiently then the transmission of DC over long distances must be pathetic.

In a setup such as this the batteries down at the gate can provide several amperes to operate the gate while the AC from the transformer may only need to be a fraction of an ampere to keep the batteries charged provided that the gate is not constantly opening and closing. With just a fraction of an ampere the number of volts lost in the wires might be just one or a fraction.
 
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Old 04-22-14, 02:22 PM
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It is pathetic. DC can't be stepped up/down via transformers, so it must be transmitted at the voltage it will be consumed. The lower the voltage, the higher the current for any given load, therefore the larger the conductors must be to overcome voltage drop.That's why the power grid is AC not DC. Tesla saw the infeasability of a DC grid, Edison did not. A DC grid would require a generating station literally every square mile.
 
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Old 04-22-14, 09:44 PM
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Be sure to use a solar battery charger controller to keep from overcharging the batteries and frying the electronics.
 
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Old 04-23-14, 11:33 AM
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"Be sure to use a solar battery charger controller to keep from overcharging the batteries and frying the electronics."

Thanks - I'm not using a solar battery charger controller. Had no idea I should use one. Any tips on a good one that's also small?
 
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Old 04-24-14, 08:49 AM
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I see a load of them om eBay. Shouldn't be critical which one you get.

24V Solar Battery Charger | eBay
 
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Old 07-31-14, 02:48 PM
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Now maybe I have a real problem. The two lawnmower batteries running in series died. Bad sign, I know. Anyway, I replaced them with two new internal-to-the-control-box batteries and for the time being ran the gates off of A/C power through the 24VAC transformer. After 6 days of use these batteries died. Now I know that the control box isn't charging the batteries, whether the power source is solar or A/C. Did I already fry the electronics with my 37 VDC solar panel? The board in the control box indicates (via lit LEDs) that there is power and that "charge is ok." Is there a way for me to troubleshoot whether the board is fried (as far as battery charging is concerned)?
 
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Old 07-31-14, 03:15 PM
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If you tied your solar panels directly to the batteries with no solar charger/regulator then you may have fried the control board.

There are a lot of LED's on that board. Are they showing the system as working ?
 
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Old 07-31-14, 04:41 PM
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There are two boards: a control board and a surge suppressor board. When everything is hooked up, all the pertinent control board LEDs light up as they should. The timer LED flashes 3 times when the batteries are initially connected, as per manual page 20. The Power indicator LED and Charge OK LEDs light up. The Battery Low LED does not light up. Since there is a special surge suppressor board I'm hoping it has protected the control board. The surge suppressor board is very simple - just resistors and gas discharge tubes, as far as I can tell. I think I have attached pics of the front and back of the board here.
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Name:  board rear reduced.jpg
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Last edited by PJmax; 07-31-14 at 05:29 PM. Reason: re-posted pics
  #14  
Old 07-31-14, 04:52 PM
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I should add that the only reason I know they are gas discharge tubes is because I searched the name/number on the white barrel-like things.
If it came to it, I can test the resistors with my multimeter but don't know if I can test gas discharge tubes in a simple way.
 
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Old 07-31-14, 05:33 PM
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I fixed your pictures. The problem is not on the surge protection board. The discharge tubes are just a spark gap and shouldn't require checking. The resistors could possibly burn or open on a surge.

If you disconnect the batteries.... is the charge ok light still on ?
Can you measure the voltage on the battery leads with the batteries not connected ?
 
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Old 07-31-14, 06:40 PM
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Thx for fixing the pics. I will check for sure whether the charge light stays on when the batteries are disconnected, but can't until tomorrow (but my memory during my fiddling with it today is that the charge light went off). I will measure battery voltage then, too. Will get back. And Thank You!
 
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Old 07-31-14, 08:57 PM
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Measure the voltage coming from the board..... not the battery voltage.... with the batteries disconnected.

Then connect the batteries and check it again.
 
  #18  
Old 08-01-14, 09:12 AM
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If I disconnect the batteries (transformer plugged in) the "charge ok" LED stays on.
Voltage coming from board (batteries disconnected) = 20
When I connect the batteries the voltage is also 20.
 
  #19  
Old 08-02-14, 11:23 AM
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So anyone know what to look for on a control board that is supposed to convert 24 VAC to 24 VDC but is only converting to 20 VDC?
 
  #20  
Old 08-02-14, 11:27 AM
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In order for me to help you I'd need a large pic of at least the front of the board.
A back view would be helpful too.

You'd have to post them at a free pic hosting site or email them to me. The picture size on the board is not large enough.
 
  #21  
Old 08-03-14, 07:52 AM
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Thanks PJmax,
I got it working and if you're interested I got a circuit board forum to help. There are photos there. Amazing that it worked! Here's the link:
Control Board VAC to VDC Problem - All About Circuits Forum

Thanks for all your help! Wife says thank you, too!
 
  #22  
Old 08-03-14, 09:21 AM
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That's what I do.... reverse engineer.

I'd recommend getting a charge controller on there too if you haven't already.
 
  #23  
Old 08-03-14, 01:52 PM
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Absolutely, already bought one.
 
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