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Increase 12 VAC

Wouldyoukindly

Mar 23, 2020
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Hey people I hope you can help..

I have some electronics knowledge but not not enough to help with this challenge so I hope you can help me

I am replacing a wired Gate intercom system with a wireless system..The gate is approx 1000 yards from the property and has an existing 12 VAC supply supplied via an underground cable going back to house. The NEW intercom system requires 12 VDC so I installed a AC TO DC convertor in the existing PSU box..

The convertor has a LM7812 regulator so I thought it was a simple job of connecting the AC supply to the PCB and hey presto 12 VDC O/P ...BUT nothing.. I have checked the Spec sheet of the LM7812 and it says to achieve 12 VDC output a minimum of 14 VAC is required, I have 14.8 VAC.. Either the PCB is faulty or the regulator requires a higher voltage input..I have sent the PCB back for a replacement but I was wondering if it isn't faulty what else could it be..I cannot change the AC input/ transformer to a higher voltage as the owners have no idea where the supply comes from in their house so I only have the existing supply. Is there a way to increase the 14.8 VAC incase that is the reason for not powering the regulator. ? I am also limited to space in the PSU box so cannot add too much circuitry...I need 12 VDC - 1 to 1.5 amp
Hope you can help..Yours Hopefully Wayne
 

bertus

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Nov 8, 2019
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Hello,

Do you have a schematic of your AC-DC converter?
When you rectify 12 VAC there will be about 15.5 Volts DC on the buffer capacitor.
This should be just enough to work with for the LM7812.
Keep in mind that the LM7812 might need some cooling.
When it gets to hot it will shut down.

Bertus
 

davenn

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Sep 5, 2009
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The convertor has a LM7812 regulator so I thought it was a simple job of connecting the AC supply to the PCB and hey presto 12 VDC O/P ...BUT nothing.. I have checked the Spec sheet of the LM7812 and it says to achieve 12 VDC output a minimum of 14 VAC is required,

Some important info missing that is needed for us to help you

does this regulator board have a bridge rectifier and smoothing capacitors ?
show a photo of it

I suspect you are sticking that 12-14 volts AC straight into the 7812 ??
you cant do that it will probably kill it


The gate is approx 1000 yards from the property and has an existing 12 VAC supply supplied via an underground cable going back to house.

Is the underground cable from the house mains voltage or 12VAC ?
If only 12VAC the voltage drop will be horrific
 

dave9

Mar 5, 2017
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Did you measure the 14.8VAC loaded (as input with the regulator board attached and powering the intercom) or unloaded, with nothing attached?

I suspect you're suffering too much voltage drop. Is your 12VAC supply at the house end, so instead of supplying high mains voltage at the gate to it, you are suffering a voltage drop from 1000' feet of wire? This could be significant at 1A or more current, depending on wire gauge. Thus decreasing the wire gauge could be one option, though the most expensive and labor intensive one.

Next, instead of an LM7812, I'd just an LDO, low dropout regulator, OR a switching buck regulator. Either will have less voltage dropout than the LM7812. Next, I would consider using a rectifier bridge made from schottky diodes instead of silicon diodes, which should cut in half (or more) the dropout loss from the diodes. If your regulator board had the diodes already on it, you could wire-jumper them out of the circuit and supply your own schottky bridge rectifier. This does not need to be large and take up a lot of the limited space, that could be air wired and tacked onto the circuit anywhere, just upstream of the smoothing capacitor before the linear regulator, or if it's a switching buck regulator it likely already has sufficient capacitance on the circuit board. If it does not, having only a little ceramic cap (or whatever, less than a few hundred uF capacitance) then add a few hundred uF capacitor after the bridge rectifier.

Another alternative is simply use a higher voltage rated transformer, or use a complete switching PSU that outputs (higher than 12V to overcome the long wire run loss) DC then you don't need the bridge rectifier and again if the regulator board had a bridge rectifier circuit integrated onto it, that could be jumpered out to not suffer that additional voltage drop.

Ideally, considering available easy to find components, I'd go with a 24VAC transformer and a 12VDC switching buck PSU board. if there are temperature extremes there, i'd think about using a PSU board (or swapping them yourself) with solid capacitors rather than electrolytic, and either way if this is shoehorned into a small air-tight, weatherproof enclosure, then due to probable heat buildup, would pick a PSU module rated for significantly more current than you need so it has additional thermal margin. Otherwise, you could use a cast (so thicker than sheetmetal thickness) aluminum enclosure and mount the switching IC or transistor to it for heatsinking that sheds much of the heat external to the enclosure.
 
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Wouldyoukindly

Mar 23, 2020
4
Joined
Mar 23, 2020
Messages
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Hello,

Do you have a schematic of your AC-DC converter?
When you rectify 12 VAC there will be about 15.5 Volts DC on the buffer capacitor.
This should be just enough to work with for the LM7812.
Keep in mind that the LM7812 might need some cooling.
When it gets to hot it will shut down.

Bertus

Good morning Bertus,

Thank you for taking time to reply to my challenge.I have attached 2 images

I have attached the schematic but I cannot get the component info from the company that I bought it from..I will try and get specific values of them direct from the pcb.

Regards Wstabilised-power-supply-12vdc.jpg Diagram.jpg ayne
 

Wouldyoukindly

Mar 23, 2020
4
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Messages
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Some important info missing that is needed for us to help you

does this regulator board have a bridge rectifier and smoothing capacitors ?
show a photo of it

I suspect you are sticking that 12-14 volts AC straight into the 7812 ??
you cant do that it will probably kill it




Is the underground cable from the house mains voltage or 12VAC ?
If only 12VAC the voltage drop will be horrific
Morning davenn

It is a 12Vdc, 0.5A Stabilised Regulated Power Supply. Pre made

see images

Wayne Diagram.jpg stabilised-power-supply-12vdc.jpg
 
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Wouldyoukindly

Mar 23, 2020
4
Joined
Mar 23, 2020
Messages
4
Good morning Bertus,

Thank you for taking time to reply to my challenge.I have attached 2 images

I have attached the schematic but I cannot get the component info from the company that I bought it from..I will try and get specific values of them direct from the pcb.

The 14.8 VAC measured was under load with the PCB connected....

Regards WView attachment 47474 View attachment 47475 ayne
More i
 

bertus

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Nov 8, 2019
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Hello,

This is stated on the page of the kit:

As supplied without heatsink it is quite safe to draw 500mA continuously. This is a very conservative current rating as the circuit is capable of delivering 1 to 1.5A if the regulator IC is fitted with a suitable heatsink (e.g. Order Code HS-TO220 for loads of up to 1 Amp) and properly cooled. You would also need to up rate the input transformer accordingly.

https://quasarelectronics.co.uk/Item/smart-kit-1061-stabilised-power-supply-12vdc-500ma

I would also fit a larger buffer capacitor when you want to draw more current as the mentioned 500 mA of the kits page.
This could be a replacement for the buffer capacitor:
https://www.rapidonline.com/vishay-2222-021-17472-4700-f-20-40v-axial-electrolytic-capacitor-49-2162

You could also replace the diodes with schottky diodes as suggested by @dave9 .
These could be used and will have a larger current capability:
https://www.rapidonline.com/diotec-sb5100-schottky-barrier-rectifier-diode-100v-5a-do-201-64-9030

Bertus
 
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73's de Edd

Aug 21, 2015
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Sir Wouldyoukindly . . . . . . . . . . . . ( do what ? )

Seems like you have already been given all of the finite technicalities for the creating of your end solution.
EXCEPT for the very starting point.
You need to get meter in hand and place it in AC mode and monitor the two wires of AC at the "gate" and take a reading and drop close to a 12 ohm value of resistor across the monitored leads.
That is being JUST for the instant that it takes for a reading to come up and be made. That thereby permits the use of less than the expected 12 watt resistor that would be needed for a constant measurement of 1 amps of current consumption of the new equipment.
WHAT voltage is now present under a 1 amp simulated equipment loading . . . . with that imparted supply line resistance loss ?

Then since you didn't give us ANY info on the new equipments power drain, I would take the unit out and connect to an unlimited 12V
power source capability . . . . like your car / truck battery. This is to be done with your metering in DC current mode and in series with one of the battery supply leads.
Then you will know the actual current . . . .to be computed to wattage . . . . that is being consumed by your new equipment if you put it thru all of its functional capabilities and use worst case consumption.

I would expect that they might be:

  • Intercom talk mode
  • Intercom " RING" mode
  • Power used by the relay that activates your power gate mechanism ***

***( If this unit is being just some mousey mick "fob" RF Relay design similarity . . .it might require an intermediate relay with somewhat heftier contacts for adequate gate mechanism A.C. brute power interfacing .

SO let us know what that available 12VAC voltage, pulls down to, under a 1 A load simulation, if its being required to travel 1000 yards . . . .and that is crossing a WHOLE bunch of neighbors yards!

(And some how ? why is it ? that I don't expect them to have made that long run with underground grade Romex with 14 ga wires. )

Test and let us know now, to see which solution to use . . . . . all is hinging upon the new equipments current consumption paired up to the loaded voltage that you read on the resistor load test, with its incurred line loss.


73's de Edd . . . . .


I just now found out that you can make your water-bed much, more bouncy by filling it with spring water !


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