bypass 240v to 12v 500ma transformer and run from a 12v battery

#1
Ok my "Flyball timer" project is progressing well v0 of the console built and have interfaced with the lights and timing gates without a problem.

The timing gates currently work from a 240v to 12v transformer(rated output is 500mA) which when not at home is powered from a 12v battery to 240v inverter which seems a waste.

My questions is straight forward am I correcting in thinking can power the gates directly from the 12v battery providing I install a 24 ohms resistor....possibly I will need a 12v regulator??

Thanks in advance.
 

rossko57

Senior Member
#2
I don't really understand the questions, can you expand ...

You're thinking you might need a 12V regulator to get 12V from a 12V battery?

What's the 24 ohm resistor for, what are these 'gates' that need powering in electrical terms?
 
#3
I don't really understand the questions, can you expand ...

You're thinking you might need a 12V regulator to get 12V from a 12V battery?

What's the 24 ohm resistor for, what are these 'gates' that need powering in electrical terms?
Sorry thought it was clear:

the original setup is as follows

12V battery - 12v to 240v inverter - 240v to 12v 500ma transformer - timing gates (The gates are 8 photo electric sensors, exact model unknown as yet)

can I just go

battery - timing gates

or will I need

battery - resistor - timing gates
(resistor to drop it to 500ma
I have been told I can not just connect the battery as its pushes out say 80ah which will fry the gates as they only need 500ma hence the resistor personally this does not make sense to me)

or

battery - resistor - regulator - timing gates

the regulator was to confirm it was defiantly 12v out as the AC to DC transformer would be 12v and not fluctuates higher like a normal battery.

* I am nervous regarding the gates as each sensor is c£45 each!!
 

Haku

Senior Member
#4
A device that takes 500mA @12v will still take 500mA even with a 12v power source capable of delivering 80A.

The real problem you face here is likely to be the voltage, a lead acid battery (which I assume is what you're using here) has a nominal voltage of 12.6v and when charging, up to 14v.

You're certainly right that up converting to 240v AC and then down to 12v DC is wasteful, you want something like this DC-DC converter:
http://www.ebay.co.uk/itm/350769448474
Wide variable input voltage with a fixed (adjustable to your needs) output voltage.
 

MPep

Senior Member
#5
80Ah is a capacity measurement.
When battery terminals are short-circuited (KIDS DO NOT TRY THIS AT HOME) it is possible for 200+ A to flow.
A circuit that is designed to work from 12V will only use the current required.
I have been told I can not just connect the battery as its pushes out say 80ah which will fry the gates as they only need 500ma hence the resistor personally this does not make sense to me
Too right, makes no sense to me either. A battery cannot 'push' current out, it will supply the current that the load can handle and demand.

A wall-wart rated at 500mA, means that it can supply upto 500mA, but if the load (circuit) only requires 20mA, then that is all that gets drawn from the wall-wart.

Make sense?

The more conversions there are in a power system, the more wastage.
 
#6
The timing gates currently work from a 240v to 12v transformer(rated output is 500mA)
Is the 12 Volt output from the "transformer" a.c. or d.c ?

If d.c. then yes what you intend is possible, but add a fuse in the circuit close to the battery. Would seem like a 500 mA or at most 1 Amp is "gates" have any surge would suffice.
 

PaulRB

Senior Member
#8
Crazynight, is the transformer regulated? If you connect your dmm to it and it reads 12 with no current flowing it probably is. If it reads significantly over 12 it isn't.

Either way, you will probably be fine connecting the battery direct to the gates. Connect a 500mA quick blow fuse in line to protect your gates. If you can measure the current flowing out of the transformer when in use, you may be able to choose a lower fuse still.

Paul
 

rossko57

Senior Member
#9
You might like to add a diode to the power input of your timing gear, to protect against reverse polarity battery connection - sooner or later this will happen in the field.
 
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