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Help with power supply

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rarcher252

New Member
I'm in over my head, I built a train table for my son and I bought a kit to supply train sounds. The kit is battery powered, 2 AA batteries, but I want to connect it to the power supply from the model train transformer.

The sound kit requires a power supply of 2.4V to 6V. It says is has a standby current less than 1 uA and the operating current is 0.17 to 0.23 mA.

The model train transformer power pack that I want to connect it to is described as follows:

Power pack Input: 120V 50/60 Hz, Output: 17VDC 20VAC, Max output 7VA total. (AC is US Standard)

I was told that I could use an LM7805 voltage regulator to run the sound board on the model train power supply? Will that work? If so, how does it connect?

I'm a cop not an engineer, so dummy it down for me! Thanks!
 

MikeMl

Well-Known Member
Most Helpful Member
Read the LM78L05 data sheet (click on this), and write back if you still have questions. Use the one in the TO-92 package.
 
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Electronworks

New Member
I am surprised no one else has commented on this post, so here I am diving in with both feet...

the 7805 is a good choice of regulator to use, but you need to think about the power you are going to be dissipating in it.

The input to your 7805 is 17V and your output is 5V. This is 12V across the component. With a current flowing through it of 0.23A, this represents 2.76W.

Anything over about 0.3W and the 7805 will start to get hot

Personally, I would use a separate 5V regulated (and it must be regulated) wall cube power supply (a plug with the power supply built into it)

You could also use a 3W series resistor to drop some of the voltage (about 40 Ohms) then this will share some of the heat between the resistor and the 7805
 

MrDEB

Well-Known Member
this should get you started

the enclosure could be bigger then you could put the sound kit into same box.
suggest using gromets to run wires from wall wart to box.
or you could buy the adapter plugs (way over priced)
the heat sink may be overkill but better than getting to hot which it shouldn't.
as mentioned the leftover power 17v - 3v has to go somewhere.
here we are starting out with 9volts reducing to 5 volts.
Radio Shack has a 4.5 wall wart but then you can't use the 7805.
 

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audioguru

Well-Known Member
Most Helpful Member
The OP said the max current of the sound circuit is only 0.23mA, not 0.23A.
So the little 78L05 will dissipate only 3mW. Its idle dissipation without a load is about 65mW.
 

bountyhunter

Well-Known Member
I'm in over my head, I built a train table for my son and I bought a kit to supply train sounds. The kit is battery powered, 2 AA batteries, but I want to connect it to the power supply from the model train transformer.

The sound kit requires a power supply of 2.4V to 6V. It says is has a standby current less than 1 uA and the operating current is 0.17 to 0.23 mA.
Just get a cheapo 5V wall cube. The sound card doesn't need perfectly regulated DC, if it did the VIN range would not be so wide. You could get a 9V wall cube and feed it to a 78L05 for a regulated supply, but I doubt it will improve performance.
 
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MrDEB

Well-Known Member
I figured the op is a newbi

so off to Radio Shack for parts.
RS dosn't carry the 78Lxx regulators.
 

Mr RB

Well-Known Member
I'd re-check the operating current of that module. 0.23mA sounds absurd. I've got some tiny talking applicances here and even turning them right down (to very quiet but still audible) driving 8 ohms they still consume 10mA or so.

0.23mA average into 8 ohms is less than a microwatt?
 

MrDEB

Well-Known Member
If the regulator needs more current draw

as someone suggested, perhaps just add an LED power on indicator
 
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