help with transformer/voltage reg

[npicot2304]

Hi all

just wondering if someone is able to help me out, i have an old sharp boom box that i know still works but i want to hook it up in my 12vDC solar system, i have done a little bit of investigation with the unit and found that there is a battery compartment that accepts 8 D cell batteries and on the rating plate it says its compatible with 12vDC, so i proceeded by opening the unit up to start investigation on how i can make this happen, i found a transformer off the main power port that also connected to the battery contacts, the transformer part number is a PT-1579A to which i punched in to google to see if there was such an animal but nothing found apart from a P-1597NL, which was a transformer and DC/DC converter in one (from what i read in the datasheet).

now the issues i have are

1. my solar is regulated to 14.6V DC so i am not sure if that is going to be a huge issue or not

2. i have 3 wires that lead from the battery contact board, red, black and orange. the red and the black link up with the the main power wires going to the main board, the orange leads from the battery contacts them selves and goes to the transformer which links up with the red wire after the transformer so i think i should be able to tap in to that

3. i only have 1 shot at this and i dont want to let the smoke out of it

i have enclosed some pics so people might have more of a clue as to what i am going on about

[SeanB]

Are you going to use it on solar only, and are going to remove the transformer entirely? If so then simply connect the solar wiring to the AC inputs of the bridge rectifier and leave a cut off AC cord in the input socket to keep the switch closed so it thinks it is running off AC power.

Otherwise use a simple connection to the wires off the battery pack, and include a fuse ( 5A will be fine for pretty much any small boom box running off batteries) in the line, and a diode (1N5401) in series to prevent damage from reversed power leads.

Did the same to my radio, as the battery was already there, solar power is free once you have the equipment, the radio uses 10W as power loss even if the power switch is off ( same as yours) and it still will work when the mains goes out. Plus D cells are both expensive, have a short life in the radio and tend to be left in place till they leak.

[jitter]

1. my solar is regulated to 14.6V DC so i am not sure if that is going to be a huge issue or not

Power it from the mains and measure the voltage across the + and - pins on that big bridge rectifier. If it's closer to 14.6 V than 12 V, I'd say it's not going to be an issue. There will be some sort of regulation inside the boom box anyway.

If you disconnect the transformer and connect the solar system at the AC terminals of that rectifier you achieve two things:
- a voltage drop of about 1.2 V behind the rectifier, leaving a more comfortable 13.4 V;
- no polarity to worry about, the bridge rectifier takes care of that.

2. i have 3 wires that lead from the battery contact board, red, black and orange. the red and the black link up with the the main power wires going to the main board, the orange leads from the battery contacts them selves and goes to the transformer which links up with the red wire after the transformer so i think i should be able to tap in to that

Unplugging the mains lead from the boom box most likely operates a switch that connects the batteries instead of the transformer. So yes, that's also a way in.

3. i only have 1 shot at this and i dont want to let the smoke out of it

Well, that's for you to risk and find out...   

[npicot2304]

Are you going to use it on solar only, and are going to remove the transformer entirely? If so then simply connect the solar wiring to the AC inputs of the bridge rectifier and leave a cut off AC cord in the input socket to keep the switch closed so it thinks it is running off AC power.

 i will be keeping the AC available so if in the future i want to hook it back to 240vAC i am able to

Otherwise use a simple connection to the wires off the battery pack, and include a fuse ( 5A will be fine for pretty much any small boom box running off batteries) in the line, and a diode (1N5401) in series to prevent damage from reversed power leads.

i was thinking about doing that but as the solar panel charges the battery and the controller reaches is maximum voltage of 14.6vDC i am not sure if the transformer/voltage reg will handle it

Did the same to my radio, as the battery was already there, solar power is free once you have the equipment, the radio uses 10W as power loss even if the power switch is off ( same as yours) and it still will work when the mains goes out. Plus D cells are both expensive, have a short life in the radio and tend to be left in place till they leak.

yeah i looked and thought about that option as well and came to the same conclusion, solar + 12v battery + controller = almost completely free power

thank you for your help 

[npicot2304]

1. my solar is regulated to 14.6V DC so i am not sure if that is going to be a huge issue or not

Power it from the mains and measure the voltage across the + and - pins on that big bridge rectifier. If it's closer to 14.6 V than 12 V, I'd say it's not going to be an issue. There will be some sort of regulation inside the boom box anyway.

If you disconnect the transformer and connect the solar system at the AC terminals of that rectifier you achieve two things:
- a voltage drop of about 1.2 V behind the rectifier, leaving a more comfortable 13.4 V;
- no polarity to worry about, the bridge rectifier takes care of that.

2. i have 3 wires that lead from the battery contact board, red, black and orange. the red and the black link up with the the main power wires going to the main board, the orange leads from the battery contacts them selves and goes to the transformer which links up with the red wire after the transformer so i think i should be able to tap in to that

Unplugging the mains lead from the boom box most likely operates a switch that connects the batteries instead of the transformer. So yes, that's also a way in.

3. i only have 1 shot at this and i dont want to let the smoke out of it

Well, that's for you to risk and find out...   

think i am going to have to have a crack at it and hope it all works

thank you

[npicot2304]

Well I bit the bullet and plugged it into mains power....... Something I very rarely do due to being bitten before, and strangely the output from the connector that leads off to the main board of the unit is getting 17vdc........ So in saying that I should be fine in putting 14 odd volts onto the battery connector, but my only concern now is that if I do that it might go above that 17 volt output, looks like it will be trial and error from now on in lol

[jitter]

Well I bit the bullet and plugged it into mains power....... Something I very rarely do due to being bitten before, and strangely the output from the connector that leads off to the main board of the unit is getting 17vdc........

I kind of expected this. Small transformers tend to have quite a difference between nominal output (loaded) and unloaded output. If you were to run the boombox at a somewhat higher volume, you'll notice it'll drop closer to the expected 12 V nominal.

So in saying that I should be fine in putting 14 odd volts onto the battery connector, but my only concern now is that if I do that it might go above that 17 volt output, looks like it will be trial and error from now on in lol

Well, as SeanB and I have remarked, unplugging the mains lead from the boom box most likely operates a switch. With the lead in place it'll connect the transformer, with the lead gone, it'll connect the batteries. If you don't change this, you should be fine, it'll only have one or the other power source connected, never simultaneously.

What I might do is drill a hole in the battery compartment lid to mount a DC socket in, wire it up to the battery spring clips with a fuse and diode in series (just in case) and then connect it to your solar system using a DC plug like that on wall warts (or better isolated plugs and sockets might even be nicer).
This way you keep the boombox's circuit orginal and you can keep running it off the mains when needed.