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Old 22nd December 2007, 06:44 PM   #1
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Default Battery supply.

My turntable is fitted with a dc motor and speed control board. I use a stabilised power supply set at 7.5 volts to drive the board instead of the nasty wall wart supplied.I wish to use a 12volt battery instead of the psu so I need a circuit that will accept the 12 volt battery, supply 7.5 volts to the motor, and will auto charge the battery when it gets down to around 8 or 8.5 volts with visual indication. The battery will be a Yuasa or similar lead acid sealed type rated at 17 ah. Can anyone help or is there a ready made unit able to do this?
Regards, Si.
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Old 22nd December 2007, 09:24 PM   #2
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It's probably easier and almost certainly more functional to just alter the DC controller to run on 12.6V and use a standard commercial SLA charger.
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Old 22nd December 2007, 09:50 PM   #3
paulb is offline paulb  Canada
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An SLA charger is not that hard to build. Check out the datasheet and especially the application note for the UC3906. For the 7.5 volt regulator, an appropriate circuit depends on what current the motor draws. If it's less than 1 amp or so, an LM317 should work nicely.
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Old 22nd December 2007, 10:31 PM   #4
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Thank you both for your advice.The motor draws well under an amp. I'm an electrical engineer and enthusiastic diyer but certainly not an electronics engineer so I need someones help here. I'd like everything in the same case as this will be in my living room. So all I require is a circuit to give me a steady 7.5 volts from the 12 volt battery and another that will turn on the charger when the battery gets down to around 8 Volts. I can put it together once I get the basic outline.
Cheers, Si.
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Old 22nd December 2007, 11:08 PM   #5
gootee is offline gootee  United States
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Why do you want to use a battery? They are not perfect DC supplies. A very good regulated supply can be better, and more reliable, and probably safer, and certainly less of a bother.
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Old 22nd December 2007, 11:10 PM   #6
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Remember, you never want to discharge a 12V lead acid below about 10.5V. If left below that for a length of time, there's a good chance it will never take a charge again. I'd wire up an LM317 for the output voltage you need, and figure out the average amount of energy you'll use, based on your playing duty cycle. Then I'd just trickle charge through a resistor to exceed the demand slightly. Check the data sheet for the battery for the maximum recommended continuous trickle charge. Unless you play LPs constantly, or the battery is small, that might be be all you need. Another way is to float the battery at a fixed voltage through a limiting resistor to isolate the tt system. The battery then acts as a giant filter and current reserve. Given the current capability of a lead acid battery, I'd also include a fuse.
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Old 23rd December 2007, 12:08 AM   #7
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Thank you all for your advice. I'm convinced that my tt will be improved with a battery power supply. After all if we didn't try these daft things our hobby would be the worse for it I feel.So I shall initially build something around an LM317 and use a fully charged battery and see what the results are. If I'm suitably impressed then I'll have another look at the auto charging. I don't want any mains connection whilst the motor is running.
Happy Christmas all, Si.
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Old 23rd December 2007, 12:48 AM   #8
gootee is offline gootee  United States
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"Y' gotta do what y' gotta do.", as they say.

But take a look near the bottom of this page:

http://www.tnt-audio.com/clinica/reg..._noise4_e.html
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Old 23rd December 2007, 01:29 AM   #9
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My own Version of Mark Kelly's Speed Controller
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Old 23rd December 2007, 01:56 AM   #10
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Quote:
[i]

But take a look near the bottom of this page:

http://www.tnt-audio.com/clinica/reg..._noise4_e.html [/B]
I don't know what drugs whoever ran that trace was on but I hope they were good.

I've tried to replicate these results on ordinary SLAs and can't come close.

Feeding the SLA's output to an HP3580A spectrum analyser and dialling it to 100uV full scale I get a trace at the -60dB mark with a couple of small bumps at 100, 200 and 400 Hz which I put down to mains harmonics. That's -60 dB with reference to the 100uV full scale so we're talking 100nV, or -150dB referred to the battery's DC voltage.

Same result with the battery sending 200 mA through a wirewound resistor.
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