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Old 28th January 2011, 09:53 AM   #1
roblio is offline roblio  Dubai
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hi everyone im a beginner to this stuff i just want some help regarding transformer yesterday i found some transformer in our stock room with a rating 220vac primary and 13vac secondary @ 5 ampere i hook up a rectifier diode and did make a fullwave now i make a test in output to make it DC it reads 20vdc is this normal?can i used it to power up my car audio amplifier?because im planning to make a subwoofer with a car amplifier which only need 12vdc to power up.follow up question what should be the rated dc output with a 13vac secondary output?
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Old 28th January 2011, 11:25 AM   #2
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13Vac with full wave rectification will make 13V * SQRT(2) = 18.3V; 20V without load makes sense. Make sure your transformer can provide enough power for your amplifier. Check its 'VA rating'.
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Old 29th January 2011, 04:57 AM   #3
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Quote:
Originally Posted by mastodon View Post
13Vac with full wave rectification will make 13V * SQRT(2) = 18.3V; 20V without load makes sense. Make sure your transformer can provide enough power for your amplifier. Check its 'VA rating'.
thanks for the response
so you mean to say i can't hook this up in my car audio amplifier because the max voltage is 15.5vdc right?i got lightning audio LA-600m model i just want to make sure that my amp will not be damage when i connect this transformer to my amp.
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Old 29th January 2011, 06:19 AM   #4
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The rectified voltage will be too high,also,the current rating is a bit low.
Something like a 9-10Vac 10Amp(or more) transformer would be suitable for running a car amplifier.

Edit: That amp claims to be 600W,you're gonna need a hefty transformer for that.
Do you have any old microwave ovens around? You could possibly rewind the transformer from one and get enough power to run your amp.
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Old 29th January 2011, 09:38 AM   #5
AndrewT is offline AndrewT  Scotland
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Originally Posted by DigitalJunkie View Post
Do you have any old microwave ovens around? You could possibly rewind the transformer from one and get enough power to run your amp.
No, no, no !!!!
The level of competence of this Member has already been shown. Do not suggest he/she wind their own mains transformer.
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Old 29th January 2011, 09:41 AM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by DigitalJunkie View Post
,the current rating is a bit low.
That amp claims to be 600W,you're gonna need a hefty transformer for that.
600W of total maximum output power normally requires a transformer rated from 600VA to 1200VA.

A 10Vac transformer meeting this specification would have a current rating from 60Aac to 120Aac.

Car Audio is silly. Don't make it any sillier.
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Old 30th January 2011, 07:02 PM   #7
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Quite, for both posts Andrew.

The safer way for the OP to do it would be to use a car battery, and recharge the battery as required from a correctly rated (commercial) SLA charger. And even then using lead-acid batteries indoors is probably a bad idea - esp. at high discharge rates.


ETA: I always figure car amp ratings are for teh birds. It'll be something like ' 600w = 2 channels added, which each are PMPO into 1ohm' or about 600/8, 75w into each channel if generous. It still requires n internal SMPS, but probably limits PSU draw to somthing like 8-10A. Even so, even large car batteries (i.e non traction-rated lead-acids) will droop a lot under such loads. A big lump like the spare 90Ah unit I use (for testing some low-noise toys) will only last about an hour and a half under such a load. That's because SLA amp-hour ratings are non-linear, and usu. based on a notional '20hour' rate, Ie how much current it will supply when new to draw-down to 10.8v after 20rs... And unless it's a traction /'leisure' ie marine-use battery, such gross charge/discharge use will soon kill it. Car batteries are for 'float' use only!

Last edited by martin clark; 30th January 2011 at 07:21 PM.
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Old 31st January 2011, 11:31 PM   #8
Minion is offline Minion  Canada
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You might be able to get a transformer from a car battery charger that might work ..... my charger has a 70A setting for starting the car , but still that is a lot of current to be playing with and could be dangerous .....
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Old 31st January 2011, 11:54 PM   #9
norture is offline norture  Indonesia
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Some car audio amplifiers only contain a pair of output transistors/channel, so if you are in for a mod then it'll be better to add another pair on the unused space inside the amplifier casing, as I did with my Boss 4x250W amp (true rating btw). This will make the amp stand better against more demanding cases.
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Old 5th February 2011, 06:06 AM   #10
roblio is offline roblio  Dubai
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hi everyone i did find some 12vdc 3a transformer that's manufactured ready i did hookup the amp and it works fine the only problem is that the transformer got heats up easily its because it drops current when a low freq was draw from amp.now i'm still afraid to used this 13vac 5a transformer to hook my amp. i just need some advice if its 100% safe to used this transformer hoping for the best advice. thanks
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