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#1 stevenrob

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Posted 13 October 2019 - 10:28 PM

Hello guys. I have a boombox whose transformer seems to have heated a lot, but I have no indication on the transformer. I would like to buy onea on aliexpress. In general, what power is needed to properly power a boombox? Thank you

 



#2 Fatdog

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Posted 14 October 2019 - 09:57 AM

I am thinking it really depends on which boombox it is.  Which one seems to be dead?



#3 stevenrob

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Posted 16 October 2019 - 12:08 AM

Hi, a crown sz-5100 (15V).



#4 Superduper

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Posted 16 October 2019 - 02:21 AM

That Boombox is not that powerful, probably 30VA is sufficient. The question of actual transformer specs (i.e. volts) will depend a lot on the type and design of the power supply board. What kind of rectifier? Half wave, full wave, bridge type? Capacitor? All makes big difference in final DC volts. Don’t confuse AC volts with the DC output after rectification. A transformer with 15v ac secondary is going to be much higher and almost certainly too high DC if ac output is 15v before converting to DC.

#5 Superduper

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Posted 17 October 2019 - 12:05 AM

Here is a link to Hammond, a transformer manufacturer with guidelines regarding the use of transformer, rectifiers and the resulting voltage/current outputs depending upon the type of rectification circuit.  As you can see, the output varies greatly depending upon the transformer board design.  Maybe it will help you to better understand.  Most quality big brand name boomboxes use a full wave bridge rectifier with capacitor ripple filters, but I have also seen cheaper built boomboxes use other designs including full wave center tapped designs.  I don't know about the Crown SZ-5100 because I don't own one, but I have seen one and it is definitely a cheap built boombox, even the handle bends when you lift it.  It's fair to say that they probably cut corners or design with as little parts as possible.  You should check to confirm first.

 

The full wave bridge with capacitor type designs (common) will have an output peak DC voltage of (input-voltage x 1.41).  I have seen 12.6v transformers show over 19v dc with no or low load.

 

http://www.hammondmf...m/pdf/5c007.pdf



#6 caution

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Posted 17 October 2019 - 10:36 AM

A 35-years-old transformer isn't going to look like new, the shellac and glue inevitably turns a shade of brown.

Doesn't mean it's bad.



#7 stevenrob

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Posted 18 October 2019 - 02:41 PM

Oulala !  Thanks guys. I saw the doc of Superduper. In my case, I think it is a "full wave bridge capacitor input load". So a 220VAC to 15VAC transformer = 15x0.9 VDC ?    I must buy a 220VAC to 18VAC transformer to have enough tension? 



#8 Superduper

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Posted 18 October 2019 - 05:13 PM

Attached File  Screen Shot 2019-10-18 at 6.08.24 PM.png   178.64K   0 downloads

 

No, that's not correct.  It's the peak (x 1.41) that you need to worry about.  The reason is that the average only applies at full or high load.  At light or no load, the peak voltage is pretty much the average voltage because the capacitor is going to smooth out and keep topping off the voltage.  Remember, these are not "regulated" power supplies, they are unregulated.  If you want to install a robust enough regulator IC to keep voltage at a constant 15VDC, you can, it's simple but will reduce power efficiency and batteries won't be able to supply the overhead needed to operate those IC's.  Many semiconductors have a max operating voltage limits.  Exceed those and they could go poof.



#9 stevenrob

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Posted 19 October 2019 - 10:12 PM

OK, thanks guys. Boomboxery is "THE" forum.