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  #1  
Old 04-23-2013, 03:38 AM
davidGG davidGG is offline
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Default Transformer/Heatsink question

Hello everybody,
I have a transformer rated at 36VCT 0.8A. If I use a rectifying diode and a voltage regulator (LM 317T) to lower the voltage to 5VDC to charge my kindle fire rated at 5V 1.8A, will the transformer be able to handle this current?
Also, the rectified voltage is 10VC which means I will be dropping 5V, so the total power dissipated will be 9W! Will a standard heat sink suffice or should I design this project differently?

Thank you for the help

Transformer link: http://www.allelectronics.com/make-a...SFORMER/1.html

Heat sink: similar to
http://www.ebay.com/itm/2-Pcs-TO-220...item337cba997a


edit: yes I know I can just buy a wal-wart and simplify everything but I'm trying to use the items I currently possess before buying anything new.
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Old 04-23-2013, 03:44 AM
gerty gerty is offline
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The transformer you want to use isn't a good fit, voltage is too high, current too low. Maybe this'll help answer some questions.
http://www.hammondmfg.com/pdf/5c007.pdf
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Old 04-23-2013, 04:02 AM
davidGG davidGG is offline
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Quote:
Originally Posted by gerty View Post
The transformer you want to use isn't a good fit, voltage is too high, current too low. Maybe this'll help answer some questions.
http://www.hammondmfg.com/pdf/5c007.pdf
Thank You.
Just to clear things up for the future, if the transformer is rated at 36V 0.8A, I can NOT draw 1A at 5V? Meaning 0.8A is the absolute maximum I can draw (without dieing)?
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Old 04-23-2013, 04:47 AM
gerty gerty is offline
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Quote:
Originally Posted by davidGG View Post
Thank You.
Just to clear things up for the future, if the transformer is rated at 36V 0.8A, I can NOT draw 1A at 5V? Meaning 0.8A is the absolute maximum I can draw (without dieing)?
That's correct .8 amps is max. And to regulate the voltage down to charge a 5 v battery, you'll have to drop about 40 volts across the regulator, which is way too much.
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Old 04-23-2013, 08:35 AM
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crutschow crutschow is offline
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1.8A @ 5V from a 36V 0.8A source can be done using a switching buck regulator. The practical maximum including the switcher efficiency and proper transformer derating would give about 3A @ 5V.
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  #6  
Old 04-23-2013, 07:46 PM
gerty gerty is offline
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Quote:
Originally Posted by davidGG View Post
Hello everybody,
I have a transformer rated at 36VCT 0.8A. If I use a rectifying diode and a voltage regulator (LM 317T) to lower the voltage to 5VDC to charge my kindle fire rated at 5V 1.8A, will the transformer be able to handle this current?
Also, the rectified voltage is 10VC which means I will be dropping 5V, so the total power dissipated will be 9W! Will a standard heat sink suffice or should I design this project differently?

Thank you for the help

Transformer link: http://www.allelectronics.com/make-a...SFORMER/1.html

Heat sink: similar to
http://www.ebay.com/itm/2-Pcs-TO-220...item337cba997a


edit: yes I know I can just buy a wal-wart and simplify everything but I'm trying to use the items I currently possess before buying anything new.


I was trying to keep within the posters specs,,,
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Old 04-23-2013, 10:00 PM
Potato Pudding Potato Pudding is offline
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Using a switcher as either a preregulator or for all of the regulation will help you get your voltage down AND your current up.

That is efficient.

Basically you can drop a switched pulse of 1.8A current across an Inductor, down from 50 to 5V.
An input filter capacitor will smooth out that 1.8A to less than 0.8A continuous from the transformer. In fact that input current will likely average around 200milliamps.

The magnetic field in the inductor will have enough power at a low duty cycle to provide the 1.8amp current during the off switched cycle.

RB has a two transistor switcher design on his website.
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Old 04-23-2013, 10:49 PM
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With what you have, the bottom left drawing in the Hammond document is the best way to go analog. You will get .8A DC with about 24V on the first filter capacitor. That's 3 times the voltage you need, demonstrating that this transformer is really not right for your needs.
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