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Author Topic: voltage regulator using LM 317T problem. mind helping here ?  (Read 1289 times)


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voltage regulator using LM 317T problem. mind helping here ?
« on: December 02, 2005, 02:14:46 AM »

i just soldered a fan speed controller, and i notice something really funny.

it's not giving full currrent when it's 12V,
i tested it with 3 x Luxeon white LEDs, the output current is around 20% less compared to plugging in normal PSU's 12V.
did some testing with fans rated at 0.3A as well, still not giving enough current at 12V, only gives out around 0.2A
however, the max current output is normal

so what could have go wrong here.


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Re: voltage regulator using LM 317T problem. mind helping here ?
« Reply #1 on: December 02, 2005, 11:04:28 AM »

Just to clarify:

You say it's not giving full current, do you maybe mean voltage? Or are you operating the regulator in constant-current mode?

What values are you using for R2 and R1 (if you're in voltage regulator mode)?

If by chance you are getting less than 12V from Vout, that is to be expected. The LM317T will drop about a volt and a half to do its work (regulating the voltage), so that regardless of 1.25V * (1 + R2/R1), Vout can't exceed Vin - 1.5V (approximately).

If you are getting less current to a known load, having the lower voltage would be the reason why. When you say it is "normal" from the 12V supply, that reinforces the point. I'm just not 100% clear from you're original post what you're expecting to see and how your regulator is set up.
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