voltage drop

[ThSpeakerDude88]

hey I have a question. You cn see my amp eleswhere on the forums, its the one built into a dockers tin. I need to drop 32 volts down to 12 volts so I can run a computer cooling fan in it. I also plan on useing the circuit at 48 volts later on, but for now I'm only useing 32 volts. How do I drop the voltage simply? 4-H is coming up this weekend ( saturday) and I'd like to see if i can get it done before then. Thanks!

also, what kind of a heat sink should I use for the TDA7264? ( 25w+25w @ 22.5v +/-) I need it to fit into the dockers tin I have the amp in right now, the heat sink I'm useing is way too small I think, it gets very hot to the touch, you dont want to hold onto it for half a second.

[homer09]

if your fan does not draw much current, you can simply use a resistor of appropriate wattage/resistance.

if you're adding a fan, possibly the heat sink you have now will be enough, a fan will improve the thermal dissapation of your heat sink significantly.

[ThSpeakerDude88]

thats good news! thanks for the quick reply! so what resistor should I use, and what wattage? my power supply is 32 volts @ 940 ma ( yes i know a little low, but its all ive got for now) and the fan im useing is just a small cheap pc fan that cant draw more than .5 amps ( 500 ma) of current. hey will dropping the voltage affect in any way the voltage across the 32 volt rails that supply the amp?

[dsavitsk]

My PC fan only draws 50mA. You might want to check as it will make a big difference in the resistor you use.

But, assume .5A

V = IR or R = V/I

You want to drop 20V so R = 20/.5 = 40R

Keep in mind that P = IV

Thus, you are going to need a resistor that can dissipate over .5 * 20 = 10 Watts.

[homer09]

ok to clear some things:

your amp will run off 32V rails, parellel to your fan. refering to basic electronics, this means the fan should not affect the voltage of the 32V rails, BUT they must share the meager 940mA.

now, max draw of the fan is not important. what you want to know is, what it will be drawing during normal use. many people actually run fans at lower voltage to reduce noise. so what i say is test what speed you need for enough cooling (ie what voltage). then with your DMM measure how much current draw.

now this number, if its a significant percentage of 940mA, prepare for a noticeable handicap on your amp's current driving ability with the fan spinning. if its not to bad, using ohm's law, choose a resistor value that will drop your voltage to its desired amount. using power law, calculate how much heat disipated in watts using the current you measured above. this will determine the rating of the resistor (+ some headroom of course)

if you just want a fan in there because u feel its the only way to cool your amp enough, i highly suggest finding a larger/better heatsink. its best to avoid fans because of current draw and noise.

EDIT: took too long to post

[ThSpeakerDude88]

ok.. I dont know much about ohms law so thats why I'm asking. So does this mean mu fan is going to suck 10 watts of power out of my amp? maybe my fan does draw less current, I know that it draws very little thought. Thanks!

[dsavitsk]

Originally posted by ThSpeakerDude88
ok.. I dont know much about ohms law so thats why I'm asking. So does this mean mu fan is going to suck 10 watts of power out of my amp?

More. P = IV where the fan's current is multiplied by the rest of the voltage drop and added to the power dissipated by the resistor. Thus, more like 16 Watts. These are watts from the PS, which is not the same as watts given by the amp to the speakers.

Originally posted by ThSpeakerDude88
maybe my fan does draw less current, I know that it draws very little thought. Thanks!

I would bet that your fan fraws about 0.05A. My fan is a standard computer case fan (6cm) and that's what it draws. Thus if this is the case, and if you want to run it at ~12V, a 390R resistor is a good choice. Here, you are only dissipating 1W through the resistor which is a bit more manageable (though I would us a 2W, or 2 1W 180 or 220R resistors in parallel). Or, it looks like radio shack carries 100R/1W resistors. Put 4 in series with the fan.

Do check the current rating again. I looked at the fans at radioshack, and they all draw a ton more current than mine -- anything from .2 to over.3A.

-d

[asgorath]

I was gonna recommend a resistor divider setup for you, but it seems you have that under control.

From what I've heard however, adding a brushless DC fan on the same power rails as your amp might add some noise. You might want to consider using an AC fan connected before the transformer so it can't possibly do anything to your amplifier's power rail. It won't change the voltage, but it might add dirty noise.

Just a thought.

[homer09]

Originally posted by asgorath
I was gonna recommend a resistor divider setup for you, but it seems you have that under control.

From what I've heard however, adding a brushless DC fan on the same power rails as your amp might add some noise. You might want to consider using an AC fan connected before the transformer so it can't possibly do anything to your amplifier's power rail. It won't change the voltage, but it might add dirty noise.

Just a thought.

that is yet another valid reason to avoid fans...

listen, the best and easiest way to see how much current your fan draws is by measuring it! get yourself a few batteries or something that supplies 12V and a variable resistor (or many different value fixed ones). test your fan at different voltages until you think it spins just fast enough, max speed at 12V is most likely not necessary, it will just create more noise and waste more current. then with an ammeter in series, measure the current draw. use this to calculate resistor. <BEST WAY, anything else is a guess

[ThSpeakerDude88]

wow thanks for all this info. I understood the post by dsavitsk better now! thanks! Well this is gonna sound dumb, but I dont have a multimeter right now.. *bes embarrased to be in electrical work w/o one* planning on getting one as soon as I get the money. Oh and in responce to the last one, I'm useing a regulated power supply from a printer so theres no A/C I can hook it up to unless I use a seperate power adaptor ( which is what I did for my fisher) I think I might have some 1 watt and higher resistors in my parts box ( I took apart an old tv, its sure to have them) So 100 or 180? ( thats single ohms, not k ohms right?) aye! Wish I had a multimeter! ok lets say I want to drop the voltage to the fan to around 6 volts ( I run my fisher's antec fans at 7.5 and their practically silent) what resistance should I use? also , do you mean adding resistors like this?





_____ +
r|___
___|fan leads
__r|___ <ground