madmikejt12

Hi, I would like to lower a current from 15-17A to around 200mA is this possible, cheap and easy to do?

thanks,
Mike

Nigel Goodwin

You need to tell us exactly what you're trying to do, your post makes no sense!.

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madmikejt12

sorry, its for a in-car CD player (ive hooked it up to my speakers and a computers' PSU). I want flashing LED's that flash to the beat, i've measured the voltage at about 3V and the current 13-17A

Nigel Goodwin

For a start you don't measure the current like that - you will probably kill the PSU (and perhaps your meter?). As long as the voltage is correct (or reasonably near) the unit will only take the current it wants - as long as the supply can provide that much, or higher, it will be fine.

But NEVER place an ammeter across the output of a PSU, it's not measuring anything useful at all!.

Where you should place the ammeter is in series with the +ve lead to the player, it will then measure the current it's drawing.

Think of the PSU as a barrel of whiskey!, you only drink it slowly, a small glass at a time - you don't have to drink it all down in one quick swallow!.

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madmikejt12

it was the speaker outputs i put it across (+connection and -connection)

the current was from the speaker output from the cd player, not the power supply, it is different isnt it? If i connected an LED with current limiting resistor, the LED would blow wouldnt it?

Nigel Goodwin

That's an even worse place to put it, and is even more useless - and is VERY liekly to blow the amplifier.

No it wouldn't, you've just said CURRENT LIMITING RESISTOR, that will limit the current to the value you design. But in this case as well, any current capability of the output is completely irrelevent - it's the VOLTAGE you need to know. But neither that, nor the current can be measured by a DMM.

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madmikejt12

if its the voltage i need to know, but cant measure it..... does this mean that i can either risk it or not try it?

is it safe to put a large resistor and an LED directly to the amplified speaker output?

Nigel Goodwin

Use ohms law - presumably you know the claimed output of the amplifier? (which will probably be far higher than it actually is), and the speaker imedance - it's simple to calculate the peak or RMS output voltages.

Yes - however, why do you want to do this? - it works really badly, you need to filter the audio and use a monostable to give a decent flashing LED effect.

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madmikejt12

it says on this PDF i have:
Allowable input : 45 W or more
Impedance : 4 – 8 Ω

So...
V=P/I
V=45/8
V= 5.625 (maximum voltage??)

is this correct?
and the current will only be drawn as needed?

the monostable circuit.... do you mean having a short time delay with the trigger connected to the speaker output (where an op-amp or shmitt trigger would be requited presumably?) and what about a filter? what type of filter do i need?

if this is completely wrong, do you have a circuit/block diagram please?

(sorry I am only at GCSE standard with my electronics, just finished the course, ive only ever used low power devices and have not really used much audio electronics)

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I would like to say thank you to everyone who has helped me, I enjoy learning new things and eventually, i would like to be really good at electronics

Mike

audioguru

Most LEDs have a max allowed current of 30mA and a max allowed reverse voltage of only 5V. Therefore you need to limit its current and rectify the audio.

The rectified audio will light the LED very dimmly.

Make this very simple rectified monostable circuit:

Attached Images

LED music blinker.PNG (3.8 KB, 5 views)

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Uncle $crooge

madmikejt12

ok, thanks i will have a go at that soon

__________________
I would like to say thank you to everyone who has helped me, I enjoy learning new things and eventually, i would like to be really good at electronics

Mike

madmikejt12

it gives the same effect as what the bulb does on its own (but the bulb is brighter)

__________________
I would like to say thank you to everyone who has helped me, I enjoy learning new things and eventually, i would like to be really good at electronics

Mike

madmikejt12

I have decided to go with normal bulbs.... I would like to use 4 12v bulbs (1 per speaker output) and connect them how the current bulb is connected (it is a 12V bulb from one of those bright lamps that get really hot)

Would these work: http://www.maplin.co.uk/Module.aspx?...0bulb&doy=17m6

and most importantly, would it be safe? The power supply I am using to power the CD player is around 350w

Edit: at +12v, the PSU delivers 18A max

__________________
I would like to say thank you to everyone who has helped me, I enjoy learning new things and eventually, i would like to be really good at electronics

Mike

audioguru

The 12V light bulb is 20W when it is operating at about 2000 degrees C. But it is about 200W when it is cold and then it might damage the amplifier that is rated for only about 14W per channel.

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Uncle $crooge

madmikejt12

ooh, didnt know it was only 14w per channel.... didn't know what it was to be honest!!

how about 10w Halogen bulbs??
or these bulbs (5W) http://www.maplin.co.uk/module.aspx?...4&doy=17m6#faq

or a relay http://www.mutr.co.uk/catalog/produc...e86de32bfcd00e
http://www.maplin.co.uk/Module.aspx?...relay&doy=17m6
http://www.rapidonline.com/productin...moduleno=29524

what voltage/current relay do i need? 6V or 12V
Does the current need to be low? so it switches at say 2 or 6A? or higher than the max. output?

Really sorry to be a pain, hope you dont mind all the questions.
Mike

__________________
I would like to say thank you to everyone who has helped me, I enjoy learning new things and eventually, i would like to be really good at electronics

Mike