Current limiting

[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!.

 

[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!.

 

[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]

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

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

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?

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.

 

[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]

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?

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.

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

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.

 

[madmikejt12]

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.

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)

 

[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:

 

[madmikejt12]

ok, thanks i will have a go at that soon

 

[madmikejt12]

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

 

[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: **broken link removed**

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

 

[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.

 

[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) **broken link removed**

or a relay **broken link removed**
**broken link removed**
**broken link removed**

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

 

[kjennejohn]

Hi, madmike.
Let's get a few things settled:
A. In the formula you used, V=P/I, I is the current, not impedance. So dividing 45 by 8 is not correct.
B. A speaker's impedance is called that because resistance is not the proper term for a device that handles changing frequencies. Impedance is the measurement of "apparent" resistance to a changing signal. For speakers, their impedance is calculated while driven by a 1KHz signal. Since audio largely falls in the range of 20Hz to 20KHz, the impedance is different throughout the range. Since a speaker is largely a long thin wire wound around a form of some kind, it is basically a short circuit, so avoid DC across the speaker.
C. A DMM set to ammeter function is pretty much a dead short. If you put this ACROSS an amplifier output you will either blow a fuse (in the meter or amp) or blow the output stage.

Using a single LED per channel isn't very satisfactory for tracking audio output. An LED operates in a very limited voltage and current range. It's not good at showing power output throughout the entire power range. That's why most audio power meters you see on stereo amps use multiple LEDs.

Using a bulb is probably the better idea. However, they're tricky in that a cold filament is a dead short. After you apply power and let it heat up this resistance goes to a safe resistance range, though it's different from bulb to bulb. You'll have to guard against that. Perhaps you need to apply a small "startup" current to preheat the filament while isolating the amp from it. Or, better yet, put a power transistor to drive the bulb. This largely isolates the amp's output from the bulb, but not entirely.

Hope that helped.
kenjj

 

[madmikejt12]

Thanks for your reply,
A) oops!! silly mistake
B) You learn something new every day
C) I was lucky

Using a bulb is probably the better idea. However, they're tricky in that a cold filament is a dead short. After you apply power and let it heat up this resistance goes to a safe resistance range, though it's different from bulb to bulb. You'll have to guard against that. Perhaps you need to apply a small "startup" current to preheat the filament while isolating the amp from it. Or, better yet, put a power transistor to drive the bulb. This largely isolates the amp's output from the bulb, but not entirely.

What about using an op-amp to the bulbs?? would this be safe for the amp?

Are there any circuits I could use where the bulb is not directly connected to the amp? (like a relay circuit or potential divider?)

 

[audioguru]

A power transistor makes a pretty good light bulb driver, without mechanical parts that wear out in a relay.

 

[madmikejt12]

Thanks could you possibly advise me on what type I require??

and is it used like a normal transistor where the input goes to the base, the device (bulb in this case) goes from the +Ve to collector and the emiitor to 0v??

and I presume I would need one for each output?

Would the attatched circuit work?

 

[audioguru]

Thanks is it used like a normal transistor where the input goes to the base, the device (bulb in this case) goes from the +Ve to collector and the emiitor to 0v??

and I presume I would need one for each output?

Would the attatched circuit work?

The DC output of each output wire is about 6.5V. So your transistor would be turning on the light bulb all the time, and the bulb will flicker at the audio frequencies which is too fast to see any variation.

You would need to use an opamp to shift the DC level and drive the power transistor.