Current drawing. Addition or series? - Page 1
Forum section: electronics / general

author: ElectronicFanatic1
author_rank: Contributor
timestamp: March 10, 2012, 12:53:03 am
content: Okay. ???I have a question that feels simple, but I can't seem to wrap my head around it. If I had two audio amps that say on the data sheet "Maximum current output (into speakers) 4.5A".. does that mean that: a) thats the maximum current it'll be drawing from the p.s. & b) if a is true, that if I have two of them in parallel, will it be drawing then 9A? and If I had a 1A usb port for charging on it, would it then be 10A?-Mitch

author: IanB
author_rank: Super Contributor
timestamp: March 10, 2012, 01:08:03 am
content: I think there is insufficient information.It depends on the configuration of the amplifier and its power supply. A reference or datasheet would help.Audio amplifiers output audio frequencies, so what exactly does 4.5 A mean? Peak? RMS?Also in the simplest case, 4.5 A into an 8 ohm load (typical speaker) would require 36 V. Does the amplifier have a power supply voltage greater than 36 V? If not, how would it get 36 V?Too many questions, too little data.

author: amspire
author_rank: Super Contributor
timestamp: March 10, 2012, 01:17:43 am
content: Amplifiers cannot normally be put in parallel unless they have been designed for it. Simpler to just have a speaker for each amplifier. Or do you just mean two channels of a stereo amplifier running off the same supply?Now with the current spec, it is hard to know if they are talking about peak DC current to the speakers, transient RMS current or continuous RMS current.If it is continuous RMS current, then yes 4.5 RMS out would equate to roughly 4.5ADC from the supply, and two amps would draw double that. If the amplifier has a dual supply ( + and - voltage rails) then each rail will be half that - 2.25 A per amplifier.Now an amplifier that could output 4.5A RMS into an 8 ohms speaker may have a supply of +/- 60VDC or more. you wouldn't want to regulate 60V DC down to 5V at 1A with a linear regulator. That would be 55W of power to get rid of.So is there a lower voltage supply available? If not, why not just take the circuit out of a standard mains to USB power adapter, and you will draw no power from your amplifier supply.Richard.

author: T4P
author_rank: Super Contributor
timestamp: March 10, 2012, 02:42:29 am
content: Anyway a linear regulator will blow up if you input more then 30 volts DC .It could be that the speaker might be 4ohms , guys , no ?And the class of operation , Class AB or D = 50-70% efficiency for AB and D = 80-100%

author: Pentium100
author_rank: Frequent Contributor
timestamp: March 10, 2012, 03:38:46 am
content: Quote from: Dave.S on March 10, 2012, 02:42:29 amA transistor with a zener diode won't. Well, as long as you have a heatsink big enough.

author: T4P
author_rank: Super Contributor
timestamp: March 10, 2012, 04:02:45 am
content: Quote from: Pentium100 on March 10, 2012, 03:38:46 amAnd that is just huge .Most people wouldn't go to the expense of building an external regulator , and the 78 series is only 35V .

author: amspire
author_rank: Super Contributor
timestamp: March 10, 2012, 04:23:09 am
content: It doesn't matter how much voltage an IC regulator takes. It is just that an IC regulator can only supply 1A with a fairly low voltage in. If you buy a $30 heatsink, you might be able to get a TO220 packaged 7805 regulator up to 20V DC in for 1A out.A switching power pack costs $5 to $10, and so if you pull the circuit board out of one of them, you eliminate the need to pull an extra amp from the transformer, you eliminate the need for an extra heatsink on the back of the amplifier, and you eliminate a fair bit of time building the supply. Also, it means the USB supply is electrically isolated from the power supply circuitry, and that is a very good thing, particularly if any of the USB devices get connected to the amplifier or preamplifier.Richard.

author: T4P
author_rank: Super Contributor
timestamp: March 10, 2012, 04:43:34 am
content: I know you are taking things into context but 5C/W heatsinks aren't really $30

author: amspire
author_rank: Super Contributor
timestamp: March 10, 2012, 04:57:10 am
content: Quote from: Dave.S on March 10, 2012, 04:43:34 amA sub 1C/W can be. Remember the package is already 4C/W and any externally touchable heatsink should be kept below 70degC so people to not get burnt. With a 5 C/W heatsink, and a 30 deg allowable heatsink temp rise, you could run the 7805 with anything up to about 11 volts input for 1 A output! The very last thing you need on a power amplifier is a requirement for even more heatsinks when it is totally unnecessary.It doesn't really matter what the numbers are really - the idea is just nuts.Richard.

author: T4P
author_rank: Super Contributor
timestamp: March 10, 2012, 05:33:21 am
content: In any case it would just be cheaper to hack a 5v switchmode plug pack ...

author: siliconmix
author_rank: Frequent Contributor
timestamp: March 10, 2012, 07:33:40 pm
content: what are the amplifiers called ?

author: T4P
author_rank: Super Contributor
timestamp: March 11, 2012, 10:39:09 am
content: Quote from: siliconmix on March 10, 2012, 07:33:40 pmIt's one of those audio amps .

author: ElectronicFanatic1
author_rank: Contributor
timestamp: March 14, 2012, 11:21:57 pm
content: to clear up all confusion, here's the data sheet for the TDA2050 amp. when i mean in parallel im taking about p.s. wise. for stereo application.http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1461.pdf4 ohm load±25 max i believe5A peakMitch

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 15, 2012, 02:42:02 am
content: That kind is basically a power opamp. They are very inefficient in typical audio applications, but are very easy to use.They are increasingly being replaced with digital amplifiers (which can be truly all digital or "hybrids") that are basically PWM power inverters. Those, like the buck converters they resemble, can output more current than they pull from the supply.

author: ElectronicFanatic1
author_rank: Contributor
timestamp: March 15, 2012, 05:48:31 pm
content: All I need to know is if I put 2 of those in parallel for the POWER RAIL if it'll just do 5A for both of 10A and if I had a 1A USB charger if it'll run of of that too or be 11A drawing currentMitch

author: mariush
author_rank: Super Contributor
timestamp: March 15, 2012, 08:11:01 pm
content: EF, if you read the datasheet, you will see that it says that the amplifier you link to is typically running at up to +/- 25 v or a single power supply of up to 50 volts.The higher the voltage the power supply delivers, the higher power the amplifier can do. On page 7 at the bottom you have a graph there in the bottom left corner there that shows how much power it can deliver, at a certain voltage.A USB charger will give you about 5-5.6 volts at 1A (but in reality it's less than 1A, it will probably melt if it stays for a very long time at 1A) - you can see in that graph on page 7 that this chip needs at least +/-5v or 10v to output 2 watts of audio power.With 5v from an USB charger, it won't even start or it will be very unstable. You may be able to make it work using a regular computer power supply because you will then have 12v, but even then this chip will only be able to deliver about 3 watts of power. It's simply designed to be more efficient when it receives 30-40 volts, with very low voltage it's lousy.Let's say you want to get 20 watts out of this chip. You look at the graph on that page 7 and you can see it can do 20 watts when the voltage is +/- 20 volts ... so if it runs from a single power supply, you would have to give it 40 volts DC.You start from 110v or 230v AC and you use a transformer to convert this to a lower AC voltage and then you use a bridge rectifier to convert the AC power to DC. When the AC to DC conversion is done, the voltage is multiplied by 1.41.So 40v /1.41 = 28 volts.So let's take a more common voltage you would easily find ... 24v ... 24v x 1.41 = 34v , which going back to the chart will give you a maximum of about 18 watts of power from each amplifier chip.Now you have to keep in mind that this kind of amplifier has about 60-70% efficiency, so in order to generate those 18-20 watts of power, the transformer must actually be capable of delivering about 40-50 watts of power to each amplifier chip.So the transformer must be rated for 50 watts / 24 volts = 2.1 A - to be on the safe side, your transformer needs to be 2.5 ASo for two of these chips, you will need a 24v / 5 A transformer to get 2 x 20 watts audio amplifier.A transformer that would do this is for example:http://uk.farnell.com/schneider-electric/abt7esm010b/transformer-100va-1-x-24v/dp/2070218So i hope it's clear... my advice is to start practicing with a smaller amplifier that you would be able to run and get a decent amount of power using a regular computer power supply ... one that would get run better at +/-6v or 12 volts thats easily available on atx power supplies..For example TDA2040 will run at a minimum of +/-4.5 v so it will run at 12v definitely and will do about 4 watts @ 4ohm load when running from 12v, as you can see on page 5:http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000130.pdfTwo of them will do 4 watts per channel, at about 60% efficiency it means you need about 10 watts on 12v (~ 1A ) for each channel, which is easily achieved by any atx power supply.Or you could get a 19v laptop power adapter... you can easily find one that does 19v @ 3 A... that gives you about 1.5A x 19v = 28 watts per channel, so you'll get about 10 watts of audio per chip.PS. Also it's very important to remember you need LARGE HEATSINKS for these chips. Here's an amplifier using TDA2040 that I pulled from a Phillips TV :You can see the large heatsink - I used a +/- 15v transformer (on the right), the square thing is a bridge rectfier and the two capacitors are 3300uF 63v capacitors I had at that time for smoothing (normally you would use bigger capacitors and better quality ones, not Teapo I had). It's not "professionally" made, but it worked great to output about 12watts per channel. The board says in corner 2x35w but that is doable only with distortions and a larger transformer ( +/- 25-30v).

author: T4P
author_rank: Super Contributor
timestamp: March 15, 2012, 08:56:52 pm
content: The TDA series are special chips , i heard somewhere .But anyway , such shockingly low distortion is very unheard of , in this real world currently .Anyway since at 4amps peak output that would mean somewhere around 4amps 4ohms would be 16volts and 16*4 = 64watts REALLY ?But let's be fair , 35W at 10% ... you'd want 0.03-0.5% that would be 28W , at 28W would mean something around 9.8W dissipation .Ahh yes , 9.8W , for a 25C rise , the chip internally is 3C/W already so 9.8*3 is 29.4C internal rise29.4+ambient(25) is 54.4CSo if we allow it to rise 25C it would be 79.4CNot bad , quite a lot of margin .2.55C/W is the minimum heatsink requirements , that's not very big nor very smallso we shall size is up to 2C/W , okay , 2.5C/W is rather expensive at 10$ per heatsink ,i think 2C/W is a safe heatsink to use .