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How do I reduce 12v to 9v?
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Xed
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Joined: 25 Oct 2007
Posts: 4

PostPosted: Thu Oct 25, 2007 12:43 pm    Post subject: How do I reduce 12v to 9v? Reply with quote

Hi everyone!

I hope I've come to the right place, and if I've made any mistake, please forgive me.

I'm in the process of rewiring/tuning certain things in and around my PC. At present, I'm trying to eliminate the need of an extra external adapter to drive my PC speakers.

The amp circuit needs a 9v (not sure how many amps) supply, and I thought if I could connect it to the 12v rail of my SMPS along with a resistor, there would be no need for me to use an seperate adapter, and I would be saving some space on my power-strip.

So the question is, how do I figure out how many Ω resistor should I use?

The ampl. ckt doesn't specify the current requirement or the power rating. (It was pulled from an old pair of Creative speakers). My 350W SMPS rates the +12V -> 12.0A, -12V -> 0.5A.

What I personally feel is that I should find out the resistance of the amplifier and once I get that, the resistance I require would be = 1.33 * R(amp). -or- the other option is to measure the current passing through the amp and divide 12 by it to get the value. Either ways, I'm not sure how to go about it.

I have a Digital Multimiter, but I've no experience in using it to measure current.

I'll be grateful if anyone can be patient enough to guide me through this.

Thanking you,
Xed
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Turd
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Joined: 18 Aug 2007
Posts: 274
Location: Saskatchewan, Canada

PostPosted: Thu Oct 25, 2007 2:36 pm    Post subject: Reply with quote

Use a LM7809 voltage regulator Very Happy


eric
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Project_Nightmare
HG Master


Joined: 12 Oct 2007
Posts: 119
Location: Gig Harbor, WA

PostPosted: Thu Oct 25, 2007 8:44 pm    Post subject: Reply with quote

If you want to have more fun, you want to save more power, you don't care about cost, and don't have that high of current requirement, try looking into Buck converters Cool
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Xed
Newbie


Joined: 25 Oct 2007
Posts: 4

PostPosted: Fri Oct 26, 2007 4:19 am    Post subject: Reply with quote

Thanks for the quick replies!

I've been doing some reading and I'm in a much better understanding of what I need now.

As far as the solution to my resistor problem is concerned, I think it can be solved if I use a voltage divider circuit, with a 1ohm res between Vin and Vout and a 3ohm res between Vout and GND (please correct me if I'm wrong). But whether this is practically implementable or not, I don't know. Can someone please explain if this would work, and if not, why it would fail or what the limitations are?

--

Now practically speaking though, I learnt that voltage-divider circuits waste energy since the resistors convert the excess voltage to heat. I'm totally against wasting energy so practically, I would lke to use some other method.

That means, although the LM7809 suits my needs well (easy to use too), I don't want to use it since it wastes energy.

Therefore, I believe, a buck converter is what I need. So can anyone please recommend me a good one (which doesn't need an external inductor or has an integrated inductor)? It'll be great if its as easy to use as an LM78xx regulator. I'm looking at something within the range of $5, but I don't mind spending a few extra if it's worth it. Smile
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Project_Nightmare
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Joined: 12 Oct 2007
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PostPosted: Fri Oct 26, 2007 8:46 am    Post subject: Reply with quote

you can't have in inductor in an IC (unless your willing to pay around $20) and the shipping costs are going to be higher than $5. In addition, we need to take how many amps being drawn into acount. I post as soon as i can find one that converts to 9V

(As u can see, I'm very into dc-dc converters Twisted Evil )
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Turd
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Joined: 18 Aug 2007
Posts: 274
Location: Saskatchewan, Canada

PostPosted: Fri Oct 26, 2007 12:06 pm    Post subject: Reply with quote

This or this should do Very Happy

You could even hack a cigarette lighter adapter from a cell phone. All you need to do is change one resistor to change the output voltage Very Happy


eric
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Project_Nightmare
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Joined: 12 Oct 2007
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Location: Gig Harbor, WA

PostPosted: Fri Oct 26, 2007 5:32 pm    Post subject: Reply with quote

Thank you Turd Very Happy Looking at the IC for bucks, i cant find a value for 9v. But if you want to try making one, I have a schematic that I'm working on that is adjustable and if you have some of the parts lying around, cheap.
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Turd
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Joined: 18 Aug 2007
Posts: 274
Location: Saskatchewan, Canada

PostPosted: Fri Oct 26, 2007 5:45 pm    Post subject: Reply with quote

I'd like to see the schematic. Does it use the 34063 dc-dc converter chip?
The 34063 can be hacked to make a great flashlight Very Happy


eric
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Project_Nightmare
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Joined: 12 Oct 2007
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Location: Gig Harbor, WA

PostPosted: Fri Oct 26, 2007 9:34 pm    Post subject: Reply with quote

Not at all, it's so simple it doesn't use any uncommon IC to make it work Very Happy It's also adjustable and I'm just troubleshooting it now. It seems to have problems with sudden resistance changes causing the capacitor store large voltages that can damage ICs (around 24v my simulator says). This only would happen if you have an extremely low load (I'm testing with 10 ohms) and suddenly disconnect it. The solutions that i can think of is to use a resistor or zener diode to drop the voltage to 9v again Rolling Eyes and I'm trying to use the same components so our friend doesn't have to have left over parts.

This circuit that I designed is so flexable, I'll test it myself and make a seperate post to explain how it works Razz

I'm also going to need to buy some Mosfets from my overpriced local Radioshack and find some wire to make a coil lying around
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Project_Nightmare
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Joined: 12 Oct 2007
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Location: Gig Harbor, WA

PostPosted: Sat Oct 27, 2007 10:38 am    Post subject: Reply with quote

I found how to prevent the spikes (quite a simple solution and I'm suprised that I never saw the circuit online), but another problem I found is the voltage drops. It's probably not going to be a problem since not many circuits loads have a resistance under 100 ohms Cool

I feel odd triple posting so I'm just going to add onto this one:
I'm not sure that my circuit is going to save power considering my most recent modifications require it uses 5 op-amps and needs several different voltages to funtion properly which i took care of with voltage dividers with high ohm ratings (they are used on the op-amps for reference, not to run) Rolling Eyes But it sure looks cool Twisted Evil
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Project_Nightmare
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Joined: 12 Oct 2007
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PostPosted: Sun Oct 28, 2007 4:04 am    Post subject: Reply with quote

If you want to look at a range of voltage regulators check here: http://forum.hackedgadgets.com/viewtopic.php?t=1508

Xed, if you want something simple, and from what I can tell, try the op-amp one. It's simple and will respond quickly to what ever load you hook up to it. (A pity that I didn't think of it sooner) Cool
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Xed
Newbie


Joined: 25 Oct 2007
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PostPosted: Sun Oct 28, 2007 9:39 pm    Post subject: Reply with quote

Thanks for the advise guys. Smile

@Project_Nightmare:

Will the Op-amp one waste energy? Because if it does, then I might as well as use an LM78xx Smile

Anyways, I repeat my earlier question: How do I use a buck converter, and exactly which IC and what components would I need?
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Project_Nightmare
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Joined: 12 Oct 2007
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Location: Gig Harbor, WA

PostPosted: Sun Oct 28, 2007 10:30 pm    Post subject: Reply with quote

Well, if you use a LM741 with a 10k potentiometer, the waste current is around 1.8 mA (this is including the voltage divider current usage). The datasheet for the LM78xx says that it can waste up to 8mA (Both without a load ofcourse). Like I mentioned in my article, a benefit of using my op-amp design is that it can be adjusted using a voltage divider.

The buck converter is a lot more complex than I first thought. First off, the circuit needs to adjust itself for any changes in load. This means it needs to drain the excess voltage generated from the inductor, spikes in voltage from resistance increases, and recover from drops when resistance decreases. Because of this fact, the circuit needs to continuously check the load.

My buck converter design is made with a lot of parts and will most likely use more current then the conventional linear regulators (LM78xx). So it would be better using a programmable PIC or AVR when it comes to simplisaty building, effiency, and voltage drop/spike adjustments.

If you want to buy vender buck IC for the regulation, you might face a problem considering the places I know that sell the chips don't sell them for 9 volts using a 12 volt supply. This means that you have to buy an adjustable one which would require more parts than the pre-set ones (refer to the device's manual for the parts). In addition, you need to know the max amp requirements for the device being connected.
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Turd
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PostPosted: Sun Oct 28, 2007 11:33 pm    Post subject: Reply with quote

Turd wrote:
This or this should do Very Happy


Those are both adjustable Very Happy


eric
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Project_Nightmare
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Joined: 12 Oct 2007
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PostPosted: Mon Oct 29, 2007 8:35 am    Post subject: Reply with quote

But mine is cheaper, simpler, and you can get the parts at a local electronics store like your local overpriced Radioshack Twisted Evil

And if anyone is interested in my semi decent buck converter, I will post a tutorial for how it works, but like i said in my previous posts, I'm trying to improve its quality Razz It will also require a lot of typing and time, which school hasn't given me yet
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