Author Topic: power supply  (Read 3541 times)

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Offline M0BSW

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power supply
« on: May 08, 2013, 08:05:25 pm »
 Ok here we go, I'm building a 6 volt fixed power supply got the rectifier, now I need some advice smoothing capacitor, the one I have to had is a Rubycon 47 micro farad 35 volt, is that  OK, if not what would you suggest.
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Offline c4757p

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Re: power supply
« Reply #1 on: May 08, 2013, 08:16:36 pm »
The capacitance you'll need depends on how much overhead you have over the regulator's dropout and how much current you will draw, but it's unlikely that 47uF will be enough.

In general, the output ripple will be approximately IOUT/(2fC) for full wave, and double that for half wave. Choose C so that the minimum peak output voltage minus the ripple voltage still gives you enough headroom for proper regulation.

Example:
10V minimum output from rectifier
60 Hz, full wave
6V 0.5A out
2V dropout
----
Maximum ripple = 10 - (6 + 2) = 2V
2V = 0.5A / (2 * 60 * Cmin)
Cmin = 2083 uF.
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Offline M0BSW

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Re: power supply
« Reply #2 on: May 08, 2013, 08:28:43 pm »
The capacitance you'll need depends on how much overhead you have over the regulator's dropout and how much current you will draw, but it's unlikely that 47uF will be enough.

In general, the output ripple will be approximately IOUT/(2fC) for full wave, and double that for half wave. Choose C so that the minimum peak output voltage minus the ripple voltage still gives you enough headroom for proper regulation.

Example:
10V minimum output from rectifier
60 Hz, full wave
6V 0.5A out
2V dropout
----
Maximum ripple = 10 - (6 + 2) = 2V
2V = 0.5A / (2 * 60 * Cmin)
Cmin = 2083 uF.

Goodness me I think I'd better spend sometime learning this,As you been so kind as to reply so quickly,I'll tell you what I want it for, I have a small pulse generator of 5 volts in, and a fast edge pulse generator, with a max of 5 volts in, design I got from W2AEW Alan, & I want to box both up together, and make them stand alone, that 's the plan, so when it comes out of the rectifier, I then  I want to put it through a regulator, to drop it to the 5 volts,  sorry a bit long winded, I thought it would be nice to explain what I'm trying to achieve, and of course I have other things I want to build that run on a similar voltage & the fun of learning.
Paul
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Offline kfitch42

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Re: power supply
« Reply #3 on: May 08, 2013, 08:52:53 pm »
10V minimum output from rectifier

I am a noob, so please excuse me if I am totally misunderstanding this. Isn't the MINIMUM output of a rectifier 0V ? Should this say maximum?

In a 'normal' simple power-supply we have a transformer followed by a diode bridge, right? We are talking about the smoothing capacitor that goes across the output of the diode bridge, right?

So, my next step was to draw this up in LTSpice to play with it ... I got this far (see attached), and I was a bit confused about how to connect things.

My intent was for V1 to represent the output of the transformer. Is this the right approach?

Does that orphaned line going to the left get connected to ground? But, isn't the '-' output of the rectifier also ground? Don't think those two should get connected together.
 

Offline jaycee

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Re: power supply
« Reply #4 on: May 08, 2013, 09:02:16 pm »
Connect the -ve end of the voltage source to the junction between D4 and D2. Put your ground node at the junction between D3 and D4, since is this is the 0V point on the DC side.

For simulating a transformer secondary you want a SINE voltage at 50Hz, and it must be the peak value. So for example a 6V AC secondary would be 6*1.42 = 8.52 v p-p. It is also a good idea to add some series resistance to approximate the winding resistance, otherwise you'll have a secondary that can provide unrealistic currents.
 

Offline c4757p

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Re: power supply
« Reply #5 on: May 08, 2013, 09:05:40 pm »
I am a noob, so please excuse me if I am totally misunderstanding this. Isn't the MINIMUM output of a rectifier 0V ? Should this say maximum?

Sorry, minimum peak. Due to line voltage and load variations, the (min,peak) voltages could range from, for example, (0,10) to (0,15). In this case, 10 is the minimum peak.
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Offline c4757p

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Re: power supply
« Reply #6 on: May 08, 2013, 09:07:34 pm »
Here's how you'd connect it in LTspice.
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Offline c4757p

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Re: power supply
« Reply #7 on: May 08, 2013, 09:09:14 pm »
It is also a good idea to add some series resistance to approximate the winding resistance, otherwise you'll have a secondary that can provide unrealistic currents.

The output impedance of many transformers is ridiculously low, approximable to zero - if you add the correct winding resistance you're unlikely to see much difference. They really can deliver incredibly huge peak currents. Until saturation, of course, but I don't believe LTspice can simulate that short of a full behavioral model.
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Offline mariush

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Re: power supply
« Reply #8 on: May 08, 2013, 09:14:24 pm »
If you want it to run from mains, you need a transformer and that big capacitor to smooth out the output after the voltage is rectified using the bridge rectifier (or diodes). This will give you some DC voltage above 5v.
Alternatively, you can make everything run from a 9v battery or from 3-4 AAA / AA batteries. Naturally, as the batteries get depleted, the voltage slowly drops. The nice part about this is that you don't need big capacitors and rectifier, because the voltage coming from batteries is already DC.

So in  both cases, you need a linear regulator to give you a stable voltage from whatever input you give it.  Each linear regulator has a particular voltage drop, which is the minimum voltage ABOVE 5v that needs to preset at the input for the regulator to give stable 5v output.

This value is listed in datasheets but keep in mind this value is often specified for a current draw close to the maximum current the regulator will output, for example 2v at 1A. If your projects only require 100-250mA (and I think that's the case here), the regulator will most likely be stable even with less than 2v reserve.

With the option to power it from mains using a transformer, you can use any basic linear regulator with a voltage drop as high as 2v (therefore demanding at least 7v to output 5v) and simply size the transformer and capacitor so that the regulator will always see that 7v.

With the 9v battery, same story. The 9v battery will die at about 6.5-7v so you don't really have to take too big precautions - a regulator that needs 1.5-2v voltage above 5v will work.
A 9v battery however has little capacity, usually about 150-200mAh, so depending on how much you use those gadgets, you'll run through batteries like crazy.

It's more cost efficient to use 4 AAA/AA batteries, because these usually have 1500-2000mAh capacity. But, you have to pay attention to the voltage. Rechargeable aa/aaa batteries have about 1.35v when charged, 1.15v when discharged... 4 of them are too close to 5v to get a regulator to output 5v.

However, if you go with non-rechargeable batteries, those have about 1.65v each when full and about 1.35-1.4v when discharged, which gives you about 0.6v above 5v so a regulator with less than 0.5-0.6v would work fine.

http://uk.farnell.com/texas-instruments/lp2950-50lpre3/ic-v-reg-ldo-5-0v-to-92-3-2950/dp/1262363  up to 100mA, up to 0.38v drop,
http://uk.farnell.com/on-semiconductor/lm2931dt-5-0g/ldo-reg-37vin-0-1a-5v-3dpak/dp/2102542  up to 100mA, up to 0.16v drop
http://uk.farnell.com/infineon/ifx25001tf-v50/ic-ldo-reg-5v-400ma-to252-3/dp/2215565  400ma, 0.25 v

etc etc

 

Offline kfitch42

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Re: power supply
« Reply #9 on: May 09, 2013, 12:39:26 am »
For simulating a transformer secondary you want a SINE voltage at 50Hz, and it must be the peak value.
50Hz!!! What do you think this is, Austraaaaaalia? :)  I'm in the good ole US of A, thank you.

And,  yes I was being quick and dirty, and left off the rms to peak conversion.

Seriously everyone here is awesome. Really helpful to us noobs. When I get some time tonight I will play with this.
 


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