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Topic: Power Supply Question (Read 2705 times) previous topic - next topic

floresta

#15
Jun 25, 2011, 09:06 pm Last Edit: Jun 25, 2011, 09:13 pm by floresta Reason: 1
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... but is there anything I could do that doesn't involve finding another voltage regulator since the current one can only handle 1 A?

Look in your data sheet for an example of a 'High Current Voltage Regulator'.   These circuits typically use the 78xx regulator along with a substantial transistor to handle the higher currents.

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Will any of the components fry from too much current or will they only draw what they need?

The devices that are connected to your power supply (except LEDs) will draw the current that they need.  As far as the power supply supply is concerned it will (try to) supply all the current that is required by the devices that are connected to it.  If this is more than it's rated current then it will fry.

Don

guitarboy667

So figure 22 here:
http://www.farnell.com/datasheets/76220.pdf
With this transistor:
http://www.newark.com/jsp/search/productdetail.jsp?SKU=25M7729&CMP=AFC-GB100000001
?

Thank you for your help!

floresta

That looks good.  Don't forget that the transistor will need a heat sink.  Also check to see if the tab of the transistor is isolated.  If it is connected to pin 2, as many are, then you will have take some more precautions.

Don

guitarboy667

So is the voltage rating of the capacitor important in this case? What does it mean? Is that the max voltage it's rated for or does it actually build up that difference?

guitarboy667

Well I found a 50V radial, so that should be fine, right?

guitarboy667

Alright I have a 12V, 2 A power supply and I need to power an Arduino Mega and 4 digits of a 7 segment display:
http://www.us.kingbright.com/images/catalog/SPEC/SA18-11EWA.pdf
that take 6V. (They're controlled with shift registers by the Arduino.)

If this the correct method then:
Oh I meant to ask about the resistor too. In Figure 22 there is a value of 3 OHM underneath the resistor, but there is a calculation involving R1 = ... to the side. Do I need to calculate the value of this resistor or is it 3 Ohms?


http://www.farnell.com/datasheets/76220.pdf

floresta

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...that take 6V.

LEDs do not 'take' any specific voltage and therefore you do not 'apply' any specific voltage to an LED.  An LED is rated by the amount of current that should be flowing through it which you must limit with a series resistor.  The voltage that ultimately winds up across the LED is the result of how much current there is flowing through it.  You should probably design your circuit for around 10-15 mA per segment.  

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Do I need to calculate the value of this resistor or is it 3 Ohms?

They used the values for their transistor and their current requirements to come up with 3 ohms.  You will have to put your values in in order to find out what resistance you need for your version of the circuit.  Data sheets are essential, but no one ever said they are easy to deal with.  They are written to provide a competent engineer with enough information to get the device functioning without blowing it up, but not much more.

Don

floresta

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The datasheet says that it is designed for 30mA per segment (including the decimal points).

That's the 'Absolute Maximum Rating'.  I typically use half that or less, but the data sheet shows 20mA for their 'Test Conditions'.

Don

guitarboy667

This shift register:
http://www.sparkfun.com/products/734

The Arduino supplies the 5V to each of the registers and sends out the bits and those are passed through each shift register by Ser out-ser in.

So the 12V goes directly into pins 1 an 5?

Grumpy_Mike

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So the 12V goes directly into pins 1 an 5?

Only if you want to blow it up.

guitarboy667

That's what I thought, but that's what the other posts seem to be saying if the resistors are only for each segment.

Grumpy_Mike

They are, you can't use resistors alone. You need resistors on each segment and then you need a transistor to drive the segment under the control of the shift register. I don't think any one suggested directly driving it from the shift register. What sort of shift register are you using?

guitarboy667

This shift register:
http://www.sparkfun.com/products/734

The Arduino supplies the 5V to each of the registers and sends out the bits and those are passed through each shift register by Ser out-ser in.

The shift register is already all figured out, I just need to figure out how to power the digits.

guitarboy667

The first aforementioned shift register is connected to the Arduino by a clock, data, and latch pin. All the latch and clock pins are connected together. The data is sent from Ser out to Ser in on the next shift register. The shift register is connected to Arduino's 5V at its Vcc and SRCLR and the SRCK, G and GND are connected to ground. The segments are connected to Drain#'s with a resistor between. (Resistors are based on old power supply)

I've gotten it to work with a different power supply, but I'm trying to get it to work with this power supply.

The last power supply was just 5 AA's. I'm currently trying to get it to work with a wall plug-in power supply that gives 12V 2 A DC.


Grumpy_Mike

That type of shift register has an open drain FET output and so is the one type that it is OK to connect an LED and resistor to with the other end at a high voltage (up to 50V).

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