Converting 5v to 12v using an LM2577 - all i get is 4.8v?

Hi,
I am trying to use an LM2577-12 as a step up converter to convert 5v into 12v. I have used a LM7805 and am getting a solid 5.01v input, but all i get out is a 4.8 - 4.81v output. I'm sure that everything is connected as said in the example in the datasheet (www.national.com/ds/LM/LM1577.pdf). I am however using a 1/8 watt resistor for the 2k2 resistor. Could this be doing it?

Otherwise, could I have burnt out the IC or something? I got one that's in a TO220 package if that's any help?

Thanks
John

If you have a scope is simple to see in pin 4, you must have like figure 5 (pdf manual).
Don´t forget the inductor must be Q equal or more than 20.

Edson

Sorry i'm a bit confused as to what that means?

Could you please try and explain a little more simply?

Do you have an oscilloscope? If so, do you know how to use it?

It sounds like the device is not oscillating. Are you sure the GND pin is connected? What 100uH inductor did you use? 1/8w rating will be more than adequate for the 2k2 resistor.

I'm not sure what you mean by " I have used a LM7805 and am getting a solid 5.01v input, but all i get out is a 4.8 - 4.81v output." Where does the LM7805 fit into your circuit? If you mean that you have a 7805 with 5V and are only getting 4.8V...that's normal.

--
The Basic Motor Driver: simple, inexpensive motor driver for 1 stepper motor or 2 DC motors

Your LM2577 circuit is either not wired correctly, or wrong support components being used, or some component is defective, as clearly the LN2577 is not working and just allowing the +5vdc pass on through to the output terminals. Getting a switching DC converter regulators working is not a trivial task, many people just recommend buying a premade module.

Lefty

The inductor that I am using is a 100uH RF choke (http://www.jaycar.com.au/productView.asp?ID=LF1534). This is what I was sold when I asked for all of the components on the schematic, but perhaps its the wrong one?

Unfortunately because im building this into a perfboard style arduino, a shield is not practical.

I am using the LM7805 to convert 6v into 5v and it is solidly outputting 5.01 - so im getting the input into the LM2577-12 right. Thats the only reason that I mentioned that.

idiotjohn:
The inductor that I am using is a 100uH RF choke (http://www.jaycar.com.au/productView.asp?ID=LF1534). This is what I was sold when I asked for all of the components on the schematic, but perhaps its the wrong one?

That's a small-signal inductor, not a power inductor. It almost certainly won't be capable of storing the energy needed. Inductor design for boost converters is quite complex, for example see http://www.metglas.com/downloads/apps/pfc.pdf - although the cores described there are designed for much higher power converters than you are using.

I suggest you take a look at the inductor selection guide in the LM2577 data sheet http://www.national.com/ds/LM/LM1577.pdf.

I see.

Would http://www.jaycar.com.au/productView.asp?ID=LF1270&keywords=100uH&form=KEYWORD be more suitable?

John

That's much more likely to work. What's missing from the spec is the saturation current, but as it's rated at 5A, it's quite likely that the saturation current will be high enough. My other concern that it has a powdered iron core, which may not be as good at the switching frequency as a ferrite one.

[EDIT: I see that some of the inductors mentioned in the LM2577 data sheet have powdered iron cores, so maybe that's not a problem.]

You also need to work out what value inductor you need because it depends on output current and input voltage. See the "Application Hints" section of the data sheet at http://www.national.com/ds/LM/LM1577.pdf.

Would this one be better? http://www.jaycar.com.au/productView.asp?ID=LF1272&keywords=100uH&form=KEYWORD

Im planning on inputting approx 4-5v, so L100 would be the most suitable inductor.

The table reads: L100 | 67127000 | PE - 92108 | RL244

Would one of the above linked ones work? I would prefer not to have to buy anything online, and Jaycar is the only read electronics distributor near me.

Thanks for your help
John

After replacing the inductor with the one above (first link) I am still getting the same 4.8v output - could I have blown the chip?

idiotjohn:
After replacing the inductor with the one above (first link) I am still getting the same 4.8v output - could I have blown the chip?

Unless you have miswired the chip at some point, I would be surprised if you had blown it. Running it with your original RF choke would be more likely to blow the choke than the chip. Are you certain you have the chip connected correctly? Perhaps you can post a photo of your wiring?

Could the type of capacitors that i'm using affect it?

When I asked in jaycar I was given a 1000uF (instead of the 680uF) and tolds that would be alright - and from my little experience thats correct, isnt it? That capacitor is an electrolytic.

The 0.1uF cap that im using is small, orange and says 104 on the side (which works out at 0.1uF?).
And the 0.33uF cap im using is dark grey and rectangular prism in shape.

Are these all right?

Ill have one more try at fixing my wiring and if it doesnt work ill post a photo.

John

idiotjohn:
Could the type of capacitors that i'm using affect it?

When I asked in jaycar I was given a 1000uF (instead of the 680uF) and tolds that would be alright - and from my little experience thats correct, isnt it? That capacitor is an electrolytic.

The 0.1uF cap that im using is small, orange and says 104 on the side (which works out at 0.1uF?).
And the 0.33uF cap im using is dark grey and rectangular prism in shape.

Are these all right?

Yes, 1000uF in place of 680uF should be fine. 104 is the correct marking for the 0.1uF cap, it means 10 x 10^4 pF. If the 0.33uF cap were not correct, you should still see more than 5v at the output.

It still sounds to me that the chip is not oscillating, which could mean a few things: wiring seriously wrong (so the chip is effectively not there), GND pin of the chip not connected, or blown chip. I presume you have pin 2 (feedback) connected directly to the diode and output capacitor?

dc42:
It still sounds to me that the chip is not oscillating, which could mean a few things: wiring seriously wrong (so the chip is effectively not there), GND pin of the chip not connected, or blown chip. I presume you have pin 2 (feedback) connected directly to the diode and output capacitor?

Yep the ground pin is definitely connected, and pin 2 is connected to the stripe side of the diode and the positive of the output capacitor.

Here are some photos - hopefully they will help.

More pictures...

I can't tell for sure that the electrolytic capacitor is wired correctly, but otherwise it looks to me that you have the correct connections. However, the 0.1uF capacitor should be placed right next to the regulator, between its Vin and GND pins. Also the -ve pin of the electrolytic capacitor should be connected as directly as possible to the GND pin of the regulator, not through long wires and the GND bus as you have it - I suggest you also put it right next to the regulator if there is room, between the GND and feedback pins.