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DC-DC Boost Converter Trouble

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Firefox

Thu Jul 24 2008, 11:54PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

I just finished building Uzzors' boost converter (click here for schematic) to charge a small 330V capacitor bank I use for making Ni-Chrome wire explode and it works great (with 8 AAs!) up to 500V.
However, I have yet to get the comparator section of the circuit working. I have the voltage divider on the negative end of the op amp set to about six volts, 1M to 20k, which should disengage the 555 when the bank reaches ~300 volts. Unfortunately for one of my test caps, it just kept going right up to 500 volts. Any ideas on what I am doing wrong? I'm using an 741CN op amp.

cjk2

Fri Jul 25 2008, 04:33AM

Registered Member #51 Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263

If you haven't already, try playing with opamps/ comparators on a breadboard to get an idea of how they work. In simple terms, if the + input is greater than the - input, the output will be high and viseversa. Check all of your wiring too, and post a picture if you still can't get it to work. Someone might be able to quickly see your mistake.

Antonio

Fri Jul 25 2008, 01:01PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

I assume that you also connected the power supply pins of the 741, not shown in the schematic. The 741 is not a comparator as the 311, as the output will not go close to the negative power supply (stops at ~1.5 V from it). But this should be enough to stop the 555. Experiment with the circuit at low voltages first.

uzzors2k

Sun Aug 03 2008, 11:53AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Did you get it working?

As Antonio pointed out the 741 can't swing to ground. From what I remember while playing with that circuit is that you had to pull pin 5 below 0.6V or so. I used a voltage divider to get the voltage low enough when I first made the circuit, back then I only had some TL071 op-amps.

Firefox

Sun Aug 03 2008, 03:49PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

Thanks, Uzzors, I got it working. I used a quad comparator (now that I know the difference) from Radio Shack, to fix the circuit, and it works great now. All I need now is a better transistor that doesn't start to bake at 220V with a 1440uF cap bank. I'm looking into a 24V 280A MOSFET from Mouser that states in its datasheet that it is meant for DC-DC converters. With an Rds on of 1mOhm, I think it will do

, and it only costs 3 dollars.

Also, I am thinking about using higher value divider resistors and the quad comparator to get voltage lights at 100, 200, 300, and 400V.

uzzors2k

Sun Aug 03 2008, 05:43PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Sounds nifty. I'm glad you got it working.

Remember that the mosfet has to withstand the full DC voltage of the charger, one of several disadvantages of this simple driver. You need high current and high voltage devices. If the switching losses become too much of a problem then try a transformer based push-pull charger (I've seen a Mazzilli and ATX xfrmr used for this, check out kipmans site IIRC) as then you can use low voltage high current mosfets. But of course for some casual charging the boost converter works.

Firefox

Sun Aug 03 2008, 09:18PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

Oh, so it does have to take the full voltage? Darn. I thought only the diode and inductor had to be able to take the full voltage of the inductive kick. Do you have any suggestions for low Rds on transistors that can get to 400-700V? I would like to be able to put this in a nice finished box, but I cant do that when the MOSFET hits 100C after 240V. I am currently using a IRF740 from a computer monitor

uzzors2k

Mon Aug 04 2008, 08:21AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

The IRF460 maybe, or just browse for better ones where you get parts from.

TheMooseGoesMoo

Tue Aug 19 2008, 04:28PM

Registered Member #1576 Joined: Sun Jul 06 2008, 12:48AM
Location:
Posts: 13

I just built one of these, and the LED turns on once i give the circuit power. I can only get it to charge up to 150V, and the 555 never turns off. Does anyone happen to know what is wrong? I am using a LM339N instead of the LM311 specified in the circuit.

Firefox

Tue Aug 19 2008, 08:43PM

Registered Member #1389 Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346

Probably the same thing as me, you need to use a comparator, not an op-amp. You must also connect the +V and Ground pins on your comparator chip for it to work correctly.

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