Re: [TCML] op amp wiring

["Scott Bogard" <sdbogard@xxxxxxxxx>]

Here is a picture of what I had in mind
http://bogard.110mb.com/op-amp%20driver.bmp
as you can see the beginning looks just like the standard 3055
circuit, except the BJT drives the non-inverting pin, and the
inverting pin is fed a steady 6 volts.  This should give a square wave
output to feed the MOSFET.  If this won't work, I can always drive a
BJT which would drive the MOSFET like is done with the 555 timer
schematics.  Let me know what you guys think.  Thanks.

Scott Bogard.

On 1/15/09, Gabriel Cardona <cardogab7341@xxxxxxxxx> wrote:
> Scott,
>
> Driving a power MOSFET from an op-amp is not recommended. Power MOSFET's
> require about 10V higher voltage at the gate than the source to be fully
> turned on, and there is a significant amount of stray capacitance on the
> gate that needs to be charged before the gate voltage will get to that 10V
> above Vsource. Op-amps typically cannot supply the instantaneous current
> required to drive a power MOSFET gate. You can use the op-amp to drive a
> MOSFET driver. It looks like you are using the op-amp as a comparator, you
> may want to look at a stand-alone comparator such as the LM111 or Linear
> Tech LT1011. You will still need a MOSFET driver after the comparator.
>
> Regards,
>
> Gabe Cardona
>
>
>
>
> ________________________________
> From: Scott Bogard <sdbogard@xxxxxxxxx>
> To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
> Sent: Thursday, January 15, 2009 11:22:08 AM
> Subject: Re: [TCML] op amp wiring
>
> I'll put a schematic up someplace later it may be some time as I don't
> have internet in my apartment and school internet frowns on uploading
> files; but basically this will be a modification of the single
> transistor feedback winding schematic, the op amp will have a steady 6
> volt fed the the non-inverting pin? and the sinusoidal output (which
> would normally be the base for the 3055 transistor) will go to the
> inverting pin?.  The idea is to have the op amp flip between 0 and 12
> volts to drive a mosfet for a square wave output, when the mosfet is
> on, it will choke the input to the inverting pin (via the feedback
> winding) until the op amp switches off, and then the cycle begins
> again when the field in the feedback winding dissipates.  This is
> probably an over complicated system, but I want something that is
> square wave and self resonant (the standard 3055 circuit seems to
> never utilize the full 12 volts input unless I'm reading the
> oscilloscope wrong, and I don't like the 555 timer circuits as they do
> not dynamically adjust to load.)  Also as best I can tell nobody does
> it this way and I wanted to make something original; then again maybe
> they done do this because it won't work, I don't know.  Any help would
> be appreciated.
>
> Scott Bogard.
>
> On 1/15/09, Gabriel Cardona <cardogab7341@xxxxxxxxx> wrote:
>> Scott,
>>
>> Can you explain what you're trying to do?
>>
>> 1. What will the output of the op-amp be connected to and what voltage
>> levels are needed at the op-amp output?
>> 2. What voltage levels will be at the input to the op-amp?
>> 3. What is the op-amp you're using (manufacturer and part number)?
>>
>> With this information, it will be easier to help.
>>
>> Regards,
>>
>> Gabe Cardona
>>
>>
>> ________________________________
>>
>>
>>
>>
>> ________________________________
>> From: Scott Bogard <sdbogard@xxxxxxxxx>
>> To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
>> Sent: Wednesday, January 14, 2009 1:17:51 PM
>> Subject: [TCML] op amp wiring
>>
>> Hey guys,
>>      I am in need of some assistance, I am doing an experiment with
>> flyback drivers, and the circuit I designed uses a Op-amp in a
>> comparator configuration.  The only problem is I know nothing about
>> actually wiring an Op-amp except for the extreme basics (inverting
>> pin, non-inverting pin, output) which I learned in school.  I do not
>> know how to connect it to the power supply, if I need negative
>> voltage, what the "offset" pins are for, what Vcc or Vss is, etc.  Any
>> help would be appreciated, there are 8 pins and I only know for sure
>> what 3 of them do, thanks.
>>
>> Scott Bogard
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