Author Topic: Are resistors really required for this dc/dc circuit?  (Read 1723 times)

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Offline z.s.tar.gzTopic starter

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Are resistors really required for this dc/dc circuit?
« on: August 03, 2011, 04:59:33 PM »
http://www.instructables.com/id/Simple-Flyback-inverter/step6/The-schematic/
I'm trying to lower the input voltage as much as possible but the resistors essentially divide my voltage going to the mosfet by two.
This guy somehow seems to both know what he's talking about and be completely clueless at the same time so I'm a bit skeptical.
Save yourself the typing. Just call me Zach.

Offline lrmall01

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Re: Are resistors really required for this dc/dc circuit?
« Reply #1 on: August 03, 2011, 06:04:04 PM »
My vote is yes.  I'm not quite sure what you are proposing as an alternative.  If you replace them with shorts, then you have 12v to 0v which will overload your input.  If you replace them with opens then you have no voltage applied to the mosfet to turn it on. 

Perhaps change the values so it doesn't divide 1:1 ?

Offline Soeren

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Re: Are resistors really required for this dc/dc circuit?
« Reply #2 on: August 03, 2011, 09:51:39 PM »
Hi,

I'm trying to lower the input voltage as much as possible but the resistors essentially divide my voltage going to the mosfet by two.
This guy somehow seems to both know what he's talking about and be completely clueless at the same time so I'm a bit skeptical.
Just look at the schematic... He knows enough to be dangerous - period.

Besides, you don't want quite as high a voltage as this will generate.

For the lowest possible drive voltage, forget MOSFETs, a BjT will run circles around the MOSFET here.
One transformer, one BjT, one resistor and one cap and you can get your coil gun powered up ;D

The higher a voltage you start with, the higher the efficiency will be, if that matters to you.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
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Offline Conscripted

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Re: Are resistors really required for this dc/dc circuit?
« Reply #3 on: August 05, 2011, 10:55:09 AM »
WOW. I just spent 15 minutes at that schematic and I don't understand it at all. You are feeding DC power into a transformer and gettting high voltage out the other side?  I could really use a walk through for this circuit if someone would be willing?

Thanks
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Offline z.s.tar.gzTopic starter

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Re: Are resistors really required for this dc/dc circuit?
« Reply #4 on: August 05, 2011, 07:09:42 PM »
I'm not 100% sure myself but the general theory of operation is this:
You don't need "real" AC to run a transformer, you only need to change the current.
AC does this by alternating current back and forth and (probably) gets the highest efficiency.
This can be done with DC by the way of a transistor switching the current on and off. Current is moving, but always in the same direction. (Imagine a rectified sine wave)
Of course the transistor needs to be switched at the right time and that's where things get hairy (for me at least)

@soeren: can you point me to an example of the "one cap, one resistor, one transformer and one bjt" setup?
« Last Edit: August 05, 2011, 07:15:50 PM by z.s.tar.gz »
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Offline Soeren

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Re: Are resistors really required for this dc/dc circuit?
« Reply #5 on: August 05, 2011, 07:51:12 PM »
Hi,

WOW. I just spent 15 minutes at that schematic and I don't understand it at all. You are feeding DC power into a transformer and gettting high voltage out the other side?  I could really use a walk through for this circuit if someone would be willing?

Here's the circuit for reference:

This is close to the simplest oscillator possible. Even so, or rather despite that, he got it seriously wrong and mutated it into an unsafe circuit, by feeding the HT side into the primary, which besides the obvious dangers of someone touching the "low voltage side" in good faith, might create flashover in the primary that probably isn't wound with enough (if any) isolation between layers, or any other precautions (like making sure there's no large potential differences in close by windings/layers), that are usually taken in HT circuits. Further, I wouldn't like to be the MOSFET in that circuit.

It's called a blocking oscillator and the simplified explanation is like this:
Assume power is just applied.
The transistor is opened (conducting) due to the upper resistor, starts drawing current through the primary winding.
As the magnetic field changes along with the electric field, the flux changes and the changing flux induces an electric field in whatever other windings there is on the core.

At some point determined by the resistance and the inductance, the core cannot hold more flux (i.e. it is saturated) and no matter how much further the current is drawn through the primary, the flux cannot increase.
No flux change equals no electric fields generated, so the secondary winding shuts off the transistor. This would normally be done in a separate feedback winding, so the galvanic isolation keeps the voltage of the secondary side from venturing back into the primary side.

When the transistor closes (stops conducting), the magnetic field collapses for an even faster change of flux, generating a higher voltage in the secondary than before (even with a 1:1 transformer, the secondary voltage will be higher than the primary, as the sec. voltage is a matter of how fast the flux changes).
The feedback will now be negative and blocks the transistor (guess where it got its name).
After some time the field has decayed and the circuit is back to where it is turned on by the upper resistor and the cycle repeats.

It's the cornerstone of (nah, the entire) circuits for single cell LED boosters, whether they steal the lame name invented by the guy who stole the circuit from a reader supplied column called "Ingenuity Unlimited" that ran in the then named Practial Electronics* back in the eighties. (This reader didn't invent the blocking oscillator, just presented it used to light an LED from a single battery cell).

Practical Electronics later assimilated two other mags, "Electronics Today International" (which in its golden days was a brick of more than hundred pages mostly of quality stuff) and "Everyday Electronics" (which was always a lousy rag, only of value to the true beginner).
They promised to keep the colo(u)rs of E.T.I. flying up to the merger, but lo and behold, the E.T.I. stuff (and probably staff) was gone in 2 or 3 issues   >:(

But I digress... In the LED circuits that actually doesn't steal a thing, there is just the primary and the feedback windings and it's the flux collapse that boosts the primary (with inverted polarity of course). It s easy to add a secondary for a substantial higher voltage and that's how I'd do it for the coil gun, even if it has got a low efficiency (around 60% to 70% max due to the constant power take up, but it's cheap and easy to change as experiments goes). For the coil gun, it wouldn't take much extra to turn it off automatically when the caps reach a given voltage and that would lift the efficiency.

In another (even older) issue of P.E. was a similar circuit for driving a Weller WTCP50 (i.e. 24V/~2A)from a 12V car battery and turn on/off controlled by the irons Magnastat switch. This used a pot core around 2" in diameter (plus a 2N3055) for a 50W output- If a suitable secondary was wound on a core that size, those caps would be charged really fast  ;D


Well, this must be enough words for the simple explanation    ::)
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Soeren

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Re: Are resistors really required for this dc/dc circuit?
« Reply #6 on: August 05, 2011, 08:41:32 PM »
Hi,

@soeren: can you point me to an example of the "one cap, one resistor, one transformer and one bjt" setup?


Add a primary of 100 or more windings (experiment) and rectify that, like you would in a regular PSU.

Experiment with the number of windings (primary and feedback as well), as you'll probably use another core than what was used in the schematic.

If you cannot get the Zetex transistor, select one that can handle your primary current with the lowest possible Vce drop.

This is where the Zetex transistors really shine, the ZTX851 has got a max Vce voltage drop of 250mV max. (200mV typ.)... At 5A!... In a TO92 case!! With a peak current of 20A, although this is the max. ratings.
You can find some Japanese transistors that are quite good in this respect as well, but I'm not aware of any US transistors that can match it.

Cores found in PC supplies would be good candidates. Toroids are good, but tedious to wind, so better look for a pot core.

There's a lot of magnet wire to grab, if you can locate an old discarded TV set (a fat screen, not a flat screen). We recently had a fierce thunderstorm that took out countless TV's (including the external input on our TV :(), so just sit back and follow the weather reports ;D  (People don't repair old TV these days, as a new flat screen is cheaper than the labor of a repair).


If you want more circuits to study, just google "Blocking oscillator circuit schematic" or something like that - should give you plenty to do this weekend.

A small main transformer with two secondaries may be used (at low frequencies only though), where the secondaries are used for primary and feedback windings. The original primary (mains side) will then be your secondary (high voltage) output to rectify.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Conscripted

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Re: Are resistors really required for this dc/dc circuit?
« Reply #7 on: August 05, 2011, 10:23:30 PM »
Thank you Soeren for taking the time to explain that circuit. I'm going to read your description a few more times to make sure I get it.

When the transistor closes (stops conducting), the magnetic field collapses for an even faster change of flux, generating a higher voltage in the secondary than before (even with a 1:1 transformer, the secondary voltage will be higher than the primary, as the sec. voltage is a matter of how fast the flux changes).

Is that why the back EMF from a DC motor can be higher then the applied voltage?

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Offline Afroman

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Re: Are resistors really required for this dc/dc circuit?
« Reply #8 on: August 06, 2011, 05:36:43 PM »
Instructables is the largest source of incorrect, half-assed, or flat out dangerous circuits on the internet. Unless you know exactly what you are doing I would always recommend searching for other schematics before trying something from instructables.

Offline z.s.tar.gzTopic starter

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Re: Are resistors really required for this dc/dc circuit?
« Reply #9 on: August 07, 2011, 10:46:24 PM »
I know, but their collection of almost-seems-plausible circuits offer 300% efficiency and only require you to sell your soul to work! (Plus sign up for an otherwise useless account.)
How is reality supposed to compete with that?
Save yourself the typing. Just call me Zach.

 


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