Re: [CAD_CAM_EDM_DRO] Re: Mariss' power supply circuit

I think I see. C1 charges up through R2 linearly thus forming the sawtooth
wave portion of the modulator.

I am building a new servo supply for my machine. The current one is not
stiff enough and causes following error estops with the high acceleration
high speed moves I want to do. I will incorporate this circuit in the supply
I think.

I need about 30 amps cont at 85v and the 50 or so pounds of transformer
iron laying on the floor says I will get it. I am using stepdown isolation
transformers at half voltage so that iron won't be at the full flux density
but there is still plenty of room. I will nominally go from 120v to 60
so I should get 60*1.414= 84.8vdc. My amps turn off at 92v and have
100 wv caps in them.
Since my line voltage is notoriously high I may put a few bucking turns
on the transformers. I read why power companies like higher voltage-
revenue goes up as the square of that voltage! Too bad they can't control
it.

As for sizing R7 in your circuit- well lets see.... my 25 joule of max
regeneration
energy could attempt to pump up the 20,000uf supply cap from its normal
72 joule @ 85v to 72+25=97 joule. That would be 98.5v. It would be some less
from losses but that is a maximum. With your circuit at 100% duty cycle
dumping
25 joule in say .05 sec would require 500 watts as a linear approximation.
So the value would be about 20 ohms. I use Dale 10 ohm 25w wirewounds
to short the servos in my estop circuit and they don't even get warm
so 25w should be fine. I could use a lower value.. if I chose the resistor
current
to be the ps rated current it would be about 4 ohms but if I go too low
I think I will estop on every power flicker.

As a check for power failure 3*R7*C= 1.2 seconds ; long enough. My estop
will short the servos in a sustained failure anyway.

Les


Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html
engineering page:
http://www.rabun.net/~leswatts/shop.html

----- Original Message -----
From: "mariss92705" <mariss92705@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Monday, June 17, 2002 8:09 PM
Subject: [CAD_CAM_EDM_DRO] Re: Mariss' power supply circuit


> Les,
>
> I appreciate you taking the time to understand the circuit. The PWM
> modulation is actually a linear function of the over-voltage seen by
> the supply. The amplitude of the 120 Hz sawtooth (R2,C1) is set to
> about 2V peak-to-peak, this being the "regulation" range of the
> circuit. The "trough" of the full-wave rectified reference clamps the
> negative excursion of the sawtooth to the difference voltage.
>
> The IRF540 has an Rds of about .07 ohms. It always operates as a
> switch, the current being limited by the 10 to 20W shunt resistor.
> The resistor must be wire-wound because wire-wound resistors have a
> superb overload capability, typically 10 times their rated wattage
> for 5 seconds, 50 times rated for 200mS.
>
> In an undervoltage situation the circuit would attempt to adjust
> itself to the new, lower voltage. It would be abley assisted by the
> load in this endevor.
>
> Like all silicon, MOSFETs invariably fail shorted if abused. The
> consequence would be the shunt resistor would rapidly overheat and
> fuse open, clearing the circuit. Everything else would fail
> harmlessly and would leave the basic supply uneffected.
>
> Another thing about the circuit is the component value choice for the
> +12VDC supply. The size of C3 is chosen so that the circuit remains
> functional for .5 second after AC power removal to fully discharge
> the supply.
>
> Thanks again for your comments; my pleasure is designing minimalist
> circuits the way others might enjoy working crossword puzzles.
>
> Mariss
>
> --- In CAD_CAM_EDM_DRO@y..., "Les Watts" <leswatts@r...> wrote:
> > I finally found the .pdf file and had a look.
> > As I understand it the circuit will perform shunt regulation
> > using PWM at 120 hz. Duty cycle will vary from 0-100% over a
> > couple volts diff between the filter cap and transformer secondary
> > peak. I think it will modulate with a raised cosine law. Right?
> >
> > Question: what is the on resistance of the Fet you selected? Just
> > want to calculate heat sink thermal resistance. Also, what happens
> > in a line power undervoltage scenario?
> >
> > And as a little FMEA thing, what would be the most likely part to
> fail?
> > Would it be the fet if say the primary mains went very high? Would
> it
> > fail open or shorted? (I currently have a floating neutral argument
> with
> > my Georgia Power supplier and their electrical feed)
> >
> > I want to publicly thank you for publishing this as open source.
> > I appreciate your weekend efforts.
> > These things cost about $300 from other suppliers.
> >
> > Les
> >
> > Leslie Watts
> > L M Watts Furniture
> > Tiger, Georgia USA
> > http://www.rabun.net/~leswatts/wattsfurniturewp.html
> > engineering page:
> > http://www.rabun.net/~leswatts/shop.html
> > ----- Original Message -----
> > From: "mariss92705" <mariss92705@y...>
> > To: <CAD_CAM_EDM_DRO@y...>
> > Sent: Sunday, June 16, 2002 3:54 PM
> > Subject: [CAD_CAM_EDM_DRO] Re: Power Supply Circuit
> >
> >
> > > Hi,
> > >
> > > OK, here's my Saturday's project. I posted a "Power supply
> > > circuit.pdf" in the files section of this list.
> > >
> > > It wound up being different than what I started with. Sensing
> current
> > > was too crude. Instead this circuit compares the power supply
> voltage
> > > against the xformer secondary voltage. It pulse width modulates
> the
> > > dump resistor and has a very nice proportional response. The PWM
> goes
> > > from 0% to 100% as the power supply voltage climbs to 2V above
> the
> > > nominal voltage. It has been tested and works very well.
> > >
> > > Mariss
>
>
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