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'MOSFET as a current limiter?'
1999\02\23@070639 by Harrison Cooper

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I've been following with at least one eyeball, the discussion of firing
MOSFET's from a PIC.  Got thinking about something I've been wanting to play
with, but it deals more with a current ramping.

Lets say I want to inject a DC current into a pair of motor windings (this
is basically called dynamic braking).  I've done this in the past, but
always just dumped it into the windings for a set period of time.  Taken a
110 VAC signal, ran it thru a 25A bridge, and then a power resistor (big one
with the aluminum heatsink) to limit the current, and then ran it through a
relay for a second or two.

But I wonder....is it possible to ramp the current with something?  I'd need
a device capable of probably somewhere around 10A, and I want to ramp the
current down to zero, or maybe even at least half of what it starts with.
Measure the current using a A/D input, calculate the ramp and then....insert
magic device.....and then ramp it down to some calculated value.

So, any of you smart folks know of a device that I can ramp the current,
probably based on some voltage on the gate (so now we need a D/A, but thats
ok).  I think what I am looking for is something that the resistance of the
source to drain can vary as a function of the voltage on the gate?

1999\02\23@071712 by Jochen Feldhaar

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Hi Harrison,

this sounds just like an application for a VMOS FET device. I do not know
if any of your Drain or source leads will be tied to a fixed potential. If
this is so, look at the HV082 (also called STHV82...and so on) in the
Thomson website. These guys do more than 10 Amps and more than 400 Volts
(but not at the same time...;-).
Another thing to observe: The gate capacitance of such a device is above 1
nanofarad, take care when switching, it needs current to alter the
potential of the gate. The electrode the gate is referenced to is the
Source!
There are some other devices available, their codes begin with IRF....

Greetings, hope this was some help,

Jochen DH6FAZ
spam_OUTjfTakeThisOuTspamdetektor.de

1999\02\23@083416 by Michael Rigby-Jones

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Another thing, the power dissipation is going to be big.  At half current,
say 5 amps, you'll be dropping  half of the supply voltage accross the
MOSFET giving 55 * 5 =275 watts.  This will probably be too much for one
device, unless it is some hugely expensive ultra high power item.

In actual fact the resistance of most motor windings is so low I guess
you'll still need the power resistor to limit maximum current so this will
add to the dissipation.

Regards

Mike Rigby-Jones
.....mrjonesKILLspamspam@spam@nortelnetworks.com

{Quote hidden}

1999\02\23@083838 by Harrison Cooper

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In actual fact the resistance of most motor windings is so low I guess
you'll still need the power resistor to limit maximum current so this will
add to the dissipation.



yes....I believe that in series, there will still need to be a power
resistor to limit the
overall current in the circuit. At least at startup, prior to any ramping.

1999\02\23@090134 by Tom
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The HT0740 by http://www.Supertex.com works great to drive an IRF640 Mosfet. It
generates the voltage necessary and the input draws only 200ua.

Tom Adams
tomspamspam_OUTcomtronix.com

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