Voltage controlled rheostat?

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This is sci.electronics.basics, where there is no such thing as a
stupid question. :-)

Heh. Yeah. That was the first thing I thought of. I did many extensive Google searches but came up with nothing that really fit my needs. ( or my wallet )

Thanks. I was using "voltage controlled" power supply. Maybe I'll get more relevant results with this.

Many thanks. :)

Jonathan

While the voltage-controlled power supply already mentioned will fill the bill, we might be able to suggest a different (less expensive, etc) approach if you gave us the "big picture" of what you are trying to do.

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

...especially the *no assembly required* part...

Bob Masta wrote:

Ah. The magic phrase.

Yup. There's lots of ways to skin a cat--if you have details of the cat.

*-*-*-must-be-a-variable-resistance+multiplying-DAC-*-*-*-*-FET+duty-*+unless-designed-for-it+switch-between-two-*-*-voltage-values+PWM+*-divider+LPF-*-*-*+FET-array

I've done numerous Google searches with many variants on "voltage programmable power supply" but almost all the results are irrelevant as they're "high voltage -- programmable power supply" . When excluding "high" there are only a few hits, mostly patents, that refer to "voltage controlled power supply" but don't specify any model. The only real info I got was from this message :

:

"I'd look at something like a used HP 6266B, which will give you 0 to

40V at 5 Amps. You can program it from an external voltage or current source (or resistance source, with opt. 002). Programming speed isn't too fast, but it is sufficient for most ATE apps. "

When I looked at the specs of said supply on eBay, it indicated it was "analog programmable" and had "sense" inputs. So, I searched again but this time I used "analog programmable" -- got a truckload of hits. Unfortunately when the search is restricted to only those hits that have a service manual, then it's down to only a few hundred. Even in the service manuals though, I've yet to find a complete description of the "analog programming" process, only a mention that the device supports "analog programming" .

So, is this what I'm looking for?

Thanks,

Jonathan Busby

I have an automatic DC fan controller that can vary the voltage depending on a temperature sensor between 0 and 12 volts while being able to handle 20 watts of power. The problem is that the fan I need uses 24 volts and over 70 watts at max voltage.

Hope this clarifies things.

Jonathan Busby

Well, it's just that I hate soldering. ;) Although solder paste and an oven I can tolerate. ;)

cat.http://groups.google.com/group/sci.electronics.design/browse_frm/thre...

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Maybe.

What \'analog programming\' is referring to is that the supply\'s
output voltage can be controlled by (instead of cranking the knob(s)
on its front panel) connecting a voltage source, a current source,
or a resistance to some terminal blocks on the rear panel of the
supply.

From your more recent post: 

"I have an automatic DC fan controller that can vary the voltage
depending on a temperature sensor between 0 and 12 volts while
being able to handle 20 watts of power. The problem is that the fan
I need uses 24 volts and over 70 watts at max voltage."

and from an earlier post:

I\'m looking for a device ( preferably no assembly required ) that
functions as the hypothetical device in the subject : something that
takes a control voltage of 0-12 volts and adjusts the output from a
voltage source ( a power supply ) from 0-24 volts linearly w.r.t the
control voltage while handling up to 80 watts of power.

It appears that what you\'d like to be able to do is use the 0 to 12V
output from the controller as a control signal to drive the larger
power supply needed to drive the 70 watt fan.  

Something Like this: (View in Courier)

MAINS>----------------------+
                            |
         +----------+       |
MAINS?>--|          |       |
         |          |       |       
    +----|          |   +---+---+   +---+
    |    |          |---|      +|---|+  |
[SENSOR] |CONTROLLER|   |SUPPLY |   |FAN|
    |    |          |---|      -|---|-  |
    +----|          |   +---+---+   +---+
         |          |       |
MAINS?>--|          |       |
         +----------+       |
                            |
MAINS>----------------------+

where the connections to the supply voltage control terminals are
the outputs from the controller?

Yes that's basically it. :)

Jonathan Busby

Brain Fart. Replace "Service Manual" with "Data Sheet" .

Jonathan Busby

Please allow me to stimulate some other ideas out there:

If you really need an "analog" output to drive your fan motor, you might want to consider using a power op-amp (Apex at

is a good source). They can be used just like regular op-amps (i.e. be configured as voltage followers, or with a DC gain of 2 -- to convert your 0-12V input into a 0-24V output. Bear in mind that the op-amp supply voltage would probably need to be a few volts higher than the required op-amp output voltage. Apex apps or sales engineers could probably suggest an appropriate device and circuit.

However, your existing DC fan motor might be happy being driven via a "PWM" (pulse width modulator) driver. PWM drivers are more compact, require a much smaller heat sink (i.e. are much more efficient), and would be much cheaper. If your motor can be driven via PWM, that is probably the most sensible way to go. The easiest approach is to simply buy an appropriate "motor driver", available from most motor distributers. Many will accept a DC control signal of 0-10V (which can easily be derived from your 0-12V signal via a resistor or two.)

G. Duchene

--
OK, what do you want to do now, and how much money do you have
available to throw at getting the problem solved?  Material, that
is.  As long as I don\'t have to start wielding a soldering iron
I\'ll be happy to help you for free, :-) 

BTW, do you have a schematic or a link to the controller\'s data
sheet?

I think my price cap is around a hundred bucks.

Nah. But I want you to come over to my house and complete the construction of my 20 ft tall Tesla coil -- I have completed the first steps : ordering the plans. ;) :P

Thanks a lot for all your help!

Jonathan

Oops. Forgot the fan :

Part # : FFB1224XHE :

( The one at the top )

Jonathan

On Tue, 1 Jan 2008 11:08:36 -0800 (PST), Jonathan Busby wrote:

--- OK, since it's a brushless fan it's not really designed to be pulse-width modulated and since you've got a $100 cap on the project, here's what I'd do: (View in Courier)

A1 +-----+

120AC>-+------|~ +24|-----+ | | | | 120AC>-|-+----|~ -24|--+ | | | +-----+ | | | | | | | | +-----------+ | | | | A2 | | | | +-----+ | +-|-|--|~ -12|-----+-----------------+ | | | | | +-|--|~ +12|--+ | | +-----+ | | | | LT | 0-12VDC>---|--[4K7]---|+ \\1636 D | +---|S >--+-[1000]-+---G IRFZ34E | | +-|- / | |K S | | | | [10K] [1N4744A] | +------+-|--+ | | | | +--------+ +-----+ | | |+ | [10K] [FAN] | | | GND>-------+-----------------+--------------+

A1 is a Kepco RKW 24-4.5K, $79, (or an RKW 24-6.5K for $99) from Kepco at:

A2 can be pretty much any 12V wall-wart you might have laying around as long as its output stays under 15V under high line and no load conditions.

You'll probably need a heat sink for the MOSFET, but I haven't got a good handle on what current VS voltage looks like for a brushless motor. However, assuming that the relationship is roughly linear means that with 24V across the fan pulling 3.25A through it, 14V (at the low end of its range) will pull:

14V ----- = 0.583 of 3.25A, 24V

So:

0.583 * 3.25A = 1.89A.

With the supply at 24V and the fan at 14V means the MOSFET is dropping 10 volts with 1.89 amps through it, so it's dissipating:

P = IE = 1.89A * 10V = 18.9W

Quite a bit of power.

From IR's thermal data for the MOSFET we have:

Tj max = 150C

Rtjc = 2.2C/W

From:

Tj - Ta Pd = -------------------- Rtjc + Rtcs + Rtsa

we rearrange to solve for the thermal resistance of the heatsink surface to air:

Tj - Ta Rtsa = --------- - (Rtjc + Rtcs) Pd

150C - 50C 2.2C 0.5C = ------------ - ( ------ + ------ ) 18.9W W W

= 2.59C/W

Which is a pretty substantial convection-only heatsink.

Here's an example:

Finally, here's a little simulation so you can see how the thing works:

Version 4 SHEET 1 880 832 WIRE -224 112 -944 112 WIRE -512 192 -832 192 WIRE -464 240 -608 240 WIRE -336 240 -384 240 WIRE -512 272 -512 192 WIRE -224 272 -224 112 WIRE -608 320 -608 240 WIRE -544 320 -608 320 WIRE -336 352 -336 240 WIRE -336 352 -400 352 WIRE -272 352 -336 352 WIRE -544 384 -704 384 WIRE -512 464 -512 432 WIRE -464 464 -464 432 WIRE -464 464 -512 464 WIRE -224 464 -224 368 WIRE -944 512 -944 112 WIRE -832 512 -832 192 WIRE -704 512 -704 384 WIRE -608 512 -608 320 WIRE -944 640 -944 592 WIRE -832 640 -832 592 WIRE -832 640 -944 640 WIRE -704 640 -704 592 WIRE -704 640 -832 640 WIRE -608 640 -608 592 WIRE -608 640 -704 640 WIRE -512 640 -512 464 WIRE -512 640 -608 640 WIRE -224 640 -224 544 WIRE -224 640 -512 640 WIRE -944 704 -944 640 FLAG -944 704 0 SYMBOL voltage -944 496 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 24 SYMBOL voltage -704 496 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 3 24 104 Invisible 0 SYMATTR Value PULSE(0 12 0 .1) SYMATTR InstName V2 SYMBOL res -240 448 R0 SYMATTR InstName R5 SYMATTR Value 8 SYMBOL nmos -272 272 R0 SYMATTR InstName M1 SYMATTR Value IRF1312S SYMBOL Opamps\\\\LT1636 -512 352 R0 SYMATTR InstName U1 SYMBOL res -624 496 R0 SYMATTR InstName R1 SYMATTR Value 10k SYMBOL res -480 256 R270 WINDOW 0 69 58 VTop 0 WINDOW 3 69 56 VBottom 0 SYMATTR InstName R2 SYMATTR Value 15k SYMBOL voltage -832 496 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value 36 TEXT -856 672 Left 0 !.tran .2

-- JF

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Nice one! :-)