Digitally Controlled Power Supply with Digital POT, Keeping the Digipot from being fried

[Meek the Geek]

Hi, My name is Mike, I'm a mechanical engineering student

I'm building a digitally controlled electromagnet for a project,
the system needs to be controlled between 0V to 24 Volts DC with up to
4 Amps of current and I do not think that PWM will be a suitable
control on this devce for a variety of reasons (it gets complicated,
but trust me though, no PWM).

I found a design on a website for an adjustable power supply, and
given that for the purposes of experimentation we will be using a
lab-bench DC power supply set to provide 30V DC power, and then
regulate that current. It means that we can eliminate the transformer
and bridge rectifier for now. The design I am proposing involves using
a digital potentiometer to replace the manually controlled
potentiometer shown in the design below

http://www.aaroncake.net/circuits/supply.htm

My problem right now is that the digital potentiometers that I
have seen seem to be limited to only 5VDC. At the voltage which I
intend to use, I am very sure that I will be blowing it up when I reach
certain voltage levels. Is there a way of buffering the digital Pot
such that I can avoid barbecuing it, while still being able to use it
to provide digital power control. I was provided with an x9c103
digital potentiometer from Xicor/Intersil (10K Ohm, with 100 wiper
positions) http://www.intersil.com/products/deviceresults.asp?i=10763

Is there a simple way to buffer this digital Potentiometer (use
OP-Amps, a transistor, magic faerie dust) so that even though it only
has a 5 Volt survival range, I can use it to control the larger
voltage?

Thanks in advance

 

[Ken Taylor]

Hi, My name is Mike, I'm a mechanical engineering student

I'm building a digitally controlled electromagnet for a project,
the system needs to be controlled between 0V to 24 Volts DC with up to
4 Amps of current and I do not think that PWM will be a suitable
control on this devce for a variety of reasons (it gets complicated,
but trust me though, no PWM).

I found a design on a website for an adjustable power supply, and
given that for the purposes of experimentation we will be using a
lab-bench DC power supply set to provide 30V DC power, and then
regulate that current. It means that we can eliminate the transformer
and bridge rectifier for now. The design I am proposing involves using
a digital potentiometer to replace the manually controlled
potentiometer shown in the design below

http://www.aaroncake.net/circuits/supply.htm

My problem right now is that the digital potentiometers that I
have seen seem to be limited to only 5VDC. At the voltage which I
intend to use, I am very sure that I will be blowing it up when I reach
certain voltage levels. Is there a way of buffering the digital Pot
such that I can avoid barbecuing it, while still being able to use it
to provide digital power control. I was provided with an x9c103
digital potentiometer from Xicor/Intersil (10K Ohm, with 100 wiper
positions) http://www.intersil.com/products/deviceresults.asp?i=10763

Is there a simple way to buffer this digital Potentiometer (use
OP-Amps, a transistor, magic faerie dust) so that even though it only
has a 5 Volt survival range, I can use it to control the larger
voltage?

Thanks in advance

Look up R/2R Ladder Networks, or mosey on down to:
http://www.csse.monash.edu.au/courseware/cse2111/tut6.pdf

and extrapolate from tehre.

Cheers.

Ken

 

[Rich Grise]

I'm building a digitally controlled electromagnet for a project,
the system needs to be controlled between 0V to 24 Volts DC with up to
4 Amps of current and I do not think that PWM will be a suitable
control on this devce for a variety of reasons (it gets complicated,
but trust me though, no PWM).

I found a design on a website for an adjustable power supply, and ....
http://www.aaroncake.net/circuits/supply.htm

My problem right now is that the digital potentiometers that I
have seen seem to be limited to only 5VDC. At the voltage which I
intend to use, I am very sure that I will be blowing it up when I reach
certain voltage levels. Is there a way of buffering the digital Pot
such that I can avoid barbecuing it, while still being able to use it
to provide digital power control. I was provided with an x9c103
digital potentiometer from Xicor/Intersil (10K Ohm, with 100 wiper
positions) http://www.intersil.com/products/deviceresults.asp?i=10763

Is there a simple way to buffer this digital Potentiometer (use
OP-Amps, a transistor, magic faerie dust) so that even though it only
has a 5 Volt survival range, I can use it to control the larger
voltage?

Are you absolutely constrained to the LM338K? There are lots of adjustable
power supply designs out there on the web, that are not too much more
complicated, and do exactly what you need. Google up "Adjustable power
supply" or whatever, and poke around.

Good Luck!
Rich

 

[[email protected]]

Hi, My name is Mike, I'm a mechanical engineering student

I'm building a digitally controlled electromagnet for a project,
the system needs to be controlled between 0V to 24 Volts DC with up to
4 Amps of current and I do not think that PWM will be a suitable
control on this devce for a variety of reasons (it gets complicated,
but trust me though, no PWM).

I found a design on a website for an adjustable power supply, and
given that for the purposes of experimentation we will be using a
lab-bench DC power supply set to provide 30V DC power, and then
regulate that current. It means that we can eliminate the transformer
and bridge rectifier for now. The design I am proposing involves using
a digital potentiometer to replace the manually controlled
potentiometer shown in the design below

http://www.aaroncake.net/circuits/supply.htm

My problem right now is that the digital potentiometers that I
have seen seem to be limited to only 5VDC. At the voltage which I
intend to use, I am very sure that I will be blowing it up when I reach
certain voltage levels. Is there a way of buffering the digital Pot
such that I can avoid barbecuing it, while still being able to use it
to provide digital power control. I was provided with an x9c103
digital potentiometer from Xicor/Intersil (10K Ohm, with 100 wiper
positions) http://www.intersil.com/products/deviceresults.asp?i=10763

Is there a simple way to buffer this digital Potentiometer (use
OP-Amps, a transistor, magic faerie dust) so that even though it only
has a 5 Volt survival range, I can use it to control the larger
voltage?

As Rich Grise says, the LM338 is not a good choice for your job. He
should have mentioned that if you really want change the driving
voltage from 24V down to 0V, the LM338 falls short because you can't
pull the outptu voltage lower than 1.25V (check out the data sheet at
http://cache.national.com/ds/LM/LM138.pdf).

The thermal overload protection built into the LM338 is a very nice
feature, but you can now get this in some power MOSFETs. Farnell lists
"Philips Intelligent switches" where the BUK106-50L and the PIP3206R
look quite attractive, the equivalent "fully protected MOSFET
switches" from International Rectifier, and OmniFETs from ST where the
VNW100N04 looks nice
(http://www.st.com/stonline/products/literature/ds/4593.pdf).

Since you want a linear circuit, you are going to have to dissipate up
to 37.5W in the MOSFET (assuming a 30V supply and a coil resistance of
6 ohms) which is going to mean a big heat sink and very good themal
coupling between heat-sink and MOSFET.

If you assume a maximum junction temperature of 150C, and a maximum
ambient of 40C
you are going to need at total thermal resistance of less than 2.9
degrees C/Watt - of whixh at last 0.6C/W is going to be the junction to
case termal resistance of the MOSFET (VNW100N04) another 0.5C/W between
the MOSFET and the heat sink leaving you needing a 1.8C/W heat sink,
which is on the large side (see Farnell order code 414-438, but I don't
know if the central gap is big enough for a TO-247 package).

Once you've got your series element, you've then got the fun of
relating the voltage drop across the solenoid, which has to have one
end tied to the +30V supply rail, and the other end tied to the drain
of your MOSFET, to the votage that you et up with some kind of A/D
converter, which will probably be referred to the 0V rail.

You can use a differential amplifier (otherwise known as a subtractor)
to generate a 0V-referenced voltage in the range 0V to 5V that is
proportional to the voltage drop across the solenoid, and then you can
use a standard op amp circuit to adjust the gate voltage of the MOSFET
to product the desired voltage drop across the MOSFET. Note that the
input capacitiace of the VNW100N)4 is big - at around 12nF - and you
either need to drive with an op amp that will tolerate a large
capacitative load on its output, or you have to isolate the output from
the gate with a resistance (see the op amp data sheet for the
appropriate value).

All this will make the feedback loop relatively slow, so do a Bode plot
for the loop as a whole at the very least ...

 

[Meek the Geek]

I thank you all for your input

Just so it is known, it is easy enough to incorporate a computer
controlled switch at voltage close to 1.25V such that I can shut it
off. I do not need to keep voltages of this low a value, because this
is below an initial power value needed to produce any sort of results
(in short, if I have power in this range, odds are I will be getting
little if any result from the system).

I am now seriously starting to think that maybe I would be better
off with an array transistor/resistor simple DAC type network instead,
coupled with an off-switch. Would this fare me much better? One of
the problems is that getting weird IC components costs me too much and
I have a limited timespan.

What might serve as a better type of controller?

 

[Meek the Geek]

I thank you all for your input

Just so it is known, it is easy enough to incorporate a computer
controlled switch at voltage close to 1.25V such that I can shut it
off. I do not need to keep voltages of this low a value, because this
is below an initial power value needed to produce any sort of results
(in short, if I have power in this range, odds are I will be getting
little if any result from the system).

I am now seriously starting to think that maybe I would be better
off with an array transistor/resistor simple DAC type network instead,
coupled with an off-switch. Would this fare me much better? One of
the problems is that getting weird IC components costs me too much and
I have a limited timespan.

What might serve as a better type of controller? cheap DAC
resistor transistor network, or x9c103 digital pot with only the +5V
limit before burnout? I sometimes cannot get everything where I am.

 

[[email protected]]

I thank you all for your input

Just so it is known, it is easy enough to incorporate a computer
controlled switch at voltage close to 1.25V such that I can shut it
off. I do not need to keep voltages of this low a value, because this
is below an initial power value needed to produce any sort of results
(in short, if I have power in this range, odds are I will be getting
little if any result from the system).

Why is 1.25V magical?

I am now seriously starting to think that maybe I would be better
off with an array transistor/resistor simple DAC type network instead,
coupled with an off-switch. Would this fare me much better? One of
the problems is that getting weird IC components costs me too much and
I have a limited timespan.

What might serve as a better type of controller? cheap DAC
resistor transistor network, or x9c103 digital pot with only the +5V
limit before burnout? I sometimes cannot get everything where I am.

Don't futz around with either, but use a proper DAC - one with a
built-in voltage reference.

They aren't expensive, and the last thing you want is to re-invent the
wheel.

None of the componets I mentioned were weird - you can buy them all off
the shelf from Farnell, or any other broad-line distributor (like
DigiKey). I don't know where you are, so I don't know how fast Farnell
delivers in your area. In Europe, they seem to claim that if you order
before about 4.00pm, you will have the parts in your letter-box the
following morning.
E-mail me if you need more details.

 

[Rich Grise]

I thank you all for your input

Just so it is known, it is easy enough to incorporate a computer
controlled switch at voltage close to 1.25V such that I can shut it
off. I do not need to keep voltages of this low a value, because this
is below an initial power value needed to produce any sort of results
(in short, if I have power in this range, odds are I will be getting
little if any result from the system).

I am now seriously starting to think that maybe I would be better
off with an array transistor/resistor simple DAC type network instead,
coupled with an off-switch. Would this fare me much better? One of
the problems is that getting weird IC components costs me too much and
I have a limited timespan.

What might serve as a better type of controller? cheap DAC
resistor transistor network, or x9c103 digital pot with only the +5V
limit before burnout? I sometimes cannot get everything where I am.

From your original application, it sounds as if what you're really looking
for is a digitally-controlled current source. You want regulated AMPS
through your solenoid, not necessarily regulated VOLTS, right?

A current regulator isn't hard at all. Do you want it on the high side
or the low side? Are you good enough of a designer to do it with PWM?
Ever built an R-2R ladder?

If you can afford the power budget, then one mongo transistor on a hefty
heat sink, a current sense resistor, a feedback amp, and you control it
any way you want to.

Good Luck!
Rich

 

[Joseph2k]

I thank you all for your input

Just so it is known, it is easy enough to incorporate a computer
controlled switch at voltage close to 1.25V such that I can shut it
off. I do not need to keep voltages of this low a value, because this
is below an initial power value needed to produce any sort of results
(in short, if I have power in this range, odds are I will be getting
little if any result from the system).

I am now seriously starting to think that maybe I would be better
off with an array transistor/resistor simple DAC type network instead,
coupled with an off-switch. Would this fare me much better? One of
the problems is that getting weird IC components costs me too much and
I have a limited timespan.

What might serve as a better type of controller?

I have no idea what others have said yet. Get the manual for that Lab
supply and look for the characteristics of the voltage and current control
interfaces. Post that info and you will get better results.

 

[Joseph2k]

I thank you all for your input

Just so it is known, it is easy enough to incorporate a computer
controlled switch at voltage close to 1.25V such that I can shut it
off. I do not need to keep voltages of this low a value, because this
is below an initial power value needed to produce any sort of results
(in short, if I have power in this range, odds are I will be getting
little if any result from the system).

I am now seriously starting to think that maybe I would be better
off with an array transistor/resistor simple DAC type network instead,
coupled with an off-switch. Would this fare me much better? One of
the problems is that getting weird IC components costs me too much and
I have a limited timespan.

What might serve as a better type of controller? cheap DAC
resistor transistor network, or x9c103 digital pot with only the +5V
limit before burnout? I sometimes cannot get everything where I am.

For that matter if it is really a digitally controlled power supply is the
control interface RS-232 (serial) or IEEE-488 (or something else)?