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

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

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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)

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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

Reply to
Meek the Geek
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Look up R/2R Ladder Networks, or mosey on down to:

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and extrapolate from tehre.

Cheers.

Ken

Reply to
Ken Taylor

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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

Reply to
Rich Grise

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

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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

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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 ...

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Bill Sloman, Nijmegen
Reply to
bill.sloman

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?

Reply to
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.

Reply to
Meek the Geek

Why is 1.25V magical?

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.

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Bill Sloman, Nijmegen
Reply to
bill.sloman

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

Reply to
Rich Grise

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.

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JosephKK
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Reply to
Joseph2k

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)?

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JosephKK
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Reply to
Joseph2k

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