Variable PS Design

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EPA III
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Variable PS Design

Post by EPA III » Thu Sep 29, 2005 9:20 am

Anybody know of a good design for a 0-30 Volt variable supply? About 3 Amps and regulated, of course. Variable current limit would be a nice plus. <p>Don't need to supply schematic, just a general idea and where to look. <p>Paul A.
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philba
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Re: Variable PS Design

Post by philba » Thu Sep 29, 2005 9:37 am

Do you want a linear or switcher? For some uses, a switcher is not desirable but much more efficient. Also, a linear to cover that range will have some issues with heat dissipation when you are at the low end of the range - you'd need approx. 32V input to get 30V output and when you turn it down to 1.25V, you will dissipate almost 30V. At 2A, that's 60W.<p>I'd take a look at Discover's Variable Power Supply section - they have lots of circuits there.<p>Also, there are lots of good ap notes at National and Linear.<p>Finally, did you really mean 0 to 30 or 1.25ish to 30? That will drive some of your design.<p>Let us know what you come up with.<p>Phil

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Chris Smith
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Re: Variable PS Design

Post by Chris Smith » Thu Sep 29, 2005 11:33 am

The LM series linear VRs can be used as constant current regulators as well as voltage regulators.<p> If you use one for the voltage, and a second one for the current, you should have all your bases covered. <p>A great book By Forrest Mimms covers all these concepts but Im not sure if its still in print?<p> Radio shack used to sell all of his books, so that would be a great place to start. <p>And they are cheap.

EPA III
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Re: Variable PS Design

Post by EPA III » Fri Sep 30, 2005 6:57 pm

Thanks guys! <p>RE: Linear vs Switcher. Yes, I know from experience. 33V (you need a little headroom) - 0.001V times 3 Amps = 100 Watts. That's a small room heater. If you do the research on heat sinks, you find that a switcher is real attractive. I'm not committed to it, but it is most likely a necessity. <p>RE: 0.000 Volts vs. 1.25V. Again, yes 1.25 is a LOT easier than a true zero. It almost requires a negative bias supply to allow the head (foot) room that the regulator needs to operate. I already have a circuit for a -5 Volt supply for the negative regulator voltage. <p>I'll checkout the reference. Thanks again.
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philba
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Re: Variable PS Design

Post by philba » Fri Sep 30, 2005 8:01 pm

One thing to consider is simulation of your design. Linear supplies a really nice SPICE simulator called switcher cad and provides models of many of their switcher chips. You can take one of their canned designs from the datasheets, similate it and then tweak it around to you liking. It was really great for me when I was learning about switch mode power supplies. You can see the effect of things like ESR in caps on the quality of the output, for example.<p>In general, its a good spice implementation - I use for lots of circuit simulations. It really helps to get the circuit right.

Robert Reed
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Re: Variable PS Design

Post by Robert Reed » Fri Sep 30, 2005 9:11 pm

By using a tapped secondary on your power transformer, you could switch voltage ranges for the level you wish to work at. This will reduce wattage dissipation tremendously. Even a common center tapped transformer can be quite effective, just switch the tap desired into a full wave bridge for the range desired.Remember to size The 'lytics to maximum out put voltage. Switchers are nice for their intended pupose (lite weight, efficient and relatively cheap), but --Ohhh the RFI !
For a bench transformer, these qualitys are not a big consideration, at least for the power levels you're using.

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Re: Variable PS Design

Post by k7elp60 » Sat Oct 01, 2005 8:15 am

ROBERT REED has a good suggestion. It will certainally reduced the dissipation by the series regulator.
Having built many linear power supplies here are some guidlines or rules of thumb for linear powersupplies.
Use 3000 uF of filter capacitance for each amp of load current.
Use rectifiers of 3X forward current rating for full load current.
If power supply is continous duty then use transformer secondary current/0.6 for rated load.
If power supply is for intermittent duty then use transformer secondary current for rated load.<p>The reason for the higher current diodes is that the peak charging current may be as high as 3 to 5 times the average load current.<p>The higher rated current for the transformer for continous duty is that the peak charging current will cause extra heating on the transformer. So if the secondary current is used on continious basis the transformer will overheat.<p>I would refrain from using adjustable 3 terminal regulators for the voltage adjustment because most of them have internal peak dissipation shut down. In other words when the input output differential and the load current exceed internal references the regulator shuts down, even thou the regulator is on a big heat sink and isn't warm.<p>I recently built an adjustable power supply similiar to what Paul A needs and had problems with 3 termial adjustable regulators as described
below.<p>There are some high current regulators like the LM338, and LT1083 that are excellent for a fixed voltage, but when used as a wide range regulator they will not work because of this internal charastic.<p>I have found the best solution is a LM723 and an external series pass transistor.

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Edd
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Re: Variable PS Design

Post by Edd » Sat Oct 01, 2005 4:09 pm

OK….. I have dug deep into my archives of circuits that I had used successfully for
Voltage and Current adjustable linear power supplies in the past, with total
satisfaction in their performance.
Note their voltage referencing ……..such that either unit has the capability to
swinging to zero on their low end voltage adjustment.
The first power supply schematic below is a design in the order of what I used back
in the ’68-70’s that was using discrete components and a set of op amp IC’s.
It worked great.! The reference is to a site that gives all of the info on its PCB also, if
you want to use it. <p>Discrete Power Supply with Adjustable Current Limit and Output Voltage
http://www.electronics-lab.com/projects ... index.html<p>I had to dig to find an old Roto-Mola Data sheet site with their LM 150-350 series of
adj Voltage Regulator IC’s …..Scan down to page 8 and you will find the schematic,
with the component count resplendently diminished in this design : <p>Laboratory Power Supply with Adjustable Current Limit and Output Voltage
http://www.datasheetarchive.com/semicon ... et=1134611 <p>
73's de Edd
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;) ;) <p>Yesterday I had to go in for a urine test……but I certainly wasn’t sweating it, as earlier, I had written all of the answers on my hand.

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Re: Variable PS Design

Post by rshayes » Sat Oct 01, 2005 10:48 pm

Sorenson made a line of lab supplies that minimized the dissipation by ganging a variac with the output voltage control. This was arranged such that the rectifier output was always a few volts above the output voltage. This limited the power dissipation to a reasonable value. Between the varial and the power transformer these were pretty heavy units.<p>A variation of this idea would be to use a switching preregulator followed by a linear regulator. The preregulator could be a simple buck regulator is AC isolation was obtained with a power transformer. The preregulator output would be set to about 3 volts higher than the desired output voltage. The linear regulator should be able to remov most of the switching noise.

bodgy
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Re: Variable PS Design

Post by bodgy » Sat Oct 01, 2005 11:12 pm

Apart from the ROTH (Rule of Thumb) method mentioned by K7elp60, which is often sufficient, you can also use the following formula.<p>C = I*t/Vrip<p>C= capacitance
I = the maximum current to be passed/supplied
t = discharge time of cap in seconds (actually milliseconds for a mains supply)
Vrip (in this case) = Vrectifier - Vmin regulator.<p>So I live in a 50Hz country if using full wave rectification t= 10mS<p>A standard 12v regulator (78 series) needs 15v to work properly so Vmin = 15v<p>A 15v transformer into a bridge rectifier will give a Vrect of 15 * 1.414 = 21.21v - 1.2v (Dvrect) = Vrect = 20.01v<p>Vrip = 20.01 - 15 = 5.01v<p>plugging everything in you have<p> I * t = 3 * 0.010 = 0.03

C= 0.03 / Vrip = 0.03/5.01 = 5988uF . NPV = 6800uF<p>Two things to note, you'd need to substitute 1/60 * 2 for t and Vrip here is high and you'd probably want a lower figure - which would bring the cap up to somewhere near 10,000uF may be more if your power supply is going to drive inductive devices such as motors.<p>Colin
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philba
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Re: Variable PS Design

Post by philba » Sun Oct 02, 2005 2:00 pm

Nice post, colin. A couple of points, though.<p>I believe that Vrip is the ripple coming off of the rectifier and is independent to the min input voltge of the regulator. (basically .414Vac - rectifier drop in this case). The numbers work out the same in your example. I base this on a Nat Semi app note:
AN 556, Introduction to Power Supplies This is why the filter cap size remains the same even though you can use a much higher input voltage to the regulator.<p>Also, this presumes a full wave rectifier (which is the right way to go IMO). If you use a half-wave rect, you use the full period (1/60th or 1/50th of a sec) for t instead of 1/2 the period.<p>Also, if the source is a wall wart, there very well may be some filtering in it already so you can reduce your filter cap size. I've looked at several of them with an oscope and there appears to be pretty significant filtering going on.

Robert Reed
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Re: Variable PS Design

Post by Robert Reed » Sun Oct 02, 2005 4:57 pm

One thing I haven't seen mentioned yet is in regards to regulated supplys. If you leave enough head room in the V in to the regulator (or pass transistor) you don't have to be concerned with ripple as much.. For example: Vout = 10 volts; Vin = 15 V: Vrip = 4Vp-p. Regulator input sees 13V to 17 volts at its input terminal. Inspite of this large ripple. Vout is unaffected, because it still has the head room to meet minimum voltage differential across the chip. I always take this into consideration when designing with these devices as it greatly reduces the filter capacity size. Remember, these chips can have very high frequency response so keeping up with 60 (or 120) hertz variations is a" day at beach" Also as mentioned FWB with its 120Hz ripple will cut your filter size in half. The only draw back to this method is increased wattage dissapation of the chip.
But you wouldn't want to have much less because aside from load variations, you have to consider line variations.

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Re: Variable PS Design

Post by grant fair » Mon Oct 03, 2005 11:07 pm

If you go to National Semiconductor's website and get the datasheet for the LM117/317 High Voltage regulator datasheet, there is on page 8 a schematic for a 5 amp adjustable voltage and current power supply. Note: the LM317 HV datasheet is fine, but the ordinary LM317 datasheet has an error in the schematic. Also you can't use any old op amp in the circuit, you need the TL071 or 301. <p>I've built it and it works very well. For some reason it doesn't seem to dissipate much heat.<p>Grant
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perwer

Re: Variable PS Design

Post by perwer » Mon Nov 14, 2005 6:20 am

Maybe you have schematic for 0-30 linear regulator??? I don't know how make true zero in output...Help me :)

Robert Reed
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Re: Variable PS Design

Post by Robert Reed » Mon Nov 14, 2005 5:20 pm

The regulator common connection must be taken somewhat below 0 volts (-2 or 3 volt). Google up any of the regulator chips for data sheets. Most of them will show this variation and also have plenty of power supply designs.

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