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Hero999

Design new 0 to 30V power supply.

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Herro the 88.8 is the defualt value it just prints out like that, no both pots work fine if I have looked at the drawing correct you added 2 100k resistors iwill look at r24 bit later and let you know

Sorry this was after the new year party, I will explain better now

Herro the +88.8 is greyed out when the simulator is not running(this is there default state), Once I start the simulator they then read the correct voltage. I've re-looked at the drawing and made the correct modifaction to my drawing added C5,R23&R24, C5 is 1N and R23 is 100k, I changed the value of R24 to 680R. Yesterday when I tested it the sim did not run so I put it down to adding the above parts but I went through I sim log and found a couple of errors with s 100N cap I used and reinstated the above part and it ran fine, I placed a current porpe on the output with the pot set to zero the prope reads 1.70693e-05 coming output side of U2 is 0.096MV at 2% the probe reads 0.008A the output voltage U2 is .098MV and at 3% the probe reads 0.1398A and the voltage is 0.14875MV.

So I'd say if your having problems I'd put it down to LTspice. I hope this explains it a bit better for you

Steve

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I've start to look at part and The supplier I use I can't get the 22000uF caps but can get these
10000UF 50V 105C HE SERIES SNAP IN RC  Rapid part number  11-3165
from here
http://www.rapidonline.com/Electronic-Components/Capacitors/Radial-Electrolytic/Low-profile-snap-in-105-deg-C-2000hr.-electrolytic-capacitor/80204
I find these are cheaper than RS on most of there stuff but the draw back is not has much components at RS, If the crunch comes to the crunch I will order the correct ones, Rs always seem to have a SSM quanity but rapid most of there stuff is one off's
 

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Maybe we can look at charge pump inverter for the op-amp supply, like the one attached. This config offers a -Vin and +2xVin.The only problem is tha te maximum Vin is 20VDC so some sort of preregulation will be needed from the V+ line.

Edit:  Wouldn't adding a resistor from Iout node to ground form a pot. divider with R23? Maybe you can change the gain of the dif amp.

post-48894-14279144039812_thumb.jpg

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Can you create a negative voltage without using a doubler with the way the centre tap is connected?
D3 and D4 route the negative half cycles to ground potential and the centre tap is connected to V+.
I dont think it's possible. Using a doubler to create just about -0.7 volts sounds overly wastefull to me..

[glow=red,2,300]EDIT:[/glow]

I have added the -9 supply and to me makes it worse,


That would exceed the LT1014 dif supply voltage by 5 volts!

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Can you create a negative voltage without using a doubler with the way the centre tap is connected?
D3 and D4 route the negative half cycles to ground potential and the centre tap is connected to V+.
I dont think it's possible. Using a doubler to create just about -0.7 volts sounds overly wastefull to me..

I did do that in the end I reduced the 39V zener to 36V and used a 4.7V zener and with no load the pot now can go to 0% and still obtain the voltage out, If  used 33V zener with 4.7V you don't obatin the full voltage out, I placed the current porbe on the output it reads 0.001A with the pot set to zero.

So far everything seems to be working great in the SIM,

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I had a feeling you lot would jump on me for posting that circuit with a 9V zener. ;D

That was never my intention, it was just a circuit I've already drawn.

I'm using an ordinary silicon diode to provide a negative voltage of 0.7V.

I might look into using a cheaper op-amp as there's no point in using a single supply omp-amp with a negative supply.

Alex,
A voltage doubler is required to get a negative voltage because one side of the rectifier is connected to 0V.

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I thought you posted that as a topology example as you had also already mentioned using -0.6V. But PICMaster copied that in his simulator. Maybe that's why you got even worse result PICMaster.

So we have decided to include a small negative supply to get the output to zero. Hero, will you look into a balanced positive/negative or +V+6 and say -0.7V?

The question now becomes whether there is a way to minimise the component count of the power supply's power supply.

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The recovery from a short circuit at the higher voltage settings is slow and ugly because it takes awhile for the filter capacitor to charge when it's switched to the higher voltage. This is an inherent problem with reducing the input voltage at lower output voltages. I don't think it matters, the short ircuit characteristic is more important.

Here's a plot. The supply starts off short circuited, the current is interrupted for a time before being chort circuited again. The short circuit is simulated by connecting a diode in series with a 40V voltage pulse, LTSpice got funny when I started playing around with MOSFETs, I'll post the .asc tomorrow.

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This is great Hero, as you say the charging of C1 is evident as it is during a normal star-up.
I agree, the recovery profile is not too important. I see that the under/overshoot is minimal.

Could you also run a sim with the load changing from 10% to 100% and back? That would be important. Also, please post the asc you are working on.

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Here's the latest .asc

I've experimented with a current limit delay by increasing the value of C5 to 100nF. Connecting C5 to 0V just resulted in oscillation.

The short circuit curve is not pretty, first there's a short transient spike caused by C7 and C5, then an exponential decay with some ripple superimposed on top of it. The oscillation is worse at lighter loads, I've tested it with a short circuit and it still died away so I don' think it's a big problem.


The delay also slows down the recovery time which is no big surprise. Incidentally with C5 at 1nF and the output set to 13V the recovery time is nearly instantaneous which is to be expected as the rectifier voltage doesn't change much.

0_to_30V_5A.asc.txt

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You're right, attachment edited.


You mentioned simplifying the rectifier part of the supply; I don't think this is possible because two voltage doublers are required. I think it's probably best to just minimise the size of the capacitors. I don't see any point in going below 10μf, for the negative supply, because it's a fairly cheap and common value but I should be able to reduce C2 and C3 considerably. I'm also questioning the added value of having two separate capacitors for the main filter rather than one large capacitor.

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The MCU with LCD 6/8 push buttons and may be 2 digital pots I would say no more than 100-200MA may be well lower, I would say if you allowed a max of 500ma this should more than enough,
I will look into it for a more accurate figure, but the 5V supply would need to be filtered well.

I will try altering the Load resistor and run some more graphs just to compare

I like the idea of adding a boost transistor so that it can be rum from a 5V source, Now that I've got the latest drawing I will compare it agaisnt mine aswell

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Don't forget that the higher the current the larger the transistor and heat sink needs to be. I would like to use the same heatsink as used for the MOSFET but I don't want to use long leads for a voltage reference.

I'll rate the reference power supply to 250mA for now which will allow a cheap transistor such as the TIP31A to be used and a small clip-on heat sink.

I've noticed that I've grossly over-sized C1, I managed to reduce its value to 8000

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allowed a max of 500ma this should more than enough,

I'll rate the reference power supply to 250mA


You will probably find that less current will be needed. Most of this will be going to the LCD backlight, about 20mA. Digipots will draw about <1mA (excluding resistor ladder). Dual or more versions can be found. The MCU itself needs not run off 5V (the LCD accepts TTL signals). It can run at 3.3 and at relatively slow speeds like 1MHz. 20mA will be a conservative guess.

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20mA means I could happily remove the booster transistor or do you think that would be a bad idea?

Here's the latest schematic, I've added the current delay in.

I'll probably precede to the final version and the BoM soon.

I'm not going to worry about getting exactly the right models for the MOSFETs and diodes that I'm actually going to use, I don't see the point.

0_to_30V_5A.asc.txt

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Oh no I see that we have lost 3 pages of the topic What's going wrong ?

I no this is jumping the gun a little but I've been playing around with the Digital part well laying out the GLCD for volts and amps, I shall try the bigger fonts and aslo the standard font, If we use the standard font there is a lot more room to play around with.

What do we need to display apart from the voltafe/current readings, We could have a max set volts/current .

Herro how is the final sechematic and parts list looking ?

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I'll get the final parts list and schematic finished tomorrow.

I'm still not sure it even needs the negative supply, I know LTSpice didn't like it without it but that doesn't mean anything. I'll include it on the schematic and ask whoever builds it to try connecting it to 0V to see if it still works and the output goes down to 0V.

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