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Old 18th November 2010, 12:43 PM   #31
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A worthy addition to the supply is a simple shunt regulator before the LM317:

LAB SUPPLY SCH.1.png

This does a few good things:
- reduce the voltage at the LM to allow it to run cooler.
- reduces ripple at the LM, thereby reducing ripple considerably at the output.
- further isolates the LM from the raw supply, making it more immune to voltage and current fluctuation in the raw supply.
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Old 18th November 2010, 02:45 PM   #32
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Updated schematic:

LAB SUPPLY SCH.png

Some changes include the new pre-reg shunt. Some resistor values have changed - had an extended tinkering session last night and further refined this.
Nothing written in stone yet, all is subject to change.

New board:

LAB SUPPLY BRD.png

This I will etch and use as my new prototype.
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Old 18th November 2010, 09:34 PM   #33
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No one sees my little mistakes...
The board layouts posted above have the neat trick of reversing the diodes in the voltage doubler.

New prototype:

NPX_848.JPG

Getting ready to power it up now. Should be ok (I hope)
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Old 18th November 2010, 11:56 PM   #34
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Success! 2 small things that needed to be changed:
- R9 from 220 to 1K to set the correct reference voltage (ideally ~10V).
- R1 from 10K to 22K, this was getting too hot, dropping a maximum 70V at ~7mA. Even running 2 to 3mA at the base of Q1 is enough for well over 2A at the output.

Everything else was roses, once it was "dialed" in. Constant current (set in this instance at 1.6A) deviation was just 20mA when driving an 8 ohm load from 30V down to 15V. This is actually much better than I expected.
As configured, the maximum current into any load at any voltage is ~2.5A and since the raw supply has more capacity than the earlier version, I will leave that current ceiling as is. I also want to go back and add the second pass transistor back in to spread the current and dissipation over 2 devices instead of one. I have a bunch of MJL21194's anyway, just waiting to be used.
So, back to the layout program to do the final board revision.
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Old 19th November 2010, 01:53 AM   #35
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Final schemematic:

LAB SUPPLY SCH.png

Added 2nd pass transistor and emitter resistors.

New layout:

LAB SUPPLY BRD.png

Just a bit bigger.
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Old 19th November 2010, 03:13 PM   #36
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I'll try to make this psu on prototype board
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Old 19th November 2010, 05:29 PM   #37
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Quote:
Originally Posted by tome View Post
I'll try to make this psu on prototype board
It is possible for sure. It doesn't have stability problems so layout isn't critical.
I tried to breadboard it but gave up - too many parts for me. I hate breadboard anyway, I'd much rather spend 30 minutes or so making a board. At least then I can get parts soldered in properly and keep things organized.

Refined the layout a bit more:

LAB SUPPLY BRD.png

And made a pair:

NPX_850.JPG

Ready to populate. Boards are 10cm x 12cm.

Last edited by MJL21193; 19th November 2010 at 05:32 PM.
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Old 19th November 2010, 05:59 PM   #38
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Nice design, I think IŽll build one :-)

I also make my own PCBs and I was wondering: how do you get the screenprint on to the component side?
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Old 19th November 2010, 07:01 PM   #39
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Quote:
Originally Posted by thelasse View Post
how do you get the screenprint on to the component side?
I use double sided board (I have a stack of this I got cheap) and etch the silkscreen in:

Picture 694.jpg

I mostly only do that on the final revision / finished product. I like having as much information on the SS as possible, to avoid mistakes. Component values, etc.
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Old 19th November 2010, 11:07 PM   #40
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Quote:
Originally Posted by MJL21193 View Post
Some of the features I would like to incorporate into the design:
- Adjustable voltage. As planned, 0 - 30VDC
- Isolated dual supplies, independently adjustable that can be used on their own or put in series to form either a split 30VDC supply or a single 60VDC. When put in parallel, the 2 supplies can give double the current.
- Adjustable current limit for both supplies.
- Built in analogue voltmeter and ammeter on both supplies.
- Built in current limited receptacle. Current limiter in this case is a regular incandescent light bulb in series with the mans. This is for first powering up external power supplies. The bulb will have a bypass switch.
Another feature I want to include in the power supply case is a series of high power dummy loads, for load testing external circuits (amps, etc.). Something like 2, 4, 8, 16 and 32 ohms, each capable of handling at least 100W. I think this is a convenient place to have it, as it won't take up much space and it seems like the most logical place to put it.
This could be banks of cement resistors or another possibility is nichrome wire. I have some from a burned out heatgun that might be enough.
One possibility is a 5 position rotary switch to select the impedance and 2 binding posts to connect to the circuit being tested. Another way is a series of 4 single pole switches, each can put an additional load across the posts (there would already be a 32 ohm load permanently attached to the posts). Button for "16 ohms" would put a 32 ohm load in parallel, "8 ohm" button would put a 16 ohm load, and so on. I like this approach better than the rotary one for a couple of reasons:
1. Better power handling in single pole switch. High current rotary switches are not cheap.
2. As the load goes lower, the power handling goes up, due to the added resistors.
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