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DIY Stompboxes => Building your own stompbox => Topic started by: Kevin Mitchell on July 02, 2019, 12:04:50 PM

Title: Adjustable power supply for your bench
Post by: Kevin Mitchell on July 02, 2019, 12:04:50 PM
I've got quite a few projects I need to wrap up but really should get this one out of the way. I've got some voltmeter modules on the way and plan to box this one up so I can stop stealing my Pedal Power from the pedal board.

Features 4 adjustable supplies - 3 positive and 1 negative.
Should output something like 1 to 15 VDC per channel.
Circuit is per the LM317 & LM337 datasheets
(https://i.postimg.cc/Gm5Khdfp/Bench-Supply.png) (https://postimg.cc/CRbkcTSy)

Voltmeter modules (https://www.ebay.com/itm/10PCS-DC-0-100V-3-Wire-0-36-LED-Digital-Display-Voltmeter-Voltage-Tester/253995306849?_trkparms=aid%3D111001%26algo%3DREC.SEED%26ao%3D1%26asc%3D20160908103841%26meid%3Db4483609b8524b8dae354cf6085455bd%26pid%3D100227%26rk%3D1%26rkt%3D6%26sd%3D253995306849%26itm%3D253995306849&_trksid=p2054502.c100227.m3827)

-KM
Title: Re: Adjustable power supply for your bench
Post by: amptramp on July 02, 2019, 04:49:27 PM
It looks like you only have halfwave rectification for each polarity.  The result is you have current going in both directions through the transformer but these currents are likely to be unequal and a bridge would give fullwave rectification which offers cancellation of the 60 Hz waveform at the output.  This would require a centre-tapped transformer.

Also, where are the electrolytic filter capacitors?  You need one at the input to each polarity of rectifiers and it looks like you only have ceramics there.  The output theoretically can do without but usually you add an electrolytic at each output as well.

Draw up an actual schematic because it is hard to trace out a circuit from a layout diagram and I may be missing something.
Title: Re: Adjustable power supply for your bench
Post by: Kevin Mitchell on July 02, 2019, 04:59:44 PM
Thank you for your input. Indeed I did leave out filter caps! For the time being at least. Regarding the input voltage - I had not considered any sort of cancellation since I've used this configuration in other bipolar power supplies where the current pull was typically far from equal between the rails. But that was with L78XX, L79XX type regulators. I'm not sure if that makes a difference.

Here's a snap of the datasheet;
(https://i.postimg.cc/QdxW8tvL/ps-schem.png) (https://postimg.cc/9rKf1WW1)


If I goofed up the rectifying I'll just use a CT transformer  :icon_redface:

-KM
Title: Re: Adjustable power supply for your bench
Post by: PRR on July 02, 2019, 06:20:54 PM
> Here's a snap of the datasheet

This *assumes* the guy in the other department who designs raw DC supplies put proper fat caps on the end of his work. The chip-maker tells you, the regulator-man, that unless you are "near" those big caps, you "must" have little caps AT the chips to swamp inductance in the possibly long raw-DC feed wires.

In this case I would extend the left end of the board an inch and have two 1,000uFd caps wedged-in between. Then the 0.1u caps may not be essential, but are too cheap/small to scrimp.
Title: Re: Adjustable power supply for your bench
Post by: Rob Strand on July 02, 2019, 08:15:53 PM
Quote
LM317 & LM337 datasheets
Those are good choices form simple adjustable supplies.
You don't *have* to use a 240 ohm resistor, a 220 ohm or 270 ohm is fine although it will change the maximum output voltage a small amount.

Quote
In this case I would extend the left end of the board an inch and have two 1,000uFd caps wedged-in between. Then the 0.1u caps may not be essential, but are too cheap/small to scrimp.
Yes, it's not good to skimp on those.    Especially today where the parts don't quite behave like the originals.

An interesting thing about the input caps, not just for the LM317 but for others, is the distance from input caps is sometimes not specified and sometimes is it is.  Like the National semiconductor datasheet indicates the input cap isn't required when the distance is less than 6 inches from the input caps, whereas On-Semi and Fairchild only say an "appreciable distance".

Another finer point is the negative regulators often have different input and output  cap requirements than their positive counterpart.  I haven't checked but in the back of my mind I'm quite sure the LM337 is like that.  You should check it.
Title: Re: Adjustable power supply for your bench
Post by: antonis on July 03, 2019, 05:24:48 AM
Good work, Kevin..!! :icon_wink:

Some strictly personal notes:
In case of pedal build/test/troubleshoot it should be more convenient to use standard (fix output) regulators, like 78XX/79XX, to save pots, resistors and voltmeter modules..

Of course, an adjustable benctop PS is more flexible but to also look (and be..) more "professional" it should exibit some extra abilities, like current limiting (a MUST for experimental circuits..) and "all down to zero volts" output..
(brute versions of both the above abilities could be easily obtained with rather simple circuits..)

P.S.
Lugs 1 & 2 of all pots should be shorted.. :icon_wink:
Title: Re: Adjustable power supply for your bench
Post by: Kevin Mitchell on July 03, 2019, 09:20:28 AM
Okay okay! I've revised the layout considering the comments.
-Filter caps added
-Input caps placed closer to regulators
-(not mentioned) added space for heatsinks

The datasheet doesn't mention placing the input caps close but does suggest this for the 240 ohm current set resistor  :o  But I've seen these supplies all around where this was not done so I'm not sure how crucial it would be. Perhaps the layouts are small enough for it to suffice.

I'll reconfigure the AC input if necessary. If my drawn method doesn't work out I'll do what amptramp suggested - a CT transformer and diode bridge.

(https://i.postimg.cc/02S6fY4s/Bench-Supply.png) (https://postimg.cc/RNMVCH9s)

Better?  :icon_lol:
-KM
Title: Re: Adjustable power supply for your bench
Post by: Rob Strand on July 03, 2019, 06:45:57 PM
Quote
The datasheet doesn't mention placing the input caps close
It doesn't spell it out but by implication of them being required when far from the caps is they should be close.  Usually the closer the better.

The LM337 should have an output cap.  The datasheet says 1uF tant or 10uF electro (upto 1000uF electro).  It also specs an input cap of 1uF tant.  Tants on the input side tend to fail so an electro say 10uF will be better.

Quote
but does suggest this for the 240 ohm current set resistor  :o  But I've seen these supplies all around where this was not done so I'm not sure how crucial it would be. Perhaps the layouts are small enough for it to suffice.
You can put a wide range of values there.  There is a small current coming out of the adjust pin.  The smaller the output resistor the more current flowing through it.  If the current is large it swamps the small adjust current so the adjust current can't affect the output voltage.  The small current isn't constant it can have a small hum component (actually input voltage dependency).  The adjust current varies with temperature which affects drift, which is not important for your case.

One thing about your layout is the ground track on the top left is bad.  It is connecting the regulator ground to a noisy ground pretty much adding hum to the output of that channel.  You don't want any connections to the line between the common AC input and where the two electros join.  So for example flip the top cap so the common AC line goes between the two caps first.  This puts the AC connections on the left.  Ideally the wire between the caps is fat.  Then the circuit ground connects on the right.  You can see the idea in the PCB layout at the bottom of this page.   You will have to move the ground connection for the top left regulator.

https://xtronic.org/circuit/amplifier/symmetrical-power-supply-for-potency-audio-amplifiers/

Title: Re: Adjustable power supply for your bench
Post by: antonis on July 04, 2019, 05:12:19 AM
https://xtronic.org/circuit/amplifier/symmetrical-power-supply-for-potency-audio-amplifiers/

<off topic on>
Plain enough, Rob but, IMHO, unacceptable use of Watts (instead of VA) for transformer power rating..  :icon_cool:
<off topiv off>
Title: Re: Adjustable power supply for your bench
Post by: Rob Strand on July 04, 2019, 05:26:42 AM
Quote
Plain enough, Rob but, IMHO, unacceptable use of Watts (instead of VA) for transformer power rating..
To be honest I didn't read that page at all.  I only wanted a pic showing the idea of separating the input and output power routing.
Title: Re: Adjustable power supply for your bench
Post by: Kevin Mitchell on July 08, 2019, 09:47:47 AM
The LM337 should have an output cap.
It does. It is lower on the board than the other ones but shouldn't be a problem moving it closer to the wire.

One thing about your layout is the ground track on the top left is bad.
That's a good point - I definitely overlooked the ground at that spot. I've moved it and now everything connected to ground happens after the filter caps.

(https://i.postimg.cc/g2Bs3QXH/Bench-Supply.png) (https://postimg.cc/FY304PxY)

                                 (https://i.postimg.cc/Y9QxKSXd/Bench-Supply-Traces.png) (https://postimg.cc/xJTmL9Vm)


-KM
Title: Re: Adjustable power supply for your bench
Post by: ElectricDruid on July 08, 2019, 10:18:22 AM
I built a "single channel" version of this circuit to enable me to test pedal circuits at lower voltages and also up to 20V or so. Very handy.

For bonus points, I bought a cheap 2 wire digital voltmeter on eBay and stuck that on the output so I could see what I was getting. It works down to about 3.5V or 4V before it gives up, but that was probably lower than I needed for a pedal power supply anyway, so that was fine. Definitely worth the couple of quid it cost to add. This type of thing:

https://www.ebay.co.uk/itm/0-56-Color-DC-AC-5V-30V-500V-Digital-Voltmeter-Voltage-Panel-Meter-LED-Display/273406755547?hash=item3fa85002db:m:mYh6JcO0qZTGzVc9Qu7vMTw (https://www.ebay.co.uk/itm/0-56-Color-DC-AC-5V-30V-500V-Digital-Voltmeter-Voltage-Panel-Meter-LED-Display/273406755547?hash=item3fa85002db:m:mYh6JcO0qZTGzVc9Qu7vMTw)
Title: Re: Adjustable power supply for your bench
Post by: Kevin Mitchell on July 08, 2019, 10:27:54 AM
For bonus points, I bought a cheap 2 wire digital voltmeter on eBay and stuck that on the output so I could see what I was getting. It works down to about 3.5V or 4V before it gives up,

That's why I went with the 3 wire ones  ::)

-KM
Title: Re: Adjustable power supply for your bench
Post by: ElectricDruid on July 09, 2019, 06:19:22 PM
For bonus points, I bought a cheap 2 wire digital voltmeter on eBay and stuck that on the output so I could see what I was getting. It works down to about 3.5V or 4V before it gives up,

That's why I went with the 3 wire ones  ::)

Fair enough. I might next time too. But as I said, for a pedal supply it doesn't matter much since I don't need voltages below what it can measure.

Which voltage did you use for the meter supply? Did you have to include a another regulator for that? (like LM78L05 or something?)
Title: Re: Adjustable power supply for your bench
Post by: Kevin Mitchell on July 10, 2019, 08:42:27 AM
Which voltage did you use for the meter supply? Did you have to include a another regulator for that? (like LM78L05 or something?)

The specs on the listing has some conflicting information. The title says 0-100v but the description suggests staying under 30 to avoid damaging the module.

For power 3 to 30 volts should work. Less than 3 volts the LED will start to dim.
Though I wont need low voltages often I'd rather have the option than not. Other than that the 2 wired ones are probably just as well. The link to my modules are in the first post.

-KM
Title: Re: Adjustable power supply for your bench
Post by: Rob Strand on July 10, 2019, 06:42:44 PM
If you want to get to zero volts there's some nasty tricks like connecting the adjust pot to a negative bias voltage (1.25V).  Perhaps leave the adjust cap going to ground.

Here's the basic idea,
https://www.circuitlab.com/circuit/7k69b4/screenshot/540x405/

But I don't recommend the method with the diodes for quiet supplies as some input ripple appears across the diodes.  You could improve this with a two stage shunt regulator.   Ideally you would use another LM337 as the -ve "bias" for the LM317's.

While I know this scheme works I can't guarantee the reliability and freedom from output glitches when you turn on the power.   Power glitches would be more prone with the doubler type input rectifier you are using since one input rail will *always* come-up before the other.   A small amount of insurance would be to put a 100 ohm load on the output of LM337 bias regulator so it pins the output if the negative rail isn't up.

Interesting trick anyway.

Title: Re: Adjustable power supply for your bench
Post by: printer2 on July 10, 2019, 09:12:16 PM
The EBay volt-amp meter with three wires needs a separate ps if you want to measure below 3.5V. It needs between 3.5-30V to run the module, hooked up to the red and black wire. The yellow/white wire  is the one you hook up to your voltage you are measuring, which could be up to 100V.
Title: Re: Adjustable power supply for your bench
Post by: mozz on July 12, 2019, 10:22:19 AM
Much easier to pick up a HP supply at a hamfest for $5, they will last another 50 years.
Title: Re: Adjustable power supply for your bench
Post by: Kevin Mitchell on July 12, 2019, 02:55:57 PM
The yellow/white wire  is the one you hook up to your voltage you are measuring, which could be up to 100V.
Quote
Notes:
1. When the voltage is lower than 3V, the display brightness will darken but it will not affect the correct measurement.
2. Measuring voltage higher than 30V may damage this product.
Included:
10 x Panel Voltage Meter 3-Wire Lead Connector

To note - they did not come with the panel mount as the picture indicates. Those bastards.

-KM
Title: Re: Adjustable power supply for your bench
Post by: Kevin Mitchell on August 31, 2021, 08:12:51 AM
Keep forgetting to post here. I did eventually throw a small variant of the circuit together. A simple ajustable bipolar supply on vero. There's a couple jumpers hidden underneath. I'll post the layout later.
(https://i.postimg.cc/mcD7QPR2/20210813-174909.jpg) (https://postimg.cc/mcD7QPR2)

(https://i.postimg.cc/06BDZ92G/20210813-174936.jpg) (https://postimg.cc/06BDZ92G)


Title: Re: Adjustable power supply for your bench
Post by: antonis on August 31, 2021, 04:12:50 PM
I should call it "internaly trimmed" rather than "adjustable".. :icon_wink:

P.S.
I know you like to get yourself better Kevin so I strongly suggest you to study Chapter 25 in below:
https://nvhrbiblio.nl/biblio/boek/Self%20-%20Small%20signal%20audio%20design-2e.pdf (https://nvhrbiblio.nl/biblio/boek/Self%20-%20Small%20signal%20audio%20design-2e.pdf)
(especially about mutual shutdown circuitry for bipolar supplies..)
Title: Re: Adjustable power supply for your bench
Post by: Kevin Mitchell on August 31, 2021, 04:35:10 PM
Woah where did that come from?
I could have used this a long time ago thank you so much! I'll keep a copy with my early 1970s electronics handbook. Though this would prove to be far more useful  :icon_lol:

I have seen that sort of implementation of that circuit for "mutual shutdown" before, perhaps in the datasheets. Personally haven't seen it used in the real world. It really should be included in a production design as one should expect anything and everything to go wrong - within reason.

Thanks again!
Title: Re: Adjustable power supply for your bench
Post by: Rob Strand on August 31, 2021, 07:13:24 PM
Quote
(especially about mutual shutdown circuitry for bipolar supplies..)

What should be done is an interesting topic.

A lot of professional Lab quality power supplies have a tracking mode which typically is used when you want a bipolar +/-V supply.  If you haven't used one of these before the voltage setting on the master supply controls the outputs of both slave and master DC outputs.  The pot on the slave channel is disabled.   In non-tracking mode both channels behave independently and identically and the master/slave labeling means nothing.

In one configuration the second channel (slave) tracks the first (master) by literally tracking the voltage on the output terminals of the master.   When you overload or short the master the voltage drops and the slave will follow.     However when you overload or short the slave, the slave voltage drops but the master does not follow.     The behaviour is asymmetrical.

In a second configuration the second channel (slave) tracks the first (master) by following the voltage *pot* setting.   When you short or overload either channel only the voltage on that channel is affected.

I reviewed a number of professional power supplies and both behaviours are out there.   As far as I can rememeber there's none which pull both rails down if *either* one of the rails drops, like a mutually tracking idea.   No doubt that's because of the messy analog electronics required to handle this off the radar case.    (There was one Tektronix power supply which had a lot of funky stuff going on but I can't rememeber if it did mutual tracking at the end of the day.)

All being said you might not want or care if the supplies to mutually track!

Anyway the take home message is there's no standard for this behaviour so there's no point enforcing such requirements on a simple supply because not even the big-boys are consistent.

Having a tracking mode can be convenient.
Title: Re: Adjustable power supply for your bench
Post by: akc1973 on August 31, 2021, 09:18:47 PM
I should call it "internaly trimmed" rather than "adjustable".. :icon_wink:

P.S.
I know you like to get yourself better Kevin so I strongly suggest you to study Chapter 25 in below:
https://nvhrbiblio.nl/biblio/boek/Self%20-%20Small%20signal%20audio%20design-2e.pdf (https://nvhrbiblio.nl/biblio/boek/Self%20-%20Small%20signal%20audio%20design-2e.pdf)
(especially about mutual shutdown circuitry for bipolar supplies..)

Not meaning to butt in but thanks for that link. What a great book!
Title: Re: Adjustable power supply for your bench
Post by: anotherjim on September 01, 2021, 03:52:12 AM
I think there were some of those synth IC's (can't recall if it was Curtis or SSM?) that were prone to self-destruction if the bi-polar supplies were not always equally present. This was fixed in the new replacement chips.
Title: Re: Adjustable power supply for your bench
Post by: ElectricDruid on September 18, 2021, 08:01:56 PM
I think there were some of those synth IC's (can't recall if it was Curtis or SSM?) that were prone to self-destruction if the bi-polar supplies were not always equally present. This was fixed in the new replacement chips.

I haven't heard about it with the CEMs (although it wouldn't surprise me) but the SSM2164 quad VCA is famous for this. Removing the negative supply while the positive is powered up fries the chip permanently, detailed here:

https://www.njohnson.co.uk/index.php?menu=2&submenu=2&subsubmenu=3 (https://www.njohnson.co.uk/index.php?menu=2&submenu=2&subsubmenu=3)
Title: Re: Adjustable power supply for your bench
Post by: Rob Strand on September 18, 2021, 10:34:59 PM
Quote
I haven't heard about it with the CEMs (although it wouldn't surprise me) but the SSM2164 quad VCA is famous for this. Removing the negative supply while the positive is powered up fries the chip permanently, detailed here:

https://www.njohnson.co.uk/index.php?menu=2&submenu=2&subsubmenu=3

I guess the main point is if you want to avoid problems the protection needs to be on the device or on the board.

As far as a bench/lab power supply goes there's no point making it a mutually tracking for these special cases.   You could just as easily pull one of the power supply leads out of the power supply which will instantly remove one of the supply rails.

There's worse mistakes than that:   connect the supply leads around the wrong way.   accidentally dial up 30V on a 3V3 rail.
(I noticed the Rigol lab supplies have a programmable upper voltage limit setting which overrides the normal voltage adjustment setting to stop silly mistakes.)    Pulling out the ground when you have 3V3 and -30V will cause problems I'm sure!


If you use special chips (or you only have one part left  ;D) it's always wise to put protection on the breadboard.   It still doesn't stop a rogue wire flinging off and falling on the board.