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Adjustable Power Supply

FredF

Sep 16, 2014
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Hello Everyone!
I was hoping to get some information to help me with a project I am working on. I am in the process of building a table top cnc milling machine, using an arduino uno and a GRBL 3.0 cnc shield to control 3 stepper motors. I am currently in the testing stage of the electronics, using small bi-polar steppers salvaged from old scanners, which are rated somewhere around 6v dc 800ma. I am looking at eventually running 3-287 oz. nema 23s rated at 8.6v dc 1.0A/phase. I would like to build an adjustable power supply from an old computer power supply that I could use through all stages of this project, from start to finish. I have several on hand, ranging from 250 watts, up to 400 watts. Would I be correct in assuming that I would need to adjust both voltage and amperage? Is this something that can be done? Any information or helpful links would be greatly appreciated. Thanks.
Fred
 

Arouse1973

Adam
Dec 18, 2013
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If the load stays the same adjusting the voltage will naturally change the current. You would only need to adjust the current if you were operating the load in constant current mode or current limited mode of some kind. The load will only take what current it needs to satisfy V=I*R (derived from Ohms Law) when a certain voltage is applied. There is no reason why you can't limit the current if you feel the need, but this might effect the motors start up time.
Adam
 

KrisBlueNZ

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I would like to build an adjustable power supply from an old computer power supply that I could use through all stages of this project, from start to finish.
Computer power supplies are great for this kind of thing. But what output voltage range do you need? Generally people use the +12V output, and you will be able to vary the voltage on that rail somewhat, but not over a very wide range. You should be able to drop it down to 8.6V and 3A won't be a problem.
Would I be correct in assuming that I would need to adjust both voltage and amperage? Is this something that can be done? Any information or helpful links would be greatly appreciated.
As Adam said, you don't "adjust" the current; the load draws a certain amount of current. The power supply just needs to be able to comfortably deliver the amount of current demanded by the load. If the load is mainly resistive (a stepper motor is mostly resistive), adjusting the voltage will affect the amount of current it draws. The 12V rail on a PC power supply is usually rated for at least 8A; sometimes 20A or even more!

You could start by choosing a power supply to modify. If you have two or more of the same model, that would be a good choice so you can have a spare. Take some photos of both sides. See https://www.electronicspoint.com/resources/how-to-take-photos-of-circuit-boards.6/
 

arcQuisumbing

Sep 18, 2014
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hello guys. i have a similar issue so i would just like to continue here instead of opening another topic.

i have a very limited electronics knowledge so please bear with me.

just to elaborate, please let me know if my understanding is correct.

Having a 12V 30A power supply would normally produce 360 watts. My question is, if i will have a load of 12V with 72W rating, will it be alright to use the 12V 30A? Or will it fry my circuit.

I am hoping that the 12V 30A is the supply's maximum and that it can easily power up my load as it will only draw the current that it require.

your expert advise will be highly appreciated.
 

KrisBlueNZ

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A "12V 30A" power supply will produce 12V at its output, and can supply UP TO 30A. The amount of current it actually supplies depends on the load. So "30A" is a maximum rating for the power supply.

Wattage is voltage multiplied by current, so again it depends on the amount of current flowing.

So a 12V 30A power supply will not damage a cicuit that only draws 72W (6A).

The only case where a high-current power supply will damage a low-current load is when the power supply is not regulated, and its voltage is higher than it should be when it is not sufficiently loaded. PC power supplies are regulated, but the regulation is applied on the 5V rail or the 3.3V rail, and is not as tight on the 12V rail. So you may find that the 12V rail is a bit high - perhaps 12.5V - when the load is light. But this small difference isn't a problem.
 

arcQuisumbing

Sep 18, 2014
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A "12V 30A" power supply will produce 12V at its output, and can supply UP TO 30A. The amount of current it actually supplies depends on the load. So "30A" is a maximum rating for the power supply.

Wattage is voltage multiplied by current, so again it depends on the amount of current flowing.

So a 12V 30A power supply will not damage a cicuit that only draws 72W (6A).

The only case where a high-current power supply will damage a low-current load is when the power supply is not regulated, and its voltage is higher than it should be when it is not sufficiently loaded. PC power supplies are regulated, but the regulation is applied on the 5V rail or the 3.3V rail, and is not as tight on the 12V rail. So you may find that the 12V rail is a bit high - perhaps 12.5V - when the load is light. But this small difference isn't a problem.

Thank you very much for the quick response.

More power to you.
 

FredF

Sep 16, 2014
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Hey Kris & Adam!
Thanks for the very informative, not to mention, quick response to my post. I, like arcQuisumbing, am a novice at electronics, so your incite is greatly appreciated. Any way, from doing some research on the nema 23 power requirements, it seems that a 12v-6A output would work, but a 24v-6A configuration would be optimal. That being said, I went through my inventory of computer power supplies ( I'm a shameless thrift store/flea market junkie :) ) and found that the 300 & 400 watt supplies range from +12v-16A to +5V-30A. I also found a Radioshack regulated power supply rated at 13.8VDC-3A. Darn thing weighs twice as much as the other power supplies. I posted a couple pics of it. As far as making the output adjustable, would this be done with a series of resistors for each prospective output? Or could this be done with a variable resistor, like a potentiometer or rheostat? Again, if these ideas seem ridiculous, bear with me, I'm new and want to learn. lol. Thanks again everyone.
FredR.S. Power Supply.jpg R.S. P.S..jpg
 

Arouse1973

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Resistors would probably generate too much heat and waste energy depending on how much current you are supplying. They also won't keep the voltage that stable if you are drawing different amounts of current from one of the resistors.

You need something with feedback to regulate the voltage. Fortunately you have all you need to make an adjustable power supply. I wouldn't recommend modifying the existing power supply as you don't yet have the experience.

However you could use this as the power supply and build a small adjustable regulator circuit that connects to the output. I would have thought with the right components you could make a 12 Volt adjustable power supply. I'll have a look and see what I can come up with.
Adam
 

FredF

Sep 16, 2014
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Thanks Adam. That would be awesome! I looked around online awhile back, but didn't find anything that I thought I could try. My schematic reading skills are a little lacking at this point I'm afraid. lol.
Fred
 

Arouse1973

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Here is a simple low voltage drop adjustable regulator based on the LT1085. It would be a good idea to use a heat sink also for the device to prevent over heating. P1 is a 1W standard linear POT and is used to adjust your voltage. This design only has variable voltage and not current so be careful. If you want variable current then that's a bit more complicated.
Adam

LT1085.JPG
 

FredF

Sep 16, 2014
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Thanks for posting the schematic Adam. I'm going to try putting it together this weekend.One question though. When you suggest putting a heat sink on the "device", were you referring to the power supply, or the device it would be powering? Also, I hate to bother you, but could you post the schematic for a variable current circuit? It might come in handy further on down the road. Thanks a million.
Fred
 

(*steve*)

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In that circuit,the LT1085 will probably need a heatsink, but it all depends on your load and the voltage you set it for.

Assuming the input voltage is 13.8 and you set your output voltage to V, and the current you require is I, the power that the LT1085 needs to dissipate is (13.8 - V)*I

If V it 6V, and I is 0.25A (You always state these in volts and amps for the purposes of the equation, although 0.25A may be more often called 250mA)
Then (13-8 - 6) * I = (13.8 - 6) * 0.25 = 7.8 * 0.25 = 1.95

That 1.95 is the power (in Watts) dissipated by the LT1085. If this exceeds about 1 you will need a heatsink. This is a very rough rule of thumb based on the presumed package that the LT1085 is in.

If you need a heatsink, you need to think about "how big" and "how do I connect it", both of which require some additional knowledge that we can go into if you need us to.
 

FredF

Sep 16, 2014
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Thanks for the explanation Steve. Once I get this circuit operational, I was considering either putting it inside the power supply enclosure, or if there isn't enough room, build a separate enclosure with a cooling fan ( I'm in the sheet metal fabrication business) just to keep things tidy. Is a cooling fan an option, or is the heat sink the best way to dissipate the heat? Thanks again.
Fred
 

Arouse1973

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Dec 18, 2013
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Hi Fred

Go for a heatsink. You might need some heatsink compond or heatsink material to help transfer the heat from the device to the heatsink.

Adam
 

FredF

Sep 16, 2014
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Hey Guys!
Having trouble locating the LT1085 in the local stores. Radioshack is a joke! The kid had no idea what I was looking for! He tried to sell me a cell phone and an RC car! lol. The other store is a "big box" electronics store which usually has what I need. But unless it is listed under another number,( they're an NTE distributor,the cross reference didn't recognize the LT1085) they didn't have it either. Looks like I might have to order it online, unless someone can suggest an appropriate substitute. I do have an L7805CV and an LM317T on hand. Thanks.
 

(*steve*)

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The LT1085 is a newer and better version of a LM317.

You could use an LM317 in this circuit, however you will be limited to a lower current, and the maximum output voltage you could reliably supply would be lower (it would be 11V vs 12.5V).
 

davenn

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Thanks Steve

I wasn't familiar with the LT108x series

actually WOW LT1083 - 7.5A; LT1084 - 5A; LT1085 - 3A

outstanding

Dave
 

FredF

Sep 16, 2014
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Thanks Steve. I think I'll order the LT1085, breadboard the circuit with the LM317 just to see if I can get it to work, then swap it out for the 1085 when it arrives. Thank you to everyone for your help.
 

(*steve*)

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Just be sure to check the pinout as they may be different.
 
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