Author Topic: DC Variable power supply  (Read 1656 times)

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Offline omkarTopic starter

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DC Variable power supply
« on: January 02, 2012, 08:53:07 PM »
Hi All, I am looking to purchase a DC Variable power supply as more i get into this robot stuff, more it seems i need one. I am looking at following :
1. http://www.ebay.com.au/itm/DZ-PS305D-Variable-30V-5A-DC-Power-Supply-Lab-Grade-/320684622216?pt=LH_DefaultDomain_0&hash=item4aaa4b0188
2. http://www.ebay.com.au/itm/Linear-Adjustable-Variable-DC-Power-Supply-30V-5A-Digital-Power-Supply-PS305D-/360396453994?pt=AU_Components&hash=item53e94d786a
3. http://www.ebay.com.au/itm/ATTEN-APS3005S-Variable-DC-30V-5A-Power-Supply-/220840336138?pt=BI_Electrical_Equipment_Tools&hash=item336b1c1b0a

I have read that desirables are : needs to have a +ve and -ve voltage apart from the ground, it needs to have a range starting from zero. Any others? Any views on these options listed above?

Cheers
Omkar

Offline Soeren

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Re: DC Variable power supply
« Reply #1 on: January 03, 2012, 09:34:57 AM »
Hi,

I am looking at following :
[...]
Basically, they're all the same voltage and current, but the last one has got more specs (if they hold, it sounds really good, perhaps too good) even if in a semi-funny machine translation. Don't let the "free gift" of one lousy alligator clip lure you into buying #1, they're dirt cheap.

Apart from that, calculate the true price (i.e. including shipping to your home address) and pick the cheapest - not much difference anyway.


I have read that desirables are : needs to have a +ve and -ve voltage apart from the ground, it needs to have a range starting from zero. Any others? Any views on these options listed above?
Not sure if that's what you mean, but none of the supplies have a negative output. They have 0V, +V and an earth connection (connected to the housing if metal, that you'll only have a use for if your mains supply connector carries a ground pin (not all do) and even then, I wouldn't let that be a deal breaker - a 4mm socket in the rear, wired to the metal housing (or the transformers iron core) , is both cheap and easy to install.

People claiming that starting from 0V is important is usually the type that think that we should all be driving a Lamborghini, even when shopping groceries.
Lab supplies either start from 0V or from 1.25-1.50V.
Most amateurs (and a good number of pros) will never need less than 1.5V (and the unlikely event a diode or two in series with the output will get it down).


What's important is that the current limiter goes down to a very small current, preferably a real CC mode. Some high current supplies are lousy in setting low currents, like eg. 2mA in a 5A unit.

Most people will be just as well off with a stepped voltage control. Some people just goes overboard on the dial, insisting that 12V is 12.00V (= pure insanity) - [12V 1% means a range of 11.88V to 12.12V and most people will do excellent with a tolerance of 5%] and that worry is removed with a stepped control.
From that philosophy, I built a supply for my brother. It has got steps for 1.5V, 3V, 4.5V, 6V, 9V, 12V, 15V and 18V and he's never needed anything besides those values (during ~15 years) and no voltmeter is needed.

Stability (step response and load-/line regulation) is an extremely important parameter (or rather a set of parameters), but most first time buyers neglect this, of ignorance or because the numbers aren't always available.

Anybody serious about electronics should have (at least) 2 lab supplies:
One for high currents which can have a stepped voltage selector (or not) and a lower current unit eg. 1A max. with a continuously variable and easily settable current limiter and here the voltage should be continuously variable and going from 0V will help if/when using it for reference voltages (if it is stable enough).
The latter is an obvious choice for DIY, but large current supplies have become so cheap, that you cannot get the parts and box alone at a competitive price (unless you're able to design and build a switch mode PSU).
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline omkarTopic starter

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Re: DC Variable power supply
« Reply #2 on: January 04, 2012, 02:19:34 AM »
Thanks for the detailed reply. Here is another one i found that fits the bill ( i thnk):
http://www.ebay.com.au/itm/New-HQ-Variable-Regulated-PS305D-30V-5A-DC-Lab-Power-Supply-240V-/140674491892?pt=AU_B_I_Electrical_Test_Equipment&hash=item20c0da65f4

1. 0-30V variable (description doesnt talk about accuracy - whether 12 or 12.0 or 12.00V - but I guess you are saying thats not important)- i think this is a xx.0V display.
2. "Current limiter goes down to very small current" : not sure from description which says "Output Current: 0-5A, 0-999mA (adjustable)". Cant tell whether this means it can set 2mA current in this unit or not.
3. Stability: Current Stability: 0.1% + 3 mA
Load Stability: 0.2% + 3 mV
Is this good enough?

I see your point with having two supplies (one for current and other for voltage) but rt now I need to start with something. Thouth I dont understand the bit about -ve and +ve voltage terminals apart from the ground (0V) terminal. I thought if u connect the ciurcuit betn the -ve and ground (0V) terminals then the current will from in the circuit from 0V to -ve volts and vice cersa for +ve/0V combo.

Offline omkarTopic starter

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Re: DC Variable power supply
« Reply #3 on: January 04, 2012, 02:23:31 AM »
Last line should read:
I thought if u connect the ciurcuit betn the -ve and ground (0V) terminals then the current will FLOW in the circuit from 0V to -ve volts and vice cersa for +ve/0V combo.

Offline Soeren

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Re: DC Variable power supply
« Reply #4 on: January 04, 2012, 03:40:40 AM »
Hi,

Thanks for the detailed reply. Here is another one i found that fits the bill ( i thnk):
[...]
Is this good enough?
Yes. More expensive, but that includes shipment within Australia, so might be the same as the other 3 supplies (I didn't calculate the totals for them).


I see your point with having two supplies (one for current and other for voltage)
More like one for higher currents and one for lower currents that can be set with greater accuracy (but where there's no need for high currents).


but rt now I need to start with something.
Yes, and either of the supplies you linked to will be very, very, very fine as a first supply.

I started out with a cigar box (literally) with a neon indicator, a small mains transformer, rectification and a large cap for my first mains supply, to replace a number of D-cells in a carton tube - I used that for quite a lot of experiments ;D  (That was around 1968..1969, where allowance was small and electronics components were hard to come by though).


Last line should read:
You can use the "Modify" link to edit your posts


I thought if u connect the ciurcuit betn the -ve and ground (0V) terminals then the current will FLOW in the circuit from 0V to -ve volts and vice cersa for +ve/0V combo.
Not quite so.
There are three terminals (4mm sockets), one, often the middle one (yellow in the supply you linked to), is connected to the metal box and the mains earth (if used). If the supply has screw terminals I usually short it to 0V (the black socket), as it helps in some (noise sensitive) circuits to have the 0V connected to Earth (only do this if your mains connector has the Earth connection though).
There should be no potential (=voltage) difference from 0V to Earth, but if your mains supply is not connected to earth, a weak current at half the mains voltage may be present on the Earth (like on a PC), due to 2 noise decoupling caps mounted from mains phase to Earth and from mains 0 to Earth.


You can have dual rail supplies with +V, 0V and -V, but most of them give a non-adjustable output voltage, like +12V, 0V and -12V. You can view such a supply as a 24V supply with a center tap at 12V, if you use the -12V as your ground reference (hope I don't confuse you here).

You won't need a dual rail supply unless you're going to use very special components in very special circuits (like high end audio or instrumentation).

The supplies you're looking at, are all single voltage and that's what you need.
In most circuits, probably all you'll be building for a very long time, you can ignore the earth socket and just go with black (0V) and red (V+).

Did that clear it up a little? :)
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline omkarTopic starter

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Re: DC Variable power supply
« Reply #5 on: January 08, 2012, 05:03:25 PM »
Hi yes ... thats perfectly clear now. Thanks so much. I have purchased this power supply now just awaiting delivery.

 


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