Bipolar benchtop power supply, and why are they so hard to find.

[fabgar]

There's something about bench power supplies I don't quite understand.

A lot of the time you need a positive and negative voltage. So why is it so hard to find an affordable benchtop power supply (like these popular sub $100 ones from LaVolta, Korad, Voltkraft, Velleman, .. for instance) that offers this? It would be so convenient. The circuit to get a bipolar voltage is pretty simple, yet you're always advised to get two or a multi output supply and put them in series.

What about an extension box that splits a unipolar into bipolar voltage? Or would this be a bad idea for some reason, since I've never even seen something like it.

I feel like I'm missing something obvious here ?

And btw, what is the correct term to use, because there seems to be confusion about this sometimes. Or terms are just used randomly. Is it a bipolar, dual rail, split rail, dual channel .. power supply?

[Aodhan145]

Whats wrong with a multi output supply in series?

[Coceth]

Maybe it's so easy to get two power supplies in series and reference the ground at the midpoint, so companies don't bother manufacturing a tracking supply.
I think that is what they are called, right? Tracking supplies, since the negative channel tracks the magnitude of the positive channel.

[fabgar]

Whats wrong with a multi output supply in series?

they are expensive, especially if you are starting out.

And since it's not that hard to do or expensive (basically two voltage regulators), I find it odd why you don't just find a simple, affordable biopolar bench supply.

[jpb]

Most supplies have floating outputs so that if you have two outputs you can use them in series with the middle connected to earth.

Tracking is handy - two of my three supplies are tracking (TTi PL32 and Agilent 3631A) but neither of them were cheap (the TTi cost me £160 as new old stock and the E3631A £300 on e-bay).

But I agree, this is getting rarer. I think modern circuits use op amps much less and so don't need bipolar supplies in the same way (everything is digital).

I don't know how adjustable bipolar supplies need to be. An alternative is to get an encapsulated fixed linear supply at +- 15V say and putting it in a case. I recently bought a new (old stock) Calex 52212A off e-bay for around £25. It does up to +-15V at 500mA but has a screw adjustment to set the voltage anywhere between 10V and 15V. Because everyone uses smps now, there are some bargain linear supplies around.

[_Wim_]

they are expensive, especially if you are starting out.

And since it's not that hard to do or expensive (basically two voltage regulators), I find it odd why you don't just find a simple, affordable biopolar bench supply.

If you only need a bipolar power supply with current limiting and any other feature, I agree this would be more cheap to make (but also ideal as a DIY project for a beginner)

But if you need feature like CV and CC and OVP and OCP, then making the supply bipolar limits its usefullness, and the only money saved will probably be 1 banana terminal because the commons will be tied togeter internally.

[David Hess]

What about an extension box that splits a unipolar into bipolar voltage?  Or would this be a bad idea for some reason, since I've never even seen something like it.

That can be done.  Audio amplifiers make good rail splitters but they have to be modified to drive capacitive loads and implement constant current limiting if desired.  They are only a good option if a bipolar or dual supply is not available.

I think that is what they are called, right? Tracking supplies, since the negative channel tracks the magnitude of the positive channel.

A bipolar or dual supply may or may not include tracking.  I like the old Tektronix implementation where the bipolar outputs are individually controlled so that the ratio between them and the maximum output voltage of each may be set and then the tracking control adjusts both.  HP did it a different way with one tracking control for the ratio and one control for the output voltages.

And since it's not that hard to do or expensive (basically two voltage regulators), I find it odd why you don't just find a simple, affordable bipolar bench supply.

For a simple design where the two sides share a common, the cost of a tracking supply is greater than two individual supplies because it is not simply a matter of replication although mirroring is possible.  For a dual floating design which could be replicated, the interconnection to support tracking between the floating supplies is complex.

The thing which puzzles me is that there are no good kit designs for single and tracking bipolar output CC/CV power supplies.

But if you need feature like CV and CC and OVP and OCP, then making the supply bipolar limits its usefullness, and the only money saved will probably be 1 banana terminal because the commons will be tied togeter internally.

How does making a power supply bipolar limit its usefulness?  Either side or both together may be used in place of a unipolar output.  Current limiting becomes trickier to use when both are used but the output voltage is doubled.  All designs I am familiar with implement the current limits for each side completely separately which is inconvenient but not fatal.  A "tracking" current limit is an interesting design exercise.

One thing I like about floating bipolar tracking supplies is that they may be used as single power supply of twice the voltage so my +/-20 volt bipolar tracking supplies also serve as 0 to 40 volt single power supplies.  From a design perspective, this is advantageous because it divides the total power dissipation across two power devices.

[_Wim_]

How does making a power supply bipolar limit its usefulness?

Because you cannot use is anymore as 2 separate sources with separate grounds. The way I understood the op was that he did not like the fact that to make it bipolar, he had to make a connection from one ground to the plus of the other channel. By tying them together internally, you have a bipolar supply, but because the connection is always there, its limits your options...

[David Hess]

How does making a power supply bipolar limit its usefulness?

Because you cannot use is anymore as 2 separate sources with separate grounds. The way I understood the op was that he did not like the fact that to make it bipolar, he had to make a connection from one ground to the plus of the other channel. By tying them together internally, you have a bipolar supply, but because the connection is always there, its limits your options...

Making a tracking dual power supply without a common is a lot more complicated although it does allow paralleling the outputs for twice the current.  A digitally controlled implementation has this easier because the galvanic isolation can be done on the digital side but then the complexity starts out much higher.

For an analog design, it is probably easier to just make two completely separate bipolar tracking supplies than it is to make a dual floating output tracking supply.  The later is not impossible even in a fully analog design but I am dubious that it would be better.

[FlyingHacker]

Something like this is an excellent supply. It can easily be made to be tracking, and the supplies are floating you you have have +/- split supplies/ I have two of these beasts, and they are rock solid and quiet.

http://www.ebay.com/itm/Lambda-LPT-7202-FM-Triple-Output-Regulated-DC-Power-Supply-/311959415327?epid=1401816455&hash=item48a23aea1f:g:7l8AAOSwfaZZuW3m

Lambda makes some of the finest supplies on the planet. They make many of the supplies used by most of the big boys (Keysight/Agilent/HP, Tek etc.) Dave has repeatedly referred to the as "the best in the business," and for good reason.

It is possible a used one you buy off eBay will need a re-cap. It is quite easy, though.

[rdl]

This probably doesn't help much because used equipment is not so easy to find outside the U.S. , but you might keep an eye out for one of these. They are fairly common on ebay for around $100 US. Mine is almost constantly in use.

[fabgar]

Thanks guys, exactly what I wanted to know, really appreciate it!

[David Hess]

If you check the Tektronix CPS250 schematics, then you can see what I meant by extra complexity.  Besides that 4 pole 3 throw switch, there is an extra set of control circuits to enforce tracking and current sharing.

[andyturk]

This probably doesn't help much because used equipment is not so easy to find outside the U.S. , but you might keep an eye out for one of these. They are fairly common on ebay for around $100 US. Mine is almost constantly in use.

I've got the same unit, but with a BK Precision name tag on it. It's a great little supply: totally quiet (no fan) and doesn't take up much bench space either. The analog meters aren't the most accurate though.

[capt bullshot]

  All designs I am familiar with implement the current limits for each side completely separately which is inconvenient but not fatal.  A "tracking" current limit is an interesting design exercise.

There's an interesting dual bench supply from Oltronix, the model B202 (you can google its schematics). This one outputs a positive and a negative voltage with common ground. In tracking mode, if one output goes into current limit, the other output reduces its voltage in the same ratio. Never seen another supply doing it this way. IMO this behaviour is quite handy if you use this supply for any kind of amplifier circuit for protection and testing purposes.

[rsjsouza]

If your split rail demands are low power, you can always use the TLE2426 - I find it useful on my opamp circuits.

[TopLoser]

The cheap and nasty Tenma dual output supplies can be switched between independent, serial or parallel connection. In serial or parallel the outputs track each other, in independent mode they can be controlled individually.

http://uk.farnell.com/tenma/72-10500/power-supply-2ch-30v-3a-adjustable/dp/2251948

I always have a stack of these going cheap.

[xani]

Out of curio

The cheap and nasty Tenma dual output supplies can be switched between independent, serial or parallel connection. In serial or parallel the outputs track each other, in independent mode they can be controlled individually.

http://uk.farnell.com/tenma/72-10500/power-supply-2ch-30v-3a-adjustable/dp/2251948

I always have a stack of these going cheap.

Out of curiosity is it KORAD rebadging TENMA or TENMA rebadging KORAD? I've got a very similiar one. (just 5A and have 3rd 5V/3A constant output) but with binding post.

Mine survived all I can throw at it from electrical perspective, doesn't overshoot and has clean power but on the other side power switch survived maybe a week and I had to replace it

[TopLoser]

Out of curio

The cheap and nasty Tenma dual output supplies can be switched between independent, serial or parallel connection. In serial or parallel the outputs track each other, in independent mode they can be controlled individually.

http://uk.farnell.com/tenma/72-10500/power-supply-2ch-30v-3a-adjustable/dp/2251948

I always have a stack of these going cheap.

Out of curiosity is it KORAD rebadging TENMA or TENMA rebadging KORAD? I've got a very similiar one. (just 5A and have 3rd 5V/3A constant output) but with binding post.

Mine survived all I can throw at it from electrical perspective, doesn't overshoot and has clean power but on the other side power switch survived maybe a week and I had to replace it

Tenma rebadging Korad I think.

Most of the Tenma ones come with binding posts now, except the 60V ones. They can't seem to make 2 units the same to be honest though.

I think they're great for the money, especially at half price. The packaging they come in is shoddy for such a heavy linear supply and a lot do get damaged in transit. A lot of LED failures as well, the actual LED's and the TTL drivers.

[bd139]

I've actually got my eye on one of them. Do they blow up if you run them flat out? I've noticed a lot of cheaper supplies do.

[hermit]

Re: Bipolar benchtop power supply, and why are they so hard to find.

Lithium? 

[xani]

I've actually got my eye on one of them. Do they blow up if you run them flat out? I've noticed a lot of cheaper supplies do.

I dont think I have enough power resistors to test that, sorry. But I did successfully charge 3000F supercap from it.

Dave did a review of it if you're interested, just google for it. It had few problems but they seem to be fixed:



For a price it is bargain IMO

[bd139]

Thanks - will watch

[David Hess]

  All designs I am familiar with implement the current limits for each side completely separately which is inconvenient but not fatal.  A "tracking" current limit is an interesting design exercise.

There's an interesting dual bench supply from Oltronix, the model B202 (you can google its schematics). This one outputs a positive and a negative voltage with common ground. In tracking mode, if one output goes into current limit, the other output reduces its voltage in the same ratio. Never seen another supply doing it this way. IMO this behaviour is quite handy if you use this supply for any kind of amplifier circuit for protection and testing purposes.

I think that Tektronix example does that also.  It is pretty easy to do with integrated regulators if you use the right circuit and it is one of the features I would design into a tracking power supply.

[Zero999]

I made my own, many years ago when I was a student. I used transformer with two 15V secondary windings, which I connected in series, with the centre tap going to 0V on the schematic.


Note that this design is marginal. I'd use higher value smoothing capacitors, rated to 35V, if I did this again.