Bipolar benchtop power supply, and why are they so hard to find. - Page 1
Forum section: electronics / beginners

author: fabgar
author_rank: Contributor
timestamp: October 08, 2017, 11:36:02 am
content: 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?

author: Aodhan145
author_rank: Frequent Contributor
timestamp: October 08, 2017, 11:40:13 am
content: Whats wrong with a multi output supply in series?

author: Coceth
author_rank: Contributor
timestamp: October 08, 2017, 11:43:18 am
content: 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.

author: fabgar
author_rank: Contributor
timestamp: October 08, 2017, 11:49:34 am
content: Quotethey 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.

author: jpb
author_rank: Super Contributor
timestamp: October 08, 2017, 12:14:34 pm
content: 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.

author: _Wim_
author_rank: Super Contributor
timestamp: October 08, 2017, 02:02:00 pm
content: Quote from: fabgar on October 08, 2017, 11:49:34 amIf 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.

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2017, 09:03:44 pm
content: Quote from: fabgar on October 08, 2017, 11:36:02 amThat 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.Quote from: Coceth on October 08, 2017, 11:43:18 amA 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.Quote from: fabgar on October 08, 2017, 11:49:34 amFor 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.Quote from: _Wim_ on October 08, 2017, 02:02:00 pmHow 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.

author: _Wim_
author_rank: Super Contributor
timestamp: October 08, 2017, 09:09:58 pm
content: Quote from: David Hess on October 08, 2017, 09:03:44 pmBecause 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...

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2017, 12:28:16 am
content: Quote from: _Wim_ on October 08, 2017, 09:09:58 pmMaking 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.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 09, 2017, 01:17:13 am
content: 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:7l8AAOSwfaZZuW3mLambda 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.

author: rdl
author_rank: Super Contributor
timestamp: October 09, 2017, 06:11:15 am
content: 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.

author: fabgar
author_rank: Contributor
timestamp: October 09, 2017, 07:06:27 am
content: Thanks guys, exactly what I wanted to know, really appreciate it!

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2017, 10:29:53 am
content: 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.

author: andyturk
author_rank: Frequent Contributor
timestamp: October 09, 2017, 10:37:27 am
content: Quote from: rdl on October 09, 2017, 06:11:15 amI'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.

author: capt bullshot
author_rank: Super Contributor
timestamp: October 09, 2017, 10:41:23 am
content: Quote from: David Hess on October 08, 2017, 09:03:44 pmThere'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.

author: rsjsouza
author_rank: Super Contributor
timestamp: October 09, 2017, 10:47:32 am
content: If your split rail demands are low power, you can always use theTLE2426- I find it useful on my opamp circuits.

author: TopLoser
author_rank: Unknown
timestamp: October 09, 2017, 10:48:23 am
content: 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/2251948I always have a stack of these going cheap.

author: xani
author_rank: Frequent Contributor
timestamp: October 09, 2017, 10:58:52 am
content: Out of curioQuote from: TopLoser on October 09, 2017, 10:48:23 amOut 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

author: TopLoser
author_rank: Unknown
timestamp: October 09, 2017, 11:07:18 am
content: Quote from: xani on October 09, 2017, 10:58:52 amTenma 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.

author: bd139
author_rank: Super Contributor
timestamp: October 09, 2017, 11:27:36 am
content: 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.

author: hermit
author_rank: Frequent Contributor
timestamp: October 09, 2017, 11:39:10 am
content: Re: Bipolar benchtop power supply, and why are they so hard to find.Lithium?

author: xani
author_rank: Frequent Contributor
timestamp: October 09, 2017, 12:16:00 pm
content: Quote from: bd139 on October 09, 2017, 11:27:36 amI 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

author: bd139
author_rank: Super Contributor
timestamp: October 09, 2017, 12:51:41 pm
content: Thanks - will watch

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2017, 06:45:39 pm
content: Quote from: capt bullshot on October 09, 2017, 10:41:23 amI 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.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2017, 09:12:40 pm
content: 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.

author: David Hess
author_rank: Super Contributor
timestamp: October 09, 2017, 11:27:44 pm
content: Quote from: Hero999 on October 09, 2017, 09:12:40 pmThat is the straightforward way to do it. It is possible to do without the operational amplifier as shown below. EDN publishedanother example.Driving the virtual ground to the feedback network as shown in the EDN example has the virtue of maintaining tracking symmetry from both sides. I might combine that with the example shown below. I think I have seen another design which uses two operational amplifier to cross connect the feedback networks achieving the same thing.

author: Zero999
author_rank: Super Contributor
timestamp: October 10, 2017, 12:28:34 pm
content: Quote from: David Hess on October 09, 2017, 11:27:44 pmThat should be better than my design. It should also be more stable. Others have said that my op-amp based design might oscillate but I've not managed to make it do that on the bench, even though no compensation capacitor is used.

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2017, 05:11:03 pm
content: Quote from: Hero999 on October 10, 2017, 12:28:34 pmThe simplified tracking design does not track with line and load changes and does not track in current limit. I think its major advantage is simplicity. Adding a single operational amplifier to balance the output as shown in the EDN example solves that.I have never had a problem controlling an integrated regulator with an operational amplifier. If the operational amplifier is slow enough, then frequency compensation is not a problem. If frequency compensation is needed, then it is not difficult to add.None of these designs are suitable for a variable output bench supply unless extreme simplicity is desired and an integrated design is unlikely to need universal tracking.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: October 11, 2017, 10:18:09 pm
content: There are also some specialized ICs for building symmetric power supplies:the RC/XR4194 (Raytheon/Exar) and the SG1501 (Silicon General/Unitrode), albeit the latter does not go below +/-10V. Both have Current limiting and use external transistors for higher currents.Also see LM125 (NS, see AN82). The circuits in the datasheet of the CA3094 (RCA/Intersil) andthe applications for the MC1461/1463 also have some circuits.I just went though my index of appnotes and articles: all build-your-own ones with one knob controlling two outputs use a double pot or related, so they are not tracking each other. NS AN127 gives a tracking regulator using OPA's (LM143).

author: b_force
author_rank: Super Contributor
timestamp: October 12, 2017, 12:13:48 am
content: What power do you need?If i am right I read between the lines you're just starting?So why not making one yourself? It's not that hard.In general, why don't people make stuff themselves anymore?

author: Shock
author_rank: Super Contributor
timestamp: October 12, 2017, 12:30:59 am
content: Tenma = Farnell, Element 14Korad = Shenzhen Korad Technology

author: Awesome14
author_rank: Regular Contributor
timestamp: October 12, 2017, 02:59:48 am
content: I use a Sorensen quad 15-4. It's cheap, digital, bullet-proof, does parallel and serial, so pos/neg voltage no problem. I have 2 of them, $300 ea. off eBay.

author: David Hess
author_rank: Super Contributor
timestamp: October 12, 2017, 03:54:51 am
content: Quote from: Neomys Sapiens on October 11, 2017, 10:18:09 pmMost tracking supplies do not track each other including the example in National Semiconductor application note 127. It like most has a second output configured as a gain of -1 inverting power amplifier. Maybe call that half tracking?In practice however the distinction is almost always irrelevant and even less so with bipolar supplies. With multiple output supplies, it can be very important; some circuits will destructively fail if the higher supply voltage fall below the lower ones. Sometimes protection diodes are included in the design to prevent this from happening.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: October 12, 2017, 10:15:31 pm
content: Quote from: David Hess on October 12, 2017, 03:54:51 amConcur on this. I did not take a look at the circuit. I also think that I saw another, more interesting circuit somewhere (which was, inexplicably, not in my index). A possibility that was not mentioned was using a power OPA for creating a virtual ground.Also, it becomes obvious why all those circuits are not made up of 'regular' LPS, that is, CV/CC ones with a rectangular characteristic, as the control rule would become tricky.The function described for the Oltronix B202 (and probably our PS503s) is one possibility (and maybe the one that makes the most sense). If we assume two PS tracking each other in CV mode with the same CC limit setting and the load drawing different currents out of the two, nothing happens as long as both stay below the CC limit. When one reaches the limit, forcing the other supply into CC mode too, it would try to force the same higher current into its load, maybe damaging it. Or should the function be implemented in such way, that the lower current is tracked? But that would be two tracking current sources only, or did I get this wrong?

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2017, 01:51:54 am
content: Quote from: Neomys Sapiens on October 12, 2017, 10:15:31 pmI have never seen tracking applied to the output currents of a bipolar output.With the Tektronix PS503A, each current limit only applies to its side and the output voltage of both sides track a reference voltage. (1) So the two sides do not interact at all; it is just two power supplies permanently wired in series with one combined tracking voltage control, two individual voltage controls which set the maximum voltage of each side and the ratio between them, and two current controls. When tracking mode is not enabled, it is the same as if the tracking voltage control was set to maximum. (2) This way of operating may seem weird but it closely duplicates how some tracking regulators work in circuits.In that EDN example where the outputs are balanced against each other, each sides tracks the other and current limiting on one side causes the voltage on that side to drop. The other side now tracks the *voltage* of the side which is being current limited so it also drops but it still uses its own current limit. The advantage of this over how the PS503A operates is that when the PS503A is used as a single 0 to 40 volt power supply, the lower current limit causes both outputs to drop simultaneously all the way down to 0 volts. With a PS503A, one side drops down to 0 volts (actually -0.6 from the protection diode across that output) and then the other side only drops if its current limit is exceeded. The disadvantage is that you might prefer only one output to lower its output voltage when current limiting occurs.Which way is better? The balanced method will be better for maximizing power supply rejection in some circuits making it better as part of a built in power supply. The tracking method used by the PS503A is probably better for a bench power supply used for development and other work except in the case where it is being used as a single 0 to 40 volt output power supply.(1) This allows the PS503A to be remote programmed.(2) The way this works is that with tracking off, each output of the bipolar supply is set independently. Now when tracking is turned on, both outputs may be adjusted from 0 to the previous output voltage with the single tracking control and they always maintain the same ratio. This allows low voltage circuits to be powered without risk of applying a higher voltage during adjustment if only the tracking control is used.