Dual power supply from single - Page 1
Forum section: electronics / beginners

author: crash2140
author_rank: Contributor
timestamp: December 13, 2015, 10:21:49 pm
content: Hi guys,I've been researching on ways to get dual power from a single power supply, until I found this handy circuit, which I did not know about and that I think it could be very useful to some people, so I'm sharing it here:I've tested it and it works like a charm. The op amp supplies/sinks very little current, altough the transistors heat up depending on the load. Even with 10 ohms in one of the rails and 10kohms in the other, it keeps a nice stable +-12V. Also, the input voltage doesn't necessarily need to be 24V.It works fine at lower input voltages.

author: ataradov
author_rank: Super Contributor
timestamp: December 13, 2015, 10:56:34 pm
content: Can you attach LT-Spice project?

author: Ian.M
author_rank: Super Contributor
timestamp: December 13, 2015, 11:11:39 pm
content: Now try it with a pulsed load.Due to the large capacitance on the emitter followers output + them being in the OPAMP feedback loop, it is likely to be extremely unstable and may even be oscillating.

author: Seekonk
author_rank: Super Contributor
timestamp: December 13, 2015, 11:42:15 pm
content: I thought the 10 ohm was a typo till I read the text. Really, 1.2A through a 3906 with 12V C to E.

author: dom0
author_rank: Super Contributor
timestamp: December 14, 2015, 12:05:19 am
content: Well, that version is (a little) better than most variants of that circuit floating around.Still...https://www.mikrocontroller.net/topic/258266#2675651

author: MrSlack
author_rank: Frequent Contributor
timestamp: December 14, 2015, 09:35:06 am
content: Not a terrible variant. Most of the versions of that I see have no feedback at all. May oscillate at certain frequencies and those poor little transistors will let the magic smoke out if you load it heavily though. Might be worth putting something a little more hefty on the output drivers as 2n39xx Ic is only ~200mA and they're unstable when very hot. It will currently blow up if you build it for real with a 10 ohm load unless the output impedance of your source saves your bum which it sounds like it may be.

author: Ian.M
author_rank: Super Contributor
timestamp: December 14, 2015, 10:28:20 am
content: Lets consider what the input impedance of the emitter followers looks like.First, a simplifying assumption - due to asymmetries, it is highly unlikely that the OP07 output will ever remain in the Vbe crossover region.Due to the load asymmetry Q1 will be cut off and Q2 will be conducting and act a a simple emitter follower. If the load asymmetry was reversed, so would be the transistor states. If the load was symmetrical the transistor states would be determined by the sign of the OPAMP's offset.Considering Q2, I will refer to theFairchild 2N3906datasheet.If the load asymmetry is greater than 4mA (i.e R3<2.1K), from Fairchild fig. 13 the input impedance at the base with respect to the emitter will be of the close order of 50R or less.Now lets look at the load the OPAMP sees in more detail.From I=C*(dV/dt), it can be shown that, fror the purposes of AC analysis, the emitter circuit capacitance of 200uF (C2||C3), can be transferred to the base circuit by dividing by hFE. The best case hFEis 400, and the worst case is 30.200uF/400 is 0.5uF. However theAnalog Devices OP07datasheet states on page 12:QuoteWhoops! 0.5uF and less than 50R in series.Considering the worst case hFE, it is obvious that a capacitance of >15nF at the emitter risks instability. A similar analysis can be done for Q1 if the asymmetry is reversed, with similar results.

author: Ian.M
author_rank: Super Contributor
timestamp: December 14, 2015, 10:37:32 am
content: @dom0: I assume you are referring to the OPAMP free schematic posted by ArnoR (Gast) at 14.05.2012 11:44 in the German www. mikrocontroller. net topic?

author: dom0
author_rank: Super Contributor
timestamp: December 14, 2015, 11:08:20 am
content: That's where the link leads (or should lead, with the #-part of the URL), yes.The circuit is remarkable mainly for beingalmostunconditionally stable. Offset and output impedance are higher as an op amp based circuit, OTOH output impedance is more constant over frequency, while op amp based circuits start to roll off quickly (beyond a few Hz).

author: MAntunes
author_rank: Regular Contributor
timestamp: December 14, 2015, 10:50:47 pm
content: I too am looking for a stable virtual ground circuit. I'll stay tuned on this topic!

author: crash2140
author_rank: Contributor
timestamp: December 15, 2015, 07:48:53 pm
content: Here is the LTspice file:https://mega.nz/#!G0pimByS!SqfkmEOX95GN-bpIyFpmI5-HiYjhPny6PEOP1EDb3ZYI loaded the circuit with 10ohm on +12V and 10k on -12V to simulate a worst case scenario, that is, the power drawn from each of the rails is very asymetrical. I've tested it on the breadboard (with lighter loads, but balanced) and it worked, with close to no ripple. The 100uF caps maybe aren't necessary, I just put them in there to reduce possible ripple on the output. I chose the 2N3904/2N3906 because they were the transistors I used to test the circuit, they would need to be replaced by power transistors on an heatsink if more current is drawn. Also, the OP07 was used just as a general purpose op amp. I used a 741 on the breadboard.Quote from: Ian.M on December 14, 2015, 10:28:20 amI haven't got enough knowledge about the circuit yet to follow your explanation, so I can't comment on that.

author: MAntunes
author_rank: Regular Contributor
timestamp: December 29, 2015, 09:05:46 pm
content: Hello, what is the best way to test this kind of circuits?

author: dom0
author_rank: Super Contributor
timestamp: December 29, 2015, 09:55:49 pm
content: Like you would test any power regulator?

author: pelule
author_rank: Frequent Contributor
timestamp: December 29, 2015, 11:34:35 pm
content: I am using the TLE2426 (TI) "Rail-to-Rail-Splitter", an easy and precise solution, unfortunately the 3-pin package is obsolet. For 4V...40V up to +/-20mA a easy and convinient solution.Data sheet:http://www.ti.com/lit/ds/symlink/tle2426.pdfBRPeLuLe

author: DimitriP
author_rank: Super Contributor
timestamp: December 30, 2015, 01:12:48 am
content: Quote from: pelule on December 29, 2015, 11:34:35 pmMouser has a few lefthttp://www.mouser.com/Search/Refine.aspx?Keyword=TLE2426

author: tooki
author_rank: Super Contributor
timestamp: December 31, 2015, 12:53:45 am
content: Quote from: pelule on December 29, 2015, 11:34:35 pmHuh? That datasheet says the TO-92 is still in production. Only the military spec versions appear to be obsolete.

author: Ian.M
author_rank: Super Contributor
timestamp: December 31, 2015, 01:34:07 am
content: Mouser have got 1600 of the TO-92 package one with 6 weeks factory lead time if you need larger quantities.http://eu.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyTLE2426CLPThere are several To-92 package variants listed as active on T.I's site:http://www.ti.com/product/tle2426/samplebuyI reckon pelule got a bit confused and was paying over the odds for TLE2426MLP (unless he actually needed the extended temperature range M variant)

author: 3141592
author_rank: Regular Contributor
timestamp: January 04, 2016, 10:21:51 am
content: The LTC3260 / LTC3265 also seem like good and easy solutions for light loads, IMO.

author: pelule
author_rank: Frequent Contributor
timestamp: January 04, 2016, 09:23:42 pm
content: QuoteYepp, you are right, I got confused.Only the extended temp. versions (-55 to 125 °C) are obsolet.BRPeLuLe