Question on resistor label. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 24, 2017, 09:13:14 pm
content: Just found this, and thought it would make a reasonable entry level LM723 based psu build. If anyone can help me, my question is, what is RSC on the resistor of Q1 emitter ? And is this likely a ceramic power resistors.Thanks for reading, and advice appreciated.

author: The Soulman
author_rank: Super Contributor
timestamp: June 24, 2017, 09:59:21 pm
content: RSC = Resistor Sense Current ?Anyway it is the current sense resistor, power dissipation is dependent of your maximum output current.Example: 0,068 Ohms at 10 Amps = 0,68 Volt (drop) times 10 Amps = 6,8 watts dissipated.Don't forget to calculate power dissipation on that upper 2n3055 and give it cooling accordingly.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 24, 2017, 10:07:53 pm
content: Thank you for the reply, and help. I did Google a bit for it first, cheers for that. Yes I will take the final current demand on the last 2N3055.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 24, 2017, 10:16:08 pm
content: I'm not sure if this kind of resistor at that rating has to be hand made, lowest value I can find is 0.1 ohms. So not sure where to go from there. Do they even make 0.065 ohm resistors at say 10 watts, I'm not sure.

author: Zero999
author_rank: Super Contributor
timestamp: June 24, 2017, 11:04:17 pm
content: Quote from: davelectronic on June 24, 2017, 10:16:08 pmI don't see why not. If you can't get 0R065, then you'll almost certainly be able to find one close: 0R06 to 0R07. Another thing you can do is use multiple resistors in parallel to get same value, for example six 0R39 1.5W resistors in parallel would give you 0R065 @ 9W.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 24, 2017, 11:57:37 pm
content: Quote from: Hero999 on June 24, 2017, 11:04:17 pmMany thanks for the help, I will try and find a combination if I fail to find that exact value. Cheers for your help.

author: Ian.M
author_rank: Super Contributor
timestamp: June 25, 2017, 12:35:55 am
content: That design's got problems.The transformer rating is inadequate - for 10A DC out you need 16.2A AC RMS into the bridge rectifier.The use of a 2N3055 as the driver transistor in the Darlington pair is suspect - its unduly expensive and too low gain. A 2.5A 0r 3A rated NPN transistor would be more suitable here. Also why not modernise it with a TIP3055 output transistor that's easier to mount to a heatsink than a 2N3055?At full output current and minimum voltage, the output transistor will dissipate about 110W, which is going to need a *BIG* fan coolled heatsink as a 2N3055 is only rated for 115W with a case temperature below 25 deg C. Its crying out for at least two paralleled output transistors (with current sharing resistors in series with their emitters, that can also be used as the current sense resistors - just link all the emitters to CL (pin 2) via individual 100R resistors), to allow the heatsink to be run hotter so it can be smaller require less drastic fan cooling..The 10K feedback pot for voltage control is also wrong for a general purpose PSU - as wired, it wont let the output go below 7V and as you increase the voltage will stop regulating properly and start letting ripple through with about half of the pot travel remaining. If the objective is to make a 13.8V 'battery eliminator PSU' it should be reduced in value and 'end stop' resistors added to limit its range to say 10V to 15V. If a general purpose PSU is wanted, you'd do better to look at other LM723 circuits.N.B. a high current linear lab-grade PSU is not an easy project and is definitely *NOT* entry level. You'd do better to limit your first PSU to only an Amp or two out.

author: MK14
author_rank: Super Contributor
timestamp: June 25, 2017, 01:41:09 am
content: As already stated, a single 2N3055, would probably overheat (the Safe Operating Area of the 2N3055 is probably violated as well!).Which is why they usually have a number of 2N3055's in the final output stage (ignoring the one that drives the bases), which then spreads the heat dissipation between multiple transistors, so their junction temperatures can be lower. To give them much longer service lives and meet the datasheet specifications (I doubt a single 2N3055 does, as it will exceed the maximum junction temperature, towards full load, with real life heatsinks/fans).http://www.rason.org/Projects/powsupply/powsupply.htm

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 25, 2017, 02:07:41 am
content: I've built a few power supplies now, although they've been modest, with just the regulator supplying the current. I've built a couple of units with series pass transistors added to LM317, and 7812 regulators. When I said entry level, I was really referring to my first LM723 psu. I know the source of the schematics, although was confused over the current sense resistor.I have to finish my MOT psu first, so just tyre kicking some ideas at this stage. Thank you for the input and help! Including the drawings. I have looked at that last one, it looks ok. I won't be taking anything to its maximum limit anyway, 75% of what it's rated for would do me. Thanks again for the help.

author: MK14
author_rank: Super Contributor
timestamp: June 25, 2017, 02:16:05 am
content: Quote from: davelectronic on June 25, 2017, 02:07:41 amEven 75%, on just a single 2N3055, is probably not going to last very long. If it even works, at all, without popping. At such huge loads. When it goes, it may go short circuit, and blow up (overvoltage), anything connected to the output.Also there are better (and still fairly cheap), output transistors, you can use. With significantly better power dissipation capabilities. E.g. The 2N3773, but it is NOT as cheap as I originally thought.EDIT:I assume it is a learning exercise, so you want to keep it as simple as possible. At lower output load currents, especially below 5 Amps. It should work (OP circuit), reasonably well. But remember to keep the 2N3055 really cool.

author: eblc1388
author_rank: Frequent Contributor
timestamp: June 25, 2017, 07:03:01 am
content: The circuit suggested by MK14 is a better designed one than your original.You can use 3x0.2 ohm 5W resistor in parallel for RSC, as these resistors has a tolerance of 5% so you are good to go.Remember also to use the type of bridge rectifier with a heat sink tab so you can affix it to the heatsink as well.The 723 is old but its performance is not bad. The last power supply I've built using it, the output voltage 13.88V drops by only 3mV from zero load to 15A load.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 25, 2017, 09:38:43 am
content: Yes its a learning curve, time to move on to an LM723 based psu. My last high current 3 terminal regulator below, been in use for about 5 weeks now. The cream colour case on the floor right side of the picture. Its using an LM317K and 4 x MJ4502 power transistors.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 25, 2017, 09:46:25 am
content: Not the best picture above, but yes time to have a go at an LM723 psu. Thanks for the help an diagrams, i will take all your advise in to count when building this.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 25, 2017, 10:36:00 am
content: What do you think about this circuit, i know it says 3 Amp bridge, but what would be this circuits maximum current ? And is it a viable looking circuit. Thanks for any advice.

author: Ian.M
author_rank: Super Contributor
timestamp: June 25, 2017, 01:37:57 pm
content: Take the current sense limit voltage as a nominal 0.65V (See fig .9 in the T.I. LM723 datasheet), divide that by the 0.15R sense resistor and you get 4.33A. Better use a 5A bridge!Also, I don't believe the 0-30V claim. The LM723 inverting (INV) and noninverting (N.I.) inputs go directly to the bases of a long tailed pair so there'd no voltage offset between them. The nominal minimum output voltage for the LM723 is 2V when in the Figure 4. Basic Low Voltage Regulator circuit configuration which means you will run into problems when INV and/or N.I are below 2V due to lack of headroom for the current mirror feeding the tail of the long tailed pair. Certainly, by the time the inputs get down to 1V the loop gain will have gone to pot.Taking the voltage on pin 6 (Vref) as its nominal 7.15V, the bottom end of the 10K pot will be at 0.34V and the top end at 6.42V. The (15K, 3.9K) potential divider divides the output voltage by 4.85, so even if the inputs were rail-to rail, that would give a minimum output voltage of 1.65V. However due to the long tailed pair input voltage limitations I would expect significant problems below 5v and degraded performance below 9V. The maximum output voltage calculates as 31.1V.To solve the output down to zero problem, you need to offset the feedback from the output to N.I. so at 0V out N.I. doesn't go below 2V.You may want to study a commercial LM723 based PSU. I suggest the Philips PE 1535/00 40V 0.5A PSU. Oldway introduced its schematics to the linked thread and I recreated them as a LTspice sim:https://www.eevblog.com/forum/projects/how-do-make-a-current-limiting-knob-using-lm723-for-linear-reg-power-supplies/msg1212923/#msg1212923It would be fairly easy to beef up the output stage to make a 3A PSU - above that the dissipation gets pretty ridiculous and difficult to deal with even with fan cooling unless you add some sort of a pre-regulator or tap changing.IIRC you've had problems getting LTspice running under WINE on LINUX - electronics design is no place for O.S puritanism as many manufacturers design tools and utilities are Windows only, so get yourself a Windows VM or PC up and running and on it, install the design and simulation tools you need to evaluate circuits and reject the crazy ones without having to breadboard them!

author: SeanB
author_rank: Super Contributor
timestamp: June 25, 2017, 02:26:27 pm
content: Quote from: davelectronic on June 25, 2017, 10:36:00 amMax would be around 2A, as the drive and output transistors are going to be limited to this level in any case with the 723 available drive current. The sense resistor as well will only protect against short circuits for a while, as short circuit current is around 4A, which will blow up the bridge rectifier and transistors with a prolonged short.Will do a true zero volt output, as it divides the output voltage so it essentially only gives 0-7V to the comparator. A 470R resistor across the base emitter of the 2N3055 and the BD135 will help keep it stable with high dissipation, I did a similar one years ago, designing it for a 60A current, using a massive section of aluminium extrusion and a dozen 2N3773 transistors ( I had 2 trays of them come in, I ordered 2 transistors, but they interpreted that as 2 trays, and I really was not going to complain or send them back either, good replacements for 2N3055 with better ratings all round, and I also had some UPS units in for repair which used 48 per unit as drive, so the tray of 200 transistors was a great thing to fix them, as any fault blew 24 transistors minimum, but not the 300A fuse) but was limited by the wiring in the wall to 25A, and did not want to rewire to the distribution board with the roll of 00 gauge wire I found in stores, known as my foot stool.

author: Ian.M
author_rank: Super Contributor
timestamp: June 25, 2017, 03:19:15 pm
content: Quote from: SeanB on June 25, 2017, 02:26:27 pmLook at the internal schematic of the LM723 (T.I. datasheetpage 15), especially the section around Q9-Q12. Its obvious that when the base voltage of Q11 or Q12 go below a threshold set by the voltage drop across R11 + one Vbe drop, they will be biassed off. The long tailed pair comparator *DOES* *NOT* *WORK* with 0V on its inputs!Quote from: SeanB on June 25, 2017, 02:26:27 pmI agree with your estimate of the max continuous current, but not your reasoning.Considering the pass transistor S.O.A. and dissipation, a practical limit for a 0-30v single 2N3055 pass transistor PSU without preregulation is somewhere in the 2A to 2.5A region depending on how well you heatsink the 2N3055. 3A out is right on the bleeding edge and will require a massive fan cooled heatsink.What's wrong with your reasoning? Well, the minimum HFEof a 2N3055 is 20 @IC=4A, and a BD135 is 25 @IC=0.5A, giving a total minimum gain of 500 for the Sziklai pair. A LM723 has a max output current of 150mA dissipation limited by the PDIP package to 660mW, so pin 10 can sink about 15mA with 35V in and the output grounded. x500 gain, that's 7.5A so the output current is *NOT* limited by the LM723's drive capability and the transistor gain. It is limited by the S.O.A. of the 2N3055 - its only good for fractionally over 3A IC@ VCE=35V, @ 25 deg C, and it would be a real bitch keeping the 2N3055 case cool enough to get that current rating.

author: SeanB
author_rank: Super Contributor
timestamp: June 25, 2017, 04:27:25 pm
content: 2N3055 is a lovely transistor, just that the specmanship in the ratings is really never achievable, it really is a good 2A transistor or a 5A saturated switch, but not good for anything more. there are better devices around, even from the same era, and it really was just that the construction was cheap, as there originally was no heat spreader on the small die, limiting it quite a lot. The original also had a steel plated TO3 plate, which also is not great. You want good in TO3 you want a BUX20, which has a lot better performance, but at a massive price premium for the kovar spreader and the larger die, and the much improved thermal resistances of the thick housing.Like I said, limited in use, SOA for sure, but for most applications it does the job cheaply, and millions are still in use planet wide, and I have some in power supplies that have been toasting there for over a decade, after I added something the original manufacturer did not include, an actual heatsink instead of the thin steel case alone. this alone kept case temperature below boiling, and was an old PC motherboard heatsink that was close to hand, around the right size to fit the case area and surplus, plus I had a drill to make the single new hole and a bit of white thermal paste spare. 16V in, 13v out and around 2A, and runs at over 100C on die I guess, but still, for a cheap linear supply ( coincidentally also rated at 5A peak, 3A continuous and using 1N5401 diodes in the bridge) that was bought years ago it still works fine.I designed a few myself, using a 723 and really derating the whole lot, so they are well in the limits, and the TIP3055 transistors run barely warm, though I put current limit at 1A, as it is a battery float charger that I wanted to survive a shorted battery indefinitely. Those 2 have been running for 2 decades plus, I built them in high school. Gone through a lot of batteries in there, they last around 5 years, so I must be doing something right, even with using cells from dead UPS units a few times as replacements.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 25, 2017, 05:44:56 pm
content: Thanks for your replys, that's a lot to get my head round lol. I should have said that I would be using a fixed voltage, somewhere between 12.5 Volts and 13.8 Volts, so fixed bench supply use. Primarily for radio gear, I think the minimum current I would be using is about 3.9 Amps with a small amplifier and radio, up to 10 Amps for a more punchy amplifier and radio. And maybe a bit more for a future bigger amplifier still. If the circuit above with the BD135 driver could supply 4 Amps at a maximum of 13.8 Volts, it's would be viable for my small amplifier and radio combination. But I think I'd be better off building an LM723 based psu with at least two output power transistors.

author: Ian.M
author_rank: Super Contributor
timestamp: June 25, 2017, 06:11:34 pm
content: The nearly fixed output voltage makes it a lot easier because the maximum drop across the output transistors, and thus the dissipation is lower. The Sziklai pair pass transistor LM723 circuit will work down to 3V headroom, so for 13V out, the input can be as low as 16V. That's 160W when shorted with a 10A current limit, but it will be a bit more as the average input voltage will be higher as the 16V is the minimum tolerable trough for the ripple. Therefore for a 10A 13.8V PSU I'd use four TIP3055 transistors (three is marginal - you'd have difficulty keeping them cool enough). The BD135 can provide enough drive for all of them. Use 0.27R emitter resistors and tap across one of them for the current limit to limit each to just under 2.5A, for a total of 10A out. If you want a 5A PSU leave out two of the TIP3055 transistors and their emitter resistors.TIP3055 are much easier to mount to a heatsink. The *ONLY* reason to use TO-3 package 2N3055 transistors is if you already have them N.O.S from a western manufacturer, with a big pre-drilled heatsink and all the mounting kits.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 25, 2017, 08:41:42 pm
content: Thank you for the added info Ian. I've got a lot of TO3 hardware as I bought a lot of it. But I've been buying heatsinks as I go, just 6mm thick plates as heatsinks for the MOT project. I will post the result of that in the "overhead voltage" thread when it's finished. Yes back to the LM723, I've not built one before, so would be my first using that regulator. As I only use strip board, it's keeping the pathways heavy enough to take the current. I use the FR4 gold plated stuff, be it more expensive. But two 2N3055's should do for a first build, three transistors if you count the BD135. Thanks again for the help. I could do with a scope at some point, rather than poking around in the dark.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 25, 2017, 08:45:17 pm
content: Quote from: Ian.M on June 25, 2017, 06:11:34 pmFor a full range voltage and current supply, I think the only way to go is taps switched in or out depending on what voltage and current range you need. Bit beyond me at the moment.

author: Ian.M
author_rank: Super Contributor
timestamp: June 25, 2017, 10:37:21 pm
content: Quote from: davelectronic on June 25, 2017, 08:41:42 pmI think you'll need something better than a flat plate heatsink for that project. IIRC you've got quite a few watts to dump. I'd recommend a manual reset thermal trip mounted on the heatsink to cut the power if the PSU is shorted for too long.Quote from: davelectronic on June 25, 2017, 08:41:42 pmThere's no real problems with building a high current LM723 PSU using TO-3 pass transistors with the LM723 on stripboard. Nearly all the high current parts should be chassis mounted and hooked up with point-to-point wiring, (including the bridge rectifier and the reservoir capacitor) so nothing on the stripboard carries more than a few hundred mA. For the current sharing (and sensing) emitter resistors, you can either use an insulated turret terminal next to each transistor with the resistor mounted between the terminal and the emitter pin, or you can group the resistors together on twin row tag strip.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 25, 2017, 11:43:57 pm
content: These aluminium 6mm plates for the MOT project are 200mm x 100mm each has just one single MJ11015 on it, positive air pressure from the fan on the case top will flow over the plates via many slots already cut in the case sides. I don't expect more than 10 Amps maximum at 12 Volts and typically running a cb radio and RM KL203P linear amplifier. Combination draws about 8 Amps.Yes point to point is really my best option there with circuits, although I did solder some heavy tracks on some FR4 gold plated copper strip board. Mid you I did get through a lot of solder doing that capacitor board, 90000uf on it 9 x 10000uf capacitors. Tested tonight, all good. Won't go off topic, but 14.5 Volts AC, 21 Volts rectified and filter capacitor bank. I will post final voltage outcome result in the other thread. Thanks again to everyone.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 26, 2017, 11:28:50 am
content: My eyes glazed over when I saw this was Yet Another Thread on the infernal '723...But, I did notice that no one correctly answered the OP's actual question: the "sc" in Rsc stands for "short circuit". This resistor is sized to produce around 0.6V of drop when the desired maximum current flows through the pass transistor, at which point it will provide enough forward bias to turn on another transistor that starts shunting away current from the pass transistor, resulting in automatic overcurrent protection. For example, to trip at 5A you would select a 0R12 resistor for Rsc.The trip level is roughly defined and responds somewhat linearly to overcurrent (that is, acts like a CC regulator during moderate overloads) so it is best to set the trip level well above the max operating current for the regulator but, of course, well within the SOA of the pass transistor(s).Also note that the presence of Rsc automatically increases the output impedance of the regulator so voltage feedback sampling should be done after it.edit: some seriously messed up grammar in 2nd sentence...

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 26, 2017, 06:06:18 pm
content: Quote from: MagicSmoker on June 26, 2017, 11:28:50 amSo not sense current ? Seems an odd label to give it, although I understand your explanation.

author: Ian.M
author_rank: Super Contributor
timestamp: June 26, 2017, 08:33:57 pm
content: Either - However, expanding RSCas RSense_Currentmakes more sense from a practical viewpoint as one of the pins it connects to is called 'CURRENT SENSE', and from a semantic viewpoint as it will cause the regulator to limit on all current overloads (if implemented properly) not just on short circuits

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 26, 2017, 08:48:16 pm
content: This is more of a semantic argument, than a technical one, but historically speaking, Rcs refers to a current sense resistor, while Rsc refers to the resistor which sets the short circuit trip current in the classic two-transistor current limiting circuit, which is precisely what is used in the infernal '723.To put it another way, when the resistor supplies a signal that an error amplifier uses to regulate current you might call it Rcs, but when it is sized to drop ~0.6V at a certain current to then turn on a transistor which shunts base drive away from the pass transistor, you call it Rsc, because then its purpose is to protect the pass transistor from shorts.But, like I said, this is more of a semantic argument.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 27, 2017, 10:10:20 am
content: Quote from: MagicSmoker on June 26, 2017, 08:48:16 pmQuite a bit to get my head round, but very interesting, I'm picking up vibes, your not an LM723 fan ? I know its been extensively used over the years, even in retail units. Do you think its problematic ? Or is it just "it gets everywhere" curious on your thoughts of the chip.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 27, 2017, 02:26:51 pm
content: Quote from: davelectronic on June 27, 2017, 10:10:20 amI don't have any real problem with the '723 (where ' is replaced with LM, or UA, or about a dozen other manufacturer prefixes) it's just that it is - literally - a 50 year old IC that isn't all that impressive by today's standards. All it does is combine a mediocre reference with a slow and drifty error amplifier, a paltry 150mA pass transistor, and another transistor for short circuit protection. For a 13.8V/10A PSU it will get the job done, I just cringe a little when I see people try to make a "lab" power supply with it.

author: davelectronic
author_rank: Frequent Contributor
timestamp: June 27, 2017, 06:16:11 pm
content: Quote from: MagicSmoker on June 27, 2017, 02:26:51 pmYes I can see what you mean, it's output to drive other transistors is quite low. And I can remember seeing it in bench cb psu's back in the 80's. Every thing is smps now days, so I guess not much call to design another high tech linear regulator really.