Sense resistor value. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 25, 2020, 01:50:33 pm
content: I'm looking for a through hole current sense resistor of value 0.056 There doesn't seem much available where I am in the UK. Is it possible to uses 2 X 0.1 Resistors in parallel to achieve a current sense resistor. The circuit it's drawn in is below.Thanks for reading, any thoughts appreciated.Credit to Tony Van Roon for this schematic diagram.

author: george.b
author_rank: Frequent Contributor
timestamp: December 25, 2020, 02:27:45 pm
content: Is it 0.056 or 0.065 ohms? The schematic says 0.065.If you check the datasheet for the LM723, you'll find that the current limit equals 0.65 divided by Rsc. That means a current limit of 10A for a 0.065 ohm resistor. With two 0.1 resistors in parallel, the current limit would be 13A, which, for one, exceeds the specified transformer rating.

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2020, 05:20:57 pm
content: Its no good - the current limit circuit shown provides inadequate protection and the 2N3055 will blow if the output is shorted. As the supply is supposedly capable of 10A@13.8V, its obvious that the transistor will have at least 15V across it when shorted.However, look at the SOA graph (fig.2) in the OnSemi2N3055 datasheet. The highest voltage drop that you can have across a 2N3055 passing 10A without instantly destroying it is about 12.5V, and that will take heroic active cooling to maintain the mounting surface at or below 25 deg C.TLDR: PSU designed by monkey fingerpainting with their excrement!The *ONLY* hope of a high current single 2N3055 pass transistor PSU surviving a short is if it implements foldback current limiting vicious enough to stay within the DC line of the SOA and fast enough acting not to violate the 1ms pulse line either. See fig. 6 of the T.I.uA723 datasheetfor the required current limit network. Chose Rsc for the foldback current @0V then choose the ratio of R3:(R3+R4) to subtract enough voltage from the drop across Rsc to get the desired knee current @13.8V.

author: bdunham7
author_rank: Super Contributor
timestamp: December 25, 2020, 05:51:28 pm
content: Quote from: Ian.M on December 25, 2020, 05:20:57 pmBut it isn't a single transistor. Q2 is also a 2N3055.

author: geggi1
author_rank: Frequent Contributor
timestamp: December 25, 2020, 06:06:24 pm
content: There is some room for improvement here, put in two 2N3055 transistors in stead of Q1 that will give less power on the transistors. RS components got 0,068 and 0,07 ohm in several wattsages.

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2020, 06:30:07 pm
content: Quote from: bdunham7 on December 25, 2020, 05:51:28 pmQ2 is the driver transistor, not thepasstransistor.Quote from: geggi1 on December 25, 2020, 06:06:24 pmYes, considerable room for improvement. Parallel the two 2N3055s' collectors and bases, with individual current sharing resistors in series with the emitters to the common output. Use the same resistors for the current sense by connecting each to the uA723 CURR LIM pin via a 100R resistor with a Schottky diode in parallel (e.g. 1/2 BAT54C), so it normally 'sees' the average of the two, but if one transistor fails open, the limit for the other does not double! If using foldback limiting, connect the top end of R3 to the junction of the 100 ohm averaging resistors and Schottky cathodes, and size R3 so the effective resistance of the averaging network does not grossly affect the knee current.The nominal hFEof a 2N3055 is about 30@5A, and min. 20 @4A, so to get 10A from two of them you need at least 0.5A base drive, far more than the uA723 can relibly deliver, so a driverr transistor is required. If you are staying with a Darlington configuration a2N4922looks like a good candidate. The driver transistor has significant dissipation at full output current, so must also be heatsinked. The dropout voltage can be significantly reduced if you use a PNP driver with the NPN pass transistor(s) to implement a PNP Sziklai pair instead of a NPN Darlington. See fig. 5 in the uA723 datasheet.Incidentally, I posted a LTspice sim of a Philips PE 1535-00 (0V - 40V, 0A - 0.5A) single output PSU, which uses a LM723 with a Sziklai pair pass transistor, a few years back:https://www.eevblog.com/forum/projects/how-do-make-a-current-limiting-knob-using-lm723-for-linear-reg-power-supplies/msg1212923/#msg1212923Although its only a 0.5A supply, its a well tested design and would be a good starting point for 'beefing up' the design with multiple pass transistors to get a high output current adjustable current limit '723 based supply.However even if you limit a '723 based linear PSU to 20V max output, without input tap switching, its going to have to dump at least 250W if shorted in constant current mode @10A so its going to need heroic heatsinking or active cooling and/or a massive array of pass transistors and will make a very nice room heater!

author: bdunham7
author_rank: Super Contributor
timestamp: December 25, 2020, 07:10:55 pm
content: Quote from: Ian.M on December 25, 2020, 06:30:07 pmYes, I was laughing at the circuit not you. I'm imagining finding a device like this with two 2N3055s next to each other on a heat sink and thinking it they were in parallel (and thus adequate) only to find out they were wired like this.

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2020, 07:15:40 pm
content: .... which is one of the many reasons that I accused the designer of being a sh!t daubing ape in my reply #2 TLDR!

author: srb1954
author_rank: Super Contributor
timestamp: December 25, 2020, 08:45:45 pm
content: Quote from: davelectronic on December 25, 2020, 01:50:33 pmQuite apart from the sense resistor value there are a few other problems with this PSU design:(1) The selection of a 2N3055 for the driver transistor Q2 is perhaps not the best as it it will have lowish gain at the typical emitter current it is expected to deliver to the output transistor. A smaller transistor with higher gain at lower currents is more appropriate here. Something like a TIP31A would be better suited although this would require more detailed design work to confirm.(2) When the PSU is very lightly loaded the leakage current through the 2N3055 driver transistor Q2 will cause problems in that the output transistor Q1 may not be able to fully turn off and the regulator chip will lose control and the output voltage will rise above its set value. A resistor or not more than 470between the base and emitter of Q1 will bypass that leakage current so that Q1 can be fully turned off. Using a smaller transistor such as a TIP31A as a driver transistor will help alleviate this problem but you will still need a base-emitter resistor of not more than 2k2to bypass the leakage.(3) It is not clear if this circuit is intended for a fixed output of 13.8V or if it is intended to be a variable output supply. The circuit symbol for adjustment pot is shown as a front panel variable type which implies that the output is to be fully adjustable. This circuit configuration is not suitable for this as the output voltage is only adjustable from a low of around 7V to the maximum unregulated input to the series pass transistor i.e. the regulator chip will no longer be controlling the output voltage. A different circuit configuration is required if the output voltage is required to be fully variable - refer to the 723 datasheet.If the output is intended to be fixed at 13.8V then the adjustment pot should be drawn as a trimpot. Fixed resistors should be added top and bottom of the trimpot to limit the adjustment range. Suggested values are 10Kfor the top end of R2 and 12k for the bottom end. These will still allow trimming out of tolerances in the circuit whilst restricting the range such that the 723 is still in control of the output voltage.(4) The biggest problem is the rating of the power transformer. There is no way a 10A rated transformer is going to deliver sufficient current for a 10A DC output without severe transformer overheating. The transformer current rating needs to be 15-18A AC to be able to deliver a 10A DC output. The transformer voltage is also a little too high for this application although this may have been an attempt to compensate for excessive transformer voltage drop due to inadequate transformer current rating. With a more suitable current rating the transformer voltage could be dropped to around 16-18V AC but this would require more detailed design work to confirm exactly.

author: srb1954
author_rank: Super Contributor
timestamp: December 25, 2020, 09:02:13 pm
content: Quote from: Ian.M on December 25, 2020, 05:20:57 pmWhen you consider the excessively high transformer secondary voltage the SC power dissipation of the 2N3055 becomes even worse.The rectified DC voltage could be as high as 20V under load which means the 2N3055 collector current needs to be kept under 6A for it to survive and that is with the extreme heatsinking measures. With a more typical extruded aluminium heatsink that collector current needs to be reduced to something like 3.5A for the transistor not to overheat.

author: Ian.M
author_rank: Super Contributor
timestamp: December 25, 2020, 09:32:24 pm
content: Yep. However the no-load input voltage is an even worse problem. By the time you've figured in the effect of high line (mains supply) conditions, and transformer regulation, the raw DC voltage could be over 35V, and with a 10000uF bulk capacitor it will only drop 1V/us @10A load current, so without foldback limiting you could be pushing the limits of the SOA for long enough to fry the pass transistors unless you use four in parallel. Three's marginal!There are however two things to be said for the obsolete TO-3 package - its large mounting surface area makes it easier to get the heat out and its *usually* tough enough to contain the explosion of the die when you push one to destruction in a high energy circuit.However for powering 'mobile' equipment on the bench, float charging a lead acid battery at a couple of amps is a much better choice. The full power duty cycle of typical equipment is usually reasonably low, so two or three amps of charging current will be plenty and worn out car batteries are dirt cheap, and reasonably reliable if you flush and refill to remove shed electrode material so it cant build up and short the plates, and only need a small fraction of the original capacity.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 26, 2020, 02:33:44 pm
content: Thank you for your replies. I'm far from an expert, that's why I was seeking advice on the sence resistor. But I can see this circuit is far from ideal, I've read closely your thoughts on it. There are better options for the uA723 IC, in the schematic the red text should have sounded an alarm to me. I have a couple of schematics to have a go at one of them. Its the " big one " I want to build, but a lead up to that would help with the confidence to achieve that would be the way to go for me. So an interim slightly easier uA723 would be great first. I don't have to much trouble visualising laying out circuits etc, but have stuffed up a couple of times in the past. Although a closer look at my mistakes have soon corrected the fault I made. I've only got copper stripboard, so solder bare copper solid wire where the current is high in the circuits. Its not the most beautiful option, but it works.

author: Ian.M
author_rank: Super Contributor
timestamp: December 26, 2020, 02:46:09 pm
content: Maybe have a go at something like the Philips PSU I linked or other <1A, <30V LM723 PSU. Tell us what transformer you have, i.e. its nom. secondary voltage and rated current and we can suggest specs to aim for.N.B. there's a *VERY* good reason why old linear bench supplies commonly had a High/Low switch. It wired the transformer secondaries in parallel for half the voltage and double the current, and halved the current feedbck in Low mode. That vastly reduced the number of output transistors and size of transformer vs what was required to be able to deliver full current at full output voltage

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 27, 2020, 03:53:13 pm
content: Well, I have a few step down transformers. I've got toroidal and chassis mount transformers, also have four MOT's. But I wouldn't use the MOT's for an LM723 build. Two that stand out are a toroidal transformers, smallest is 100 VA at 0 - 30 Volts, the other is toroidal, 225 VA 0 - 18 0 - 18 Volts AC. Also a contender, a standard laminated transformer which is secondary 100 VA 0 - 18 0 - 18 AC. Also a couple of toroidal transformers 16 - 0 16 - 0 Volts 15 Amps each.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 27, 2020, 04:00:30 pm
content: Yes something LM723 with a single power transistor would be good. Kind of the positive step I feel I need before attempting a high power variant LM723. I didn't see your Philips PSU circuit link there. Its just an interim step for me, I'm reasonably confident I could do the "big one" but don't want to get ahead of myself. I've ordered 6 X LM723 IC's off eBay, UK seller, he's sold 128 of them, can't see any negative from his sales of them. So hopefully there good IC's, also bought IC sockets for them.

author: Ian.M
author_rank: Super Contributor
timestamp: December 27, 2020, 04:25:09 pm
content: Link rot removed the Philips PE 1535-00 / PE 1545 manual frommy post I linkedback in reply #5. Try again, I've found an alternative URL, and edited it accordingly.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 28, 2020, 03:10:16 pm
content: Thanks for the link update Ian. That looks way out of my skill set at the moment. I'd need to digest what is going on in that circuit, and I'm unsure I could in its entirety. By all accounts a very good PSU circuit design I expect. I've a couple of LM723 schematics to choose from, I will probably opt for the easiest version of 3 Amps design first. I need a clean linear power supply for a CCTV camera, so that would be great for that. Thanks for all the help and replys.