LM317 morocco - Page 1
Forum section: electronics / beginners

author: glinjik
author_rank: Regular Contributor
timestamp: March 15, 2022, 08:17:37 am
content: Hi I'm building a small variable power supply and used an original LM317I morocco I cannot find a data sheet on that chip but by accident I connected a rather large transformer 40v I thought the reg would be toast immediately but nope works fine and isn't even getting warm any ideas why this should be? Incidentally, it was recovered from an old CRT TV

author: magic
author_rank: Super Contributor
timestamp: March 15, 2022, 09:19:23 am
content: Morocco is a country, not a brandI don't know who made LM317 in Morocco (perhaps ST?), but they are all similar in terms of basic specs so any vendor's datasheet will do. You could try National as they are the original manufacturer of this part with the best datasheet.Not sure what you have done to it. The maximum input-output voltage difference is 40V (so not 56V, as in 40V RMS on the input and the output shorted to ground) but that doesn't necessarily mean that he part will blow up at 41V - it may still survive a bit more. And if the output isn't shorted and is set to at least 16V the difference is less than 40V so perfectly in spec.Negative input voltage (if it occurs) could be more of a problem, dunno if enough to kill it instantly.Negative input or input short to ground is deadly if there is large capacitance on the output. See the section on protection diodes in the datasheet.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 15, 2022, 02:32:12 pm
content: If it smokes don't inhale.

author: free_electron
author_rank: Super Contributor
timestamp: March 15, 2022, 03:29:59 pm
content: ST part. St has packaging plants in morocco : Bouskoura . They only do test and packaging. no diffusion.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: March 15, 2022, 06:06:58 pm
content: Note that the spec of the LM317 specifies it can have up to about 40V between input and output.The LM317 does not even know what GND is, There is just some resistor buggering off somewhere to the south end of the adjust pin.This means that if your adjustable power supply has been set to an output voltage of 24V, then the input of the LM317 can be as high as 24+40 = 64V and it is still within spec.But you do have to be careful with this, and normally I would not do this on purpose.One problem is if there are big output capacitors, which have to be charged on power up starting from 0V.

author: glinjik
author_rank: Regular Contributor
timestamp: March 15, 2022, 07:14:25 pm
content: the transformer has multi outputs so think i will try the 18v output see what happens funny the transformer without my circuit puts out 30v after rectification soon as i hook up the circuit jumps to 40v

author: TimFox
author_rank: Super Contributor
timestamp: March 15, 2022, 08:16:10 pm
content: That is strange. I don't know what else is present, but the voltage across an unloaded filter capacitor should be roughly (21/2) times the rms transformer secondary voltage, and go down when any load current is pulled from it. Can the rest of the circuitsupplycurrent to that capacitor? Is your rectifier a 40 V rms center-tapped transformer feeding two diodes? That would give about 30 V DC (no load). If you use a 40 V rms secondary with four diodes in a bridge, you should get almost 60 V.

author: magic
author_rank: Super Contributor
timestamp: March 15, 2022, 08:22:48 pm
content: I think a schematic would be in order before the thread drifts too far into speculation landAnd details of what measurement has been performed and how.Generally, transformers don't increase their output under load.

author: Bud
author_rank: Super Contributor
timestamp: March 16, 2022, 12:14:32 am
content: If you have an oscilloscope, switch it to AC input and check the output of your circuit for oscillation. Some voltage regulators may oscillate under certain conditions.

author: glinjik
author_rank: Regular Contributor
timestamp: March 16, 2022, 06:46:34 am
content: hi so this is the circuit as it is now but with only 2 transistors not 4

author: glinjik
author_rank: Regular Contributor
timestamp: March 16, 2022, 08:49:49 am
content: so did a few quick measurements transformer puts out 34.17v bridge rec 45.8v and under a 10k load circuit puts out 42.8v a small difference to the diagram im using 50k pots

author: Ian.M
author_rank: Super Contributor
timestamp: March 16, 2022, 09:34:14 am
content: Where did you get thatcrappymalodorously suboptimaldesign from? 💩 With 4x diode drops (including pass transistor B-E junctions), the 10K pot base resistor and emitter current sharing resistors, regulation must *SUCK*!The usual approach to boosting a LM317 without degrading regulation is to use the regulator's input current to control a PNP pass transistor which supplimets the regulator's output current. As PNP power transistors are less readily available, a PNP driverSziklai pairis generally used to permit power NPNs to be used for the pass transistor(s). Forum thread keywords:boost "lm317" Sziklai pairAlso note that correct operation of a LM317 is *NOT* guaranteed if the output current drops below 10mA. This min. load current is generally provided by the feedback divider which should therefore have a value < Vref/10mA i.e. 120 ohms for min.Vref. Your schematic has R4=240 ohms, which olly gives 5mA mi. load current. Although theN.S. datasheetsays it can *TYPICALLY* work down to 3.5mA, its prudent to design to the worst case limits!

author: glinjik
author_rank: Regular Contributor
timestamp: March 16, 2022, 09:49:02 am
content: Quite correct sir it is scrappy malodorously suboptimall but note the 10k pots were replaced with 50k and regulation is quite ok the resistor r4 correct I will replace it. It is a circuit I have been using for a long time now I just updated the original transistors and put a bigger transformer I realize it is not perfect but with a limited parts availability this is the simplest and best solution for me, I am not working with high-end delicate circuits it's just a hobby

author: Zero999
author_rank: Super Contributor
timestamp: March 16, 2022, 09:51:39 am
content: Quote from: glinjik on March 16, 2022, 06:46:34 amIt's a bad circuit because the power transistor is not in the feedback loop.Here's the correct way to do it. Tr2 can consist of many transistors connected in parallel with emitter resistors.

author: magic
author_rank: Super Contributor
timestamp: March 16, 2022, 10:17:25 am
content: Is this circuit tested?There is a similar thing in the National datasheet, but with a few differences, most notably: R5 is 5kΩ and R4 is 500Ω, which presumably decreases gain of that Sziklai pair and improves Tr1 bandwidth. I would probably go with National's values.

author: glinjik
author_rank: Regular Contributor
timestamp: March 16, 2022, 10:22:44 am
content: yes it works very well although i used 50k pots

author: glinjik
author_rank: Regular Contributor
timestamp: March 16, 2022, 10:26:21 am
content: the original transistors were 2n3055

author: Ian.M
author_rank: Super Contributor
timestamp: March 16, 2022, 10:40:17 am
content: Pass the current from Vin to the Sziklai pair's emitter through a low ohm resistor, to partially cancel out the voltage drop across R3 as the Sziklai pair pass transistor's current increases and you can even get it to limit the output current. However as the LM317 has a fairly high surge current before its SOA protection kicks in, and the pass transistors don't have SOA protection (unless you use the rare and expensive - for its ratings -LM395SOA protected pass transistor), its preferable to also add a faster acting current limit using an extra PNP of the same type as the driver to pull up the driver base when there's more than approx 0.6V across the new resistor. You still have to be careful there's enough margin to keep within the SOA if the output is suddenly shorted at max. Vin, when the pass transistors are already hot, and must mount the LM317 on the same heatsink as the pass transistors, so its SOA protection kicks in earlier if cooling is inadequate.

author: glinjik
author_rank: Regular Contributor
timestamp: March 16, 2022, 07:43:42 pm
content: thank you all for your help but after testing the circuit it simply gets crazy hot the resistors the transistors everything i put a car headlight bulb across the output and found it was incapable of regulating the current as pointed out to me the circuit is crappy so my next endevour is the circuit below any input would be welcome

author: magic
author_rank: Super Contributor
timestamp: March 16, 2022, 08:59:32 pm
content: What are you really trying to achieve?What sort of output voltage, what sort of output current?I understand you want to use this particular transformer, which gives you ~46V DC after rectification.Can you get LM317HV, which works with inputs up to 60V with zero problems?

author: glinjik
author_rank: Regular Contributor
timestamp: March 17, 2022, 05:12:45 am
content: Hi no its impossible to get parts at the moment I can use a different output from the transformer about 20 odd volts I'm trying to achive a variable output with current control Max is about 4 or 5 amps but I want to get it very low in the MA range the transformer is maybe a little too powerfull but the alternative is a 1a transformer wich is a little low I've looked at tons of scematics but usually they have parts that are obscure or the circuits just don't work I have used this circuit for a long time but with a tiny transformer but as pointed put to me it has many errors so I'm looking fir something else but maybe based in a LM series of ic's I have a few here

author: fourfathom
author_rank: Super Contributor
timestamp: March 17, 2022, 05:44:53 am
content: Can anyone explain that Q3 / Q4 / R9 / R10 / D5 circuit in the original schematic? I don't see how Q4 ever turns on, and with Q4 off, there is no current through R10, Q3 remains off, and so Q4 remains off as well. Load or no load, the output stays off. What am I missing? What is this little circuit even supposed to do?And yes, this is an awful power supply circuit.

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2022, 11:45:05 am
content: Quote from: fourfathom on March 17, 2022, 05:44:53 amWhich schematic? There was no circuit attached to the original post.Quote from: magic on March 16, 2022, 10:17:25 amNo, I haven't tested it. I just drew it from memory.I had a play with LTSpice. It's more stable with lower values and emitter resistor help to provide some very crude current limiting but I wouldn't rely on it. If in doubt, use the values listed on the data sheet.

author: Ian.M
author_rank: Super Contributor
timestamp: March 17, 2022, 01:20:35 pm
content: Try this for pass transistor current limiting. It still relies on the LM317 SOA protection overall, as its difficult to effectively add external current limiting to an unboosted LM317. I've also added the BD132 model for those using the standard BJT library.

author: fourfathom
author_rank: Super Contributor
timestamp: March 17, 2022, 01:40:29 pm
content: Quote from: fourfathom on March 17, 2022, 05:44:53 amThis is the schematic I was asking about:

author: Ian.M
author_rank: Super Contributor
timestamp: March 17, 2022, 01:59:48 pm
content: I think its actually a battery charger, not a PSU. Q3, Q4 keep the output ground disconnected unless a battery with at least a little charge is present with correct polarity. If the battery is reverse connected, D5 protects Q3 B-E junction.

author: fourfathom
author_rank: Super Contributor
timestamp: March 17, 2022, 02:10:37 pm
content: Quote from: Ian.M on March 17, 2022, 01:59:48 pmOK, that makes perfect sense! I was assuming a resistive load, but a battery with some charge will turn on that pass-transistor (Q4).

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2022, 02:22:56 pm
content: Quote from: Ian.M on March 17, 2022, 01:20:35 pmThat's better, but the idea was to monitor whether the LM317 was current limiting, rather than adding an additional one.