Zener regulator current.  - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 01, 2017, 11:11:16 am
content: I've never got round to trying a zener regulator circuit, the one i have found looks like it might be ok. Two questions really, first was, can this deliver up to 3 Amps as it claims. And am i right in thinking a 13 Volt 1.3 watt zener diode would be better to obtain 12 Volts, due to the transistor 0.7 voltage drop. Also possibly a silicone diode in series with the zener diode, on the 0 Volts side of the zener diode. But that's a maybe to gain another 0.7 Volts. Thanks for reading, thoughts on this appreciated.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2017, 11:54:20 am
content: As shown the circuit would supply up to about half an amp (as mentioned in the comments surrounding the apparent source of the circuit). Because the R1/680resistor WON'T supply enough current for the relatively low Hfe (gain) power transistor.Otherwise if the power transistor was sufficiently well cooled it could supply 3 amps.There is no overload/overcurrent/overvoltage/overtemperature etc protection and the regulation would not be that good.

author: 4CX35000
author_rank: Regular Contributor
timestamp: July 01, 2017, 12:11:25 pm
content: This is a popular simple circuit used in many textbooks which show a regulated power supply and can be used to study what the zener and transistor are doing. As mentioned not a ideal design to use. Often it is to show the temperature rise of the 2N3055 when under load and so on.You can buy a dedicated voltage regulator such as the LM138 and comes with plenty of example circuits in the datasheet (see below).http://www.ti.com/lit/ds/symlink/lm138.pdf

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2017, 12:44:56 pm
content: Quote from: 4CX35000 on July 01, 2017, 12:11:25 pmThe LM338 plastic version is around £1.50http://www.mouser.co.uk/Texas-Instruments/Linear-Voltage-Regulators/LM338-Series/_/N-1z0zls6Z5cg9gZ1yxyx8uWhereas the LM138 metal can version, is an eye watering £42+http://www.mouser.co.uk/Search/Refine.aspx?Keyword=LM138

author: 4CX35000
author_rank: Regular Contributor
timestamp: July 01, 2017, 03:19:41 pm
content: I've got LM138 on my mind at the moment. I suppose it might be to with replacing one recently. Yes LM338 and others will do.

author: madires
author_rank: Super Contributor
timestamp: July 01, 2017, 03:32:43 pm
content: TL431 plus driver transistor (BD135) for the 2N3055, creating a Darlington stage.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2017, 03:39:47 pm
content: 3Amp single 3 pin IC, at 77 pence one off price. (£0.77).Why mess around with lots of discrete components, to make a much worse circuit, as regards protections ?http://uk.farnell.com/fairchild-semiconductor/lm350t/v-reg-1-2v-to-33v-3a-to-220-3/dp/1417673Or spend 70+ pence more, for the 5 amp version (LM338).Unless it is a learning exercise, to learn how regulators work.But if so, it is not really a proper regulator, just a transistor emitter follower. Since it lacks proper feedback.But good enough for some applications, but risky at higher current/power levels, as it lacks even the most basic output protection.tl;drThe voltage in the original circuit, will tend to drift with temperature changes and not be very accurate.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2017, 04:23:51 pm
content: Quote from: madires on July 01, 2017, 03:32:43 pmThose are good ways of improving the original circuit. The datasheet even has some application circuits, to make simple power supplies.Such as this one:

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2017, 04:58:52 pm
content: Quote from: MK14 on July 01, 2017, 04:23:51 pmYes, do that, except change the transistors for something bigger, obviously.I'd recommend replacing both of the transistors with the TIP140.

author: rdl
author_rank: Super Contributor
timestamp: July 01, 2017, 05:02:53 pm
content: Just for clarity,QuoteThe LM138 is only available in the metal can (TO-3 type) package. The LM338 is available in both the metal can (TO-3 type) and the TO-220 package (plastic with metal tab).

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 01, 2017, 05:39:47 pm
content: Thank you for all your replys, I have put together various circuits with 3 terminal linear regulators, using the LM317 and LM338, also 78XX regulators. I never tried the option of trying a zener diode as a regulator. Thought it would be good to try it, so the source is in the link below. If 680 ohms is to high, what value would I need to obtain the 3 Amps maximum current ? I have ordered some 3 watt 680 ohm resistors, just going on the value of resistor from the source of the diagram. Thank you for the schematics, if the first attempt works OK (my building it) sure I'd love to try these improved circuits you've posted up.http://www.circuitstoday.com/12-v-high-current-regulator

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 01, 2017, 05:59:40 pm
content: I did find this zener calculator, but as there's a series pass transistor in the circuit, not sure the same value, with out adjustment would apply.http://www.reuk.co.uk/wordpress/electric-circuit/zener-diode-voltage-regulator/

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2017, 06:06:48 pm
content: Quote from: davelectronic on July 01, 2017, 05:39:47 pmIf it was a 12V AC secondary transformer, so about 12 x root 2 - bridge diodes (about 2 volts ) = about 15V.Min 2N3055 Hfe at around 3 Amps, is about 20 (4 amps in datasheet).So 3V across that resistor (R1), needs to supply about (3000/20 =) 150 milliamps.R = 3/0.15 = 20BUT that would not work in practice (or more accurately, it WOULD work, but the zener/resistor would get really HOT!), because the peak current (at the top of the ripple voltage, and when no output current), would be about 10 volts (across R1, estimated NOT calculated, as I don't know what capacitors or AC voltage transformer you are going to be using).Which would then be about 0.5 Amps.So the Zener would have 0.5 x 12 = 6 Watts, which is rather high. (Because the ripple voltage goes up and down, the average dissipation would be less than 6 watts. But with no output load, it would be 6 watts (est), all the time).So it is NOT a good circuit for using at higher currents, sorry.But another option is to use a power darlington transistor (instead of the 2N3055, the darlingtons huge gain would mean it needs little current to turn it on) or use a better designed circuit.N.B. Exact values, depend on the specific components you use, such as the transformers secondary, AC output voltage etc.EDIT: Figures updates. I used 4 amps, instead of 3 amps, so original figures wrong. Now corrected.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 01, 2017, 06:25:38 pm
content: I was thinking about 4700uf, yes 30 ohms is hugely less than the 680 ohms in the diagram. I've got 13 Volt 1.3 Watt zener diodes, the glass passivated type. I don't need as much as 3 Amps continuous, my load would be a 4 watt cb radio. I have noted this draws about 250mA in RX and 1.6 Amps in TX mode. So I guess what I'm looking for is at least 2 Amps at least. My transformer would have an AC secondary of 15 Volts. So this would be higher after the filtering capacitor. I'm not sure what value resistor in ohms, and power rating I need. I always struggle with the maths. Usually once I've worked it out I can build it, but the maths is my weak point, I wish I was better at it. Thank you for the help.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2017, 06:33:11 pm
content: I'd be temped as a first try, to stick with the original R1 value (680\$\Omega\$), as it runs cooler, so less voltage drift with temperature of the reference and R1. But use a power darlington, with a very high dissipation.Note that a darlington will have a much higher base voltage drop, as it has two rather than one, base diode junction voltage drops.Best to check with the datasheet, but it would be probably something like 1.5 volts or so. Almost any darlington, will probably have plenty of Hfe (gain), from the original circuit.But if you are determined to use the original circuit, you could also dramatically reduce the value of R1 (to some value, such as to 50or 100) AND use a higher Hfe (gain), (single NON-darlington, i.e. normal) power transistor, compared to the 2N3055. I.e. The 2N3055 is rather an old transistor, and does not have a very good Hfe, at higher currents. Try it and see how it works out, if you are just experimenting and trying things.N.B. The datasheet is giving the worst case guaranteed Hfe (datasheet guarantees 20, but typically it is 70, according to datasheethttp://www.onsemi.com/pub/Collateral/2N3055-D.PDF). In practice, it will probably be better than that figure. So if you try one on the bench, you will probably get better results. Just DON'T go into mass production, expecting the Hfe, to be as good as the one you try on the bench. It is best to stick with the datasheet recommendations. I just mentioned this, so that you realize why you may get, much better than expected results.

author: Zero999
author_rank: Super Contributor
timestamp: July 01, 2017, 06:45:44 pm
content: As mentioned above: you need more current gain.Another thing which will help is to increase the size of the filter capacitor to keep the ripple voltage down.One disadvantage of a Darlington pair is it increases the voltage drop. Another thing you could try is adding another transistor so it short out the base of the power transistor when the base starts to conduct, see attached. This has the advantage of being able to use a 6.2V zener diode and potential divider, which will have a better temperature coefficient than a 13V diode.

author: 4CX35000
author_rank: Regular Contributor
timestamp: July 01, 2017, 07:02:26 pm
content: Quote from: davelectronic on July 01, 2017, 05:39:47 pmI think the first time I was shown that circuit was in a school textbook, which had the transistor as a BFY51. Using that transistor I made a small PSU to provide 9 volt and it worked fine for building small circuits while at college. I built the circuit with some veroboard and placed it inside a small black box with a minature transformer and a 1 metre length of wires with clips on.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2017, 07:10:37 pm
content: Quote from: 4CX35000 on July 01, 2017, 07:02:26 pmIt is a nice circuit, for beginners, and low output currents. Especially up to about 100 milliamps.But probably NOT a good choice for high current power supplies. It lacks too much ability to drive the (base of the) power transistor, as shown in the original circuit.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 01, 2017, 09:58:51 pm
content: Thanks for the extra information and the schematics, I kind of skipped this circuit. When I started from scratch (absolute beginner) I got ahead of myself a bit and built a fare few LM317, LM338, 7812 etc. These ended up in cases as usable power supplys, even added meters to one of them. And up to 4 x PNP pass transistors on a couple of others. I'd been watching some circuits on zener diodes, and a tutorial of Dave Jones, then been kind of itching to try it out. For what I use 100mA is of little use, but 2 Amps would drive a standard 4 Watt radio. I've got a 10000uf 35V electrolytic capacitor left over from a MOT project (not finished yet) And have a couple of Darlington MJ11015 transistors, but there PNP, could order the NPN version, it's MJ11016 it's got a hfe of 1000 @20A bit over kill maybe. Tried the PNP MJ11015, that worked well in another psu I rescued off ebay. I will have a play about on a breadboard first, something else I skipped doh on my part. I just stared soldering up circuits from scratch. I should have breadboarded a lot more for the values measured in the circuits, then built them. Thanks again for all your replys and help.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2017, 10:09:30 pm
content: Here is an old UK article about these types of circuits:http://baec.tripod.com/JULY92/regulators.htmThey make some interesting extra points. Including that the extra gain from a second (or even third) transistor, improves the regulation a fair bit. By adding an extra transistor, it can do overcurrent protection (shown below). They put in a capacitor to improve the voltage stability as well.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 01, 2017, 11:43:09 pm
content: That's interesting, I've bookmarked it, thanks for the link. Some ideas to try beyond the first circuit. I have looked about at 5 watt zener diodes, do you think the 1N5350B diode would be of use, given its higher power rating ? I've got 1.3 Watt 13 Volt diodes, but wandered if the 5 watt diode would give more flexibility.

author: MK14
author_rank: Super Contributor
timestamp: July 01, 2017, 11:48:55 pm
content: Quote from: davelectronic on July 01, 2017, 11:43:09 pmThe 5W 13V zeners, seem like a good idea (if you want to try this circuit). Because at lower wattage's, they will run much cooler, and so be much more accurate (as much less temperature rise), so the voltage will drift a lot less. They will also let you try, higher zener currents, for single transistor circuits.If you end up going for the darlington circuits, the lower wattage zeners, should be ok, but the 5W ones, let you try more experimental options.You don't necessarily get the full zener rating, as the datasheet may assume a certain area of copper (PCB) cooling. Which may not apply on breadboards or verostrip. So it is best to not be too close to the limit, anyway.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 02, 2017, 12:30:47 am
content: I was kind of hoping it might give a bit more flexibility with a higher upper power rating. Yes I think I'll order a couple of the darlington transistors, the MJ11016 has a big gain, but it's TO3. I might look for a T0220 transistor. Thanks again for all the help everyone.

author: MK14
author_rank: Super Contributor
timestamp: July 02, 2017, 12:44:17 am
content: These ST Darlington TIP142's are 99p each (£0.99), 125 Watts and Hfe=1000.Quotehttp://uk.farnell.com/stmicroelectronics/tip142/transistor-darl-npn-100v-10a-to/dp/1653637

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 02, 2017, 12:57:34 am
content: Quote from: MK14 on July 02, 2017, 12:44:17 amI was just looking through a UK stores list of transistors, that's one I thought might be ok, the only thing I noticed is its 0.5 Amps Base current maximum, as opposed to 1 Amp Base current for the MJ11016, but I'm guessing a Base current limit on the devise of 500mA for this application should be ok. It's got the hfe of 1000@5 Amps. Thanks for pointing this out as a viable transistor. I'll order a couple of the 5 watt zener diodes, and a couple of the TIP142 transistors. Thanks again for all the help.

author: timb
author_rank: Super Contributor
timestamp: July 02, 2017, 01:20:45 am
content: Here's one I've used a few times in the past (and recently on the Scope Pong project):It's Zener based, only needs three transistors and provides basic current limiting. It could easily be modified to provide a higher output current by changing a few resistor values to provide more base drive and up the current limit.

author: MK14
author_rank: Super Contributor
timestamp: July 02, 2017, 01:26:42 am
content: Thanks TIMB!I wanted to see your low transistor count, regulator circuit, once you mentioned it in another thread (ping pong anyone ?).I've been curious about it, ever since.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 02, 2017, 09:35:29 am
content: I'm not sure what circuit in another thread your referring to, the only other two circuits I've built over the last few months where LM317 with single pnp pass transistor. And the bigger one, that's a 7812K with 4 x MJ4502 trasistors. Thanks for posting the scope view of your circuit, and sharing it. I did see recently somewhere (forgot where) zener diodes can be a little noisy as a regulator, andca capacitor for decoupling was recommended.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 02, 2017, 09:39:30 am
content: Quote from: MK14 on July 02, 2017, 01:26:42 amOh ok Timbs low transistor count circuit, sorry my mistake doh lol.

author: MK14
author_rank: Super Contributor
timestamp: July 02, 2017, 12:02:48 pm
content: Quote from: davelectronic on July 02, 2017, 09:39:30 amDon't worry, it is an easy mistake to make.I'm VERY impressed with his circuit. Very neat!From a quick look, despite the fact it is still "zener" based, it actually uses the combined gain of a darlington transistor AND another, plus an interesting configuration, so that it will actually close the loop (i.e. really regulate the voltage), to a fair extent (some non-linearities, obviously). It even seems to have a built in current limit.Because of its clever implementation, it is using a zener of about 6.25V, which gives really good voltage accuracy (especially as regards temperature stability, as that voltage, usually gives the smallest change per deg C, because of the semi-conductor physics of the device, because of zener/avalanche mode stuff and voltage).I presume the resistance added to the smoothing capacitor, across the zener, is to increase its ESR (virtually), to improve the stability of the power supply.Because it limits the heating effects in the zener and transistor which drives the darlington, as they have relatively little current in them, and the pot/trimmer allows most of that error to be "tuned/calibrated" out. The voltage accuracy and stability over time, should be good. Especially for a zener based, circuit.At the moment, the circuit seems to be designed for around 500mA's or so, output current. Since it seems to be limited at around the 650mA mark.(Obviously) it could use higher spec parts (especially the main output darlington), to achieve higher output currents. It may even be possible to configure it to use more than one output darlington (to share the heat dissipation), with some thought to the circuit, such as suitably valued emitter resistors, of very approximately 200 milliohms.

author: timb
author_rank: Super Contributor
timestamp: July 03, 2017, 01:01:03 am
content: Quote from: MK14 on July 02, 2017, 12:02:48 pmThanks!You got everything pretty much spot on, except for the resistors added to the large (4.7mF and 100uF) caps. I forgot to mention this when I posted the schematic: Any resistor (without a labeled value) attached to a capacitor represent the ESR of said capacitor.This particular simulation software (iCircuit) doesn't allow you to set the ESR of capacitors, so I always add a series resistor to the larger value caps, otherwise you can end up with pretty wild oscillations during simulation.But otherwise, yeah, it works pretty much as you've described. This particular implementation is good up to about 500mA (with a heatsink on the TIP110/115) and will start limiting current between 600 and 700mA. This is enough to keep quick shorts from destroying the pass transistor (and potentially whatever you're shorting). If you add an appropriately sized polyfuse to the input it would allow the supply to deal with continuous short circuits without overheating as well.It could easily upped to several amps by replacing the feedback/Zener transistor with a high gain darlington and using a larger darlington for the main pass element. (And lowering the 1 Ohm current limit resistor to to whatever drops approximately 600mV at the desired limiting current.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 06, 2017, 10:34:12 pm
content: Well I got round to trying the first posted schematic with a single transistor. I started with a 50VA 15 Volts AC transformer, 17 Volts reading no load. Rectified and filtered 25V 2200uf capacitor, no load 23 Volts, bit high. Using 14 Volt 5 watt zener diode, and trying TIP3055, and TIP142. Both worked well, I used a 100 ohm 3 watt resistor, and tried up to 140 ohms as the Base zener resistor. My off load voltage was a bit high at 14.8 Volts, dropping to 12.8 Volts under load of 1.6 Amps, 20 watt halogen lamps. The resistor dropped 7.9 Volts with no output load, and 3.2 Volts with the 1.6 Amp load. Right at the zener reference the voltage under this load was 13.8 Volts, and not far off that from the emitter of the transistor. I should have wrote some of this down, I measured around the transistor, it was fine. The TIP142 held a bit higher voltage under the 1.6 Amp load than the TIP3055. I know that input needs dropping some what to be more efficient. I was using a breadboard and 4 Amp max load hook up wire. I'd like to close the 2 Volt drop gap from no load to loaded, but I think it might not be possible due to transistor drop, and drop through output wire. Although if I built it as a power supply I'd keep leads as short as possible. Regardless of the 100 ohm 3 watt resistor, or 140 ohm resistor the voltage drop was the same at around 1.8 to 2.0 Volts, although the unloaded voltage was better at 14.4 Volts with no load and 140 ohm resistor. Thought it was worth sharing these findings.

author: MK14
author_rank: Super Contributor
timestamp: July 06, 2017, 10:46:33 pm
content: If I understand your reply correctly, 2 volts is way too high an output voltage drop. Going from no load to significant load.I suspect it is caused, because there is too little base current to properly drive the power transistor.But there will be some voltage drop in that type of circuit, because it lacks any real regulation, and the transistor as well as other circuit elements, will have varying voltages across them, as well.2 volts variation is not very good at all, as regards voltage regulation.Your experiments have shown why that type of circuit is not usually used at higher currents. Because it has way too poor voltage regulation. At higher currents, the voltage drop across the power transistor (especially a 2N3055), will get bigger and bigger. With no real increase in base current (but maybe some), to try and compensate (closed loop).Also, breadboards are not good for that sort of circuit, especially the higher currents. They tend to have too poor connections/contacts. You might find some of the 2 volt drop, is because of the breadboards excessive resistances, etc. Opinions seem to vary, but people seem to say that somewhere between 100 mA's and 1 amp, is the limit for breadboards. With 500 mA and less being the most common recommended limits.Cheap and poor quality breadboards make it worse still.Maybe you can wire the high current paths, directly with wires, to the components or something. To avoid placing high currents, through breadboards.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 06, 2017, 11:59:51 pm
content: Quote from: MK14 on July 06, 2017, 10:46:33 pmThanks for your reply, sure I understand what your saying, a bit of it might have been the transformer and my wire run was a bit long, over 6" closer to 12" I except this circuit has its limitations, if I can build a 1.8 to 2.0 Amp psu from it I'd feel I've given it a go. 80mA was the current through the 100 ohm resistor. I should have written some more details down as I did it. The no load through the zener diode and resistor ran warmer than when the transistor was under load, then the zener and the resistor ran very cool. Nothing got dangerously hot. I think if I lowered the resistor value, given the same voltage conditions input, the output would have been to high. Say 50 ohms I think the output voltage off load would have been to high. Not 100% sure. I did try the 680 ohm resistor 3 watts as in the first schematic, I only got 10.80 Volts with the same load.

author: MK14
author_rank: Super Contributor
timestamp: July 07, 2017, 12:14:52 am
content: That makes sense. The zener will get hottest (in that circuit), when there is no load. Because it will have to sink most of the current, as the transistor, is not using very much (emitter follower). But at high output current loads, the 2N3055's base, will take almost all the current from the resistor. Keeping the zener really cool.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 07, 2017, 12:47:12 am
content: Thanks a gain MK14, I will put a small project built from this circuit in the near future. Hopefully I can close that voltage differential with a tighter smaller circuit board build with minimal trace lengths and short output wires. Cheers again to everyone for the help.

author: MK14
author_rank: Super Contributor
timestamp: July 07, 2017, 01:01:15 am
content: Quote from: davelectronic on July 07, 2017, 12:47:12 amI've just tried simulating your original circuit, with a 50resistor (R1) for the zener. It seems to be able to regulate to around a 200 millivolt change, from a 1Kload, down to a 7load. Real life circuits will obviously give more accurate results.With a 680for R1, the regulation is hopeless (as expected), dropping to about 6V, from 12V. The 100R1, is almost as good as the 50R1.N.B. The simulated Hfe (gain) of the 2N3055, may be significantly different to the one you are using.200 millivolts, is not that bad. But a proper closed-loop regulator, would usually be massively better than this.The simulator (LTspice), is effectively assuming perfect wires/connectors. So some of your voltage drops are NOT included. E.g. Breadboard.I guess with the real life circuit, you can use a multimeter, to see where exactly any voltage drops, are. E.g. across the 2N3055's Collector/Emitter, etc. Which you have already been doing, as mentioned in your posts.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 07, 2017, 01:57:54 am
content: It's very interesting, would a 50 ohm 3 watt resistor get a bit hot ? Given the 7.9 Volts the 100 ohm resistor that was dropped across it. I suppose it would just be double the current, not sure that's right as the voltage across a 50 ohm resistor might be higher.

author: MK14
author_rank: Super Contributor
timestamp: July 07, 2017, 09:58:07 am
content: Quote from: davelectronic on July 07, 2017, 01:57:54 amWith no load, the 50resistor will have the full unregulated voltage minus the 12V of the Zener across it. Which you are saying is about 8 volts, which sounds reasonable. There will also be a ripple voltage, which you probably are not seeing/observing because you are not using an oscilloscope yet (if I remember correctly from other threads).So the power formula (using IV for power, then using ohms law by substituting I for V/R) is V squared over Resistance = V^2/RSo the power dissipation in R1, when it is 50, with about 8 volts across it and ignoring ripple.= ( 8 x 8 ) / 50 = about 1.3 Watts.But the unmeasured ripple may make that figure a lot higher, depending on if your measurements including an average of the voltage including ripple, or were the lowest voltage, excluding ripple.I.e. I.m NOT 100% sure what a multimeter would do if you use the DC range to measure a voltage with lots of ripple on it. (I'd use a scope). Presumably you would get some kind of average or the reading would keep on changing (fluctuating). It may vary between different types of multimeter.Anyway, when the resistor (R1) would be at its hottest, i.e. NO LOAD. There should be very little ripple, as the voltage will tend to just stay at the peak, and be held up by the relatively unloaded reservoir capacitors.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 07, 2017, 06:25:12 pm
content: Thanks for the help with the maths, yes your right I don't have a scope at the moment. I will try and invest in a modest one soon. All I can do is try the 50 ohm 3 watt resistor and see how much it drops across it in voltage, and work it out from there. I know a bit more drive to the transistor would be useful. I was lead to belive on some multimeters you can see any ripple component in a DC voltage using a low range AC setting, not sure if that would help on seeing ripple. I've got a couple of uni- t meters, one is true rms, the other is not. Also a small analogue meter and another digital non rms meter. I know a scope would be a great asset for this and so much more.

author: MK14
author_rank: Super Contributor
timestamp: July 07, 2017, 07:33:19 pm
content: For that particular circuit, not using a scope is fine. It is not even closed loop, and is unlikely to oscillate.But switched mode power supplies (if you ever go that advanced), would make a much more compelling case, to need a scope. Especially if you are designing them from scratch.Linear power supplies can sometimes oscillate and suffer from short term effects (like over shooting, just after a sudden load change or initial PSU switch on), so they can also sometimes need a scope.Scopes are useful tools for learning more about electronics.But scopes are not essential. I suspect many beginners and casual electronics people DON'T have them.EEVblog often has free competitions for them (there is one running at the moment), where you can win free scopes (but import duties are probably significant into the UK).EDIT: But a scope would have allowed you to make sure that the ripple voltage is NOT getting through to the output (and check how much it has been removed, i.e. how clean/smooth the output voltage is), and maybe other possible issues.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 07, 2017, 08:41:39 pm
content: Yes I will look about for a second hand modest scope, I'm primarily interested in power supply construction, so mhz as opposed to ghz would probably be ok for what I would use it for. Taking into account maximum switching frequency of smps. I wish I could get to grips with the maths a bit better, I can work out quite a bit, but get stuck on more complex multiple equations in some circuits.

author: MK14
author_rank: Super Contributor
timestamp: July 07, 2017, 10:10:05 pm
content: On the other hand, don't feel pressured into getting a scope. Sometimes people buy things, because of EEVblog forums, such as expensive Fluke multimeters. Then they end up not using them, for various reasons.I think if you really needed one, you probably would already have one.It could end up being never used.

author: davelectronic
author_rank: Frequent Contributor
timestamp: July 08, 2017, 09:48:10 am
content: It might see some use, more on minimising output noise of different psu circuits. I will keep my eyes open for a bargain. Thanks again for all the help.

author: MK14
author_rank: Super Contributor
timestamp: July 08, 2017, 10:23:12 am
content: Quote from: davelectronic on July 08, 2017, 09:48:10 amOk, good luck on finding a bargain scope. Even the old CRT analogue 20 MHz ones, would probably be good enough, for PSU work. They sometimes pop up for free, or small amounts like £10..£15, often/sometimes more.