Simple circuit, say 9V AC to a regulated 6.3 V at at least 2 amps? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 19, 2013, 06:12:41 pm
content: Basically I understand rectifiers and just about understand regulators, but can modern regulators handle a current draw of say 2 amps at a regulated 6.3V DC, without also using a power transistor? If so, which regulators? How big a voltage delta between AC out of transformer to required DC out of regulator? Thanks. Follow on from my transformer rating thread (VA rating of a transformer). Thanks.

author: smashedProton
author_rank: Frequent Contributor
timestamp: June 19, 2013, 06:22:15 pm
content: How about two lm317 in parallel? You need to add some resistance to the output though

author: Alana
author_rank: Frequent Contributor
timestamp: June 19, 2013, 06:36:12 pm
content: LM350 would be better, 78xx and LM317 are not ment to be connected in parallel.But there will be like 6W lost to heat so switchmode regulator like L4960 or something alike may be a good idea too. Minimal delta V you can assume for most typical regulators is 2V but some datasheets for like LM317 say 3V.Datasheets:L4960http://www.st.com/web/catalog/sense_power/FM142/CL1456/SC355/PF63199LM350http://www.fairchildsemi.com/ds/LM/LM350.pdf

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 19, 2013, 07:01:52 pm
content: OK, thanks for the part numbers. I should have asked, this is for a DC supply to valve filaments, two small Russian valves with the filaments in parallel. There will be some inrush current when they switch on. How do these regulators cope with that? I see a data sheet for the LM317 shows a "slow start" circuit, I think it uses a capacitor / resistor charge time "timer". Would that help? Why do I want DC? The circuit is a driver amp for a big tetrode, designed for very low noise, the designer specs DC to reduce chances of hum etcetera. It also allows good voltage control, of course, for better valve life. Thanks. The valve data sheet says 0.55 Amps each filament, so I am playing a bit safe on speccing 2 Amps. Cheers!

author: Kremmen
author_rank: Super Contributor
timestamp: June 19, 2013, 07:17:20 pm
content: As long as you use a linear regulator circuit the voltage you drop will be dissipated in some component, and the magnitude of dissipation will be the same regardless whether you use a power transistor or not. So it might be easier to just use one and have done with it. Any number of circuits out there. Possibly even the 3 component regulator (resistor, zener, transistor) could do but if not, many other simple options exist.

author: edavid
author_rank: Super Contributor
timestamp: June 19, 2013, 07:51:42 pm
content: You shouldn't have to do anything special about inrush current.If you don't want to find an LM350, you could just use an LM317T for each tube. That's probably cheaper.

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 19, 2013, 08:29:56 pm
content: I have bought an LM350 to have a play with, and for stock, as I am in no rush for them, a very cheap pack of 5 LM317T from China, that will take a few weeks to arrive. Sound like useful regulators to have about. If the current draw is more than the spec sheets suggest I'll do something else with a power transistor. All good learning experience! Thanks again.

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 12:51:19 am
content: You should just put two 7805 regulators in parallel on the same heatsink...it will work perfect.Some people think this idea is poor design and only one of the 7805 will be active...not true. They will quickly balance the load between them and the voltage output will remain constant during this balancing period.KISS

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 12:56:02 am
content: Someone says, "LM350 would be better, 78xx and LM317 are not ment to be connected in parallel."Try connecting two or more 78xx in parallel and test this idea! If you can, mount them on the same heatsink.

author: edavid
author_rank: Super Contributor
timestamp: June 20, 2013, 02:35:51 am
content: Quote from: Paul Price on June 20, 2013, 12:51:19 amNo, it will not work perfect(ly), and it really is poor design. The 7805 with the higher voltage output will supply most of the load until/unless it goes into current limit or thermal shutdown.Now, in this case, the OP needs 6.3V out, so he could trim the voltages on the 2 7805s so they were close enough to share pretty well, especially with a little extra ballast resistance. However, since he has 2 separate loads, he could just supply them from separate regulators and avoid the issue.

author: Rufus
author_rank: Super Contributor
timestamp: June 20, 2013, 02:57:32 am
content: Quote from: Paul Price on June 20, 2013, 12:51:19 amIf it were good design don't you think some of the data sheets might show it as a suggested higher current regulator and sell twice as many 7805s? Two 7805s can have a much as 400mV difference in output and typically only 10mV of load regulation at 1.5A. The chances are one of the regulators will try to do all the work limited by its SAO and thermal protection circuits.The other will fill in but who knows what the characteristics of a 7805 trying to current limit or shut down are like. The SAO protection possibly has a negative resistance characteristic. The higher the output voltage the less dissipation in the pass transistor and the more current the SAO circuit will allow to pass. A pair of 7805s heavily loaded might just sit there oscillating.Who knows, the idea is too naff to be worth investigating.

author: c4757p
author_rank: Super Contributor
timestamp: June 20, 2013, 02:58:30 am
content: Quote from: edavid on June 20, 2013, 02:35:51 am7805 has a negative temperature coefficient of voltage, so as long as the two are reasonably close to begin with, they should end up sharing.Quote from: Rufus on June 20, 2013, 02:57:32 amI think it's a reasonable thing to dofor a hobby circuit, if you take the time to match a pair so that they begin close together. As long as the difference isn't so high that one goes into thermal shutdown before they have a chance to balance, they should equalize.

author: peter.mitchell
author_rank: Super Contributor
timestamp: June 20, 2013, 06:43:18 am
content: I'd still stick a 0.1 ohm or something on the output of each one to ensure some current sharing though.

author: richard.cs
author_rank: Super Contributor
timestamp: June 20, 2013, 09:10:40 am
content: Is there a particular reason for using a 9V transformer and d.c. heating? Would you consider running the heaters in series?

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 10:45:19 am
content: Remember what we are talking about here, heating filaments.

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 10:50:53 am
content: Finding the simplest way to accomplish an electron task can often be the best. All the remarks about output voltage matching an thermal shutdown are immaterial and simply do not happen. They just don't happen with two 7805's in parallel if the output of both total matches the demand.There is no need to create an electron egalitarian democracy.The 7805's will always share because one attempting to take more of the load will always drop in output voltage and this drop in output voltage will cause its partner to take up the slack.It makes no difference for the circuit and it will function perfectly even if one is running a little hotter than the other. The 7805 is designed to work over its range of operating temperature and current sharing will take affect well before either one approaches it's output current or temperature SOA limit.

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 10:54:16 am
content: Edavid says,"No, it will not work perfect(ly), and it really is poor design. The 7805 with the higher voltage output will supply most of the load until/unless it goes into current limit or thermal shutdown."I say this is not true, for same reasons I have stated before.

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 10:56:25 am
content: Rufus says, "If it were good design don't you think some of the data sheets might show it as a suggested higher current regulator and sell twice as many 7805s? Two 7805s can have a much as 400mV difference in output and typically only 10mV of load regulation at 1.5A. The chances are one of the regulators will try to do all the work limited by its SAO and thermal protection circuits.The other will fill in but who knows what the characteristics of a 7805 trying to current limit or shut down are like. The SAO protection possibly has a negative resistance characteristic. The higher the output voltage the less dissipation in the pass transistor and the more current the SAO circuit will allow to pass. A pair of 7805s heavily loaded might just sit there oscillating. "---------------------------------------------------Does not and cannot happen. Have you ever tried such a circuit?

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 10:59:38 am
content: C4757p says, "I think it's a reasonable thing to do for a hobby circuit"How can you be sure it not altogether the right thing to do in any reasonable circuit. The characteristics a 7805 under load cause them to share as much as necessary.A 7805 is not be used to create a laboratory standard voltage reference!

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 11:02:58 am
content: Peter.Michel wrote,"I'd still stick a 0.1 ohm or something on the output of each one to ensure some current sharing though."Why add components that are unnecessary and make more work for yourself, as I've already said they don't need to exactly match and they will anyway for the reasons I've stated already.

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 11:05:02 am
content: Edavid says,"Now, in this case, the OP needs 6.3V out, so he could trim the voltages on the 2 7805s so they were close enough to share pretty well, especially with a little extra ballast resistance. However, since he has 2 separate loads, he could just supply them from separate regulators and avoid the issue."Firstly, is there really an issue? The 7805's are already matched close enough and there is again no need for the currents to exactly match.Why create more work for yourself if there is no proven reason to.

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 11:14:02 am
content: cs.richard says, "Is there a particular reason for using a 9V transformer and d.c. heating? Would you consider running the heaters in series?"Yes, this tampers with an antique circuit and creates more work soldering than necessary with possible damage to brittle old tube socket pins and wires with rubber or fragile cloth insulation that is ready to drop off at the first toggle or breeze.Why not try just providing the current you need easily, say with two 7805's in parallel.

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 11:17:58 am
content: Why doesn't anyone try paralleling of 7805's to test their current sharing operation before commenting?Prove me wrong as well, but Is it good to give advice on how to protect a circuit that doesn't need protecting, to match devices that do not need matching, to offer advice without experimental proof, with just fear and ignorance to guide you?Has anyone forgotten that we are trying to find a simple solution to this extremely delicate and complex precision design question, heating filaments?These poor vacuum tubes have managed to cope with unregulated 6.3V AC for enough time for your grandson to enter college, somehow they have survived.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 20, 2013, 11:25:52 am
content: For a fixed load like a filament, you could just use a resistor - it will get hot, but so will a linear regulator

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 20, 2013, 11:27:48 am
content: Personally I think unregulated 6V AC would be fine, but I am trying to emulate a known REALLY low IMD design, and not experienced enough to want to go off at a tangent from what is known to work. Thanks for the interesting points of view and ideas

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 11:28:06 am
content: Rugus says,"Two 7805s can have a much as 400mV difference in output and typically only 10mV of load regulation at 1.5A. "It is extremely unlikely that you will ever encounter a 7805 within the same lot of purchased regulators to differ as much as that.Quality control from any reputable manufacturer ensures they beat this spec.I say it is not even necessary, but is there effort at all to very quickly, maybe spend 5 seconds to quickly press a 7805 into a breadboard to check it's output voltage before adding useless components or seeking a more complex or more costly alternative?

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 11:35:02 am
content: ChrisWilson,I hope I might be speaking for the all of us, that we have scared the OP away from trying to use any regulating circuit, use any of our circuit ideas, just by our comments.No one has said here that the regulators could cause damage, we are just ranting over current sharing.Must we always say, without a reasonable doubt, that for the safety of all, and without extensive proof and laboratory testing, that there will be fire and smoke and burnt filaments, that if you tried this idea if would ruin your equipment, that it is in fact so dangerous, so experimental, and that it doesn't really work well because of some assumed "sharing" problem?

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 11:49:39 am
content: Mikeselectronicstuff,Don't forget what the OP asks for,----------------------------------Re: Simple circuit, say 9V AC to aregulated6.3 V at at least 2 amps?----------------------------------Why try to find the right size and wattage resistor when you can use regulators?First, it might be harder to find the right size and wattage resistor than to find 7805 regulators.Secondly, and more likely, he might be trying to reduce AC hum in a sensitive audio amplifier or other equipment by removing the AC filament contribution to this problem in low level amplifiers.

author: Paul Price
author_rank: Super Contributor
timestamp: June 20, 2013, 12:13:42 pm
content: Ok, if there was an attempt to parallel two or more 7805's to function as a regulator in "Mission Critical" piece of equipment such as what might be used in a hospital or other medical use, or to make a 747 fly, then the most critical evaluation and test of any circuit is required.But what if you were in Zimbabwe or some other third world land and your old tube-based design of an EKG needed 6.3V at 2amps desperately to save some lives...? Would you say, "Oh No, it can 't be done! The spec sheet doesn't recommend this! "I haven't seen a spec sheet that said this should not be done.

author: Rufus
author_rank: Super Contributor
timestamp: June 20, 2013, 01:30:13 pm
content: Quote from: mikeselectricstuff on June 20, 2013, 11:25:52 amIf it is a single filament where you don't care what voltage is applied if it has failed.You could also provide some of the current with a resistor and let a smaller regulator control voltage and provide the balance.

author: richard.cs
author_rank: Super Contributor
timestamp: June 21, 2013, 12:25:40 pm
content: Quote from: Paul Price on June 20, 2013, 11:14:02 amI've clearly missed the bit where he states this is a modification to existing equiment, rather than a new design - in existing equpment I'd expect a 6.3V transformer winding specifically for the heaters. Having said that valve heaters are useually specced for +/- 10% anyway and running them 9V ac through a suitably chosen resistor should be fine, the mains should stay within 10% in any developed country and certainly so in the UK where Chris W is based. Why complicate things with a regulated supply?

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 21, 2013, 12:46:44 pm
content: The amp I am making the power supplies and antenna switching for is a Russian linear with a driver stage using 2 tetrodes in parallel. A French ham has a lot of experience with these and always uses a DC filament supply to the driver stage to reduce hum. I am striving for the cleanest signal possible, and if a DC filament supply helps, I don't mind the slight extra complication and expense. I am not experienced enough to say whether it's OTT twaddle or a viable means to gain a cleaner signal, so I am erring on the side of caution. Amp is part of the R-140 station. Just the amp is here:http://www.gatesgarth.com/r140.jpgThanks.

author: richard.cs
author_rank: Super Contributor
timestamp: June 21, 2013, 01:48:13 pm
content: Thanks for the info Chris. It's not exisiting equipment then and you can build it how you like, but there seems to be reasonable justification for d.c. heaters.I don't think you've mentioned part numbers for the valves or posted any schematics, are they directly heated?

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 21, 2013, 02:10:19 pm
content: Input valves a 2 off 6E5P in parallel, Class A.

author: richard.cs
author_rank: Super Contributor
timestamp: June 21, 2013, 02:28:48 pm
content: With indirectly heaed cathodes I'm suprised there's significant hum from a.c. heating, but ok.I would lean towards running the heaters in series then as you can keep the voltage across the regulator the same (at just a little higher than the dropout voltage) but halve the current and therefore halve regulator dissipation. It could be as simple as a single 7812 (or there's a 1.5A variant that's otherwise the same and a little less marginal) with a diode in its ground connection to bump it up to 12.6 Volts. This would need a d.c. input of about 15V which could happily come from a 12V transformer winding.

author: edavid
author_rank: Super Contributor
timestamp: June 21, 2013, 04:31:04 pm
content: Quote from: Paul Price on June 20, 2013, 11:17:58 amBecause I don't have enough 7805s on hand to pick out a worst case mismatched pair, and I don't know how to determine the worst case conditions for the test, so it wouldn't prove anything. I know it will work if the 7805s are well matched and the load is not too extreme, but I don't know what the limits are, so I would never recommend it, especially in this case, when there's no benefit to doing it that way.Anyway, I can't figure out what the OP is building... is it part of a radio transmitter or receiver? Either way, he doesn't need DC heater power in the first place.

author: Paul Price
author_rank: Super Contributor
timestamp: June 21, 2013, 08:07:51 pm
content: Why try to find mismatch to create trouble for yourself if you desired to find a simple way to create a regulated power supply quickly?edavid, you say,"so I would never recommend it, especially in this case, when there's no benefit to doing it that way."The benefit is simple, a very simple circuit, low parts count, low cost, 7805 parts likely on hand, it works!KISS

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 22, 2013, 03:18:56 pm
content: I breadboarded a circuit today using a single LM317 regulator and fed it 10V from my variable DC supply. It would regulate OK at 6.3V until it got hot, then needed a bigger differential voltage to stabilise. Perhaps another 1V. I attach photos of the heat sink used. I also measured current draw. It was 1.2 Amps. Bigger heat sink and all OK? Need to rethink or try two LM317 in parallel? Thanks.

author: SeanB
author_rank: Super Contributor
timestamp: June 22, 2013, 03:23:51 pm
content: Much bigger heatsink for sure, about 5-10 times bigger would do. If the LM317 is so hot that you cannot touch it then it is too hot.

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 22, 2013, 03:33:02 pm
content: I can just about keep a finger on it, but then again I am working on red hot engines a lot of the time, and I admit I was once called asbestos fingersWhat do I know, but to me it felt like it was too hot for long term sensible operation? The heat sinks came gratis with the regulators, so no big deal. I will find a big block of aluminium in the workshop and try it on that temporarily! Thanks Sean

author: Rufus
author_rank: Super Contributor
timestamp: June 22, 2013, 04:11:10 pm
content: Quote from: Chris Wilson on June 22, 2013, 03:18:56 pmThe valve datasheet you linked specified 600 +/-40mA so driving 2 in parallel is 1.2A - figures.Can't you split them and have a regulator each?

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 22, 2013, 05:51:30 pm
content: Rufus: I probably could, would need advice on the necessity of changing the capacitor C3 in schematic attached.But.... I have it on an ugly triangle of .75 inch thick aluminium and it's run driving the filaments for about an hour and its temperature is just warm. Will monitor for a few hours tonight and see what happens. I am monitoring current and voltage and using my "finger probe" to monitor temps, although I could get scientific and use a K type thermocouple surface probe and calibrated meter, which I use in my day job. But for sure it's not too hot at the moment, and I can't see this amp running all day, for example. I am learning quite a bit about what sort of heat sink size things need per watts dissipation and especially if they are run near the edge of their specs. I can input 9 volts now and it regulates fine, which should reduce dissipation? Thanks for all the great advice. Will see how it goes longer term now and report back! Cheers.

author: Rufus
author_rank: Super Contributor
timestamp: June 22, 2013, 06:25:36 pm
content: Quote from: Chris Wilson on June 22, 2013, 05:51:30 pmAdd another one it is just some rf decoupling.Testing with a dc power supply you don't have any ripple on the input to the regulator. With a transformer/rectifier there will be ripple and the average input will have to be at least half the ripple voltage higher. The LM317 has a data sheet minimum input to output voltage of 3v. It needs 9.3v in for 6.3 out.LDO (low drop out) regulators are available with much lower minimum input to output requirements. They would allow lower input voltage which reduces dissipation in the regulator.Thermal considerations are not too difficult to work out. 9v in 6.3v out at 1.2A is 3.24W. Heatsink data will give you a degree C/W rating so for example a 13C/W heat sink would have to run 13*3.2 C above ambient. Looking at heat sink data will give you a feel for it.Semiconductors can be run hot. The LM317 recommended maximum is 125C junction temperature. The 'junction' will be about 3C/W higher than the metal tab. It is possible to run them on a heat sink hot enough to boil water.

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 23, 2013, 08:36:26 am
content: The supply has been left running the filaments overnight, and the regulator is a sensible temperature, I can easily leave a finger on it long term. The heat sink is just slightly warm. Voltage and current stable.Thanks for all the great info.The next supply for this thing is 12.6 V DC at 7 amps. I am tempted to use a switch mode. The Chinese 12 V ones have a trimmer that gives + - 10%. The guru for the amp in question has started using them, as they are small, cheap and light, and have proven 100% reliable if you pick one a bit bigger than you should really need.....Then on to the screen and grid supplies. Luckily the HV anode supply is something I have already built. Thanks again everyone!

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 23, 2013, 11:40:29 am
content: Oh dear, sorry I am back again with another darned question!Scoping the output across the 10,000 uF electrolytic, before the regulator shows what I think is a lot of ripple. After the reg it looks OK. Measurements taken with circuit under load. I am pretty new to all this, I just normally buy a PSSo comments please. Thanks.As an aside I have tried mounting the regulator direct to the heatsink, with no insulator washer and thermo pad, or whatever the thermal transfer, electrically insulating thin rectangular pads are called. It runs barely warm to the touch now, so the pads are presumably not terribly efficient at heat transfer? I realize the tab on the reg is also the output side, and now the sink is linked to the circuit.

author: richard.cs
author_rank: Super Contributor
timestamp: June 23, 2013, 12:25:29 pm
content: 800 mV pk-pk ripple before the regulator is fine, I would have expected a bit better than the 3 mV pk-pk on the output side but it's plenty good enough for any valve heater. How much capacitance do you have after the regulator?Yes the thermal pads aren't great. The silicon ones are pretty crummy and the mica washers (with thermal grease) are considerably better. But if you don't need the isolation from the heatsink just bolt it straight on as you have with a little thermal grease. Sometimes the circuit allows it eg: grounded tabs on a 78 series regulator or sometimes you just float the heatsink from the chassis.

author: SeanB
author_rank: Super Contributor
timestamp: June 23, 2013, 12:28:24 pm
content: 800mV is not terribly bad, you would need to add more capacitance ( or a better lower ESR one than the one you have) to drop it much further.As to the insulating kit, you need to add the thermal compound there between the device and the washer and heatsink. It is not just for show, but does do a very important function to reduce thermal resistance. It is rather like a bearing will work without grease in it, but adding the grease will make it last longer and run quieter ( as you say you are accustomed to cars so this is a good analogy) and this is very necessary. If you do not have the thermal compound ( not silicone sealer or gasket maker) use a thin smear of regular GP grease instead as a temporary substitute, and go to Maplin and buy some heatsink compound ( about the only thing they stock, just buy the cheapest per volume one, you do not need fancy here) to use there.

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 23, 2013, 06:15:27 pm
content: OK, that's a relief, thanks! I have used thermal grease between the tab and the h/sink. I used the thermal pad dry, should it have a thin film of the grease each side? The heat sink is now at output positive, and the heaters are run positive grounded, so I shouldn't have to worry about the sink being bolted to the chassis of the PSU. How much capacitance would be needed to get things smoother? Is 10,000 uF not quite a lot already? For future reference... I have a scrap Tek analogue scope here, is that likely to have some suitable caps in the PSU? I would imagine a scope needs a pretty smooth supply?Also for future reference, is 3 Amps as much as these variable regulators will manage? Or can you get heftier ones at a price? Any good sources of simple circuits to introduce a transistor to use for high current applications? Are they called "pass transistors"? Thanks.

author: SeanB
author_rank: Super Contributor
timestamp: June 23, 2013, 06:27:18 pm
content: Thin film on each side, does the trick. Buy new caps at Craplins while you are there, get 4 2200uF 35V 105C electrolytics there ( they will have some, any value like 2200uF 3300uF or 4700uF will do as long as the voltage is over 25V) and use them in parallel in your circuit. Will drop it a few mV more, and the big chunk of metal will keep it cool.As to higher current, look at the LM317 datasheet and you will see how to increase the output current using an external power transistor like a 2N2955 on a big heatsink.

author: richard.cs
author_rank: Super Contributor
timestamp: June 23, 2013, 06:40:00 pm
content: 10,000uF is a lot, more than is needed. To get really low ripple it's usual to use a fairly crude pre-regulator to a voltage a little above what you want then a second regulator down to the desired voltage. No-one does it unless they really need it because of the inefficiency and additional complexity. There will also be regulators that perform better than the LM317, better in the sense that for a given value of input ripple they give less output ripple. Also you didn't mention how much capacitance you have to ground on the output side of the regulator?You've reduced the a.c. on the heater from 18 V peak to peak (a.c. powered heater) to 3 mV peak to peak. Any hum you might have got from the heater will have been reduced by 6000 times already - you really don't need it any smoother than this. Remember the heater is not electrically part of the valve circuit, it just has a very small capacitance to the cathode. At 50 Hz this capacitance looks like a very high impedance and can usually be ignored, especially in the circuit you posted where the cathode connects straight to ground anyway. Usually heater hum is only a problem when the cathode is many kilohms from ground, typical with certain audio pre-amp circuits.3A is the biggest of this type so far as I'm aware although I'm sure someone must make adjustable linear regulators up to 5-10A. But really by 3 amps you're in the region where it makes sense to build a switching supply. There are ways to add an external transistor to a standard regulator and some regulators don't include the pass transistor at all and you have to use an external one.

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 23, 2013, 07:45:31 pm
content: richard.cs: I have a small 1 uF 50V electrolytic on the output side. As per a spec sheet I was working off. Should I go bigger?Great info, you have all been very helpful and patient! Next I need 12.6V at at least 7 Amps, the amp guru who specilises in this Russian beast uses Chinese switch mode supplies for this. As richard.cs mentions, I am in the realm where a switcher should be the way to go there.I am also playing with a 50 volt SS linear, and have bought an HP rack server 50 volts at 50 or 60 amp switch mode supply off Ebay, dirt cheap. Beautifully made, they probably cost a lot of dosh new! There's a small preset pot inside to trim the voltage. I have found someone who has added some RF caps to stop RF getting in, and changed the preset to give a bit more voltage adjustment range. I am wondering if the big 1300 Watt rack server 12 volt power supplies can similarly be trimmed. There can't be a cheaper way of getting a genuine 100 Amp 12 volt supply! Anyone been inside one? I assume they all must have some sort of regulator device, which can be trimmed with either a preset or a resistor change to a preset? When I look at my Farnell 60 volt, 50 Amp supply, that I can only JUST lift, but not get on the bench on my own, and the HP one that's very compact and light, I can see switch mode is the way to go. The 50 volt HP one I have emits no detectable RF noise, either, certainly on the HF bands.

author: richard.cs
author_rank: Super Contributor
timestamp: June 25, 2013, 09:54:26 am
content: Quote from: Chris Wilson on June 23, 2013, 07:45:31 pmThese are generally minimum values for which the regulator is stable. There's no harm in going larger. It usually has a fairly small effect on output ripple (but still some effect) and a larger effect on transient response (which isn't really relevent to you with your fixed load).Many of the fixed 12V power supplies can be trimmed easily within a small range but have overvoltage protection that needs to be disabled before you can go very far. For 12.6V you'll almost certainly be fine but anyone wanting to go to lead-acid charging voltages (13.6V to 14.4V) usually has to disable or modify the overvoltage shutdown. This is common with PC power supplies and I imagine server ones would have similar protection circuitry. With all the tweaks done the 12V ones can usually cover about 8-16V output but possibly not over their full input voltage range.For a cheap 12V 7A switching supply I would be inclined to go for a small modern PC PSU, the older ones were only a couple of amps on 12V with all the power on 5V and 3.3, the newer ones deliever a lot more at 12V straight into switchers on the motherboard. There are also auxilliary ones availiable meant for graphics cards that are 12V at 10-20A without the other outputs.

author: dr_p
author_rank: Frequent Contributor
timestamp: June 27, 2013, 08:11:37 am
content: Quote from: Chris Wilson on June 19, 2013, 07:01:52 pmLM317 can supply 1.5A (min) to 2.2A(typ) if you have a low input-output differential, so that's a choice as well.