Linear regulated PSU voltage drop. - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 09, 2024, 06:11:37 pm
content: Over the years I have put together a few linear voltage power supplys. But I'm baffled why under a resistive load they behave in an expected way with regard to minimal voltage drop under load. But an inductive load drops the PSU voltage as much as 1.5 volts or more. The supply voltage is 12.70 volts DC. The circuit is a typical linear voltage regulator with multiple pass transistors to supply the current. I have power supplys with two pass transistors, and up to six pass transistors. The transformers are more than capable of supplying the current, so are the transistors. Either TIP36C or TIP2955 transistors. The voltage regulator is a 7812+ voltage regulator. The regulator is close to the output, as are the transistor collectors. I just don't understand why a high power resistive load is fine, but an inductive load sucks the life out of the PSU. Any help appreciated, thanks for reading.Below is the circuit I've used multiple times.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 09, 2024, 06:22:36 pm
content: What exactly is an inductive load ?Did you check with a scope ?A load with a phase angle and Z that changes with freq, may be enough to make the regulator loop unstable.I’ve seen 2-5 ohms in series with 0.1-10u used to help loop stability.

author: floobydust
author_rank: Super Contributor
timestamp: January 09, 2024, 06:28:28 pm
content: Voltage drops in the circuit wiring or PCB (under load) will cause unexpected problems. You need a star ground: main filter cap (-) is where the load (-) goes etc. etc.It could oscillate with the inductive load, you have no protection diodes either. Take a look with a scope.At that kind of output current, it really needs to be 4-wire with remote sense if you are expecting great regulation as well.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 09, 2024, 08:13:28 pm
content: I don't have the use of a scope at the moment, the inductive load is 100 watt linear HF MOSFET amplifier. A PSU using the circuit I posted with 6 pass transistors is powering the radio and amplifier at the moment. That's running it fine, with no more than a few tens of millivolts drop under full load. But I have two and four series pass transistors PSU's same circuit and play up dropping to much voltage. Where is the 2-5 Ohm resistor and 0.1-1.0uf capacitor placed with the regulator ? The idea of a star point ground, is that where all grounds come together ? The ground rail is very short, and they already come together from filtered input to output. I know not having a scope is like looking blind, I just don't see why a resistive halogen lamp load 100 Watts 12.00 Volts is fine, but the linear amplifier drops the PSU output to as low as 10.89 Volts DC. Yet the lamps drop just a few tens of millivolts. Thanks for your help.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 09, 2024, 11:57:52 pm
content: This is how I layout the board, using terminal blocks for the input / output, and regulator, base, collector and emitter connections. That board is for six transistors. The only other added component is a diode to raise the 12.00 volts DC by 0.7 volts, that works fine. The 7812 is no more than 4" from the ceramic capacitors. The input/ output electrolytic capacitors are 470uf. There is a full wave bridge rectifier and 30000uf in filter capacitors before it enters the circuit below. In that picture, I haven't soldered in the solid copper link up wire.

author: floobydust
author_rank: Super Contributor
timestamp: January 10, 2024, 04:59:20 am
content: If it's powering RF loads then the output and chassis ground need small disk caps like 0.02-0.1uF so the RF doesn't get back into the IC. The 10uF (electrolytic or tantalum) does not do much at many MHz. That looks like adding a ceramic disc cap across the output terminals, and another from ground to the chassis or earth ground. Or try different places and see if the problem changes.I know a star ground helps prevent the 7812 amplifying hum (on its gnd pin) and a regulation error from the wiring voltage drop causing what looks like poor regulation.That is usually the (-) of the 47,000uF filter cap. So the bridge rectifier(-), filter cap(-), 7812, output(-) wire connect together. You can use a copper bus wire too.Most important is the three high currents are at the same node - bridge rectifier(-), filter cap(-), output(-) wire together. You can run a separate gnd wire off to the 7812.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 10, 2024, 05:38:42 am
content: Quote from: davelectronic on January 09, 2024, 08:13:28 pmMaybe the peak current of the 100 watt linear HF MOSFET amplifier, (plus the RF?) is enough to disturb the 2 & 4 designs, but the 6 design has enough headroom ?

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2024, 07:33:25 am
content: The layout and low output capacitance are probably causing the whole circuit to oscillate.I would raise the output capacitance to 22 to 100 microfarads per amp, add a power diode in series with R7 to adjust the current sharing, and tighten the layout by moving the ballast resistors to the emitters.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 10, 2024, 12:50:14 pm
content: Thanks for all your replies. I will try all your suggestions, hopefully something will work. I have a few mothballed for quite some time now. The 6 x transistor variants seems to power the HF amplifier fine, I was just puzzled why the resistive lamp load is fine, but the power supply with 2 X and 4X pass transistors don't like the HF load. I'm certainly going to get a scope, but on a budget, so something for the hobbyist. There's a few of these power supply, all are at different stages of being built. I was also going to try the LM317 on some of these units.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 13, 2024, 11:48:44 pm
content: So I have added 0.1uf ceramic capacitors on all the areas recommended, terminating at the chassis for each area. And added a 4700uf 35 Volt capacitor to the final output, right behind the terminal output posts. Run out of time today to try it out, see if it's worked. The only thing I didn't do yet is add a power diode in series with the input to the regulator resistor that goes to the transistors base. Thanks for the help.

author: p.larner
author_rank: Frequent Contributor
timestamp: January 14, 2024, 08:51:08 am
content: so did it fix it?,i guess your making these to flog?.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 14, 2024, 12:41:45 pm
content: No, not selling these. I've got a bit of a compoltion to build power supplys. Those in the pictures are in various stages of building. Some have MOT's for transformers, others have toroidal transformers in them. The cases come as a single unit, some are bolted together for a 30 Amp PSU, some are single cases, there 10-15 Amps. All are 12.00 - 13.80 Volts. Some have meters some don't. I reuse things like CPU coolers for the semiconductors, and most are fan cooled. I get a buzz out of putting 240 Volts in, and seeing 12.00 Volts coming out the other. They just started as a blank case, all the metal work needed drilling and tapping to bolt up and fit to the case. I'm trying to get a PSU to perfect along the way. Hopefully more than one will get there. I will power up the offending PSU later today, to see if my efforts have fixed the unstable voltage dropping issue.

author: CaptDon
author_rank: Super Contributor
timestamp: January 14, 2024, 02:32:39 pm
content: I don't understand why your amplifier would appear inductive to the power supply? Also, if you are using this amplifier 'on air' for ham or c.b. you could be picking up r.f. even through the power cord and then it is radiating inside the power supply cabinet causing local disturbance. A metal CorCom filter firmly attached to the metal chassis of your power supply may be worth while. Are you amplifying continuous wave or is it sideband? Without a huge amount of output capacitors forming an energy storage bank a 100 watt S.S.B. signal will cause any marginal power supply to go nuts. You could be seeing current peaks of nearly 20 amps at 12 volts and depending on power supply circuit topologies the overall regulation loop speed will be far to slow for S.S.B. use. The S.M.P.S. supplies always seem to go nuts when powering R.F. equipment. Somehow the R.F. always seems to get into them somehow.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 14, 2024, 05:36:34 pm
content: Interesting, I thought the amplifier was a combination of resistive and inductive load. I understand what your saying, and with my limited novice knowledge, have added additional capacitance to the output. And single point tied the areas where it was suggested to use ceramic capacitors to the chassis case in these areas. I'm yet to try the PSU again. I have built two power supplys the same circuit, but used six power transistors. Both of those power supplys run the radio and amplifier just fine. But power supplys with two or four power transistors play up and drop voltage. It might just be that power requirements less than a full six transistors won't run it. Hopefully the measures I have implemented might work, I will post back if it is successful or unsuccessful. Thanks for your help.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 14, 2024, 07:38:59 pm
content: this schematic is very basic, and dont do well with charges variations, you dont have a good regulation feedback thru the fixed one ...

author: bdunham7
author_rank: Super Contributor
timestamp: January 14, 2024, 08:58:28 pm
content: Quote from: davelectronic on January 09, 2024, 08:13:28 pmWhat does the "100 Watt" rating on the linear amplifier mean? If that is the output power, won't the input power be quite a bit more? I'm sure the amplifier is not 100% efficient. Have you tried simply measuring its input current with a regular DMM?

author: floobydust
author_rank: Super Contributor
timestamp: January 14, 2024, 09:14:32 pm
content: Lots of 13.8V PSU info for HAM use, they took flawed Astron designs and fixed them up, QST articles etc. worth reading for OP:https://www.repeater-builder.com/astron/astron-index.htmlIt could be overloaded with the TX current or just too much RF, esp. if the transmitter is not grounded properly.

author: Zero999
author_rank: Super Contributor
timestamp: January 14, 2024, 09:25:55 pm
content: Fuses drop a small voltage, which doesn't help the regulation.Why are they on the output, rather than input? The 1A is arguably unnecessary because the LM7812 already has over-current protection, although it's not perfect, so it's reasonable idea to have one, as a last resort. The 30A is definitely required, as it's not certain the LM7812 will current limit, before the pass transistors blow.Have you measured the currents? At bare minimum the input and output, but the current through the LM7812 is also important.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 15, 2024, 12:09:30 am
content: I will take some current readings, in a day or so to see what's going on. I have to open the circuit to put a meter in series with the regulator. The linear amplifier draws around 8 Amps FM on SSB it's nearly 15 Amps, the amplifier has a modest 4 watt drive.

author: bdunham7
author_rank: Super Contributor
timestamp: January 15, 2024, 12:33:27 am
content: Quote from: Zero999 on January 14, 2024, 09:25:55 pmThat's what I was thinking--all of the base current from the bypass transistors has to go through the regulator and those transistors have minimum gains that can be as low as 5 at high currents. So given that the input voltage will be nearly 20V, even 250mA regulator current requires the LM7812 to dissipate 2 watts, which would be its limit if not heat sinked.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 15, 2024, 01:56:40 am
content: Quote from: davelectronic on January 15, 2024, 12:09:30 amDoes the supply voltage drop on FM as well as SSB?

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 15, 2024, 01:13:59 pm
content: It drops the same voltage on FM and SSB, there might be a few millivolts between the two modes under load, but it's not much. I read your thoughts on the transistor gain under full load (hfe) might not be enough for two transistors at 8 Amps or more (ssb) I have changed the base resistor in the chain, to 33 ohms at 3 watts I had laying around. Just I thought if the regulator had a bit more load, it might be less liable to ocillate. I don't know yet if that's true or not. I have powered the supply up to a 100 watt lamp load, that's drawing just over 8 Amps and the voltage drop is from 12.74 Volts to 12.35 Volts, so about 390 millivolts, which seems quite a bit of a drop on the resistive lamp load. I will try it out on the radio and amplifier later today to see what happens. I have a sneaky suspicion it's going to play up again. But I will find out in a bit. The regulator is on a substantial heatsink of its own, I use a thermal couple on a multimeter to measure temperatures of the components under load and over time. The 7812 regulator hasn't gone past 35°C at any point. But I will measure it's current drain when the full load is attached. Thank you for your help.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 15, 2024, 03:53:24 pm
content: Well, if it drops appreciably more voltage using FM mode @ 8A vs just a resistive load @ 8A, then it looks like there's RF getting into the power supply and "confusing" the regulator.The amplifier in FM mode should be a steady 8A load just like a resistive one, unlike SSB where the current varies with the modulation level.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 15, 2024, 05:17:10 pm
content: normally a fuse is the last thing to blow lollwith smps at the frequency they are running, the mosfets power stage may or will entirely blowlinear some "hard" short are needed for the fuse to blow, and in some cases many companies are putting slow blow fuses, witch is far from ideal ...

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 15, 2024, 06:29:14 pm
content: I'm driving the amplifier with 4 watts from the radio, that's well with in the safe operating use, it could handle a 10 watt drive. But I don't see the point in running it flat out. The picture below is the radio amplifier and power supply (with 7812 voltage regulator and 6 x TIP36C BJT 's) it runs it just fine. The problem seems to be two or four TIP36C BJT transistors with the 7812 voltage regulator doesn't like it. I think is RF getting in the circuit somehow. Stray RF does funny things to some other electrical things, wipe out my dab radio when it's on, my Son can hear it through he's PS5 when I key up. I pick a time of day to talk when I'm unlikely to annoy anyone, usually late. I was unsure, with two TIP36C if it can comfortably supply the the extra current above the 7812's current. As you say, If it can power 100 watt lamp load, it should be capable of running a HF linear amplifier 100 watt load. I have a scope now, to see what's going on with the waveform if it starts playing up again. Hopefully I can get some time to try it today. After I've implemented plenty of ground coupling to the chassis from all the areas that where suggested to me.

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2024, 07:05:47 pm
content: For fun, I plugged the circuit into LTSpice. I don't have an LM7812 model handy, so used the RH117K (Liner Technology's version of the LM317) set to 12V, which should be close enough.The base current is significant.I highlighted on the plot the intersect when the input current to the regulator is 1A, the rating of the LM7812, which is when the output current is just under 13A. The voltage regulation is very good, until just under 22A, when it falls off a cliff, as U1's current limiting kicks in. If the LM7812 current limits or goes into thermal protection at a lower current, the voltage regulation will fail.Another thing to note is C2 might cause a transient voltage spike above 12V on the output, when the power is first applied. I didn't perform any start-up transient analysis, because it will depend on the impedance of the transformer, wiring and rectifier, which I don't have a model for.

author: bdunham7
author_rank: Super Contributor
timestamp: January 15, 2024, 07:10:33 pm
content: What happens if you eliminate 4 of the MJE2955s? What hFE model does LTSpice use, if you know?

author: coromonadalix
author_rank: Super Contributor
timestamp: January 15, 2024, 07:29:56 pm
content: i find c2 slightly too big, you almost do a constant charge with r7for sure you'll briefly get some instabilities on the outputto reduce the current, you would need darlingtons transistors instead

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 15, 2024, 09:58:49 pm
content: This is interesting, below I will put the link to where I first found this circuit. Every other voltage regulator with pass transistors is similar for PNP transistors, but not exactly the same. I have built a power supply using a 7812 with two Darlington transistors MJ11015 that manages 12 Amps with very little voltage drop, but I never used it for an amplifier or radio. I felt the voltage drop at full load was less than the TIP2955's or TIP36C's. But I tried another circuit the same layout, with TIP147 darlington transistors, that ended up 2.0 Volts above the regulator rated output. I couldn't swear I could have made a mistake, I went over it several times, but couldn't find what was sending the voltage high. I have never gone above 20 Amps with this circuit, the highest I went with the 7812 and six TIP36C's was 19.89 Amps and wasn't dropping much more than 200 millivolts or slightly less. I didn't have any more lamps and lamp fittings, I don't have an electronic load. I measured with a clamp meter that does AC and DC Amps. The transformers where two MOT's in series with each other. The link below is where it's from. Credits to Andy Collinson.https://www.cxi1.co.uk/Circuits/Power/1230psu.htm[/urPS with reference to R7 it's originally 100 ohms, I lowered it to 33 ohms as I wanted the regulator to do more than a few milliamps in use. Hoping it would deter ocillating.

author: Zero999
author_rank: Super Contributor
timestamp: January 15, 2024, 10:26:19 pm
content: Quote from: bdunham7 on January 15, 2024, 07:10:33 pmI would suspect the hFE will drop at higher currents, so U1 aught to passlessmorecurrent*. Download LTSpice, if you don't have it, the .asc file and try yourself.EDIT*I had a brain fart. A higher base current will mean the LM7812 has to work harder, as more current will flow into its input, from the bases of the transistors!EDITQuoteThe MJE2955 model extracted from/usr/home/LTspiceXVII/lib/cmp/standard.bjtCode:.MODEL MJE2955 PNP(IS=2.37E-8 ISE=1E-9 ISC=0 XTI=3 BF=73 BR=2.66 IKF=0.8 IKF=0.4 XTB=1.5 VAF=90 VAR=30 VJE=0.65 VJC=0.65 RE=0.00856 RC=0.0856 RB=0.81 CJE=415E-12 CJC=1000E-12 XCJC=0.75 FC=0.75 NF=1 NR=1 NE=1.26 NC=1.5 MJE=0.5 MJC=0.33 TF=140E-9 TR=0.75E-6 EG=1.11 KF=1E-9 AF=1 VCEO=60 ICRATING=10 MFG=STMICRO)Refer to the following websites for an explanation of the above parameters.https://ltwiki.org/LTspiceHelp/LTspiceHelp/Q_Bipolar_transistor.htmhttp://www.ece.mcgill.ca/~grober4/SPICE/SPICE_Decks/1st_Edition_LTSPICE/chapter4/Chapter%204%20BJTs%20web%20version.html

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 15, 2024, 10:59:12 pm
content: Somehow my post script got mixed up with the link I posted. Below is the link, LT spice simulation is used by Andy Collinson in the article.https://www.cxi1.co.uk/Circuits/Power/1230psu.htm

author: bdunham7
author_rank: Super Contributor
timestamp: January 16, 2024, 05:52:57 am
content: Quote from: Zero999 on January 15, 2024, 10:26:19 pmLT Spice took alooooongtime to update and I had to add that transistor model manually, but here it is. I see the regulator starting to drop out at 16A, but this is just an example and I doubt his regulator will work as well at those currents. And of course he doesn't have a stiff 16VDC input, he has something else.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 16, 2024, 01:12:08 pm
content: I'm a newbie, and have never used LTSpice. I don't doubt your findings for a minute. The two power supply I built with 7812 and 6 x TIP36C's where powered from MOT's, so I had a very stiff voltage input to start of with, around 22.00 Volts after rectified and filtered. That dropped to 19.00 Volts rectified and filtered, under a 19.89 Amp halogen lamp load. The final output drop wasn't more than 200 millivolts. The power supply where 2 X 80mm fan cooled on a thermal switch, so monitoring temperatures, I just let it run. Nothing got above 85°C with forced air cooling. I'm not likely to run anything up to 20 Amps any time soon. The most I would be drawing in current is 15 Amps at 12.00 volts. And only 8 Amps 12.00 volts on FM. But it would be good to get a PSU that can supply a maximum of 10 Amps at 12.00 volts, with 2 X TIP36C'S or TIP2955's BJT transistors. If it's not to be, I might try the TIP147's again, as darlington transistors they should hold up the voltage better with the higher HFE.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 16, 2024, 07:39:24 pm
content: Just connected the power supply with the 7812 and 2 X TIP36C's. I was sceptical, but it works. I implemented the chassis grounds through 0.1uf ceramic disk capacitor. Changed R7 from 100 ohms to 33 ohms, and added an electrolytic 4700uf 35 Volt capacitor just behind the binding posts. The only thing I didn't do is add a power diode in series with R7, the purpose of which I don't know what that would have done. It's voltage unloaded is 12.73 Volts DC, and under load, it's 12.40 Volts DC. The current drawn by the radio and amplifier is just over 7.4 Amps DC. There is a but to all of this...Out of the suggestions made to me that I implemented, and two things I changed or added. I don't know what made it now relatively stable. I could remove each modification one at a time, until it plays up again. But that would be a real headache to do. I think it's the chassis grounding through 0.1 UF ceramic capacitors, and the extra capacitance directly at the output. I don't know if swapping out R7 100 ohms resistor makes any difference. I know it will increase the base current to the transistors, and the regulator will pass more current. But one or more of your suggestions has worked. Thank you for the help, I couldn't have sorted this out, I don't think, with out all your help. Many thanks again.

author: p.larner
author_rank: Frequent Contributor
timestamp: January 18, 2024, 03:03:45 pm
content: i would look at the amps output on a spec analyser,bet its sprogging to daylight!.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 18, 2024, 05:47:46 pm
content: Not sure what that means, I don't have a spectrum analyser. Take it means what the RF output looks like ? Being it's a cheap HF linear amplifier, noisy I would think.

author: floobydust
author_rank: Super Contributor
timestamp: January 18, 2024, 08:35:07 pm
content: Is it an AM or FM amp? The current draw will move around a lot with modulation if AM. Yell into the mic lol.Sprogseems to mean many things. Not sure how it applies to what's going on...Verbsprog (third-person singular simple present sprogs, present participle sprogging, simple past and past participle sprogged)(UK, Australia, slang, humorous) To produce children.(Australia, slang) To ejaculate, to come.Noun1. a child; baby2. (Military) (esp in RAF) a recruit

author: p.larner
author_rank: Frequent Contributor
timestamp: February 03, 2024, 11:04:32 am
content: fwiw i use sprog as in refering to harmonic output out of band.