Author Topic: Another kit: All-discrete power supply  (Read 48639 times)

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Offline kxenos

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Re: Another kit: All-discrete power supply
« Reply #75 on: January 05, 2014, 04:56:52 pm »
Excellent project Chris! And your schematic is very professional! Way to go!  :-+  :-+
 

Offline megajocke

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Re: Another kit: All-discrete power supply
« Reply #76 on: January 05, 2014, 07:22:43 pm »
I was worried that if the output is connected to a voltage source while power is off (referring to your latest schematic) C2 will stay uncharged because of the diode string D1,D5,D6, and the voltage reference which is powered from the output will together with the voltage sense resistors provide current which can flow through the BC-diodes of the voltage error amplifier input devices, breaking down the BE-diode of Q7 (and maybe Q16A-B too) in reverse. The current is limited though so this would amount to degradation of the transistors at most.

If the output is connected to a voltage source with a higher voltage than the setpoint with AC power on, if I'm not mistaken Q7 will try to saturate and pull quite high current (looks like you have enough base drive to get a few hundreds of milliamps) in reverse through the BE diodes of Q8 and Q9. Something would probably let out the magic smoke almost instantaneously.

It also looks like the current source for the current error amplifier, Q11, will start to saturate if the output voltage starts to reverse. Collector-base voltage is about 0 V when the output voltage is 0 V, right? But maybe this is not a problem? As long as you keep the externally forced current below the rating of the supply the current throuh the output transistor can't exceed the design value anyway.

Were you simulating this in LTspice? Something to look out for is that the standard transistor models don't model reverse breakdown of the BE-diode. :)
 

Offline c4757pTopic starter

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Re: Another kit: All-discrete power supply
« Reply #77 on: January 05, 2014, 07:37:49 pm »
New schematic.

Added the thermal cutoff. The thermistor is a Vishay NTCLE100E3104JB0 - 13.4k at 75°C. That puts 0.64V on Q17, switching on the two-transistor "thyristor". You must power-cycle to reset (I think that's a good thing. Not only does it keep the transistor from getting hot, but it keeps you from leaving a heavy load attached, walking away, and letting it slowly cook itself as it shuts off and comes back on repeatedly.)

The current flow is a bit unconventional - I noticed that if something were to go wrong in the current control feedback path, the voltage error amp could saturate high and actually risk damaging a few parts. I added resistors and clamp diodes (R9 and D4, which have already been there, and R33 and D10) to prevent this problem, and then took advantage of them in the thermal cutoff to save parts, by intentionally saturating the amplifier. When the Q17/Q18 thyristor switches on, the feedback path is slammed to VEE, sending the output high (this is the negative output, so "high" means "no output" here). Q5A saturates, drawing base current through Q7, through D4 and over to the thyristor - but not before it goes through a warning annunciator LED, DS1. Because this also saturates Q7, the output transistor's B-E junction would be reverse biased. I added R33 to limit current and D10 to clamp the voltage.

I also changed the output transistor to a TIP105 Darlington, which has a protection diode built-in, eliminates Q8 from the PCB (moving some of the power dissipation off the PCB and onto the heatsink), and is a bit cheaper too.

I was worried that if the output is connected to a voltage source while power is off (referring to your latest schematic) C2 will stay uncharged because of the diode string D1,D5,D6, and the voltage reference which is powered from the output will together with the voltage sense resistors provide current which can flow through the BC-diodes of the voltage error amplifier input devices, breaking down the BE-diode of Q7 (and maybe Q16A-B too) in reverse. The current is limited though so this would amount to degradation of the transistors at most.

Solved, I believe, by adding a bypass diode (D11) antiparallel to the diode string.

Quote
If the output is connected to a voltage source with a higher voltage than the setpoint with AC power on, if I'm not mistaken Q7 will try to saturate and pull quite high current (looks like you have enough base drive to get a few hundreds of milliamps) in reverse through the BE diodes of Q8 and Q9. Something would probably let out the magic smoke almost instantaneously.

Yup, found this problem already (as mentioned above). Q7 no longer commits murder-suicide when it saturates.

Speaking of high voltages, an interesting problem that does exist in SPICE but not in real life is that a forced voltage higher than the voltage across C2 will blow up D11 in SPICE, but not in real life, because in real life that voltage comes through a rectifier.

Quote
It also looks like the current source for the current error amplifier, Q11, will start to saturate if the output voltage starts to reverse. Collector-base voltage is about 0 V when the output voltage is 0 V, right? But maybe this is not a problem? As long as you keep the externally forced current below the rating of the supply the current throuh the output transistor can't exceed the design value anyway.

Correct.

Quote
Were you simulating this in LTspice? Something to look out for is that the standard transistor models don't model reverse breakdown of the BE-diode. :)

Yes, I finally gave in to the temptation of SPICE, and it bit me. I've been checking manually for B-E reverse bias but missed that one. :-[ Time to go back to pen and paper.
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Offline dr.diesel

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Re: Another kit: All-discrete power supply
« Reply #78 on: January 05, 2014, 07:52:57 pm »
How do you plan on selling your kits Chris?  Own web, tindie, eBay, crowd, etc?

I personally feel you could make decent money with the following, kits might be your calling!

 - PS
 - Electronic load
 - FG
 - Maybe a .01% DC voltage reference
 
A single FET programmable uC based electronic load could be a big hit IMO.  Dual pushable rotary encoder, on/off button and a 16x2 display with battery test/AH mode. 

Mmmmmmmmmmmmm   :-//

Offline c4757pTopic starter

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Re: Another kit: All-discrete power supply
« Reply #79 on: January 05, 2014, 08:06:58 pm »
TBH I don't know. I'm doing this actively on the forum to gauge interest and polish them a bit. And because I can't afford to sell more than a few in the beginning - hopefully there are enough people who know me well enough here to at least know that I'm not a complete idiot and probably not a scammer ;D and can deal with my bullshit a bit as I try to handle making and selling the first few while barely being able to afford it......

I'd probably blow all the money I'd make on a voltage reference on a *calibrated* (with cert) voltmeter for testing..... I have decent confidence in my 3468A because its cal only expired a couple years ago, it's been treated well and matches everything I've tested it against, but I wouldn't want to do references without at least attempting traceable verification of their accuracy. And there are plenty of them out there. And it would make an awful kit, as most kit builders wouldn't have the equipment to verify its accuracy.

One thing I've considered making for myself: a computer-controlled device tester - basically a "curve tracer" without the logic. Just a couple low-power voltage/current sources and ADC channels that can be programmed to do pretty much whatever you want. Think that could be successful? It would be somewhat simple to do, and people love scriptable shit. ;D Just have to say the word "Python"...

Thoughts? :-// How should I proceed once the kit is "complete" and I've sold a few privately here?

And by the way - caught another problem (I'm not an idiot, I swear, I just tend to post "finished" schematics too early ;)) - R15 and R33 do not provide enough base current at high output currents for particularly pathetic specimens of TIP105. Unfortunately, simply lowering their value does rather increase the power dissipation in the "forced output voltage" case, where D10 conducts, so I may need to use 1/2W resistors there. That condition can put up to 50V across each of them, as the output falls to -50V and Q7's collector sits at 0 (48.somethingV is the maximum guaranteed clamping potential of the TVS). That, or I'll just add another 3906 and make it a "triple Darlington".
« Last Edit: January 05, 2014, 08:24:20 pm by c4757p »
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Offline SeanB

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Re: Another kit: All-discrete power supply
« Reply #80 on: January 05, 2014, 08:24:27 pm »
Just make it a selection for high gain on both darlington transistor devices. Solves that issue.

I had some fun trying to explain to people that Hfe is a variable value with current, I was selecting some devices not caring about the power device ( it had a gain of under 3 at the current it was handling at best) but was selecting the driver devices based on saturation voltage instead using a small test jig which ran them at close to the in circuit operating conditions. A lot failed to meet the specs I desired, but then again I did have a big box of 2N2219A's to go through so enough were in the spec. They had a failure mode where they unsoldered themselves from the leads. Not helped by the heatsink being poor and running next to a 150W lamp that they were driving.
 

Offline blackdog

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Re: Another kit: All-discrete power supply
« Reply #81 on: January 05, 2014, 08:26:27 pm »
Hi  c4757p

Just some 2 cent remarks...
Why don't you use a reference diode like the 1n821 series?
C3 wil not do much, because the internal resistens  of the zener is less than 20 Ohms.
Mayby its better to make C4 bigger to get lower noise?

What is the what is the purpose of d10, R33?
The problem is not the low gain of the TIP105, but your design (for better gain and dissipation use a TIP147).
Remember the risistence between base and emittor of such a transitor is normaly below 8K.
I Think your design wil be verry onstable, because of the high resistance values in your design, this creates a lot of LAG...

Thermal cutoff, use a Clickson to cutoff the basedrive of Q9.

My advice, do not start with the circuitboard for the circuit have been extensively tested on dynamic stability.

Kind regarts,
Blackdog
« Last Edit: January 05, 2014, 08:30:35 pm by blackdog »
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Offline megajocke

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Re: Another kit: All-discrete power supply
« Reply #82 on: January 05, 2014, 08:34:52 pm »
How about using a diode instead of R33? Then you wouldn't need D10 either.

Putting in D11 should work, but an alternative might be to put a small signal diode in series with the positive supply to the voltage error amplifier, similar to the way the current error amplifier has one. (edit: But this is not compatible with the circuit where the reference for the current source is separate from the diode string)

If you want the supply to be able to stand a sudden application of a voltage source to the output when AC power is off, there is a risk the current error amplifier will be damaged by the current pulse charging C1 (and C2 if using D11) flowing through R21. Initially the whole (forced) output voltage will drop over R21 and especially if RV4 is set to zero the input to the differential pair will overload and cause reverse EB-diode breakdown in Q12A.

I guess it could be fixed by adding some resistance in series with the base of Q12B together with a small signal diode.
« Last Edit: January 05, 2014, 08:56:36 pm by megajocke »
 

Offline c4757pTopic starter

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Re: Another kit: All-discrete power supply
« Reply #83 on: January 05, 2014, 08:48:22 pm »
Just make it a selection for high gain on both darlington transistor devices. Solves that issue.

I could, but I'd rather not have to bin parts (and be stuck with a pile of shitty ones!). Maybe I'll search a bit more for one with higher minimum gain. Such as

for better gain and dissipation use a TIP147.

Edit: Wait.... that doesn't have higher gain! 1000 vs 2000...

Quote
I had some fun trying to explain to people that Hfe is a variable value with current,

Engineering rule of thumb: X depends on Y, for X and Y in {all properties of everything}... :)

blackdog: I'm not using a 1N821 or similar because they're expensive. This is sufficient. You're correct about C3 not doing much, but it does do one thing: ramps the reference at startup, which helps keep the output shut down while the system stabilizes. C4 is just to shunt off RFI picked up in the wires to RV2.

D10, R33 prevent Q9 from being reverse biased if Q7 saturates.

I Think your design wil be verry onstable, because of the high resistance values in your design, this creates a lot of LAG...

My advice, do not start with the circuitboard for the circuit have been extensively tested on dynamic stability.

You are correct, and after a bit more testing I added 10nF across C10. The lag caused by the feedback resistors hasn't been a problem (at 18k Thevenin, the impedance of the feedback path is not extremely high, and the amplifier is not very quick). I will admit that there are theoretically better ways to design the feedback loop, but I have thoroughly tested the circuit and have not been able to destabilize it at all. I'll test the PCB more thoroughly when I get it, before I send off revision B.

How about using a diode instead of R33? Then you wouldn't need D10 either.

Putting in D11 should work, but an alternative might be to put a small signal diode in series with the positive supply to the voltage error amplifier, similar to the way the current error amplifier has one.

If you want the supply to be able to stand a sudden application of a voltage source to the output when AC power is off, there is a risk the current error amplifier will be damaged by the current pulse charging C1 (and C2 if using D11) flowing through R21. Initially the whole (forced) output voltage will drop over R21 and especially if RV4 is set to zero the input to the differential pair will overload and cause reverse EB-diode breakdown in Q12A.

I guess it could be fixed by adding some resistance in series with the base of Q12B together with a small signal diode.

Diode instead of R33 sounds good - I will do that. And you're right about D11 - as the current amp already has one, I'd rather be consistent. At a glance, it looks like I can save a diode and just use D5 for that.

I'll add protection to Q12 as well.
« Last Edit: January 05, 2014, 08:50:09 pm by c4757p »
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Offline megajocke

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Re: Another kit: All-discrete power supply
« Reply #84 on: January 05, 2014, 08:57:54 pm »
Another thing I was thinking of was R(Q9B). I was thinking that it probably doesn't do much difference whether you put it close to the transistor or at the far end of the wire. Is placing it close to the transistor base based on having actual problems when it was far away, theory, simulation or just hearsay? I see it as a common suggestion but I can't find any theoretical backing unless the wires are insanely long (or the transistor is insanely fast). As long as it swamps the inductance of the base wiring up to the fT of the transistor or so it should be okay.
 

Offline c4757pTopic starter

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Re: Another kit: All-discrete power supply
« Reply #85 on: January 05, 2014, 09:02:15 pm »
It serves two purposes: dampening parasitic oscillation of the transistor, and low-pass filtering (with the base capacitance) any RFI picked up in the wire. The latter requires it to be after that RFI pickup. Now that the transistor is an emitter follower, it's probably totally unnecessary. :) I wanted it there when the transistors collector was at the output because of the large voltage gain. Perhaps I will remove it.
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Offline SeanB

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Re: Another kit: All-discrete power supply
« Reply #86 on: January 05, 2014, 09:20:20 pm »
Losing parts is what eBay was invented for. Sell them as selected for gain, all falling within a band. Base/gate damping resistors are better close to the device, especially for Mosfets.
 

Offline megajocke

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Re: Another kit: All-discrete power supply
« Reply #87 on: January 05, 2014, 09:25:35 pm »
Parasitic oscillation is quite likely in an emitter follower so I'd probably want to keep the option. In this case maybe the ESR of the electrolytic capacitors is enough to damp it, but I would not count on it...

The "problem" with emitter followers is that if the load is capacitive in the region where the current gain of the transistor decreases with frequency (from fT/hfe to fT) the input impedance looking into the base ends up looking like a negative resistance in this region. :)
 

Offline dr.diesel

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Re: Another kit: All-discrete power supply
« Reply #88 on: January 05, 2014, 09:27:34 pm »
Thoughts? :-// How should I proceed once the kit is "complete" and I've sold a few privately here?

This is actually something I've been meaning to hit Dave with, just haven't gathered my thoughts.  Supplying kits out of your dorm room would be a PITA, sorting bags of parts, enclosures etc, not to mention the time required.  You'd have to begin just providing the documentation and selling the PCBs, with BOMs from Digikey or something, all support via a forum.  Start with something less complicated than the FG, either the PS or the eLoad, something that can build capital, even if only a little.

Even though I've been doing this for 20 years, I still buy anything I can as a kit.  I buy tons of kits, I give them away as X-Mas present to all kinds of people.  I'd personally buy several of any kit you're willing to sell.

Find something to start with, run it through here like you've been doing, maybe a group buy?  Good web docs, YT video, get Dave to review and link to it?

I Donno, just thinking out loud.

Offline c4757pTopic starter

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Re: Another kit: All-discrete power supply
« Reply #89 on: January 05, 2014, 09:29:14 pm »
Parasitic oscillation is quite likely in an emitter follower so I'd probably want to keep the option. In this case maybe the ESR of the electrolytic capacitors is enough to damp it, but I would not count on it...

The current sense resistor will help as well. Though I will keep it. As I said, it is harmless, and at least might be useful.
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Offline liquibyte

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Re: Another kit: All-discrete power supply
« Reply #90 on: January 05, 2014, 09:36:22 pm »
Thoughts? :-// How should I proceed once the kit is "complete" and I've sold a few privately here?

This is actually something I've been meaning to hit Dave with, just haven't gathered my thoughts.  Supplying kits out of your dorm room would be a PITA, sorting bags of parts, enclosures etc, not to mention the time required.  You'd have to begin just providing the documentation and selling the PCBs, with BOMs from Digikey or something, all support via a forum.  Start with something less complicated than the FG, either the PS or the eLoad, something that can build capital, even if only a little.

Even though I've been doing this for 20 years, I still buy anything I can as a kit.  I buy tons of kits, I give them away as X-Mas present to all kinds of people.  I'd personally buy several of any kit you're willing to sell.

Find something to start with, run it through here like you've been doing, maybe a group buy?  Good web docs, YT video, get Dave to review and link to it?

I Donno, just thinking out loud.
+1  :-+ this.  I'd buy a board with a BOM for the parts and put stuff together that I have and get what I don't.
 

Offline c4757pTopic starter

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Re: Another kit: All-discrete power supply
« Reply #91 on: January 05, 2014, 09:51:28 pm »
That's what I originally wanted to do but I wasn't sure if anyone would buy it, if they had to go off to Mouser or DigiKey or wherever and get parts.  So, am I wrong? Could it be successful in the beginning as a BOM+PCB+manual kit?
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Offline liquibyte

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Re: Another kit: All-discrete power supply
« Reply #92 on: January 05, 2014, 10:02:55 pm »
Put it together and let me know how much and I'll get one.  I have an 18V toroid I'd like to use.  Do you still plan on the 20V kit?
 

Offline c4757pTopic starter

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Re: Another kit: All-discrete power supply
« Reply #93 on: January 05, 2014, 10:03:52 pm »
Yep, it's just a change of a couple resistor values and DZ1.

Once I have it fully tested, I am going to start by ordering a first batch of 20 PCBs. I ordered ten panel meters to modify with a custom scale, and I'll include those with the first ten. I will try to keep including them, as I don't want modifying things to be part of the build (well, I was planning on having a winding added to the transformer!), but I don't imagine shipping will be very fast, so if you buy a kit after the first ten and I haven't received another shipment of meters, you're going to have to source and mod your own. I will also include a printout of the manual (including the schematic, which is oversized - U.S. ledger/tabloid, 280x430mm - and rather hard to read if you shrink it and then print).
« Last Edit: January 05, 2014, 10:13:38 pm by c4757p »
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Offline blackdog

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Re: Another kit: All-discrete power supply
« Reply #94 on: January 05, 2014, 10:08:28 pm »
Hi c4757p,


Can you explane how your Voltage Reference is working...
Q4a base and collector are shorted togetter?
Schematic error or is Blackdog is a ... :-DD

OK, the 100 a 150 Ohm base resistor is always necesary, but you already now that :-)

Lets look at R23 and what is connected on the left site C5 33pf and several transistors, at least 50pf, this wil give about 25Khz -3dB.
Is you power supply fast enough for transient loads?

I dit a lot of testing on my design of a LAB powersupply, take a look if you want on this website, use google translate becaus its in Dutch, sorry...
http://www.circuitsonline.net/forum/view/110029/8

Kind regarts,
Blackdog

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Offline dr.diesel

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Re: Another kit: All-discrete power supply
« Reply #95 on: January 05, 2014, 10:12:36 pm »
Could it be successful in the beginning as a BOM+PCB+manual kit?

If you make it easy, I'd say yes.  When doing the BOM make sure and include vendor part numbers for each part.

Offline c4757pTopic starter

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Re: Another kit: All-discrete power supply
« Reply #96 on: January 05, 2014, 10:29:32 pm »
Q4A base and collector are shorted in net name only. The base runs directly to the top of the "compound diode", buffering the voltage exactly at that point (the reference and feedback are treated as a differential pair at all times). The collector is indeed at the same voltage, but an emitter follower will work just fine with the base and collector at the same potential. You just can't forward-bias the B-C diode.

Your -3db point is off - keep in mind that point sees a Thevenin impedance of 18k, not 120k. It should be closer to 170kHz. It's not extremely fast - in SPICE the transient response time is about 200us for a rising load and 1ms for a falling load; I'm not going to bother testing my prototype for that because that's going to be layout-dependent. I don't think anybody is going to be too upset that it's not fast, though. In SPICE only, again (real data will come when I have one properly assembled), the output voltage spikes by no more than 30mV with a 500us rise time on the load and 0.5V with a 1us rise time. I'd expect test lead inductance to contribute worse than that...

I'll read your website when my eyes aren't tired; Google Translate butchers it!

Could it be successful in the beginning as a BOM+PCB+manual kit?

If you make it easy, I'd say yes.  When doing the BOM make sure and include vendor part numbers for each part.

I'll make them directly compatible with Mouser and DigiKey's BOM import tool.
« Last Edit: January 05, 2014, 10:32:38 pm by c4757p »
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Offline megajocke

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Re: Another kit: All-discrete power supply
« Reply #97 on: January 06, 2014, 12:03:13 am »
But the error amplifier is an integrator -- the -3 dB point will hopefully be at a very low frequency.

With the FB- node as input and the collector of Q7 as the output, the error amplifier behaves as an inverting integrator with a gain-bandwidth product of 40 kHz (the impedance of R23 and C5 have the same magnitude at this frequency).

The gain from the FB+ input will be the same integrator (but noninverting) at low frequency up to 40 kHz, above which the gain goes to 1 until some high frequency where the error amplifier itself starts to run out of gain.

So you would end up with a loop crossover frequency of about 40 kHz if it weren't for the output capacitor (and its ESR) which forms a divider together with the current sensing resistor and output impedance of the output amplifier.

It can be insightful to look at the output impedance. It should end up looking like about 5 µH in parallell with a 1 ohm resistor and the output capacitor. There would also be some resistance in series with the fictional inductor because of the finite low frequency gain of the error amplifier.

The attached simulation shows what would happen in voltage mode with the error amplifier approximated as close to ideal. The plot shows output impedance and the impedance of the equivalent to the right in ohms. The output impedance looks well-damped enough. :)

Modifying the compensation capacitor value changes the equivalent output inductance in proportion. (edit: oops, did a brainfart there... had written inverse proportion by mistake)
« Last Edit: January 06, 2014, 12:13:39 am by megajocke »
 

Offline c4757pTopic starter

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Re: Another kit: All-discrete power supply
« Reply #98 on: January 06, 2014, 01:22:38 am »
Yeah, you are right. :)

I like that way of analyzing the circuit. I've seen plenty of more "proper" ways to test the response, but somehow I missed that. Thank you! I'll play around with it a bit.

In the real circuit, 33pF wasn't bad, but did have a bit of peaking. 470pF + (33pF || 4.7Meg) smoothed it out completely. I don't think I'll have to do that, but I will keep it in mind.
« Last Edit: January 06, 2014, 02:15:03 am by c4757p »
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Offline flano

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Re: Another kit: All-discrete power supply
« Reply #99 on: January 06, 2014, 10:27:56 am »
Chris,

Any thoughts of putting a current limit LED?

Love your work on both the Function Gen and this project.

PCB, BOM and Doco would be fine.

Thanks Mike
 


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