Basic power supply design question

I always wanted to try turning a 68W amp module into a variable power supply. Anybody done that?

Sound good? D from BC

Circuits,

It looks like a regulated output with the 2N3055 as a pass element controlled by an op amp fed back from the ouput of the supply - a typical linear regulator.

Power supplies need a reference voltage to compare the output to and tell if it is in regulation or not. The LM317 reference is a set 1.25 volts developed between the output and the ADJ pin. Once you drop below the voltage reference it can't track it to regulate the ouput and the supply goes "open loop".

All you really need is is an OP amp, a bipolar to level shift the amp so it can control the pass element sitting up at 30V, and the pass element itself. This setup gives you control over the voltage reference itself and you can bring it right down to 0V.

The supply you mentioned above works this way with the added feature of a current control amplifier. It looks like everything you need.

Traver

"D from BC"

Two devices could be made to produce dual tracking, one following and one inverting a reference DC input.

Read the specs properly - that figure is the *minimum* short cct current for * 10 mS * with a +/- 20 volt supply.

The current into a load can be dangerously higher - until the chip overheats.

Come up with adjustable output current limiting and you may have a usable idea.

Stability with capacitive and awkward loads like DC motors cannot be certain.

....... Phil

Oops..Yeah..Those Ispecs can with some fine print.. :)

I'll be easy for me to add a current limiting circuit to the amp module.. Also.. I have some notes around here concerning feedback networks and reactive loads..It might help with stability. I've been tempted to do this project.. Mostly because it's a "crazyass" use of an audio amp module.. But I'm more into 100khz switchmode. D from BC

"D from BC"

( Old Biblical quote ...... )

I have long dreamt of using a "spare" Crown DC300A as a classy dual tracking PSU !!

...... Phil

Ok...I just have many one difficulty when it comes to current sensing.. Let's say I use a 100mohm sense resistor to monitor the PS current. I'm not sure if it matters if I do high side current measurement or I do low side current measurement..?? I have design experience with both but I haven't realized the difference in benefits from a performance point of view.. D from BC

"Line regulation" isn't a component, or separately adjustable input or anything - it's more like a phenomenon - it's just a measure of the regulation relative to the input, where "load regulation" is a measure of the regulation relative to the output. i.e., they're not independently controllable, unless by circuit architecture or some such.

I have no opinion on the LM317, having never used one - but I think that they return the adj input to -1.2 relative to the output "ground", so the circuit itself doesn't really know the difference - it thinks it's putting out 1.2V, but the user sees 0V.

Cheers! Rich

The difference is involved with not only what the current limit is measuring, but with what it is trying to ignore (the output voltage, in this case). If the pass device control circuit is referenced to the negative rail, but the current sensing is taking place on the positive rail, then the sensing circuit has a lot more to do in ignoring the large possible voltage swing than it has to do measuring the small current sense voltage drop, accurately. It is a signal to noise problem, and a level shifting problem.

By the way, it may not be obvious, but it is not necessary for the pass element and / or its control circuit to be referenced to the negative rail. Many lab supplies are designed to operate completely floating, so the negative rail has no particular ground or common concept associated with it.

On Tue, 05 Jun 2007 16:58:01 +0000, D from BC wrote: ...

To me, the only time high side sensing makes sense (pun unintended, but noted) is if the load absolutely, positively has to have its negative side grounded to system ground.

The other arguments are things like level shifting, CMRR, and all that stuff - well, you know the drill. :-)

Hope This Helps! Rich

Yup... Low side sensing can be as simple as a transistor BE across a sense resistor.. Whereas..high side sensing ..ugh :P My ugliest high side solution was a battery powered op amp that drove an optocoupler.

I'm wondering if low side current sensing is typically done in high end lab supplies.. D from BC

(snip)

The sensing (on lab supplies) is generally done on whichever side of the supply is the signal common for the error amplifier. Integrated designs more often use the negative rail, but discrete designs use either, without prejudice. This makes some of their schematics look strange for people who are used to always seeing the negative rail as common.

John Popelish

Specifically, the reference is the 4.7V Zener diode. Pin 2 is driven to 0V, so the output voltage is Vo= 0V ~ 25V. The op-amps are supplied from unregulated voltage, and the line regulation will be pretty crappy, maybe 3%, because the current through the zener is not very constant and 4.7V zeners at a few mA are not great regulators. DC load regulation should be fine because there is plenty of gain.

This is a pretty cruddy hobby-level 'bench supply'. I'd expect it would overshoot a fair bit too if you suddenly remove part of the load.

He could tie the lower end of R8 and C6 to the -1.2V reference (rather than ground) with 680R to -10V. The current limit should still work with -10V on the LM301A pin4, but I have not analyzed it in detail.

Best regards, Spehro Pefhany

Thanks for the replies. I had already started building the first circuit, so I may finish and see what sort of regulation I get. I appreciate the input, Steve

Just curious, how hard would it be to add an LED for CV/CC indication to the above circuit? Any ideas?

Thanks, Steve

Replace the 1N914 with an LED.

Best regards, Spehro Pefhany

Wow. See, I told you I wasn't an engineer. Thanks! Steve

No you are a slurping retard.

Quit posting for free advice cheapskate.

I should set up a paypal account to accept donations from here.. Please donate to D from BC.. I need to buy some electronics books.. :) D from BC

Nice forgery, Lamey The ForgeTard.

Fucking get a life, you retarded twit.