[linuxworks]

probes *do* slip, in the real world. it would be great if that didn't happen, but it does ;(

so, how much of your board do you want taken down with that probe slip-up? is there any way, via current limiting, that you could also get 'damage limiting' as well? that's part of the point of knowing a normal power range of a circuit and keeping its supply inside that but not letting it dump 'infinite current' if things go way out of whack.

plus, components sometimes die. again, how much of the rest of the circuit should take a hit if some PS is pumping out more and more and more, when the circuit really shouldn't be getting over a certain amount?

if you pull a phones jack out of its socket while the music plays, SHOULD the PSU and amp keep pumping 'more and more' current thru that momentary short?

do commercial companies allow amps to 'die' when the user removes a plug?

think...

 

[Uncle Erik]

Quote:

Originally Posted by Juaquin /img/forum/go_quote.gif Sweet, I had no idea. I'll be there. Right now I'm using my Arduino for 5V and flipping resistors around in my TREAD (with a TLE for dual) to produce other voltages. I have access to all sorts of fun gear in the labs on campus but dragging projects to school is no fun.

Before I forget, here's the W6TRW page.

You might also want to visit Apex Electronics.

 

[digger945]

Quote:

Originally Posted by linuxworks /img/forum/go_quote.gif actually, I'll chime in and say the s22 is also 'too' strong for real world use. it can fry things (I did that, gulp!) and I don't agree it was good PSU design to allow an INFINITE amount of current to sink. I don't at all agree with that design philosophy!

I've made some resistors burn white hot and made transistors smoke like a choo choo using a S22.

 

[Juaquin]

Quote:

Originally Posted by linuxworks /img/forum/go_quote.gif probes *do* slip, in the real world. it would be great if that didn't happen, but it does ;( so, how much of your board do you want taken down with that probe slip-up? is there any way, via current limiting, that you could also get 'damage limiting' as well? that's part of the point of knowing a normal power range of a circuit and keeping its supply inside that but not letting it dump 'infinite current' if things go way out of whack. think...

I didn't mean to question the usefulness, only inquiring why it was necessary (the thought of preventing short circuit damage hadn't occurred to me late in the night). Sounds like I'll probably buy a clone or a used one since I haven't seen any DIY kits with current limiting.

 

[Juaquin]

Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif You might also want to visit Apex Electronics.

I've seen Apex before but still haven't made it out there. They have anything good in the way of power supplies?

 

[linuxworks]

Quote:

Originally Posted by digger945 /img/forum/go_quote.gif I've made some resistors burn white hot and made transistors smoke like a choo choo using a S22.

O or O27 scale?

oops, wrong forum

 

[holland]

you can put a fast acting fuse on the output and input or a breaker. Variable current limiting is nice, but you can do it on the cheap with a few properly sized non time-delayed fuses.

 

[nux]

Quote:

Originally Posted by holland /img/forum/go_quote.gif you can put a fast acting fuse on the output and input or a breaker. Variable current limiting is nice, but you can do it on the cheap with a few properly sized non time-delayed fuses.

Easier to use a current limiting circuit. A fuse or breaker isn't really current limiting, it is short circuit protection.

There are a few ways to do current limiting. this is one way. I had a few links to DIY power supply schematics with variable voltage and current, I'll try to dig them up.

 

[SiBurning]

The main reason to have current limiting is when you're first powering up a circuit that you may have wired wrong or designed poorly. It'll save a lot of parts, debugging, and waiting for the new parts to come in only to fry it again.

 

[amb]

The problem with DIY-ing a bench supply is that you can't do it cost effectively compared to off-the-shelf units. Not even close.

Typically for a useful bench supply you'll want to have adjustable output voltage, say, from 0V - 50V, both a positive rail and a negative rail, can supply at least 3A of continuous output current, has nice meters or a digital display showing you the voltage and current, and adjustable current-limiting function to protect the circuit you're powering and the power supply itself.

If you're implementing this as a linear regulator, then there is a problem. You'll need to design it to be capable of the maximum output voltage, which means that at low voltage settings the regulator would have to burn the excess voltage off as heat. To illustrate the magnitude of this issue, if we have 55V before regulation to allow the 50V regulated output, then when the supply is set to output 5V, the regulator has to drop 50V. At the maximum output current of 3A, the regulator will have to dissipate 50V * 3A = 150W. Per rail.

The solution is a very special power transformer with many taps, switched to match the desired output voltage, so that the pre-regulator voltage is never much more than the regulated output voltage. Such a transformer is not available off-the-shelf and a custom one will be needed. Add all the other circuitry. switches, vernier control, meters or display, casing, etc., you can imagine how much it would cost to build.

When such a bench supply can be bought, brand new, off-the-shelf, with warranty for somewhere around $200-$400, it just doesn't make sense to DIY. That custom transformer alone would probably cost more than that.

 

[Juaquin]

But where's the fun and challenge in that

 

[amb]

Quote:

Originally Posted by Juaquin /img/forum/go_quote.gif But where's the fun and challenge in that

Save the fun and challenge to nice amps, DACs, etc. I consider a bench supply to be a tool, much the same as a DMM, oscilloscope or function generator. While you could attempt to build one yourself, it's almost always not worth the money or effort.

 

[dbfreak]

I made this one out of a computer PSU laying by a garbage pile in my "no credit for you young man-creative days."

 

[linuxworks]

I should try to find an old pop-tronics article I used to build my own 'bench' supply.

it was variable voltage (0..24) and variable current limiting (0..2A). it used some weird pnp germanium pass transistor as the current limit series shunt (I think). it was all transistor and was done on a hand drawn pcb. this was from the mid 1970's era. it did not use any fancy multitapped trafos or even 3term reg chips (no chips at all, actually).

I miss the old popular electronics DIY articles from that era. no one ever said 'buy it' back then since build-it was still pretty mainstream for home hobbiests.

I wonder if the old pop-tronics articles from the 'print days' are online anywhere? I bet they are not ;(

 

[Juaquin]

There's probably someone out there who's started scanning his collection and posting them - a quick search turned this up, and I'm sure there's more out there.