DC feed question

[jplebre]

How's it going guys?

As some of you suggested in other posts, I've built a dummy box. Nothing fancy just a couple of Jacks and a 3DPT switch, LED, etc so I know all that is working and all i need to do is hook up the aligator clips to the circuit I'm building/testing.
so far I've been using batteries and whenever there's a short (have to confess that I've left the stripboard on the helping hands and they were causing a short across all strips! doh!!!!) the LED goes off and the battery just starts melting (literally).

Wasted quite a lot of batteries like that.

I was thinking of getting a 9v power supply but this is my fear - what would happen if my circuit had a short? would I damage the power supply? or worse, trip the fuse on the wall or switchboard?
The initial plan was to build one but I'm wayyyyy to scared to attempt that right now!

Also, any "verified" schematic to do 48DC down to 9v and or 12v?
Was thinking I could use the studios 48DC to power up a pedal in case somehting goes wrong with the power supply like it happened this weekend after 12 hours in the studio :S

[smallbearelec]

I have long built my own :

http://www.smallbearelec.com/Projects/SmWart/SmWart.htm

and offer both kits and individual parts for those who want to do so. I always include a resettable fuse to keep things from getting out of hand in a short-circuit situation. Another way to deal with your problem, which also avoids having to wire to the AC line, is to start with a 12 volt dc wall wart:

http://www.smallbearelec.com/Detail.bok?no=1006

Add the filter cap, three terminal regulator, etc. like in the Small Wart. No resttable fuse needed because the wall wart has its own internal current limiting. Either way, you will have a useful companion for you bench, and maybe for gigging.

[jplebre]

ha!

with a guide like this I might put my irrelevant distortion attempts to the side and get one of these made first!
For Europe, would I just need to change the transformer for 240v to 12v?

[jplebre]

So will a kind soul help me here with transformers

Can we talk about KVA?

These are my options for the above design, which one?

http://cpc.farnell.com/block/avb3-2-2-12/transformer-3-2va-2-x-12v/dp/TF01281
http://cpc.farnell.com/block/vb3-2-2-12/transformer-3-2va-2-x-12v/dp/TF01252

[smallbearelec]

I did not realize that you were across the pond.

Either would work. The second transformer is cheaper because it is 230V only. The first can be wired for either 115 or 230.

An obvious way to use the transformer would be to create two separate, regulated, limited and filtered outputs. That, and the higher primary voltage, really dictate a new design, If you go that route, for gawd's sake get some local help with planning the mechanical layout, insulation, 230 volt power inlet, PLASTIC housing, etc., etc. This is Not trivial stuff. I don't mean to discourage too much, do want to emphasize that 115 is lethal enough, and 230 is just that much bigger a potential trainwreck.

I don't offer 230 volt to 12 VDC wall warts, but I am sure that Farnell does if you would rather go that way.

[jplebre]

hmm might get together with a guy that plays with very big toys to help me with this...
I already found plastic hammonds and pcb standoffs and a cool rocket switch and all that too excited to go back...

If any question that I do here shows you that I could do damage, please stop me I'll go back to read!

do I have to wire both circuits, or just do one regulated (limited and filtered) output?
I don't see why not do both, but I'd probably do one at a time and test it... would reduce error... if this is at all possible... or would I have to change components?

[smallbearelec]

do I have to wire both circuits, or just do one regulated (limited and filtered) output?...do one at a time and test it...

Possible. But plan the layout of the entire thing before you pick up a soldering iron. Among other minefields, be sure that you will be able to easily remove and reinstall the board for purposes of testing and then building the second section. Biggest lessons I learned in doing mine: NO internal metal hardware. Also, when taking measurements with the case open:  One Hand In Pocket, and liberal use of alligator leads and electrical tape to reduce odds that you will light yourself up like City Hall.

[jplebre]

True

Spoken with an EE friend of mine apparently by law you need to have a metal casing with the earth to use as ground if you are doing a power supply in the UK, or double insulate it if you are using plastic.
Anyone knows anything on this?

[boogietone]

It is my understand that the grounding of a metal enclosure is a safety precaution in case the hot/live wire shorts to the enclosure. This prevents the enclosure from sitting at high voltage until you touch it and it goes to ground through your heart.

[jplebre]

Hey guys

So, I did it, here's the pic:


The only thing I changed was the transformer (this one is dual, and 240v, as opposed to the small bear electronics 110v 1 secondary rail).

Now, It measures 8.82v.

The electricity in my place is fairly bad (lights dim when kettle comes on), but could it be that I changed the resistor to 4.7k?
I did the LED calculation and it only needed a 300ohms, but... that would be drawing to much current no?
I used 4.7k before on LEDs (these guys are like 5000mcd and even with a 10k resistor they are bloody bright).

ideias?

[jplebre]

The pic above was just quick wired to test it, as I want to epoxy the plugs, etc (as recommended).

Some more info on the project:
I took guidance from http://www.smallbearelec.com/Projects/SmWart/SmWart.htm the simple wall wart, not the deluxe.
The LED takes about 3 seconds to power down, the multimeter shows 0v after aprox 10sec

[anchovie]

True

Spoken with an EE friend of mine apparently by law you need to have a metal casing with the earth to use as ground if you are doing a power supply in the UK, or double insulate it if you are using plastic.
Anyone knows anything on this?

A mains appliance that has an earthed metal case is Class 1. Class 2 has no earth connection and is either double insulated or uses reinforced (solid) insulation. So a PSU in a solid plastic box meets Class 2.

Class 0 is an ungrounded metal enclosure and is banned.

[jplebre]

Hey Anchovie

A mains appliance that has an earthed metal case is Class 1. Class 2 has no earth connection and is either double insulated or uses reinforced (solid) insulation. So a PSU in a solid plastic box meets Class 2.

Class 0 is an ungrounded metal enclosure and is banned.

[jplebre]

bump on the 8.81v.

Should I measure the main i/p and apply the ratio to see if that's the cause? or are there other stuff I shoul do first?

[jplebre]

so, tried a couple of things:

- reduced the resistor to a smaller size (from 3k - 10x more than the required 360ohms for my red LED) to 1k (still well above the required) and no change (rock stead 8.81v)
- Started wandering if it was because I used a capacitor tolerant to higher temperatures. There were 2 choices of 1000uF 63v 50 degree or 105, I went for the latter). Kinda thought it could be because of the cap ESR?

Is there any way to measure ESR on a cap? (is there "a secret life of capacitors" document?

[PRR]

> bump on the 8.81v.

What is wrong with 8.81V??

8.81V == 9V for any practical purpose.

If you are doing government calibration, you might have to trim the 7809. But not for audio, not for stomp-box, and certainly not for lights.

Are you just powering LEDs??

Then why regulate at all? Transformer, FWB, cap, and a many-K resistor selected for reasonable LED brightness.

The rectifier and cap are optional. You can feed raw AC from transformer through resistor to LED, but it will flicker when you move your eye quickly.

[jplebre]

Hi PRR

Thanks for the reply

Are you just powering LEDs??

Not really. The design just includes one (kind of On/off status indicator).

8.81V == 9V for any practical purpose.

hmm ok.

you might have to trim the 7809

can you trim those? I didn't notice they had anything on them! or would it be trim the input?
I'm curious now google comin' up!