Author Topic: Digital oscilloscope project/kit (Read 317053 times)

Bluechip · « **Reply #75 on:** December 30, 2015, 02:28:02 PM »

You've no business wiring up 79xx regulators arseaboutface, that's my favourite faff-up.

BTW, you should really put a 100nF to ground close to the o/p pins on 78xx and 79xx ( and any other regulator for that matter ) otherwise they will tend to oscillate ..

Happy New Year 2U and Mrs. Rob ...

Dave

RobWilson · « **Reply #76 on:** December 30, 2015, 04:32:40 PM »

Hi Dave I bet it will not be the last time I get a component arse about face

I will amend the circuit tomorrow as you suggest

, I was wondering why I had a strange ripple on the 9 volts out .

All the best to you Dave

Cheers Rob

Bluechip · « **Reply #77 on:** December 30, 2015, 04:57:43 PM »

Hi Rob

Cheers

First time I had that nonsense was years ago, making a 1MHz Colpitts Oscillator. The circuit was driven by a 7809 and it ekkled quite handsomely at 1.6MHz. No way could I get the damn thing to work properly. Eventually discovered it was the pox-etten 9V with a hideous 1.6MHz ripple ..

Back in my analogue days ... long ago ...

Dave

RobWilson · « **Reply #78 on:** December 31, 2015, 06:32:53 AM »

Something like this Dave

I think I will change the output connector to -12, 0 , +5 ,+9, +12

cheers Rob

Bluechip · « **Reply #79 on:** December 31, 2015, 09:38:45 AM »

Well, 100uF would maybe improve things, but ..... it's the high frequency instability which causes the grief. So, if you actually freakin' READ like what I have writted 100 nF ie. 0.1 uF ...

Electrolytics in general are not very good at getting shot of HF noise 'cos of their inherent construction. An el-cheapo ceramic is all you need. You can leave the 100uF caps. there. Won't harm anything.

Good day to you, Sir ...

Late lunch today ... starving ....

BTW
I think your 0V centre-tap should be grounded ??

And that is the symbol for a safety earth, ground is an inverted triangle.

What current are you drawing from these regulators anyway? The 12V ic's. will have some 25V i/p, this gives about 13V dropped across them. If I assume the max. of 1A they will dissipate some 13W which is a lot for a TO220 package without a fairly large heat-sink and/or a fan. A 15 - 0 - 15 transformer would be a bit better ...

Dave

RobWilson · « **Reply #80 on:** December 31, 2015, 10:09:22 AM »

I was full of the party spirits when I redrew it last night Dave ,,,,,,,,,,,,,you try reading with a pint and a half of vodka and coke in you

Cheers Rob , could do with some bait myself .

RobWilson · « **Reply #81 on:** December 31, 2015, 10:28:24 AM »

Quote from: Bluechip on December 31, 2015, 09:38:45 AM

BTW
I think your 0V centre-tap should be grounded ??

And that is the symbol for a safety earth, ground is an inverted triangle.

What current are you drawing from these regulators anyway? The 12V ic's. will have some 25V i/p, this gives about 13V dropped across them. If I assume the max. of 1A they will dissipate some 13W which is a lot for a TO220 package without a fairly large heat-sink and/or a fan. A 15 - 0 - 15 transformer would be a bit better ...

Dave

oops I thought there was something else missing ,its grounded

,, about 200 milli amps Dave

Have a butchers at the new new version PDF

Rob

John Rudd · « **Reply #82 on:** December 31, 2015, 10:29:11 AM »

Well, my thoughts are:

The fuse is in the wrong position ......transformer primary would be better......
Not sure if there's sufficient headroom for the 9.0 v reg to work properly.....3.0v differential between in and out....close to the drop out voltage.....

If I were designing it, I'd be looking for around 16.0v in to the 12.0v reg, then connect the 9.0v to the 16.0 v supply....

A 15-0-15 transformer would be better as Dave says....I'd even try going as low as 12-0-12 to reduce the dissipation even further.....

Item 311 512 from https://www.esr.co.uk/electronics/products/frame_transformers.htm

RobWilson · « **Reply #83 on:** December 31, 2015, 10:55:31 AM »

Quote from: John Rudd on December 31, 2015, 10:29:11 AM

Well, my thoughts are:

The fuse is in the wrong position ......transformer primary would be better......
Not sure if there's sufficient headroom for the 9.0 v reg to work properly.....3.0v differential between in and out....close to the drop out voltage.....

If I were designing it, I'd be looking for around 16.0v in to the 12.0v reg, then connect the 9.0v to the 16.0 v supply....

A 15-0-15 transformer would be better as Dave says....I'd even try going as low as 12-0-12 to reduce the dissipation even further.....

Item 311 512 from https://www.esr.co.uk/electronics/products/frame_transformers.htm

Cheers John

I will move the fuse , I should have had a question mark next to it .

I have at hand a 12-0-12 and a 18-0 -18 transformer , the simulation did not like the 12-0-12 transformed ,no head room , and the simulator says I should get 9.74v

See new new new attachment lol

Cheers for the input lads

Rob

Bluechip · « **Reply #84 on:** December 31, 2015, 11:15:18 AM »

John is right, it will work with a 12 0 12 transformer .... depends somewhat on the VA rating ...

so what is the rating on the one you have ??

Why ??

OK. So, lets imagine we have a tranny 12 0 12 out at 24VA [for no other reason that it's easy to work out]

24vac out 24VA gives 1 amp load current.

BUT ... trannies here assume the EU nominal volts in to be 230vac ... we're often nearer 245vac.

The output voltage is at the full rated load ! ie. 1 amp.

Now we come to LOAD REGULATION

On a relatively small tranny it won't be very good, at 24 VA it's probably about 12%.

Which means at no load the output volts will be 12% higher than the full load 24 vac ie. some 26 vac.

To compound the felony we are shoving more volts into it than the 230vac nominal.

The upshot of this tripe means we will get considerably more DC volts, AT SMALL LOADS, than we think.

Hence a 12 0 12 12VA tranny will be quite OK in practice even if your wretched simulator doesn't think so.

If you get one of these really titchy 3VA trannies and measure the no-load volts out you will be surprised [ at first anyway] they are often way above the advertised value

but shove a 50mA load on them and they calm down.

Dave

Somewhere I have the schematic for one I built, see if I can find it ... might be on the laptop ...

John Rudd · « **Reply #85 on:** December 31, 2015, 11:17:49 AM »

There are a couple of reasons for putting the fuse in the pri,
1) it will blow if theres a short on the secondary or primary side......

2) the big 2200mfd cap will appear as a short circuit for a brief period as it charges up....depending on the fuse size, would cause it to blow...

RobWilson · « **Reply #86 on:** December 31, 2015, 11:27:24 AM »

Quote from: Bluechip on December 31, 2015, 11:15:18 AM

Hence a 12 0 12 12VA tranny will be quite OK in practice even if your wretched simulator doesn't think so.

Dave

Whats the poor thing done to you

I will try both .

The transformers I have are 3VA

Quote from: John Rudd on December 31, 2015, 11:17:49 AM

There are a couple of reasons for putting the fuse in the pri,
1) it will blow if theres a short on the secondary or primary side......

2) the big 2200mfd cap will appear as a short circuit for a brief period as it charges up....depending on the fuse size, would cause it to blow...

Its moved John

Rob

John Rudd · « **Reply #87 on:** December 31, 2015, 11:40:47 AM »

I've not been a big fan of sims.....much prefer to see what happens if approach.....

Best thing to do Rob, is lash up the circuit on some Vero board with the 12 v transformer and measure what comes out.....

If the usual approach of V ac in x 1.414 is applied and subtract the Vdrop for the diodes, the actual output is usually different to the theoretical calculated.....

The meter never lies....

RobWilson · « **Reply #88 on:** December 31, 2015, 11:44:16 AM »

I have attached two files showing the regulator output voltages , one with an input of 12-0-12 and the other 18-0-18

The 12-0-12 gives out less than 12 volts

Rob

Bluechip · « **Reply #89 on:** December 31, 2015, 12:17:35 PM »

3VA ???

Like running 'em lean 'n mean eh ?

Don't forget to phone up National Grid PLC before you power up. Tell 'em to brace themselves ...

Found the schematic:

The bottom bit, ie. + / - 12V was the original I built.

I've added your 5V & 9V, think it's OK ...

I've upped C1 & C5 a bit to reflect greater load but I don't think its really required for your application.

John Swift · « **Reply #90 on:** December 31, 2015, 12:20:48 PM »

Hi Rob

I see you have been busy

the kits you have built should come in handy

looking at the power supply
I would fit two 2200uF capacitors

one for the positive supplies and a second for the negative supply

John

PS
I see Bluechip is quicker !!!

RobWilson · « **Reply #91 on:** December 31, 2015, 12:54:36 PM »

Cheers Dave and John S

Two much improved designs , I like the modifications you made John .

Dave , yours has resistors and diodes added , in simple English why? (anyone)

This power supply is just to run the projects I have built from Banggood , this is their recommendations for the PSU for the signal generator .

I just ~~added~~ bodged on the 9v.

I know it is poor and a much better one could be built , I could just use the three bench PSU I have . I just thought why not have ago at building it and learn a few things on the way , which I have done thanks to you lads

and reading up on each component , f&%k lot to take in

, also I am using it as a project to learn how to use the Circuit Wizard software ,its is also something to piss about with in the warm

The thing is the passed few weeks messing with these boards has got my interest up in learning electronics

,tis all up hill from here

Dave , what electronics program are you using ?

Thanks again for your input Lads

, keep it coming

All the best Rob

Bluechip · « **Reply #92 on:** December 31, 2015, 01:19:10 PM »

Resistors are to discharge the 2200uF caps. Just considered good practice.

Diodes there to protect regulators from inadvertent connection of reverse voltage from external circuitry. [ If you use PSU for experimenting, anything can happen

Done it. Also, I did use the PSU to drive a small 24VDC motor, all sorts of spikey crap comes off them ].

You should really put more diodes across the regulators to prevent damage by putting volts on the o/p with no power to i/p. Regulators don't like it up 'em the wrong way ...

I will amend schematic to show this. Also I notice some wrong component numbering ...

Program is Express PCB. Comes in 2 parts. Schematic and PCB layout. Been mentioned before in this thread, does not export full Gerber file but I etch PCBs myself so it's OK for me. Used to do Eagle but it has a size limit Express PCB does not.

http://www.expresspcb.com/

Dave

Bluechip · « **Reply #93 on:** December 31, 2015, 01:39:59 PM »

Amended schematic:

This sort of thing is quite common. They all do the same job. Take the 9V regulator.

[1] The 1N4004 across the output. If I shove a reverse voltage into the output, the diode will conduct and effectively prevent any reverse voltage greater than some 0.6V appearing across the o/p to 0V. The regulator lives on.

[2] The 1N4004 connected across the regulator, output to input. If I shove a normal polarity voltage into the output with no volts on the input, the diode will conduct and charge the input capacitor. The reverse voltage across the regulator is again limited to some 0.6V and again the regulator survives.

Regulators don't like outputs higher volts than inputs or outputs lower volts than ground. They often exhibit their displeasure by snuffing it. Bugger eh ??

If you consider it's not going to happen D1 - D8 can be left off, no problem, it'll work just fine without.

Dave

RobWilson · « **Reply #94 on:** December 31, 2015, 01:47:06 PM »

Cheers Dave ,

Getting There with this software .

It must be beer O'clock

Rob

Bluechip · « **Reply #95 on:** December 31, 2015, 01:54:40 PM »

Quote from: RobWilson on December 31, 2015, 01:47:06 PM

Cheers Dave ,

It must be beer O'clock

Rob

What a coincidence, my clock says just the same ...

Must have batteries from the same batch.

Dave

RobWilson · « **Reply #96 on:** December 31, 2015, 01:58:39 PM »

Rob

John Rudd · « **Reply #97 on:** December 31, 2015, 02:35:10 PM »

Rob,
Make the tracks as thick as they can be.....

Then fill the voids with a copper fill......etching a board wont take as long as theres less to etch away...

wheeltapper · « **Reply #98 on:** December 31, 2015, 04:17:43 PM »

Hi all
I received the smd practice board this morning and have finished most of it.
I haven't soldered the transistors or the IC yet as they play no part in the test circuit, however putting 15 volts through it lights the LED so I did something right.

I will solder them in anyway just for more practice because there is an IC on the scope board.

It was reasonably easy much to my surprise, the 0603 resistors were the hardest but they all went on OK.

I now have the confidence to press on with the scope kit and the tester kit.

I'll see if I can get a decent pic to show.

cheers
Roy.

RobWilson · « **Reply #99 on:** December 31, 2015, 04:25:44 PM »

Nice one Roy

Good to hear the SMD soldering practice went ok

you must have bloody good eyes.

Rob

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Author Topic: Digital oscilloscope project/kit (Read 317053 times)

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