Help making a CRT oscilloscope

[guskenny83]

Hi,

I am trying to make something influenced by these projects:

http://www.instructables.com/id/Fully-Functional-Television-Oscilloscope/

http://www.crackedraytube.com/pdfs/oscillographic_tv_tutorial.pdf

the idea of the project is to put a signal into the horizontal and vertical deflector coils on the CRT in order to create a visualisation of that signal

the TV i have found for it is an old HMV A3301

I have isolated the 4 wires that connect to the two coils and if i disconnect the vertical coil control wires (brown and grey) i can get a horizontal beam across the screen.. then when i connect my signal to the those wires i can get it to slightly deflect up and down with the change in signal.

according to the instructable i linked to, the horizontal scanning frequency is way too fast for the signal and the coils need to be swapped so that the 60Hz vertical wires from the board are running the horizontal coil

the only problem is that when i disconnect the red and white wires that are the horizontal coil wires, the TV wont turn on any more

it works fine when i run it with the vertical control wires not connected, but not horizontal.

the pdf that i linked to suggests that some TVs don't work unless you use a "dummy coil" to trick it into thinking that the wires are still connected so i tested the wires for their impedance and i got the following information:

brown/grey - Vertical: 56.5 Ohms

red/white - Horizontal: 3.5 Ohms

I put a 3.9 Ohm 10W wire wound resistor in to try and replicate the coil, but when i try to turn on the TV it doesnt work and the fuse next to the big capacitor keeps blowing

Is it strange that there is such a large difference in impedance beween the two coils?

could it be that i need a higher power resistor to act as the coil?

Is there anything that anyone can suggest i do that might solve this problem?

 

[Old Steve]

Is there anything that anyone can suggest i do that might solve this problem?

Perhaps you could try a coil for the dummy coil, rather than a resistor. You could possibly measure the 'inductance' of the original horizontal coil and find one of a similar value, with a similar series resistance.

With a few calculations, you could even wind your own dummy coil.

 

[guskenny83]

Thanks for your reply. I found an old transformer which has a 5.5 ohm impedance. Ill see if that works. Otherwise ill try winding a coil. Any tips for winding a coil with 3.5 ohms impedance?

 

[guskenny83]

I tried out the transformer and it works, but i think it might be running a bit too hot.. i used a multimeter with a temperature probe and after about 30 minutes running the outside of the transformer core was around 105 degrees C.. is this too hot?

the specs of the transformer i am using are:

input: 250V, 50Hz
output: 9.5V, 150mA max

here is a picture:

is this an okay running temperature? or should i use a different coil/transformer

does anyone have any suggestions as to what i should use instead?

if this temperature is okay, then does anyone have any suggestions as to how i should mount it in the TV safely?

 

[Old Steve]

Thanks for your reply. I found an old transformer which has a 5.5 ohm impedance. Ill see if that works. Otherwise ill try winding a coil. Any tips for winding a coil with 3.5 ohms impedance?

Is that impedance, or resistance? How did you come to 5.5 ohms?

You're actually looking for a combination of "inductance" and series resistance for a good match.
I can't give any tips on making a coil to suit, except to suggest that you get an LCR meter to measure the original horiz winding's inductance, a suitable core for the frequency, work out the number of windings needed to achieve the required inductance, then select winding wire with the right gauge to be the right series resistance when wound.
Knowing the relative inductance of the core is a good start, then if I remember correctly, the formula is:-
L = Al x N², where L = inductance, Al = relative inductance and N = number of turns.

Edit: I guess I did give some tips on making the coil after all.
And someone please correct me if my formula is wrong - it's been a very long time since I did any coil or transformer winding.

 

[guskenny83]

Is that impedance, or resistance? How did you come to 5.5 ohms?

You're actually looking for a combination of "inductance" and series resistance for a good match.
I can't give any tips on making a coil to suit, except to suggest that you get an LCR meter to measure the original horiz winding's inductance, a suitable core for the frequency, work out the number of windings needed to achieve the required inductance, then select winding wire with the right gauge to be the right series resistance when wound.
Knowing the relative inductance of the core is a good start, then if I remember correctly, the formula is:-
L = Al x N², where L = inductance, Al = relative inductance and N = number of turns.

Edit: I guess I did give some tips on making the coil after all.
And someone please correct me if my formula is wrong - it's been a very long time since I did any coil or transformer winding.

Thanks again for your reply.

Sorry, i meant resistance, not inductance. Its been a while since i have done any electronics theory.. Ill have a look and see if i can measure the inductance of the original coil tomorrow and see if i can work out the specs of what i have to wind..

 

[guskenny83]

I am finally able to get hold of an LCR meter today.

What kind of materials do you suggest i use to wind the coil? Does the shape of the core matter? Or gauge of wire?

Thanks again for your help

 

[Old Steve]

I am finally able to get hold of an LCR meter today.

What kind of materials do you suggest i use to wind the coil? Does the shape of the core matter? Or gauge of wire?

Thanks again for your help

Gus, can you tell what the original core was made from? My guess would be iron, for that frequency range. I can't mentally picture what the core of those coils was made from. The shape won't be too critical. I mentioned the wire gauge and the steps needed in post #5. First, determine the material the coil is made from. Next, get a core of the same material and similar size or a little larger. You'll need the relative inductance of the new core next. Wind some turns on the new core and measure it's inductance, then transpose the formula I also gave you in post #5 to get the relative inductance. For the best match, you'll want a similar series resistance to the original coil too, 5.5Ω you said. Then follow the steps in my post #5. I don't need to type it all again. You'll need to choose the gauge by it's resistance per metre, according to the length that you calculate is needed to achieve the required inductance on that particular core. Don't wind your final coil directly on the core, either. Make up a 'former', from thin cardboard or plastic that fits closely over the core. Otherwise, you new coil won't last long. And I guess you must already know that you can't just use ordinary bare wire - it must be enamelled copper winding wire, made for the purpose. (I had to say that. I've met some people who thought it was just normal, bare, uninsulated copper wire.)
That transposed formula for relative inductance is Al = L / N².

But........ I just had a sensible thought, (for once). Before you do any of that, why not get your hands on the horizontal deflection coil from another old TV and use that? Much easier than trying to create your own.
I have an old TV here. (You can have it, but delivery might be expensive. I don't know where in the world you are.)
Surely you know someone else who does too?

 

[guskenny83]

Believe it or not, it was actually pretty hard to get hold of this CRT TV in the first place. Even the charity shops all have flat screens these days! I will try make the coil myself first and then if that doesn't work i might try track another tv down for a coil..

Thanks for your advice. I got hold of the LCR meter today so i will measure the coil tomorrow and then see about trying to work out the specs for the new one.

If i use a piece of round steel bar for the core, can i just use a couple layers of heat shrink tubing as a former? Or does it need to be able to withstand greater temperatures than that?

Thanks again for your help.

 

[Old Steve]

Believe it or not, it was actually pretty hard to get hold of this CRT TV in the first place. Even the charity shops all have flat screens these days! I will try make the coil myself first and then if that doesn't work i might try track another tv down for a coil..

Thanks for your advice. I got hold of the LCR meter today so i will measure the coil tomorrow and then see about trying to work out the specs for the new one.

If i use a piece of round steel bar for the core, can i just use a couple layers of heat shrink tubing as a former? Or does it need to be able to withstand greater temperatures than that?

Thanks again for your help.

Iron might be better than steel, but I'm not 100% sure. Iron is commonly used, but I've never heard of steel being used. It would have different characteristics.

And first double-check that the original is an iron core and not ferrite or similar, in case I'm wrong. (It won't be steel.)
The core material is pretty important in a coil.

I think heatshrink should be OK, if you can get it in a large enough diameter. Don't make the core too thin - keep it about the same as the original, or slightly thicker. Cardboard for a former is easy. Do you eat cornflakes and have some tape to hold it in place?

 

[duke37]

Solid bar will have a large energy loss. Transformers with iron cores are made with thin laminations to interrupt the current path.

The iron needs to be magnetically soft and is usually alloyed with 4%silicon. Carbon steel is physically and magnetically hard and is not suitable.

Scanning coils are wound on a ferrite former. You could perhaps use a pot core with the right inductance and then add a resistance in series to match the original.

 

[Old Steve]

Solid bar will have a large energy loss. Transformers with iron cores are made with thin laminations to interrupt the current path.

The iron needs to be magnetically soft and is usually alloyed with 4%silicon. Carbon steel is physically and magnetically hard and is not suitable.

Scanning coils are wound on a ferrite former. You could perhaps use a pot core with the right inductance and then add a resistance in series to match the original.

Thanks for helping out. I really had no idea what sort of core, except that iron was often used for low frequencies. I didn't think steel was a good idea, either.
After originally saying I couldn't help with the coil design, I found myself being drawn in further and further. Separate series resistance is a good idea, too.
I've only ever wound coils for high-frequency flyback type step-up DC-DC converters, on small ferrite E-I cores.

 

[guskenny83]

Thanks to both of you for your help. I take your points about steel.

Would any of these be appropriate?

http://www.jaycar.com.au/Passive-Components/Ferrites,-Inductors-&-Suppression/Bars,-Beads,-Sleeves-&-Cores/Large-Ferrite-Suppression-Sleeves---Pk-6/p/LF1260

http://www.jaycar.com.au/Passive-Components/Ferrites,-Inductors-&-Suppression/Bars,-Beads,-Sleeves-&-Cores/9mm-x-100mm-Ferrite-Rod/p/LF1010

http://www.jaycar.com.au/Passive-Components/Ferrites,-Inductors-&-Suppression/Bars,-Beads,-Sleeves-&-Cores/180mm-x-9mm-Ferrite-Rod/p/LF1012

http://www.jaycar.com.au/Passive-Components/Ferrites,-Inductors-&-Suppression/Bars,-Beads,-Sleeves-&-Cores/L8-25x15x10mm-Toroid-(or-Ring)-Cores---Pk-4/p/LO1234

I will measure the coil today. And work out the calculations

Thanks again

EDIT:
I measured the coil properly. And the inductance of the coil is 1.786 mH, with a series resistance of 2.5 ohms.

Im trying to work out the specs for this coil, but i am having trouble working out what relative inductance is. Is that a property of the core? Should i take the LCR meter to the shop where i am getting the core from and measure the inductance of a core there and use that in the calculations?

 

[Old Steve]

Im trying to work out the specs for this coil, but i am having trouble working out what relative inductance is. Is that a property of the core? Should i take the LCR meter to the shop where i am getting the core from and measure the inductance of a core there and use that in the calculations?

You won't be able to do that in the shop. It requires winding a coil on the core if the relative inductance is not already known.
None of the ones at Jaycar that you showed are suitable, I should say.

Relative inductance seems to be an archaic term now - I can't find reference to it anywhere.
Overall, coil calculations and design is a very complex subject, but for your purposes all you need is to wind some turns on the core, measure the resulting inductance and then use the formula I gave you to calculate how many turns will be needed.
The last one you linked to is closest, but too small I would think.

You need to do some experimentation to work out what size you'll need, and what material.
I've reached the end of my knowledge here. I used to do it by trial and error.

The number of turns needed = the square root of L / Al. (Inductance / relative inductance)

Here's a couple I have laying around:-

This is a 45mm diameter powdered-iron core coil.
It's inductance is 360uH.
There are 82 turns on it, so it's relative inductance is 53nH/turn, and it would need 184 turns to achieve your 1.8mH:-


This is another, smaller powdered iron core, 33mm in diameter
It's inductance is 8uH.
There are 10 turns on it, so it's relative inductance is 80nH/turn, and 150 turns would be needed to reach 1.8mH:-



And this is a small ferrite core, (unknown type of ferrite), 25mm in diameter.
It's inductance is 410uH.
There are 10 turns on it, (RHS), so it's relative inductance is 4.1uH/turn, and 21 turns would be necessary for 1.8mH:-


A ferrite pot core, as suggested by duke37, might be the way to go.
And not all ferrites are the same. There are a wide range of ferrite types, with a wide range of relative inductances, all intended for different frequencies etc.
One thing that gets in the way is the fact that a small, very high relative inductance core could achieve 1.8mH in relatively few turns, but could saturate in operation and get hot.
Short of making an in-depth study of inductor design, you will only get there with a little trial and error as mentioned.
Aslo keep duke37's tip in mind - you can add a series resistor to boost the series resistance, so you can use slightly thinner wire than your calculations will indicate. (When selecting a winding wire, you'll still need teh resistance per metre.)

I hope this helps a bit.

Since you appear to be in Australia, check out RS Components for ferrite cores. You'll do better than at Jaycar.
(Are you anywhere near Nowra, NSW? If so, you can have my old CRT TV for it's horizontal coil.)

Edit: Just looking at RS, the datasheet for the first ferrite toroid I looked at has the relative inductance mentioned, so it's not such an archaic term after all.

 

[Old Steve]

Just looking at RS, the datasheet for the first ferrite toroid I looked at has the relative inductance mentioned, so it's not such an archaic term after all.

It actually mentions the relative inductance for the same coil in different materials. It's only 25mm diameter, so probably not big enough, but gives an indication at least.
Check out near the top of page 2 for the relative inductance data:-
http://docs-asia.electrocomponents.com/webdocs/13c0/0900766b813c0cbd.pdf

And here's a link to their ferrite rings search results. A more general search for ferrite cores might be better, though. It'll include pot cores, E-I cores etc. Still, a ferrite toroid might well do the trick:-
http://au.rs-online.com/web/c/passive-components/ferrite-cores/ferrite-rings/

 

[guskenny83]

Hi, thanks again for all your help, really appreciated.. if anything, i feel like i am learning something..hahah

what size toroid core would you suggest so that it wont get too hot?

i was out and near jaycar so i decided to just do some trial and error, i bought a straight 9mm diameter by 100mm long ferrite core and tried that (that was before i got your response showing the cores you suggested using)..

unfortunately the inductance of that coil was only about 800 uH so about 1 millihenry too little..

i tried it on the TV anyway (with a 1 ohm 10W resistor in series), and it KIND of worked, as in, it switched on and didnt blow the fuse; although after a couple of seconds the anode started sparking loudly so i very quickly turned it off.. my intuition from that is that not enough energy was being dissipated by the coil before going back to the flyback transformer and so it was discharging it through the anode to the ground wire. is that about right?

so does that mean that i am at least on the right track, and if i am able to make a coil that has the right inductance value, it will probably work?

so my next thing is to look for a good sized toroid core i suppose.. they have 35mm ferrite and 42mm diameter powdered iron toroid cores or a 26mm pot core at jaycar, would any of those be worth trying?

i know that RS is probably better, but i have a jaycar near my house so even though its a couple of dollars more, its a lot easier to go there, also because i can see what i am buying as well.. but if you think that none of those would be suitable then

anyway, thanks again so much for your help, it will all be working soon enough!

 

[Old Steve]

Hi, thanks again for all your help, really appreciated.. if anything, i feel like i am learning something..hahah

what size toroid core would you suggest so that it wont get too hot?

i was out and near jaycar so i decided to just do some trial and error, i bought a straight 9mm diameter by 100mm long ferrite core and tried that (that was before i got your response showing the cores you suggested using)..

unfortunately the inductance of that coil was only about 800 uH so about 1 millihenry too little..

i tried it on the TV anyway (with a 1 ohm 10W resistor in series), and it KIND of worked, as in, it switched on and didnt blow the fuse; although after a couple of seconds the anode started sparking loudly so i very quickly turned it off.. my intuition from that is that not enough energy was being dissipated by the coil before going back to the flyback transformer and so it was discharging it through the anode to the ground wire. is that about right?

so does that mean that i am at least on the right track, and if i am able to make a coil that has the right inductance value, it will probably work?

so my next thing is to look for a good sized toroid core i suppose.. they have 35mm ferrite and 42mm diameter powdered iron toroid cores or a 26mm pot core at jaycar, would any of those be worth trying?

i know that RS is probably better, but i have a jaycar near my house so even though its a couple of dollars more, its a lot easier to go there, also because i can see what i am buying as well.. but if you think that none of those would be suitable then

anyway, thanks again so much for your help, it will all be working soon enough!

If you keep trying coils with low inductance and low series resistance, soon you won't have a TV to turn into an oscilloscope.

duke37 recommends ferrite, so forget the powdered iron. They were just for example purposes. The pot core might be worth buying for a test, if it's ferrite, but you won't know the relative inductance in advance.
You don't have to go to RS - order online. Jaycar have a very limited range. Delivery is free, and very fast, (2-3 days by Toll courier), so you can stock up on whatever else you need too, to pad out the order. (Try to avoid just buying one thing, or their free online delivery might not last long.)

 

[guskenny83]

Fair enough. Point taken!

So what size ferrite core would you suggest from RS then to make sure that it wont overheat?

While im doing an RS order should i get some more enamelled wire? I had some 0.63 mm diameter wire here that i used on the first coil. Is that a decent size to use? Or should i try a different gauge?

 

[Old Steve]

I had to say it.

And to answer your question - I wish there was a 'shrug' smilie.
Trial and error. It has to do with a combination of mass and saturation of the core. Make one, try it while keeping an eye on temperature, and go to a larger core if it gets too hot.

You now know as much as I do about the entire subject.

And ........ good luck. You might need it.

 

[Old Steve]

I forgot - the winding wire gauge is a matter of calculation. I went through that bit in an earlier post. It's dependent on the length needed for a given core and the wire's resistance per metre.