| Author |
Message |
Philip Pemberton
Guest
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 Posted:
Mon Dec 05, 2005 1:35 am Post subject:
Of transistor amplifiers and CRTs... |
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Hi,
I'm just finishing off the design of my CRT X/Y display. I've spent ages
playing with component values and now the HT, LT and deflection circuitry
works perfectly (or as near to perfect as I can get it, anyway). Now I'm onto
the last little bit - the Z axis modulation drive circuit.
What I've done thus far is add a few resistors to various points in the
resistor divider chain. I've found out that a 220k resistor over the
brightness pot's wiper and one of its end terminals will increase the
brightness from zero to full. That resistor is dropping 32V (measured with a
DMM) and has 145.45 microamps running through it (calculated using Ohm's Law;
I=V/R).
Now, what I want to do is convert a 0V to 4V input into a current output
from 0 to about 200uA, but with some ability to vary the max. output current
from about 100 to 200uA (so I can correct for tube variations and personal
preference). Effectively what I want is for the amplifier to look like an
open circuit when its input is at 0V, and for it to sink 150uA (ish) when the
input is at 4V. I'm planning to use ZTX458 transistors, seeing as they're the
only HV NPNs I've got in my junk box.
I've had a look at the Electrohome G05 schematics, and the Z modulator used
in that is based on a single stage common-emitter amplifier. I've uploaded
the relevant section of the manual to
<http://www.philpem.castlecore.com/zamp/g05zamp.png>. I've also uploaded a
copy of the schematic for my DG7/32's resistor chain to
<http://www.philpem.castlecore.com/zamp/dg7chain.png>. The input signal is
going to be 0V-4V relative to 0V (on the right of the schematic). The 220k I
mentioned connects between "G1" and "X", and that's where I intend to put the
output of the amplifier.
Now, I've read up on transistor amplifier design theory, but all the stuff
I've got (which basically amounts to two old electronics textbooks) basically
says "Don't even THINK of calculating bias resistors based on the Hfe of a
transistor, because it varies depending on A LOT of factors", but then says
nothing about how you should design it...
Can someone please point me to a website or book that actually gives an
useful example of how to design something like this?
One of these days, I'll buy myself a copy of AoE and read it through.
Unfortunately that's not really practical at the moment ("no money") :(
Thanks.
--
Phil. | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem@philpem.me.uk | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/ | Panasonic CF-25 Mk.2 Toughbook
No software patents! <http://www.eff.org/> / <http://www.ffii.org/>
.... Star Trek XXVII - The Search for Shatner's Teeth.
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Pooh Bear
Guest
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| Posted:
Mon Dec 05, 2005 9:09 am Post subject:
Re: Of transistor amplifiers and CRTs... |
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Philip Pemberton wrote:
| Quote: | "Don't even THINK of calculating bias resistors based on the Hfe of a
transistor, because it varies depending on A LOT of factors", but then says
nothing about how you should design it...
|
Simply design as if hfe was much lower. That way, the influence of bias current
will be minimal.
Graham |
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Philip Pemberton
Guest
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| Posted:
Mon Dec 05, 2005 5:35 pm Post subject:
Re: Of transistor amplifiers and CRTs... |
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|
Pooh Bear wrote:
| Quote: | Philip Pemberton wrote:
"Don't even THINK of calculating bias resistors based on the Hfe of a
transistor, because it varies depending on A LOT of factors", but then says
nothing about how you should design it...
Simply design as if hfe was much lower. That way, the influence of bias current
will be minimal.
Graham
|
It looks like the big problem I've got is that the voltage input spans
a range of 0V to 4V relative to ground, while the transistor ("Q")
emitter is going to be about 500V below ground. Putting a capacitor
between Q.base and Vin and adding a resistor between Q.base and
Q.emitter should get rid of the "offset", but if the signal goes DC
(which is quite likely) then the beam will turn off.
An opamp would be one way to deal with the offset, but I expect opamps
that can handle 600V are few and far between (not to mention
expensive).
Later,
Phil.
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Chris Jones
Guest
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| Posted:
Tue Dec 06, 2005 1:36 am Post subject:
Re: Of transistor amplifiers and CRTs... |
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Philip Pemberton wrote:
| Quote: |
Pooh Bear wrote:
Philip Pemberton wrote:
"Don't even THINK of calculating bias resistors based on the Hfe of a
transistor, because it varies depending on A LOT of factors", but then
says nothing about how you should design it...
Simply design as if hfe was much lower. That way, the influence of bias
current will be minimal.
Graham
It looks like the big problem I've got is that the voltage input spans
a range of 0V to 4V relative to ground, while the transistor ("Q")
emitter is going to be about 500V below ground. Putting a capacitor
between Q.base and Vin and adding a resistor between Q.base and
Q.emitter should get rid of the "offset", but if the signal goes DC
(which is quite likely) then the beam will turn off.
An opamp would be one way to deal with the offset, but I expect opamps
that can handle 600V are few and far between (not to mention
expensive).
Later,
Phil.
|
I don't quite understand what you are asking for: What voltages do you need
on the CRT electrode when the beam is on, and what voltage do you need when
the beam is off? If you could post a drawing of the circuit on a website
somewhere then that would be helpful too.
Chris |
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Philip Pemberton
Guest
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| Posted:
Tue Dec 06, 2005 1:36 am Post subject:
Re: Of transistor amplifiers and CRTs... |
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In message <11p9h8hqsv0vv41@corp.supernews.com>
Chris Jones <lugnut808@nospam.yahoo.com> wrote:
| Quote: | I don't quite understand what you are asking for: What voltages do you need
on the CRT electrode when the beam is on, and what voltage do you need when
the beam is off? If you could post a drawing of the circuit on a website
somewhere then that would be helpful too.
|
OK, the voltage at the transistor emitter will be about -500V relative to
ground. The collector will be at about -400V.
The input ranges from 0V to 4V relative to ground.
This is fine - in theory. Except the transistor's base is referenced to the
emitter. There's 500V or so between base and emitter, so it's locked on hard.
What I need is for the transistor to sink between 0 and 200uA as the input
varies between 0 and 4V.
Later.
--
Phil. | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem@philpem.me.uk | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/ | Panasonic CF-25 Mk.2 Toughbook
No software patents! <http://www.eff.org/> / <http://www.ffii.org/>
.... After we pull the pin, Mr. Grenade is NOT our friend! |
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Kevin White
Guest
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| Posted:
Tue Dec 06, 2005 1:36 am Post subject:
Re: Of transistor amplifiers and CRTs... |
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Philip Pemberton wrote:
....
| Quote: | This is fine - in theory. Except the transistor's base is referenced to the
emitter. There's 500V or so between base and emitter, so it's locked on hard.
What I need is for the transistor to sink between 0 and 200uA as the input
varies between 0 and 4V.
.... |
This is a classic problem with driving electrostatic deflection CRTs.
Some CRTs even had a separate blanking electrode that could be at anode
potential to avoid the issue.
What bandwidth do you require?
AC coupling with DC restoration is a common approach but does not work
at DC. You may be able to arrange that you periodically blank the beam
even when you are displaying very slow moving traces - you probably
won't notice it. Analog scopes with on screen displays periodically
steal the beam to update the character display.
A variation on this approach is to combine it with a chopped version of
the modulating signal that is coupled by a small capacitor and
rectified to provide the low frequency component of the signal.
Another method is to use an optocoupler although this has speed
limitations and requires a 50V or so power supply referenced to the
cathode of the CRT. Obtaining linear response may also be difficult.
Yet another method is to use a high-voltage PNP transistor with it's
current modulated to control the blanking signal - this also requires a
power supply referenced to the cathode.
You can see a couple of examples at these links:
http://www.cathodecorner.com/sc100ep1.gif
http://webx.dk/oz2cpu/clock-scope/scope.htm
http://www.timefracture.org/clockdocs/VECTCLK_SCH.pdf
http://members.tripod.com/michaelgellis/scope/blanking.html
kevin |
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Philip Pemberton
Guest
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| Posted:
Tue Dec 06, 2005 9:35 am Post subject:
Re: Of transistor amplifiers and CRTs... |
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In message <1133831546.562361.281820@f14g2000cwb.googlegroups.com>
"Kevin White" <kevinjwhite@comcast.net> wrote:
| Quote: | This is a classic problem with driving electrostatic deflection CRTs.
Some CRTs even had a separate blanking electrode that could be at anode
potential to avoid the issue.
What bandwidth do you require?
|
A few hundred kHz at most.
| Quote: | AC coupling with DC restoration is a common approach but does not work
at DC. You may be able to arrange that you periodically blank the beam
even when you are displaying very slow moving traces - you probably
won't notice it. Analog scopes with on screen displays periodically
steal the beam to update the character display.
|
Looks like that's the best plan. Even if I make a scope add-on for the
display, I guess I'm still going to need to shut down the beam during a
retrace.
So now I need to work out the biasing... Time to borrow a copy of AoE,
methinks.
| Quote: | Yet another method is to use a high-voltage PNP transistor with it's
current modulated to control the blanking signal - this also requires a
power supply referenced to the cathode.
|
I haven't got any PNPs though - I ordered NPNs for the deflection circuit,
but forgot to order the PNPs for the blanking/brightness circuit.
Thanks.
--
Phil. | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem@philpem.me.uk | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/ | Panasonic CF-25 Mk.2 Toughbook
No software patents! <http://www.eff.org/> / <http://www.ffii.org/>
.... Biography: One of the terrors of death. |
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Guest
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| Posted:
Thu Dec 08, 2005 1:35 am Post subject:
Re: Of transistor amplifiers and CRTs... |
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Chris Jones wrote:
| Quote: | Philip Pemberton wrote:
In message <11p9h8hqsv0vv41@corp.supernews.com
Chris Jones <lugnut808@nospam.yahoo.com> wrote:
I don't quite understand what you are asking for: What voltages do you
need on the CRT electrode when the beam is on, and what voltage do you
need when
the beam is off? If you could post a drawing of the circuit on a website
somewhere then that would be helpful too.
OK, the voltage at the transistor emitter will be about -500V relative to
ground. The collector will be at about -400V.
The input ranges from 0V to 4V relative to ground.
This is fine - in theory. Except the transistor's base is referenced to
the emitter. There's 500V or so between base and emitter, so it's locked
on hard. What I need is for the transistor to sink between 0 and 200uA as
the input varies between 0 and 4V.
Later.
I still don't understand - it sounds like you have already chosen a circuit
and that it cannot work because transistors can't work with 500V between
base and emitter. What I was asking about is the overall function that
your circuit is supposed to perform, in case I could suggest a different
way of doing it which would be easier to bias. I am also still not really
sure what your circuit looks like so if you could post a diagram on a
website or in ABSE then I would probably be able to give you a more useful
answer.
Chris
|
Yup, the OP still hasnt made clear what V and swing are needed on what
electrodes. ac coupling and clamping is a common approach.
There is one more possible way to do it which gets dc voltages
displayed ok. compare the probe's V_in with a hf sawtooth wave, so you
get a digitised output, then use this to switch the V on the electrode.
This will give full dc response. You can either smooth the hf before
feeding to the electrode, or feed chopped as is.
No, I havent done this with CRTs, so someone else might spot a problem.
NT |
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Chris Jones
Guest
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| Posted:
Thu Dec 08, 2005 1:35 am Post subject:
Re: Of transistor amplifiers and CRTs... |
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Philip Pemberton wrote:
| Quote: | In message <11p9h8hqsv0vv41@corp.supernews.com
Chris Jones <lugnut808@nospam.yahoo.com> wrote:
I don't quite understand what you are asking for: What voltages do you
need on the CRT electrode when the beam is on, and what voltage do you
need when
the beam is off? If you could post a drawing of the circuit on a website
somewhere then that would be helpful too.
OK, the voltage at the transistor emitter will be about -500V relative to
ground. The collector will be at about -400V.
The input ranges from 0V to 4V relative to ground.
This is fine - in theory. Except the transistor's base is referenced to
the emitter. There's 500V or so between base and emitter, so it's locked
on hard. What I need is for the transistor to sink between 0 and 200uA as
the input varies between 0 and 4V.
Later.
|
I still don't understand - it sounds like you have already chosen a circuit
and that it cannot work because transistors can't work with 500V between
base and emitter. What I was asking about is the overall function that
your circuit is supposed to perform, in case I could suggest a different
way of doing it which would be easier to bias. I am also still not really
sure what your circuit looks like so if you could post a diagram on a
website or in ABSE then I would probably be able to give you a more useful
answer.
Chris |
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Philip Pemberton
Guest
|
| Posted:
Thu Dec 08, 2005 9:35 am Post subject:
Re: Of transistor amplifiers and CRTs... |
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|
In message <11peoputlkbjk95@corp.supernews.com>
Chris Jones <lugnut808@nospam.yahoo.com> wrote:
| Quote: | I still don't understand - it sounds like you have already chosen a circuit
and that it cannot work because transistors can't work with 500V between
base and emitter. What I was asking about is the overall function that
your circuit is supposed to perform, in case I could suggest a different
way of doing it which would be easier to bias. I am also still not really
sure what your circuit looks like so if you could post a diagram on a
website or in ABSE then I would probably be able to give you a more useful
answer.
|
Right, I've dragged it through SPICE and got some voltage figures. There IS a
schematic online at <http://www.philpem.castlecore.com/zamp/dg7amp.png> -
that just covers the resistor chain though.
Basically, when the Z amplifier gets a +4V input relative to 0V, I want the
equivalent of a 170k resistor placed between "G1" and "X". That provides a
200uA (ish) path, and pulls down the cathode, thus increasing the brightness
of the trace. Brightness is determined by the voltage difference between the
cathode (which is wired to the 33k/680k junction to the right of "X") and G1.
The lower G1 is relative to the cathode, the dimmer the trace gets.
OK, so for "normal" brightness with the 100K "Intensity" control at
mid-position and no resistor over G1 and X:
V(x) = -509.00V
V(g1) = -554.40V
V(ca) = -479.04V
Diff1 = 45.40V
Diff2 = 29.96V
And with the 170k across X and G1:
V(x) = -517.92V
V(g1) = -553.67V
V(ca) = -487.31V
Diff1 = 35.75V
Diff2 = 30.61V
V(ca): Cathode voltage relative to 0V
V(x): Volts at "x" relative to 0V
V(g1): Volts at "G1" relative to 0V
Diff1: Difference between V(x) and V(g1)
Diff2: Difference between V(x) and V(ca)
This doesn't take into account the effect the CRT has on the voltages - it's
just a simulation of the resistor chain.
A voltage difference of about 32V is enough to get a full-brightness trace
without causing the focus to go wildly out. About 55V is enough to shut down
the beam.
I've been using a transistor current source, as shown in AoE (emitter
resistor controls the current, NPN transistor - from page 74) with the
"ground" end of the emitter resistor wired to G1 and the collector wired to
X. The catch is, because the emitter is at -500V relative to ground, feeding
it a 0V input ends up tripping the PSU's output breaker. Feeding it either X
or G1 as an input has the desired action - either increasing or decreasing
the brightness.
Circuit as follows:
X >--------C E---[ Re ]----> G1
\ /
=======
| ZTX458 (NPN Si HV)
|
^
0-4V Z axis input
So far, the best idea I've come up with is to add a resistor between the base
and emitter and feed the input in through a capacitor. I'm not even sure if
that'll work, though...
The current passing between G1 and X needs to vary between basically nothing
(0V input) and the 200uA maximum (4V input). Ideally the 200uA figure needs
to be adjustable over a range of 50uA or so.
Later.
--
Phil. | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem@philpem.me.uk | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/ | Panasonic CF-25 Mk.2 Toughbook
No software patents! <http://www.eff.org/> / <http://www.ffii.org/>
.... Our OS which art in CPU - RISC OS be thy name ..... |
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Fred Bloggs
Guest
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| Posted:
Thu Dec 08, 2005 5:35 pm Post subject:
Re: Of transistor amplifiers and CRTs... |
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Philip Pemberton wrote:
| Quote: | In message <11p9h8hqsv0vv41@corp.supernews.com
Chris Jones <lugnut808@nospam.yahoo.com> wrote:
I don't quite understand what you are asking for: What voltages do you need
on the CRT electrode when the beam is on, and what voltage do you need when
the beam is off? If you could post a drawing of the circuit on a website
somewhere then that would be helpful too.
OK, the voltage at the transistor emitter will be about -500V relative to
ground. The collector will be at about -400V.
The input ranges from 0V to 4V relative to ground.
This is fine - in theory. Except the transistor's base is referenced to the
emitter. There's 500V or so between base and emitter, so it's locked on hard.
What I need is for the transistor to sink between 0 and 200uA as the input
varies between 0 and 4V.
Later.
|
Hopefully MUCH later...get lost, sick TROLL. |
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Winfield Hill
Guest
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| Posted:
Thu Dec 08, 2005 5:35 pm Post subject:
Re: Of transistor amplifiers and CRTs... |
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Philip Pemberton wrote...
| Quote: |
Chris Jones wrote:
I still don't understand - it sounds like you have already chosen a circuit
and that it cannot work because transistors can't work with 500V between
base and emitter. What I was asking about is the overall function that
your circuit is supposed to perform, in case I could suggest a different
way of doing it which would be easier to bias. I am also still not really
sure what your circuit looks like so if you could post a diagram on a
website or in ABSE then I would probably be able to give you a more useful
answer.
Right, I've dragged it through SPICE and got some voltage figures. There IS
a schematic online at <http://www.philpem.castlecore.com/zamp/dg7amp.png
that just covers the resistor chain though.
|
That's a blank link. You were going to step way back and let us see
the big picture, tell us what you're trying to do? It's the forest
we need to see, not all the trees.
--
Thanks,
- Win |
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Fred Bloggs
Guest
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| Posted:
Thu Dec 08, 2005 5:35 pm Post subject:
Re: Of transistor amplifiers and CRTs... |
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Chris Jones wrote:
| Quote: | Philip Pemberton wrote:
In message <11p9h8hqsv0vv41@corp.supernews.com
Chris Jones <lugnut808@nospam.yahoo.com> wrote:
I don't quite understand what you are asking for: What voltages do you
need on the CRT electrode when the beam is on, and what voltage do you
need when
the beam is off? If you could post a drawing of the circuit on a website
somewhere then that would be helpful too.
OK, the voltage at the transistor emitter will be about -500V relative to
ground. The collector will be at about -400V.
The input ranges from 0V to 4V relative to ground.
This is fine - in theory. Except the transistor's base is referenced to
the emitter. There's 500V or so between base and emitter, so it's locked
on hard. What I need is for the transistor to sink between 0 and 200uA as
the input varies between 0 and 4V.
Later.
I still don't understand - it sounds like you have already chosen a circuit
and that it cannot work because transistors can't work with 500V between
base and emitter. What I was asking about is the overall function that
your circuit is supposed to perform, in case I could suggest a different
way of doing it which would be easier to bias. I am also still not really
sure what your circuit looks like so if you could post a diagram on a
website or in ABSE then I would probably be able to give you a more useful
answer.
Chris
|
You are falling victim to the TROLL technique- where they are
deliberately vague and pretend to suffer from an always elementary
confusion that cannot be altered by the well-intentioned response. The
hapless retard working on the clock, the mystifying amplifier
requirements, vague and partial circuit descriptions, the pretense of
sincerity, the willingness to respond with even more prolific confusion
and so forth. The best possible outcome is that he will act out his
schizoid alter personality and electrocute himself. |
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Philip Pemberton
Guest
|
| Posted:
Fri Dec 09, 2005 1:35 am Post subject:
Re: Of transistor amplifiers and CRTs... |
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|
In message <dn96io01ifm@drn.newsguy.com>
Winfield Hill <Winfield_member@newsguy.com> wrote:
| Quote: | That's a blank link. You were going to step way back and let us see
the big picture, tell us what you're trying to do? It's the forest
we need to see, not all the trees.
|
I should really check links before I post them.. The working one is
<http://www.philpem.castlecore.com/zamp/dg7chain.png>, which is the diagram
of the resistor chain that generates the bias voltages for a DG7/32 CRT.
What I'm trying to do is take an analogue signal between 0V and 4V (where 0V
is full-blank, 0.5V is "black", and 4V is "peak white") and use that to
modulate the brightness of the CRT.
Generally, that's done by varying the difference between the cathode and G1
voltage. If Vg1 goes below Vcathode by about 160V, the beam is completely
blanked. If it's set to Vcathode, you get a horrendously defocussed beam
that (if focussed down properly) would be bright enough to burn the phosphor.
Vg1 = Vcathode-70V gives maximum beam output without fouling up the focus.
So what I need to do is load down the output of the resistor chain by an
amount that varies dependent on the control voltage (0V to 4V).
I've had a look at transistor current sources, which appear to be ideal for
the task. Problem is, the control voltage is referenced to 0V, so if I wire
it up to the resistor chain I end up with about 500V between base and emitter
(or collector, depending on how it's wired). Needless to say, the transistors
don't like that...
It seems the "industry standard" way of dealing with this is to put a huge
resistor (about 10 megohms) in the path between the resistor chain and the
CRT grid (G1), then use a 0.1uF (or 0.01uF) capacitor between Z-in and G1.
I'm just trying to figure out how best to do this...
Thanks.
--
Phil. | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem@philpem.me.uk | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/ | Panasonic CF-25 Mk.2 Toughbook
No software patents! <http://www.eff.org/> / <http://www.ffii.org/>
.... Cookie pieces contain NO calories, breakage leaks calories. |
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Chris Jones
Guest
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| Posted:
Sat Dec 10, 2005 1:39 am Post subject:
Re: Of transistor amplifiers and CRTs... |
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|
Philip Pemberton wrote:
| Quote: | In message <11peoputlkbjk95@corp.supernews.com
Chris Jones <lugnut808@nospam.yahoo.com> wrote:
I still don't understand - it sounds like you have already chosen a
circuit and that it cannot work because transistors can't work with 500V
between
base and emitter. What I was asking about is the overall function that
your circuit is supposed to perform, in case I could suggest a different
way of doing it which would be easier to bias. I am also still not
really sure what your circuit looks like so if you could post a diagram
on a website or in ABSE then I would probably be able to give you a more
useful answer.
Right, I've dragged it through SPICE and got some voltage figures. There
IS a schematic online at
http://www.philpem.castlecore.com/zamp/dg7amp.png> - that just covers the
resistor chain though.
Basically, when the Z amplifier gets a +4V input relative to 0V, I want
the equivalent of a 170k resistor placed between "G1" and "X". That
provides a 200uA (ish) path, and pulls down the cathode, thus increasing
the brightness of the trace. Brightness is determined by the voltage
difference between the cathode (which is wired to the 33k/680k junction to
the right of "X") and G1. The lower G1 is relative to the cathode, the
dimmer the trace gets.
OK, so for "normal" brightness with the 100K "Intensity" control at
mid-position and no resistor over G1 and X:
V(x) = -509.00V
V(g1) = -554.40V
V(ca) = -479.04V
Diff1 = 45.40V
Diff2 = 29.96V
And with the 170k across X and G1:
V(x) = -517.92V
V(g1) = -553.67V
V(ca) = -487.31V
Diff1 = 35.75V
Diff2 = 30.61V
V(ca): Cathode voltage relative to 0V
V(x): Volts at "x" relative to 0V
V(g1): Volts at "G1" relative to 0V
Diff1: Difference between V(x) and V(g1)
Diff2: Difference between V(x) and V(ca)
This doesn't take into account the effect the CRT has on the voltages -
it's just a simulation of the resistor chain.
A voltage difference of about 32V is enough to get a full-brightness trace
without causing the focus to go wildly out. About 55V is enough to shut
down the beam.
I've been using a transistor current source, as shown in AoE (emitter
resistor controls the current, NPN transistor - from page 74) with the
"ground" end of the emitter resistor wired to G1 and the collector wired
to X. The catch is, because the emitter is at -500V relative to ground,
feeding it a 0V input ends up tripping the PSU's output breaker. Feeding
it either X or G1 as an input has the desired action - either increasing
or decreasing the brightness.
Circuit as follows:
X >--------C E---[ Re ]----> G1
\ /
=======
| ZTX458 (NPN Si HV)
|
^
0-4V Z axis input
So far, the best idea I've come up with is to add a resistor between the
base and emitter and feed the input in through a capacitor. I'm not even
sure if that'll work, though...
The current passing between G1 and X needs to vary between basically
nothing (0V input) and the 200uA maximum (4V input). Ideally the 200uA
figure needs to be adjustable over a range of 50uA or so.
Later.
|
Here is my suggestion:
-600V -478.9V -327V -127.4V -18.2V
| Bright Ca ? ? ? 0V
| 133k? | 680k | 220k | 120k | 20k |
..-\/\-.-\/\/-..-\/\/\/-..-\/\/\/---\/\/\/---\/\/\/---
| | ^ | 220k |
| | | .-\/\/\/--.
| .--. ^ 680k |0V (Gnd)
| |G3 .-\/\/--.
| | |B (300V PNP)
| |G1 | ----^--- Contrast
| 100k | 560k | C / \ E 22k?
..---/\/\/\-.--/\/\/\----.----/ \---/\/\/\-/\/\/\-.---logic(0-4V)
6.8k ^ |
Logic 0: G1=555.2V | |
.---.
My suggestion is basically to keep the cathode bias voltage about constant
(assuming beam current in the cathode is fairly small), and to modulate the
G1 voltage. A 300V transistor should do, these can be obtained in TO-92
package with adequate power rating I think. The transistor is operated
common-base. I can't be bothered simulating or calculating to find the
exact resistor values you need for the correct brightness and contrast but
you can do that. If the high resistor values make the circuit driving G1
too slow for your needs, you could reduce all of the resistor values but
this will increase dissipation. You could also try putting a tiny cap
across the 560k resistor but I would try without first. Some spark gaps
might make the circuit a bit less prone to damage.
Chris |
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