Reverse-engineering and testing an x-ray filament supply--- make money online

I bought a set of x-ray equipment on eBay, which included an x-ray head unit, 50KV supply, and filament power supply. The filament supply has a 25-pin connector, and no data or markings on the case or board. I spent some time reverse-engineering the circuit, and determined how it should be connected to make an emission-controlled x-ray

system. I tested the whole thing, and it appears to be working as intended at 50KV / 1 mA.  The head unit produces a beam of about 25 degrees total angle.

hey everyone I bought another x-ray tube

set off eBay and today I finally got the

filament supply working so I think this

thing is almost ready to fire up but

first on to show you how an x-ray tube

typically works you've got an anode here

and a filament here and you put a large

DC voltage across these two and then you

heat up the filament with a typically a

high current low voltage source and it's

actually the temperature of the filament

that regulates how much power the tube

is going to consume and it's a nonlinear

relationship so as you heat up the

filament ie put more current through the

tungsten piece of wire it'll get hotter

you get more and more emission current

which is the amount of current flowing

through the high voltage supply so when

I first got this tube from ebay I fired

it up sort of a test run and was very

surprised to find out that the filament

is floating so I'll show you later but I

really was really under the assumption

that this was a grounded path in that

the high voltage would just flow to

ground but that's actually not the case

and the reason that they floated the

filament was so that you could put a

current sense resistor here and then

measure the voltage across this current

sense resistor and use that number to

control the filament temperature so this

control box here is this and I spent

quite a bit of time figuring out how to

use this thing because there's just a

big db25 connector on the back there and

there's no there's no information I

contacted the manufacturer and got a

surprisingly got a response right away

but the response was we can't help you

and I you know pleaded one more time and

they said no so I carefully looked at

the circuit and did a very crude kind of

reverse engineering sort of diagram of

it this is probably not a hundred

percent accurate but it got the job done

so the point of this circuit is to

measure the amount of emission current

going through the tube and then adjust

the amount of filament current to

achieve a specific emission and since we

have this nonlinear relationship here

the circuit has to be a closed loop and

quick reacting so that you don't get a

runaway condition

I should also point out that the circuit

doesn't just use a simple current sense

resistor there's actually an opto

isolator in there and some other

circuitry but essentially this is

basically what it does and the voltage

across this sense resistor is

essentially here in the circuit so like

I say there's other components in here

but more or less the amount of current

that is going out the filament is what's

connected to this point in the circuit

and there's two op amps in there there's

an LM 3:58 which is the dual op amp and

these are the two parts of it and this

first stage is just set up as a

non-inverting 10x stage so conveniently

the db25 connector j1 includes a tap to

the output of this op amp so as the

current in and out of the filament

changes the current or rather the

voltage here changes so zero to one

milliamp here indicates zero to 10 volts

here I have one power supply set up so

that I can push current into the

filament as if the tube were running and

I'm measuring the current with the

multimeter here and I'm measuring the

voltage at this node in the circuit with

the oscilloscope and as you can see if

we've got you know half a million of

current we've got five volts on the

scope so this is the simple 10x gate

gain stage here the second half of the

circuit is an integrator so the output

from the op amp goes through a capacitor

and one of the capacitors has a 10k

resistor across it and the other

capacitor doesn't have any resistor

across it so this functions as as close

to a pure integrator as possible and

essentially this is open loop if the

integrator doesn't have any resistance

in the feedback path this thing will

just continue making the output go up or

down without bound so the reason that

this system works is because the

feedback comes in the form of the tube

itself so as more filament current flows

through here more emission current will

flow and then the emission current will

be measured by this and will be fed back

into the

circuit so what communica confused me

for a little while was this connection

this is actually the set current and so

they're probably using this sort of

virtual ground concept so what you do is

you put a voltage here that you want

this thing to mash and the voltage will

correspond to the emission current the

desired emission current right now I

have this 1 ohm resistor taking the

place of the filament and I'm measuring

the current through the resistor with

this meter here so currently we have

three point twelve amps going through

there there's also a set a set

potentiometer on this board to set the

maximum current even when the integrator

goes all the way to its end and so I

guess it's set right up to about three

amps right now this meter is measuring

the simulated current going into the

filament so as if this were connected to

an x-ray - this would be the emission

current and I can adjust that with a

supply here so I basically just

connected my power supply through this I

think it's about a five K resistor here

into the filament loop so I basically

put a power supply in place of the x-ray

tube with a resistor so that I can

control the current going through there

so if I turn the current up now you

notice that the filament current has

gone to zero because we're now drawing

enough current where the setpoint has

been reached and if I turn this back

down now the current rises again so

instead of adjusting this I can also

adjust the setpoint right now it's about

3.3 volts if I turn the setpoint down

eventually the current drops down and so

now it's about 2.4 volts on on this node

here

okay so I've got the whole thing set up

here this is the supply that we were

just working on the filament supply and

to it I've added this little control box

the control box just has a couple of

voltage regulators a couple of trim pots

for the meters a couple of

potentiometers and a voltage follower

the follower takes the ten volt current

sense signal from here and displays it

on this meter this is a pretty crappy

meter this one's a little bit better

this one just measures the voltage sense

coming from the high voltage supply so

this is a 28 volt switching power supply

that powers the 50 kilovolt high voltage

supply for the tube the x-ray tube

itself is back here and it has a lead

plate in front of it that's kind of

cracked open a little bit with the

Geiger counter sitting next to it so we

can tell if there's x-rays coming out

I've got my current probe clamped on to

the one of the filament lines and that

will show amperage here I've got this

meter set up just to measure voltage

coming out of the current sense coming

out of the voltage supply so we actually

have two current senses here one of them

is in the filament supply and one of

them is from the voltage supply since

the voltage supply knows how much

current it's producing at 50 kV they

should be in agreement in the original

circuit they didn't use the current

sense from the high voltage supply but I

soldered a wire on we'll just check it

to make sure okay so I'm going to plug

in the high voltage supply and you can

see the needle on the gauge went up here

so we were reading about 50 kilo volts

and now I'm going to plug in the

filament supply okay nothing's happening

it because I haven't set the current

setpoint here so if I turn this up a

little bit we can see there's activity

and we're getting some readings here and

that was just a little burst so if I

turn this up almost all the way we can

get up to about one milliamp of emission

there's about a tenth of an amp flowing

through the filament and the current

meter here is also agreeing with with

the one milliamp

so it appears we have the whole thing

working properly it's very controllable

the voltage is independently adjustable

from the emission current and I'm ready

to basically box all this thing up and

then build the tube into my backscatter

detector all right hope you enjoyed that

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