How much current does a 3" CRT use ?

[lordvader88]

It's an old 19060s RCA 3AQPl 3" CRT, with -680V on the filiment, and -660V on the cathode

Whats a ball park current draw on something like these, I have no idea about CRT in general, for any size/voltage.

And asking google gets me TV/CRT power usage and electric bill non-sense

[chris_leyson]

3AQP1 probably not to dissimilar to 3BP1 which was a popular 3" tube in the UK for homebrew scope builders, about 50 years ago. Best guess only a few uA or less for beam current. Focus and astig resistive dividers would draw more current and if you find a schematic for a scope with a 3" tube... A Google search for 3AQP1 and WO-33A pops ups, no surprise lol. The current in the cathode bias chain is about 0.6mA and maybe 100uA for astig so not much really. That reminds me, I have a D7-220GH tube a need to fire up one day 

[Nusa]

Measure the power for the entire device. That'll get you in the ballpark.

However, always assume there's enough stored energy in the system to kill you if you discharge it with your body.

[Ian.M]

Unfortunately the datasheet doesn't give the max permissible beam current:
https://frank.pocnet.net/sheets/049/3/3AQP1.pdf
or even any typical values.  I assume that if you keep all voltages within their permitted limits, the beam current will be acceptable, and the actual limit will be the degree  of 'blooming' you can tolerate.

If you are attempting to estimate the EHT supply current required, larger TV CRT tubes tended to run at a beam current of the order of 1mA for peak white,  so I'd expect a 3" CRT to run at as least an order of magnitude^* less to deliver comparable energy per unit area to the phosphor.  As a scope trace is comparable to a single scanline you can further divide that by a factor of about 250, which would put the guestimated current at the order of 1uA.

As Chris points out, bias divider chain currents will dominate.

* two orders of magnitude less area BUT also an order of magnitude less acceleration voltage.

[floobydust]

The divider string (intensity, focus) total 1.18MEG at 680V so 0.58mA for that, plus the CRT.
Tried to estimate the current from the scope schematic, at the other (cathode pin 3) end, got 0.12mA if the measurements are accurate.
That CRT does not have an isolated second anode, couldn't find datasheets on any. So it would be very low current compared with only 660V.  Control grid current and focus electrodes are less than 50uA.
I would say a 1mA power supply would do.

[james_s]

The main power consumption will be the heater, the beam current likely not more than a few tens of microamps. The HV typically comes off of a multiplier ladder.

[lordvader88]

The main power consumption will be the heater, the beam current likely not more than a few tens of microamps. The HV typically comes off of a multiplier ladder.

So next to nothing then. The transformer fried and I should try making a HV transformer

[bd139]

I managed to illuminate a 3BP1 on a sub-1mA supply. I was using a homebrew inverter. This was an E-core transformer with the primary switched off an IRF510 (I think) at the time powered by a 3A CB radio supply. The secondary was stepped up to +/-270v and then hit voltage multipliers. This gave +/-1kV approx. I tried winding the secondaries with enough volts but it arced out. The heater was wound as well and I thin the anode to heater arced out. That might have been the problem with your transformer.

I have the tube somewhere. I gave up on the power supply as I had no idea what the hell I was doing back then

I should probably resurrect that project. It was going to be a scope.

[Gyro]

Yes, it's very important that the heater is at a similar DC potential as the Cathode to prevent insulation breakdown. A few well insulated turns on the same core as the inverter for the heater supply can do the trick. Otherwise you need a mains supply transformer that can withstand approx. 700V on it's heater winding.

[whoKnows]

as far as i know crts are voltage driven and not current driven devices so it should draw very little current

i recently watched a video on youtube about the nimo tube which is basicly a melange of nixie tubes and crts
there have current figures been mentioned and i belive it would basicly apply to your screen

[lordvader88]

I need to learn how these actually work.

[james_s]

CRTs? There is lots of info out there, the basic principles are not complicated. Heated cathode sprays off electrons which pass through control and focusing elements then head toward a phosphor screen. Electrostatic deflection tubes have two pairs of deflection plates inside the tube that deflect the beam.

[lordvader88]

Yeah I know the basics I guess

[tecman]

I looked up 3BP1.  The 2nd anode current range is 400 ua to 1.6 ma

paul