This project is basically a small toaster.
Perhaps to cool this thing down I should try to duty cycle the anode voltage ?
edit: or use some kind of multiplexing to light the tubes one at a time ?
I did manage to test at 100 to 115 VAC. At 100 VAC it just puts out 138 VDC (under small load)... so I might have to use the "spark me into life" circuit from Les as insurance.
I am concerned that is does say max 250v in the data sheet .. but as there is hardly any current due to the 2.2M resistor it should be ok.
The case for this is going to be "difficult" ... you are right about the ventilation but I would prefer passive cooling if possible.
Oh... the transistor I am using at the moment is a BF469 (overkill).
Hy fire,
It is bloody annoying when other things get in the way of electronics- my wife is particularly good at that.
It is not too bad- only around 200W maximum, also there doesn't seem to be any serious size constraints . It is only the transistor junction temperature and collector resistor temperature that need to be kept within bounds.
Multiplexing is a good idea in general, but not suitable for this application. The tube type and fine control that you require would not be comparable with multiplexing.
Yes, I was worried about the low secondary voltage with 100V mains input. What rectifier diodes are you using? I know you are adverse to winding transformers but why not put another winding on the transformer core to increase the secondary voltage. As far as I can see, it would be dead easy- you may have to put the Old Rosie down though and use both hands.
Agree, but Les has suggested a good way to reduce the striking power supply voltage.
In parallel with the electronic design proving, it is best to consider the case design and thermal management for the 'production' model. In my experience, most home projects fail, not so much because of the technology, but because of a failure to complete the case and mechanical aspects- I had hundreds of technically proven projects that never got to fruition for this reason. If you see the case deign as a problem, just post your requirements and I or others will suggest a deign for you to consider. I think it would be a simple design, subject to your requirements that is.
They are obsolete transistors but more than adequate for your job
spec
Hy fire,
It is bloody annoying when other things get in the way of electronics- my wife is particularly good at that.
Hi fire,
I think your basic idea in post #162 is a good one to reduce the amount of heat but you would be dropping the voltage too much with only a 55 volts transformer. You would be back with the possibility of tubes not striking and you would only get 70 volts DC on light load. You would also need to allow for a few volts across the control transistor. I think you would need a transformer with about a 70 volt secondary. As you have a variac you could test how low a secondary voltage you could go to using the existing transformer. Farnell do some 2 x 35 volt transformers which is about what you would want but the smallest power rating is 300 VA so they would be too heavy.
Les.
You wanted naked dancing Russians .. you got it.
Well one anyway.
This oneHy fire,
What exact transformer are you using?
spec
This one
https://au.element14.com/multicomp/mcta120-55/transformer-toroidal-2-x-55v-120va/dp/9530592?krypto=AxbrAmTPDak5VyAWVR+doNM1C6QhK/ktPD8WimTjG2MoaaRrjncrd3wKNVBrwsoqjxS9ZVVeeE1/ /lRm1vRG38/ej3nqCtT9WXs74+Qdgb8KEl1XG1qv8b5ps7cB34wC
Diodes are fast ones at 1000v I had lying around.
As usual up at the shed.
Don't think they are Schottky though.
Any volt drop should be eliminated though I agree.
https://www.farnell.com/datasheets/1662165.pdfGot off my bum and risked life and limb walking to the shed in the dark.
This is oz you know .. I might have been attacked by a wombat.
Anyway.
Diode is
https://www.farnell.com/datasheets/1662165.pdf
The diodes may be causing an necessary voltage drop.
spec
You do know that you could solve all your problems with an extra winding comprising a few turns.
Afraid diodes not optimum for this application- more for low power switch mode PSUs. You need high conductance types. 10A or more schotkky types would be ideal- sorry
https://www.farnell.com/datasheets/1662165.pdf Using a 100uF/400V cap that is supposedly low ESR.
Cap Sounds good. Do you have a capacitance function on the Fluke- if so check the actual value of the capacitor.
spec
Here are two more suggestions to get the right voltage. 1 Connect a 12 volt transformer (Such as this one.) in series with the 55 volts (With windings in parallel.) so that you would have 67 volts AC.
2 Sit the high voltage supply on top of the raw low voltage supply to add that voltage to the high voltage supply.
Les.
Hi spec,
Fire would probably need just a 5 volt low voltage supply so a 9 volts transformer would be about right. So if he used a 9 - 0 - 9 transformer with a bridge fed from the ends and the negative output of the bridge to ground he could feed the 5 volt regulator from the centre tap (About 12 volts peak) The positive of the bridge would give about 24 volts so if this was added ( HV supply - to this + 24 v) to the 77 volts (55 x 1.414) he would have an HT supply of about 100 volts. This would mean that if the 115 volt mains input was down to 100 volts he would still have 87 volts. I don't think you need to allow as much as 20 volts across the transistor and its emitter resistor. He would not be driving the base with any more than 5 volts so there would only be about 4.4 volts across the emitter resistor so the transistor should still work with about 8 volts on its collector. I think he should try it with the existing power supply and see how low he can turn down the voltage from his variac before it stops working.
Les.
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