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Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
Grenadier wrote ...
It would, but there are a few problems.
A) It's going to be expensive B) In order to adjust the kVp, I would have to adjust the voltage that goes into the ZVS. The only way I can really do that is with a resistive voltage divider, but large rotary rheostats are really expensive. I don't even know if a ZVS will work at say... 4V. Since it won't be continuous duty I could probably get away with a 100 watt rheostat, but even that is expensive. C) Another problem with a CW is that they tend to sag under load, and an x-ray tube is quite a hefty load. D) Finding a suitable AC flyback is a big job itself.
If you go the ZVS route, there are much better alternatives to a rheostat. A buck converter is simple, cheap, and can easily control a lot of power with very low losses. 70kV at 3mA is 210W though, so even if everything was 100% efficient you're looking at over 17A from the supply.
I would probably use a full bridge inverter in order to get the most out of the limited supply voltage. Pick some good high current low resistance mosfets, they don't need to have a particularly high voltage rating. Don't forget to bypass the DC rails with some good quality film capacitors. The voltage from a battery is clean but there will be plenty of parasitic inductance and resistance in the wiring.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
I've been trying to make a buck converter for 3 days now without any success. I've never designed a SMPS before so I have pretty much no clue what I'm doing.
I'm probably just going to use a linear regulator + a power transistor. I'm considering getting 12x 14ah NiMH cells to make a battery. Should last a little over 30 minutes realistically, and weigh only 6lbs. 30 minutes is more than enough time for me. (going to cost $120 though ).
I don't think I'm going to use a ZVS, but instead this, mainly because I can easily adjust the frequency if needed. The best FETs I have are 600V @ .059ohms @ 47A, planned on using those. Overkill I know, but it's what I got.
Still looking for that 5 ohm 2.5A pot people! Anyone have one? Ehh I might even leave it out...
Also, I finally got the multiplier parts after a month, but I just realized I'm going to need a 500 meg resistor for testing it... 50 10 megs in series I suppose. (edit: I can use the 800 meg resistor out of my HV probe, yay!) Hopefully I can pick up that old TV next week so I can get an AC FBT.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Grenadier wrote ...
I've been trying to make a buck converter for 3 days now without any success. I've never designed a SMPS before so I have pretty much no clue what I'm doing.
I'm probably just going to use a linear regulator + a power transistor. I'm considering getting 12x 14ah NiMH cells to make a battery. Should last a little over 30 minutes realistically, and weigh only 6lbs. 30 minutes is more than enough time for me. (going to cost $120 though ).
I don't think I'm going to use a ZVS, but instead this, mainly because I can easily adjust the frequency if needed. The best FETs I have are 600V @ .059ohms @ 47A, planned on using those. Overkill I know, but it's what I got.
Still looking for that 5 ohm 2.5A pot people! Anyone have one? Ehh I might even leave it out...
Also, I finally got the multiplier parts after a month, but I just realized I'm going to need a 500 meg resistor for testing it... 50 10 megs in series I suppose. (edit: I can use the 800 meg resistor out of my HV probe, yay!) Hopefully I can pick up that old TV next week so I can get an AC FBT.
The diodes have a f/v of 5.5V. Not bad actually.
I wouldn't go with NiMHs, there are newer battery technologies out there that give you more bang for your buck, here is an example of a Li-Po battery you could use. It's got a bit lower capacity than the NiMHs you proposed, but why do you need 30 minutes of exposure time from a single charge? With this Li-Po you could manage over 100 exposures at 5mA, 70kV and 4mA exposure time. An other advantage to Li-Pos is that they deliver more of their rated capacity under heavy loading. Chargers are relatively cheap too.
I agree with hvguy that it's a good idea use closed loop control for the voltage, i.e. measure the anode voltage with a divider, then feed that voltage back into the feedback input of the PWM chip, to actually get a regulated tube voltage. I found some nice resistors on eBay that would be ideal here: . I think you should use a resistive divider to measure the tube voltage even if you don't make it regulate itself, knowing the anode voltage is essential if you want to get consistent results with it.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
That lithium is cheap! I didn't know they have come down in price so much. I still don't know what voltage I'll need though, I'm hoping it'll be 12 but it could be 24. I need to get the circuit working before I buy a battery.
As for the regulation, I'm not really sure how to do that. Sure the voltage divider is easy enough, but ho do I feed the output voltage into the IC? I see an error amp in there and I'm assuming that's how, but I'm not too sure what to even do with that...
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
No. I was fearing you had used an ungapped ferrite core, like a toroid, which would not work at all.
It's all in the compensation network. This should not be too hard to design, there are tons of application notes on this sort of stuff. Or just steal the values from a similar design, which should be tons better than no compensation network at all.
Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
So, the sg3525 thing didn't work. I tried a high frequency (80khz) ZVS and there;s some problems...
If the core gap is too small it runs at 24kHz. That's not too big of a problem since the core gap is easily adjustable. The problem is that at 80kHz I can only get 1000V out of the flyback. Plenty of current but the voltage is meh. I tried both a 14 turn and a 60 turn inductor on the same toroid and that didn't change anything. I also removed a primary winding, that gave it an extra 300V but that only leaves me with 8 primary turns (4+4). At 24kHz I get 4kV, but the lo frequency will cause current issues with the multiplier.
The core also get pretty toasty. There must be a bunch of power loss in it.
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Designing a variable HT PSU
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