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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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15th Nov 2014, 2:20 pm | #61 |
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Re: Variable HT regulator circuit.
The Mosfet is a source-follower. Its gate is set by a pot across 250v of zener diodes...
So the max output voltage is going to be a few volts less then 250v, not 300. Or have I missed something? David
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15th Nov 2014, 3:02 pm | #62 |
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Re: Variable HT regulator circuit.
Well spotted David... so it should have six (282V) of the listed Zeners in the string shown, or seven
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15th Nov 2014, 3:22 pm | #63 |
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Re: Variable HT regulator circuit.
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15th Nov 2014, 3:25 pm | #64 |
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Re: Variable HT regulator circuit.
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15th Nov 2014, 4:34 pm | #65 | |
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Re: Variable HT regulator circuit.
Quote:
B
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15th Nov 2014, 6:12 pm | #66 |
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Re: Variable HT regulator circuit.
I don't think the gate vs source voltage matters much as it is only a few volts and the output voltage (the one that matters) is measured. I suppose you could put a few diodes in the earthy end of the pot and zener to compensate.
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15th Nov 2014, 8:21 pm | #67 |
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Re: Variable HT regulator circuit.
In practice, most high-voltage zeners produce a slightly higher voltage than they're supposed to. This will more than offset the gate-source voltage.
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16th Nov 2014, 4:02 am | #68 |
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Re: Variable HT regulator circuit.
I can see that the opto-isolator shorts the gate of the MOSFET to the source while the capacitor is charging, and so keeps it turned off; but shouldn't there be a resistor from HT+ to the gate to turn it on when the capacitor has charged and the LED goes out?
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16th Nov 2014, 2:39 pm | #69 |
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Re: Variable HT regulator circuit.
Do you mean in the circuit I posted? The op-amp (comparator) only latches on when it detects 2/3 of the supply rail voltage (5V here), hence also switching on the opto-transistor and so the MOSFET. And it's ok for it to stay on until power down.
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Al Last edited by Al (astral highway); 16th Nov 2014 at 2:46 pm. |
16th Nov 2014, 5:10 pm | #70 | |
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Re: Variable HT regulator circuit.
Quote:
Also, what is the circuit actually for? It seems to be a slightly elaborate way of delaying the switch on of the HT, not a regulator? |
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16th Nov 2014, 6:23 pm | #71 |
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Re: Variable HT regulator circuit.
Hi David,
If you read back down the thread you 'll see that I put the circuit up precisely in response to a suggestion that there could be a way of delaying HT switch on ... It's really clear!
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16th Nov 2014, 6:36 pm | #72 |
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Re: Variable HT regulator circuit.
yep, see my post #58 where I pose the question/suggestion of using the VV circuit to avoid slamming high, off-load HT voltage on to a valve receiver (or amp) during warm up, from a psu with silicon rectifiers. Al's delay circuit was a helpful response to that .
B
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17th Nov 2014, 10:47 pm | #73 |
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Re: Variable HT regulator circuit.
Hi again folks... if my drawing of the 8 DIP device in my sketch didn't quite make sense, this will help. The device in the circuit shown should be labelled a Dual Photovoltaic Mosfet Driver. It is indeed specifically designed for sourcing a floating turn-on voltage for MOSFETS.
FDA217 is one such device. It wasn't clear from my sketch and but late in the night, I overlooked the point. Hope the attached link - this one from IXYS -is useful. The isolation voltage is well over 3kV, useful to know. Cheers
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18th Nov 2014, 5:38 pm | #74 |
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Re: Variable HT regulator circuit.
Thanks Al
B
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18th Nov 2014, 11:59 pm | #75 |
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Re: Variable HT regulator circuit.
I'm not certain if special transformers were used, but STC, or was it GEC, used this idea on 4KV submarine cable power feeds. One step up transformer with three windings, one input, one output and third as a saturation winding. Used feedback to control the saturation to stabilise the output. I've often wondered if the principles could be applied to lower ( hundreds of volts) circuits?
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19th Nov 2014, 10:21 am | #76 |
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Re: Variable HT regulator circuit.
The thing with the saturation winding is a 'magnetic amplifier' sometimes called a transductor.
The core is designed so that the saturation winding establishes a magnetic flux at right angles to the normal flux direction, so that the transformer doesn't try to output a huge voltage via the saturation winding. If the current in the control (saturation) winding is revved-up, it starts to drive the part of the core its flux crosses up the slopes of the B/H curve and starts to block the normal transformer action flux. Special steels were developed for mag amp cores. This technique has been used at every voltage you can make a transformer for, and almost every frequency. There are ferrite versions in some switch-mode power supplies running at hundreds of kilohertz, and I remember an article on doing it to an RF coil to extend the tuning range of a voltage controlled oscillator in the MHz region. It's been around a bit, but oddly it still isn't a generally known technique. David
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22nd Nov 2014, 5:28 pm | #77 |
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Re: Variable HT regulator circuit.
I've just assembled the regulator as per Jim's circuit in post 46 and what I’m finding is that the output voltage will not exceed the volts determined by the zeners (~250V). My input voltage is around 325V and I have a Vsg ranging from ~3.1 to 3.3V across the voltage range. Looking at the circuit in post 13, our German colleague has a zener stack of 310V to enable an output of 300V. I’ve never used this kind of MOSFET before; can anyone offer me some insight? Do I have a problem or should Jim's circuit show an output of 250 rather than ~300 max?
Thanks B Further work; looks like Vmax out is always a few volts lower than Vz?
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Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch. Last edited by Bazz4CQJ; 22nd Nov 2014 at 5:54 pm. Reason: further work |
22nd Nov 2014, 6:18 pm | #78 | |
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Re: Variable HT regulator circuit.
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22nd Nov 2014, 6:27 pm | #79 |
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Re: Variable HT regulator circuit.
Sorry Bazz, the circuit I posted was just a working sketch. You're quite right; the max output voltage is about the sum of all the zener voltages. If you want a max output of, say 310V, then add another 47V and a 12V zener in series with the five I specified. Make sure that you have adequate cooling with a decently rated heatsink or heatsinks (see the other thread "HT Supply Circuit" , in particular Herald1360's comments on this, particularly with reference to paralleling more MOSFETS. I'm going to use three MOSFETS in parallel when I build this using three 2.6 degreesC/W heatsinks.) The cooling problem mainly arises if you're trying to take near-maximum current out at a low voltage.
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22nd Nov 2014, 7:44 pm | #80 |
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Re: Variable HT regulator circuit.
It might be as well to have a fuse in the circuit too before the drain(s) but after the 220uF capacitor. 1A should do the trick - just in case one of the MOSFETs commits suicide! (If you put it before the capacitor it'll blow when you switch on because of the charging surge of the capacitor).
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