02-08-2015, 03:19 PM (This post was last modified: 02-08-2015, 03:24 PM by pwdrive.)
A basic schematic idea for plotting valve curves wot I drawed, the circuit details of the regulator(s) are not shown, gives some idea for the connections to DUT, switches etc, no component values shown as such....experiment
The ref for the mains transformer is given....'scuse scrawly diagram...On the heater switch to the bridge rectifier the top position is for 11.3 volts ac in the bridge as well as the 6.3 volts marked, forgot to write that in...
All those show the LR8 followed by an emitter follower, or some variation thereof. They should work, but the voltage regulation - if that matters - won't be as good as what I had in mind. Here's what I was thinking:
The resistors surrounding the LR8 can be determined from the datasheet.
The input current flows through the resistor before reaching the LR8 - this can be chosen so that when a certain current is reached - say, 5mA - the voltage across it causes the MOS-FET to pass current. From this point, it takes over from the LS8, so it bears the heat burden.
The zener and resistor connected to the gate of the MOS-FET are optional protection components. I tend to err on the side of caution when it comes to the gate of a MOS-FET, and like to fit protection. It's optional, but for what it costs... The zener voltage would be determined from the datasheet of the chosen MOS-FET...
Finally, the BJT is there to provide current limiting. It samples the voltage across the resistor in series with the MOS-FET, and once that voltage reaches 0.6V, then it starts to rob the MOS-FET of drive voltage. 12 ohms gives around 50mA...
This sort of circuit is used all the time with voltage regulators, and works well. As shown, it's just the bare bones; protection diodes and decoupling capacitors might well be needed...
Good stuff Mark, could a variable pot (set volts) be used at the LR8 adjust pin with that schematic suggestion of yours? I'm guessing for anode/screen voltages the HT output needs to be variable from say 50 volts to 250 volts or there about's, maximum current draw needed would be around 30 to 40 ma at most for the anode supply as a guess, it probably wouldn't be used for high power PA valves, I wish my workshop was useable, I'd have a go with the LR8/series pass combination.
Yes. I left off that detail because it's all in the datasheet, but basically make the resistance to ground variable. Pots, fixed resistors and switch, fixed resistors and a pot, or a bit of all
The resistor between output and adjust should be around 6k according to the datasheet (they say 6.04k - go figure!). Just like the LM317, the regulator maintains a constant voltage across that resistor of 1.2V, and at the same time, it tries its best to not draw current via the adjust pin*. That means that whatever current exists across the first resistor (R1 on the datasheet), the same current exists in the resistance between "adjust" and ground (R2 on the datasheet).
With 6.04k as the resistance, the current is 0.2mA.
So to get 250V, we need 1.2M. That's just (250V-1.2V)/0.2mA
*Actually, I've double-checked the datasheet, and apparently "adjust" draws a constant 10uA, so that increases the output voltage for a given value of R2. In the case of 1.2M, it's 12V greater than the prediction above.
Like the LM317, there is a minimum current requirement - in this case it's 0.5mA. For such a wide output range, it's probably worth using a transistor current sink rather than a fixed resistance. Or, you might be able to reduce the values of R1 and R2 to achieve that.
I have been away for a few days but I am back again now.
Here is what I would use for the high voltage regulator with a couple of mains chopper FETs from an old computer power supply.
The three diodes and the 1K resistor set the short circuit currant and some experiments will have to be done with the value and the number of diodes or perhaps a zena diode could be used in order to get a limit that will not kill the transformer. I have left out the decoupling capacitors on the input and output for clarity.
The short circuit currant can be tested in a lower voltage circuit perhaps 24 volts or so.
Those FETs have massive gain and most of it is in negative feedback for good regulation.
It will just keep the high end of the pot at about 12 volts in the sampling circuit.
That could work well. Similar versions using bipolar transistors were the mainstay of voltage regulators for decades
I'm guessing the pot should be above the feedback sampling point rather than below? As it stands, the maximum output would be 2 x (12V + Vgs)
The resistors should all be suitable for working at high voltage. Personally, I'd go for metal oxide, but metal film might be OK. Putting two in series helps with the voltage stress. The one feeding the gate of the second MOS-FET could allow a small amount of ripple in, so if it does get split in two, the mid-point could be decoupled with a suitable cap - even 100n would help, thanks to the high values of resistance present. Similar comments apply to the 470k feeding the Zener... I'd probably reduce that a bit, just to get a bit further around the "knee" - perhaps 120k?
(I'm sure the ripple would be good enough for this application, but hopefully those comments are useful for anyone thinking of trying such a regulator elsewhere where noise is a concern?)
In circuits like these, it might be worth considering an old trick - you can drop quite a bit of voltage over a neon bulb, and that bulb provides a useful reminder that the circuit is powered and might "bite". Tek used to do that in their waveform monitor power supplies, for example. So that 470k feeding the zener could be replaced with a neon in series with something in the 100k region...
I would be tempted to add a protection zener across the gate of the error-amplifier MOS-FET, but it's probably not needed in practice.
So Nick, plenty of ideas to consider, and I'm sure there will be more to come. When the time comes, it'll be good to experiment (carefully!) - you'll learn a lot in the process
Yes that was just a scratch pad drawing. The gate of the error amplifier should be at the other end of the pot so that it varies from about 12 volts to 132 volts this being 11 times the value of the zena diode.
A 250K pot would give 25 times the zena voltage.
