How to shift a signal with an optocoupler? - Page 1
Forum section: electronics / beginners

author: Artlav
author_rank: Frequent Contributor
timestamp: October 11, 2014, 08:19:41 pm
content: I have a microcontroller producing a signal at 0 to 5V.I have an output that should go between 1000 and 1200V, according to the signal from the uC.Question is - how do i get the signal a 1000V up?Roughly speaking, it's something like that:The most straightforward idea i had is to use an optocoupler that can take 200V in place of the NPN transistor (i.e. TLP627), but then i have no clue what sort of response will i get.Another idea is to use a regular optocoupler (i.e. 4N35) to drive the base of the HV transistor somehow, but once again i'm clueless about how to make a controllable circuit out of it, and it sounds like it would need an A+5V tap to work.Any help pleas?Perhaps there is a better solution entirely?

author: SeanB
author_rank: Super Contributor
timestamp: October 11, 2014, 08:26:51 pm
content: What sort of speed and response time do you need? If slow is fine you can use a photomos relay to do the switching, directly replacing the transistor in the circuit.Datasheet here, and they are quite cheap now it seems.http://docs-europe.electrocomponents.com/webdocs/1314/0900766b81314264.pdf

author: Simon
author_rank: Unknown
timestamp: October 11, 2014, 08:28:49 pm
content: Your opto-coupler is like a relay, firstly be careful of isolation at those voltage differences. You need to look at what voltage the opto-coupler output photodiode or phototransistor can take, yes maybe use a high voltage transistor.

author: Artlav
author_rank: Frequent Contributor
timestamp: October 11, 2014, 09:24:30 pm
content: Quote from: SeanB on October 11, 2014, 08:26:51 pmQuote from: Simon on October 11, 2014, 08:28:49 pmThe entire problem is that i don't need a relay - an on-off switch - but need a continuous range.The uC gives out a certain voltage between 0 and 5V (or PWM if that's easier to shift over), and the output should be proportional to it, between 1000 and 1200V. Whether it's inverting or not does not matter.Speed required is in the range of a few KHz.

author: Simon
author_rank: Unknown
timestamp: October 12, 2014, 06:57:59 am
content: there are opto's that will do that I was refering more to the insulation and wiring but there are analog ones.

author: max_torque
author_rank: Super Contributor
timestamp: October 12, 2014, 11:18:43 am
content: Question is - how do i get the signal a 1000V up?Answer - Very carefully!!!I am always amazed by the number of posts in the beginners sections that say "help i know nothing about electronics, but i want to play with 1000Vdc".......... I know nothing about bare knuckle cage fighting, but i have no intention of giving it a try at the weekends either ;-)However, more usefully, if the minimum output voltage is 1000Vdc, you could "float" your micro at this level (ie 1000vdc becomes "ground" for the micro) do the isolation on a digital input signal with conventional opto or whatever, and then use the micro to generate the 0-200V output, which will be a lot easier to do!In any case, be VERY careful!!

author: Artlav
author_rank: Frequent Contributor
timestamp: October 12, 2014, 02:10:59 pm
content: Quote from: max_torque on October 12, 2014, 11:18:43 amHm.Nice idea, but it avoids the problem rather than solving it.It might be possible this time, but what if it wasn't?One more solution i can think of is to use PWM, and filter it on the high side.However it's a lot of extra components and i'm not sure an optocoupler can be fast enough.Quote from: max_torque on October 12, 2014, 11:18:43 amSure!

author: c4757p
author_rank: Super Contributor
timestamp: October 12, 2014, 02:34:42 pm
content: Is this a "safety" 1000V (as in, 1000V maximum), or is it actually exactly (within reason) 1000V? You could use a tallcascodeto level-shift the output of a current sink, if the voltages are actually fixed. If you use FETs instead of BJTs, this could be quite accurate, though BJTs will have their base current (quite a lot, as HV BJTs have typically low beta and you'll have a stack of some four-six of them).Something like this:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2014, 02:16:48 am
content: You're better off doing it with a current sink (like that) or another 1000V source (see most any 400 or 500 series Tek scope manual for how they did Z axis) .Tim

author: Artlav
author_rank: Frequent Contributor
timestamp: October 13, 2014, 10:55:28 am
content: Quote from: c4757p on October 12, 2014, 02:34:42 pmIt's for deflection plates on a CRT.The anode voltage is about 1000V (exactly on the one i have, but can be in range of 700 to 1400V), and the plates should be brought above that to deflect the beam electrostatically.So, a cascade is not exactly the most usable solution.Basically i want to be able to draw vector graphics on a CRT.

author: c4757p
author_rank: Super Contributor
timestamp: October 13, 2014, 11:08:48 am
content: A cascode could still be usable there, just design it with plenty of voltage margin. Possibly a few HV MOSFETs?But - since when do the plates have to be above the anode? That's not how oscilloscope CRTs work, the deflection plates are usually near (~40-80V) ground, with the anode above ground and the cathode below. What kind of tube is this?

author: Artlav
author_rank: Frequent Contributor
timestamp: October 13, 2014, 12:14:19 pm
content: Quote from: c4757p on October 13, 2014, 11:08:48 am5LO38I,Anode voltage: 1000V, from 500V min to 1100V maxFocus voltage: 138V to 300VModulator voltage: -90V to -30VDeflection sensitivity: 0.09 to 0.14 and 0.11 to 0.16 mm per VBoth the stated voltages and the diagram seem to imply that the anode and deflection is above 0V, and cathode is at 0V.And, deflection plates are shown above the anode, so it made sense to drive them with above-anode voltage.Only relevant schematic i was able to find is a CRT clock here -http://www.webx.dk/oz2cpu/clock-scope/scope.htm, but he used a 300V tube, and so was able to cheat by using 400V transistors.There the cathode is at 0V as well, and the deflection is from above the anode.It actually sounds like a good idea to just shift the 0V to anode and make the cathode negative, but i haven't found any example where it was done this way, and i don't like the idea of having to do an isolated 0.6A heater supply.Nor have i found any practical design descriptions for working with CRTs, other than "voltage applied to the plates deflect the beam".Therefore, the reinvention of the bicycle.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 13, 2014, 01:23:46 pm
content: Quote from: Artlav on October 13, 2014, 12:14:19 pmEvery oscilloscope I've seen has the cathode at a negative potential and the anode and deflection plates close to ground (may be +100V ish). Grab a schematic for any low-bandwidth scope (20MHz or less, i.e one without a Post Deflection Anode) and it will almost certainly be using a negative cathode with the heater tied to the same potential. It's much easier to generate the heater power at an offset voltage than it is to run the deflection plates at an offset.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 13, 2014, 03:46:51 pm
content: ^^It's 2-4 times easier to couple blanking "down below" than deflection "overhead". That is, depending on just how many plates you're intent on driving. (Hint, you do want balanced drive, even for such a simple CRT. The results are pretty poor otherwise.)I will note, this depends somewhat on your priorities. In the ooooooold days of TV (circa 1930s), electrostatic CRTs were common, and because the video amplifier was the critical path, they had the cathode end near GND, so it could be hooked up better. Which meant all the anode voltage (6kV or so) also appeared on the deflection plates. They used modest values of rather high voltage capacitors to do the job -- since DC coupling is a non-issue to TV deflection, AC coupling worked just fine.Conversely, in oscilloscopes, vertical deflection at least is the critical path, and usually horizontal is almost as important. Blanking is almost an afterthought, and can be AC coupled, or if DC coupled, it's only one channel at least, probably at modest bandwidth. Hence, the deflection plates end up around GND, and the cathode end gets sunk down in the -1 to -3kV range, say. (Most later scope tubes still had an even higher final anode voltage, which was arranged using internal plates or screens, to keep the fields good, without having to push deflection voltages up as well.)Tim