Generating 250V pulse with 1ms linear fall time  - Page 1
Forum section: electronics / testgear

author: iraquois
author_rank: Contributor
timestamp: October 09, 2020, 04:24:19 pm
content: I am planing to design a circuit if I can complete it in short time like maximum 1 week.The circuit I am going to design should capable of supplying a pulse like in the expendable thumbnailit is decreasing from 250 v to 40V linearly in 930 uS.The first thing that came to my mind is using a simple capacitor discharging but it won't be linear. (I don't know if it is possible to modify it to have more linear discharge with maybe current source or controlled load)And second was to use an IGBT or MOSFET and again controlling their gates to have adjustable fall time.But I am not sure of these ideas. Since I don't have much time to try them all what can you suggest me.Thank you for reading.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 09, 2020, 06:47:56 pm
content: One can get a linear slope with a capacitor and constant current source.Depending on how complicated the whole signal needs to be, I would consider to first generate the signal at a lower more comfortable voltage level and go to the high voltage like 0-15 V and only as the last step do amplification (e.g. 20 x).Be careful with 250 V - better do experiments at a safer level.

author: Cerebus
author_rank: Super Contributor
timestamp: October 09, 2020, 07:07:09 pm
content: Take a look atthis postover in the TEA thread.It's a quick write up I did after figuring out how the Tektronix 151 pulse generator produces trapezoidal waves with nicely adjustable rise and fall slopes. Basically it uses a capacitor, a constant current source/sink, a diode clamp, and two differential pairs of transistors as current switches. If all you need is a controlled fall, then you only need half the circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2020, 09:39:52 pm
content: What's the load? Is it buffered?You say it's a pulse, but don't show the rise time. Is it instantaneous (as SPICE would hold)?Tim

author: vk6zgo
author_rank: Super Contributor
timestamp: October 10, 2020, 01:46:32 am
content: Use a 12AT7 twin triode vacuum tube connected as an astable multivibrator to get the voltage range.The "fall time" (as normally measured) will be less than 1ms, but is hardly "bleeding edge", & with a bit of "tweaking", should be readily achievable.

author: David Hess
author_rank: Super Contributor
timestamp: October 10, 2020, 10:22:47 pm
content: Generate the ramp at a lower voltage using a capacitor and current source, or for simplicity an integrator, and then amplify it to the final level. A single transistor with shunt feedback could amplify it.I might copy the circuit configuration of a CRT z-axis amplifier which converts a low current input to a high voltage output.A 1 millisecond transition time is slow enough that lots of options are available. Driving the load might be the most difficult part.

author: iraquois
author_rank: Contributor
timestamp: October 12, 2020, 06:54:28 am
content: Thanks a lot for all answers. Sorry for delay. I will look at all of these.Quote from: Cerebus on October 09, 2020, 07:07:09 pmAs I understand from this circuit it takes differental of the waves between "V5 & V6-V7" and generates triangular wave. But Didn't quite understand why you choose to use double bjt in each side. (is it like a totem pole?). So I might use it and amplify it am I correct?Quote from: vk6zgo on October 10, 2020, 01:46:32 amOkey thanks I will give it a shootQuote from: David Hess on October 10, 2020, 10:22:47 pmYes actually I have a signal generator and I will try to amplify as you suggest. But I am not sure exactly how to do this. Can you give an example about shunt feedback? Is it like common - emitter collector feedback configuration ?

author: iraquois
author_rank: Contributor
timestamp: October 12, 2020, 06:56:44 am
content: Quote from: T3sl4co1l on October 09, 2020, 09:39:52 pmActually I didn't show the rise time since it is not important. You might think it as triangular wave pulse with 30 uS delay and 970 uS linear fall.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2020, 07:41:39 am
content: Okay, but... that was one answer, for three questions?Tim

author: TurboTom
author_rank: Super Contributor
timestamp: October 12, 2020, 08:05:34 am
content: Quote from: iraquois on October 12, 2020, 06:56:44 amRead above: That's the most important question that will define if a shunt amplifier, a "small" push-pull configuration or something really beefy is required. Unless you provide an answer to this, don't expect to get a detailed answer.

author: iraquois
author_rank: Contributor
timestamp: October 12, 2020, 10:48:00 am
content: Quote from: T3sl4co1l on October 12, 2020, 07:41:39 amThe load is 50 ohm. And it is instantaneous spike. I Just want to see this signal on 50 ohm load.Buffered ->I found a function generator. I will try to amplify its ramp function mode if I can. Possible problem is that : it has 30uS delay time (on 250V) before starting to decrease linearly.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 12, 2020, 11:02:47 am
content: Oh well that's a shitton of power, 230V into 50 ohms is 4.6A and over 1kW! You can't do this with jellybean parts (not even 12AT7s).So the waveform, you just want it to go up to 230C over some indeterminate period of time, then following a trigger I suppose, drop linearly to 40V in that time? And the load is exactly 50 ohms resistive, no funny business, no coupling caps or induction coils or anything?Tim

author: iraquois
author_rank: Contributor
timestamp: October 12, 2020, 02:57:25 pm
content: Quote from: T3sl4co1l on October 12, 2020, 11:02:47 amYes it is exactly 50 ohms resistive part.

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2020, 08:40:23 am
content: Quote from: iraquois on October 12, 2020, 06:54:28 amIt is essentially the single transistor version of a current-to-voltage converter or transimpedance amplifier.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 14, 2020, 08:07:44 am
content: Quote from: T3sl4co1l on October 12, 2020, 11:02:47 amNo, but this might do it!https://culture.teldap.tw/culture/index.php?option=com_content&view=article&id=1396:6166-diode-vacuum-tube&catid=148:a-slice-of-wonder&Itemid=209Unfortunately, the website got things a bit wrong, labelling this seriously large tetrode a "diode".Quote

author: vk6zgo
author_rank: Super Contributor
timestamp: October 14, 2020, 09:14:57 am
content: But wait! there's more!Quite apart from joking, back in the day, before "IF modulated" transmitters became universal, 10kW TV transmitters made by Marconi, AWA, & others, used 6166 tubes in the final RF output stage which were "grid modulated" with around 1kW of analog video signal, using lesser tubes (although they were still serious power devices).Those modulators would have had little problem with the OP's signal.In more modern times, large FETs were used in parallel groups to produce RF power in the multiple kW region at both VHF & MF regions, so modern devices with the power hadling capabilities are out there.Because the signal needs to be as specified, I would suggest power devices designed for linear operation, rather than the ubiquitous switching MOSFETS.As soon as the OP mentioned the power & load impedance requirements, the thought of "slapping something together on the bench" went out the window.This thing will have to be properly engineered as a fairly long term project.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 14, 2020, 09:28:34 am
content: Sure, easy when you have 10kV of plate voltage to work with. OP might rightly complain about the efficiency though.I'd have reached for a (or a few of) 6AS7 or EL519 (or 6S33S) first...Well, after noticing I have a dozen types of MOSFET on hand that would do.Tim

author: AndersJ
author_rank: Frequent Contributor
timestamp: October 14, 2020, 09:30:44 am
content: What type of application needs a 250V pulse like this, into 50 Ohms?

author: Cerebus
author_rank: Super Contributor
timestamp: October 14, 2020, 01:31:04 pm
content: A bug-zapper for RF bugs?I'll get my coat...

author: Someone
author_rank: Super Contributor
timestamp: October 14, 2020, 09:35:57 pm
content: Quote from: AndersJ on October 14, 2020, 09:30:44 amNonsense specifications derived by a person who doesn't understand the actual application?