Did you miss that the load varies by ±20%?

The load is "0.5 ohms +/- 20%", but I don't know if simple resistance or a coil.

1) If the coil was 0.5 ohms (about) and resistive I could just jam 10V across it to get 20A. or 0.6 ohms needs 12V. or 0.4 needs less volts. There would have to be a calibration to know what voltage is needed.

No I did not miss that.

that's just a 1 mH inductor.

I did not bring this up because WBahn did and because that is what I do at work. I make and sale high current high Q inductors. I have a 1mH coil made with Litz wire. It is air core because (what I need) but for this probably could be a ferrite core. I stroke it with a GAN-MOSFET.

Because we can't talk directly to the faculty member we don't know what is in his thoughts. From some experience he does not know that is needed. Only after it fails will we find out that _______. Why didn't you know that?

As to speed, 10uS is not fast in my world. There is no specification for speed, other than 10uS. My guess is they don't understand that two feet of wire is a real problem if the edges are faster than 100nS. Don't worry about speed until they understand what that costs.

We will never learn about what this is or why.

 

"R. S." has certainly nailed it in post #21. And also mentioned the solution that is much more current than my idea. Both can work, his is probably less expensive.
The problem described by "Y" in post #20 is certainly true, but not limited to those institutions. It has also been a issue for custom equipment manufacturers, of which several have been my employers over the years. Sales people who believe that they are much smarter than others are able to create concepts in violation of the laws of physics, often on a routine basis. Fortunately most customers are more focused on what results they require, rather than how to achieve those results. Those folks were always the best to deal with,

 

Turns out the load (R4) is a bolometer, which are mostly (entirely?) resistive with very little if any inductance.

On the other hand, bolometers also don’t use constant current, and usually operate in the mA region for current—not tens of A.

That makes this very confusing. The bolometer is an NTC or PTC element whose varying resistance is proportional to incident energy (e.g.: light, radio, &c.). It is used to detect (usually) very low power radiant energy.

If it was designed to do something in plasma physics or some other high powered application, there might be a clever way to use the high powered pulses. But there are strange, or at least unaddressed, issues with the idea.

Since the bolometer depends on a differential between the current-heated state and the added energy of the radiation, it would seem that the slew rate of the heating would render measurement impractical. The measurement could only be taken after the current heated the element and immediately after—or so it seems.

The other problem is that constant current in the ordinary bolometer would (seem) to defeat the mechanism that allows measurement which is detecting the resistance of the element. Sometimes the bolometer is a leg in a Wheatstone Bridge which might compensate for that… I think.

I really think the TS needs more information to understand what it is the supply must do,

 

I am wondering now if the original requester for the supply did not understand the "Milli-" in the requirement. Was the need for 20 MILLIAMPS?. I have seen that sort of error previously. Some totally non technical "typist" ignoring a handwritten prefix. Confusion results!!

 

No I did not miss that.

Go look at my post again, Ron. It was not directed at you. I quoted a post from @MisterBill2.

 

Reminds me of this classic project mismanagement and poor communication.

 

On the other hand, bolometers also don’t use constant current, and usually operate in the mA region for current—not tens of A.

Good observation!
mA vs A is important. (removed my joke)
If this is a heating element, then the amount of "power" is important, but the waveform and speed is not.
I think we should just relax. Many good solutions have been proposed. If we are off by 1000:1 then I need to find something different to work on.
Bolometer

 

The nature of this problem—not the pulse circuit, the one facing the TS—is very familiar to me.

In my experience there are two kinds of faculty members, with varying intensities of each kind:

1. The kind that approaches the support staff with the problem they are trying to solve, first describing the goal and then discussing the way(s) in which the staff may be able to help—in other words, treating the staff as colleagues and not creating a silo isolating them from the ultimate goal of the project/experiment/project.

2. The kind that, although they lack the knowledge/experience/skill to create a solution themselves, approach support staff with a specific, abstract requirement based on their mistaken beliefs that 1. they understand what the solution to a sub-problem should be, and; 2. their description will be sufficient to deliver it as they imagine it.

Been there.
The problem is exacerbated by the fact that most PhD types have big egos, and the assistants are usually undergraduates or recent graduates.
As such, the assistant is in an underprivileged position to argue with the PhD.
One would have to be a Niels Bohr to talk back against preposterous assumptions.

 

Been there.
The problem is exacerbated by the fact that most PhD types have big egos, and the assistants are usually undergraduates or recent graduates.
As such, the assistant is in an underprivileged position to argue with the PhD.
One would have to be a Niels Bohr to talk back against preposterous assumptions.

Thanks to all for your many thoughtful replies/suggestions in this thread!

Just some additional context: The original requestor who contacted me is a new postdoc in a very successful research lab. He has sent me a paper describing the kind of activity relating to his interest.
https://www.osti.gov/servlets/purl/1145734

In this paper, Sec II B describes instrumentation used in the kind of measurements he is interested in (using resistive bolometers). I plan to meet with him again to try and tease out his primary concerns and just why the stated requirements (voltage, current, timing...) are what they are. He (requestor) is very amenable to such a discussion. I admittedly don't understand much of the physics involved in this experimentation. Any additional thoughts you may have would be much appreciated.

Thanks again for the responses.

 

I read that paper and while there was one rather detailed assembly drawing I did not see any level of detail in other areas, such as actual circuits and actual numbers. All I saw was rather basic block diagrams. AND, AS ALWAYS, the problems come with the details.
And certainly the claimed repeatability is not nearly tight enough for industrial production testing systems. And I must have missed the part about how the bolometer is actually used to measure X-Ray power.

 

guess see Page 6: A. Resistive Bolometers
continuing on to page 7.

Sounds like 18V @ 36A for 7.5 uS and watch for 100 mV increase from x-rays...

Makes me wonder if it could be improved via an analog (differential) front end
before the A/D so looking for a change in the 18v rather than a 100 mV step
on 18V.

 

Certainly the bolometer described is much more massive than the ones I am familiar with. And certainly most temperature measuring schemes use kelvin connections to remove the effects of changing lead resistance in the actual measurement. So really there is a whole lot of information missing.

 

Certainly the bolometer described is much more massive than the ones I am familiar with. And certainly most temperature measuring schemes use kelvin connections to remove the effects of changing lead resistance in the actual measurement. So really there is a whole lot of information missing.

I'll be talking with the postdoc today and I'll quiz him on some of the aspects of the experiments/measurements that have come up. It's basically a plasma-confinement "Z-pinch" experiment https://en.wikipedia.org/wiki/Z-pinch looking at the temperature rise from the bolometer resistance change. But as MisterBill points out, the cited paper alone does not contain all the information. I'll see what the postdoc has to say. More later.

 

If the measurement of the X-RAY intensity is quantified by the relationship of voltage drop and current in the bolometer sensor, then it should be a simple matter to record both the voltage and the current constantly during the measuring interval With any sort of current technology computer that should provide a continuous readout of the X-RAY intensity.
That is, presuming that the sensitivity of the bolometer is repeatable, and known, for the various current levels.