DC current sources

Indeed. That's partly why he chose this career :). Normally there's mains power available, and they're installing wired long-term monitoring. Not this time though.

The reason for the adjustable constant current supply is to avoid the need for daily monitoring.

Clifford Heath.

There are, but usually in the context of also installing long-term monitoring equipment (e.g. in an old skyscraper with concrete cancer). This is new work that won't be monitored continuously.

Not sure about that Ian - it's the high pH of concrete that protects the steel. Concrete cancer results when water leaches the pH down from 13-ish to about 8.5, esp. when chloride is present (e.g. marine air). More lime should help protect the steel, but the most important things are to (a) stop the rusting process, (b) passivate existing rust and (c) prevent water and chloride ingress with appropriate coatings. If you haven't done that, that's when you need to call in the experts to passivate, seal and monitor the rusted steel.

Clifford Heath.

Is this a one off? stick a 7.5V battery of the desired charge capacity on each electrode. when the batterys go flat you're done.

Not that I know much about concrete, but if you what are trying to measure is the concrete's drying time is it possible that ESR may have a role? My experience is one area shows that ESR can tell you how well a lead acid or ni-cad battery is behaving based on its past ESR when fully charged and that value changes as the battery ages. So I'm sitting here wondering exactly what your clients are trying to measure and if there is a way to do so. This might give you a variable that could control the battery voltage. Such as the resistance starts as X and thus the voltage should be Y, and as the resistance increases/decreases the voltage should change in a linear/exponential/inverse/? rate.

The fact that we sell the Bob Parker ESR kit has little to do with my suggestion, it is more that you need to be able to measure the properties if you want to automate the voltage setting. And ESR is as good a place to start as any...

John :-#)#

Not trying to measure anything. Trying to convert rust on newly- installed reinforcing. The amount of rust per section has been calculated (surface area, duration and type of exposure), so the required charge can be approximately calculated. All that's now required is a sane way to deliver that charge. There are various series resistances, some with parallel capacitances, and most of the parameters vary during treatment and with dampness, so voltage-mode doesn't work to control total charge.

Perhaps, but it's not relevant here.

(a) they don't expect to automate it and (b) it's a current setting.

There is a bit on line about the how and why of sacrificial anodes are used in contaminated concrete:

Some patents too...

Interesting.

John

Yes. There's a lot of experience too - which this company has already, since it's been their primary business for decades. Galvanic protection, impressed current protection, $30,000 penetrating radar equipment (to find re-bar in old structures), post-hoc passivation, remediation and monitoring, they do it all.

The specific problem I mooted is one they don't encounter often and didn't have a standard solution for. Surprisingly.

Clifford Heath.

Problem gets a lot easier if you ignore the initial statement of requirements and reduce the quantity by a factor of 40. ;-)

Current is a fine measure of delivered electrons as long as the voltage at the desired current never wants to go over the max voltage setpoint. If that happens, your delivered electrons won't.

Stated another way, if you want 3 amps and 9V, you'd better not have more than 3 ohms in the loop at any time in the process.

Good thing the vendor gave you a schematic so you'd understand that the current limit sense resistor is between the battery negative and the output (-). Watch out for issues if somebody decides to put more than one on the same battery...or if current from one can be induced into (-) of another thru some rebar.

If my math is right .1A x .05 ohms is 5mV controlling the current. Not much margin for noise. Gets better at 3A. A nearby lightning strike can make a mess of things. I'd put real fuses in series.

I'd do some serious QC on those Chinese clones of Chinese clones that can be delivered to your door for under $5, at a profit.

My point is to pay close attention and don't assume you can just have any technician set it all up in any situation.

I'd be interested to learn the range of series resistance encountered in this "plating" process.

Provided the total charge required is within reason, this might be not a bad idea at all. 6V alkaline lantern batteries or similar, if the capacity is right.

Can I borrow their ground penetrating radar? I want to see the ICs used on some old potted chip modules to 'read' them out. (ducking).

John ;-#)#

The rust is pretty difficult to get at until it starts to expand to the point where chunks of concrete drop off.

That depends on your polarity. These units have the current sense resistor in the negative return line, so if you're running several off a battery, and the ground return goes to the negative terminal, the battery negative terminal and the 'negative' output pole are interconnected. If you use a battery for TWO units, the negative ground stake will ensure that neither gets an accurate current reading. So, to run your twelve units, will take twelve batteries.

I think that if you have positive ground, the situation is worse; because all the positive outputs connect together, and so do all the GND (to the battery); it still takes twelve batteries to run (all the battery terminals have to float).

Yes. I had suggested an efficient buck converter to about 9.5V to set the Vmax compliance range, then LM350T, etc., as individual current sources. If there's a short fault on a channel, the '350T will thermally limit, you probably want it to do that anyway, well below 2A to save power. Just a simple clip-on heat 2W heat sink for each one.

I guess that's why these people have a business. They can passivate that rust electrolytically. It does involve some fancy gear to find the reinforcing and ensure it's all electrically connected, without demolishing the entire structure. After treatment with the impressed current, it still needs some cathodic protection, but that's as often galvanic (zinc anodes, etc) than powered. If powered, it's monitored.

That's good information, thanks. The negative will be connected to the reinforcing, the positive to the temporary anodes. If the anodes are interconnected, it'll cause dramas, as you say.

• Is not the specified (implied max) limit 9V? It is fairly easy to get at the 9V tap in some lead-acid batteries,particularly liquid electrolyte types.

• Current is not charge...you are not exactly working with capacitors.

Quoting

The coulomb (symbolized C) is the standard unit of electric charge in the International System of Units (SI). It is a dimensionless quantity, sharing this aspect with the mole. A quantity of 1 C is equal to approximately 6.24 x 1018, or 6.24 quintillion.

In terms of SI base units, the coulomb is the equivalent of one ampere-second.

On a sunny day (Fri, 05 May 2017 00:18:47 -0700) it happened mike wrote in :

Q = C.U = i.t

A lot of them delivering up to 3A is going to waste a hell of a lot of energy heating up linear regulators.

When I wanted to build a switch-mode Ni-Cd charger, I found current control was difficult to implement and had a tendency to go bang.

My solution was to use a diode-capacitor charge pump, which provides a half-decent approximation to constant current. Usually you have to draw some DC load current to keep the PWM doing something useful. That current will make up some proportion of the total current - you select the charge pump capacitor against the switcher frequency to make up the difference.

If you modify an off the shelf SMPSU - it will already have the voltage regulation circuitry that you can adjust for the maximum allowed voltage.

You leave that in anyway - it stops the SMPSU going bang when there's no load.