Anyone have a schematic for a Constant Current DC load tester?

alec_t said:

No worries there. The TS has said "Fortunately I'm not working on a budget, whatever it costs will be my budget." ! That must be a first for ETO .

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First for me... ever.

The closest is when I used to work evenings and weekends in a local garage and a chap came in with his beloved Austin 1300: polished every Sunday, never been driven in the rain, and never driven faster than 50 MPH. He was most concerned because the transmission had suddenly started whining- it was a dreadful noise.

The clock read the usual 45K miles, so I did the standard test on those cars and unscrewed the oil drain plug. Sure enough, there was a single roller bearing stuck to the magnet. Back at reception the owner was pacing up and down nervously. I gave him the bad news that major repairs/replacement were required. His reply was, 'Yes, but can it be put back to what was?'. I said "Certainly, but it will be very expense." His only reply was," Yes, but are you sure it can it be put back to what it was?".

We pulled the engine and gearbox and fitted a brand new factory replacement: it cost the earth but the customer could not have been more happy. Paid on the nose too.

spec

spec said:

Sure, but in practical terms doesn't that amount to the same thing. Maybe I should have said, 'lead acid battery (LAB) voltage profile is complex'.

LAB, mass, construction, chemistry, electrolyte specific gravity, age, cell temperature, degree of sulfation, etc also influence the terminal voltage.

Could you suggest a single cut-off voltage that could be reasonably used for all LABs, or do you see a manually adjustable cut off voltage as the answer.

I suppose you could implement a universal cut of voltage of around 10.75V at any current which would at least be better than nothing.

spec

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The electronic load that I use for testing batteries has a programmed function for that purpose. I set the discharge current and the stop voltage and let it go. I'd choose the current to match what my intended load is, and choose the cutoff voltage by going to the datasheet for the battery that I'm testing and seeing what the manufacturer recommends for THAT battery at my chosen load current.

ElectronsFlow,
The cutoff voltage, if any, really needs to come from you. And may need to be based not only on what the manufacturers recommended cutoff voltage is, but what is the pass/fail criteria for your test? Also, does the test only need to match a single operating condition, or does it need to encompass over, under or fault conditions?

ChrisP58 said:

The electronic load that I use for testing batteries has a programmed function for that purpose. I set the discharge current and the stop voltage and let it go. I'd choose the current to match what my intended load is, and choose the cutoff voltage by going to the datasheet for the battery that I'm testing and seeing what the manufacturer recommends for THAT battery at my chosen load current.

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Ah, right. So you are describing a constant current load that has adjustable current and an adjustable cut off voltage. That makes sense and should be reasonably straight forward to do.

spec

ChrisP58 said:

Will this operate unattended?

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It should never be unattended. Though it would be a great safety feature it's probably one of those non-essential options. Besides the budget is being blown on more important things such as titanium trimmings and walnut cabinet

Thread split!! The "new" topic can be found here https://www.electro-tech-online.com/threads/help-with-5v-power-supply.149277/

ElectronsFlow said:

It should never be unattended. Though it would be a great safety feature it's probably one of those non-essential options. Besides the budget is being blown on more important things such as titanium trimmings and walnut cabinet

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Nice call. By the way there are a number of different walnuts: European is expensive but more appropriate for an equipment of this class.

I will see a about including a cutoff circuit, but today I am shopping in Bristol with Mrs spec.

spec

spec said:

Here is a rough draft schematic of the linear constant current load (version 4):

POST ISSUE 05 of 2016_10_17

View attachment 101616
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ERRATA
(1) Q5 should read Q5-1. Q5-3 should read Q5-16
(2) All opamps should be shown as type LM358 (may be changed to type, OPA2192)
(3) All opamp Vee pins should connect to 0V. All opamp Vcc pins should connect to the 24 V line.
(4) N-8A should read N1-8B (N1-8B pin numbering is incorrect)
(5) C4 lower should connect directly to R6 lower
(6) All 8 opamp packs should have a 100nf disk ceramic capacitor connected directly between their Vee and Vcc pins.
(7) Connect a 1N4007 diode, cathode to three terminal regulator (TTR) input and anode to TTR output (to protect TTR from reverse voltage)
(8) Remove D4 and replace with a trace. D4 not necessary.
(9) Provide gate voltage protection: to be defined (thanks Ron Simpson)
(10) Redraw schematic to show 0V star point (thanks ChrisP58)
(11) Redraw schematic to show 48V star point (thanks ChrisP58)
(12) R7-1 thru R7-6 should read 8R not 10R (typo)

NOTES

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I would add like a 2k resistor to the negative input of the op amp. Then a small cap from the op amp output to the negative input. I used spice to get a damped response to a step input.

Here **broken link removed** are some of the issues. What you want is an active or electronic load and you can buy them. They typically can do constant R, constant I, constant P, sink and source.

Here https://www.bkprecision.com/products/dc-electronic-loads.html is one company that makes them.

ronv said:

I would add like a 2k resistor to the negative input of the op amp. Then a small cap from the op amp output to the negative input. I used spice to get a damped response to a step input.

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Hi Ron,

Thanks for suggestion.

I see you are considering the open loop response of the opamps and NMOSFETs.

My plan in that respect is to use the considerable effective input capacitance of the NMOSFETS and the series resistors in the opamp outputs to form a dominant pole for single unit slope roll off. I suspect that introducing another pole will reduce the loop stability by adding more phase lag.

The opamp will be rolling off at unit slope from a low frequency too, so this aspect needs a bit of analysis. It may be better to use an opamp with a higher break point, like the OPA197/OPA192.

spec