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

[spec]

Yes.

For a previous dummy load project, schematic and results, see https://www.electro-tech-online.com...emp-control-fan-load-bank-pwm-circuit.128753/

Thanks Dougy: some useful information in that (long) thread.

spec

 

[spec]

Linear is simple. I just don't like to dump 1000s of watts into silicon. Probably because I spend hours every day trying to not put heat into silicon.

30 years ago I made truck/bus alternator testers. (12/24 volt) Used car head lights as a load. Could not stand the heat. Pointed the light up and heated the sealing. Kept me cool. At that time head lights were cheaper than resistors.

Like the car headlight load.

Taking that a step further you could have an array of lights, say purple at one end- going through blue, green, red, orange, yellow then white- that came on according to the amount of current drain from the battery.

I know what you mean about dumping watts into silicon: somehow goes against the grain as you say.

I can remember, when 2N3055s cost the earth, reading an article in a trade magazine where somebody suggested using a 2N3055 as a controllable heater element inside an equipment- I was thoroughly disgusted on moral grounds.

I did a test set once that would dynamically test a multi-output power supply (like a PC PSU) during environmental and life testing. We never had a problem with the power supplies but the test set was another matter. I learned a lot from that exercise.

spec

 

[ElectronsFlow]

Then you could get really fancy and measure the actual battery voltage and then do all sorts of calculations in the Arduino

I've had the same thoughts exactly! Since we will have an arduino on board, why not add some additional features!

What's the consensus?

I'm going to step down from any option, just simply because dealing with this many watts isn't my day job. However the 4x 600w resistors approach should be pretty bullet proof. The tester wont be sitting idle on a shelf, it will be used quite frequently so i suppose the most reliable approach will be the best from my point of view

I imagine these are the 600w 3.9R resistors you had in mind Spec? https://nz.mouser.com/ProductDetail/TE-Connectivity/TE600B3R9J/?qs=HLeXuo0aL2oiRCz64WhESA==

 

[spec]

Hi EF,

Whichever approach we use will be bullet proof (I hope I don't have to eat my words).

Yes, those are the sort of resistors, but with the pulse width modulation approach (version 3) the resistors will need to be low inductance types which would probably double the price.

spec

 

[ElectronsFlow]

Hey Spec,

I've got a friend who has some 1 ohm resistive load coils that come out of a commercial load tester. I've yet to have one in my hands so I can't verify any specs on the units apart from apparently they are 1 ohm. He tells me they resemble a stove top element in appearance. When I get my hands on one I'll upload a pic to show you. May or maynot be useful?

 

[spec]

Hey Spec,

I've got a friend who has some 1 ohm resistive load coils that come out of a commercial load tester. I've yet to have one in my hands so I can't verify any specs on the units apart from apparently they are 1 ohm. He tells me they resemble a stove top element in appearance. When I get my hands on one I'll upload a pic to show you. May or maynot be useful?

If, between us, we decide to go for a linear approach, as in version 4, the resistor types become much less critical. Coil type resistors, while they are reliable and good for heat dissipation, tend to be rather inductive.

I'm working on a detailed version 4 circuit, but today my missus was being unreasonable and expected me to take her shopping and stuff like that.

spec

 

[ElectronsFlow]

Hey Spec, Haha that's fine! Though your missus could have been a touch more considerate and asked me if that was okay to go shopping . Joking aside, have you had a chance to figure out more of the layout/components? I don't want to imply any pressure for a time crunch, I'm more like a young boy in a candy store type excitement

 

[spec]

Hey Spec, Haha that's fine! Though your missus could have been a touch more considerate and asked me if that was okay to go shopping . Joking aside, have you had a chance to figure out more of the layout/components? I don't want to imply any pressure for a time crunch, I'm more like a young boy in a candy store type excitement

Sorry EF,

I got distracted with all sorts of other goings on. I will look at your circuit tomorrow, domestic commitments, permitting.

spec

 

[spec]

POST ISSUE 03 of 2016_10_18

Hi EF,

I have done a bit of investigation into your constant current load. Here are my conclusions and outline considerations. Let me know what you think.

(1) APPROACH
(1.2) Suggest go for a linear approach. The PWM circuit is attractive because of its apparent simplicity and would probably be the way to go for a production model. But, by the time you sort out the switching issues, the switching circuit would not be so simple. There is also the concern about taking pulses of current from the battery and the EMC aspects. On the other hand, a linear approach has non of these problems and represents a lower risk and shorter development time, in my opinion that is.

(2) CASE
(2.1) Case: dimensions: 540mm wide by 140mm high by 300mm deep.
(2.2) Heat sinks transistor: 16 of aluminum sheet panels, 140mm high by 140mm deep by 4mm (minimum) thick
(2.3) Heat sinks resistor: 16 of aluminum sheet panels, 140mm high by 140mm deep by 4mm (minimum) thick
(2.4) Fans: 4 of 12VV, 140mm square fans mounted vertically across the front of the case.
(2.5) The four 12V fans are connected in series to run off 48V, unless 48V versions can be procured at a reasonable cost.
(2.5) The heatsink panels mount vertically front to back on 30mm centers. The transistor heatsinks mount at the front. The resistor heatsinks mount behind the transistor heatsinks with a 20mm air gap between the transistor heat sinks and the resistor heatsinks (so that the resistor heatsinks do not heat the transistor heatsinks)
(2.8) The NMOSFETs mount on the front heatsink panels.
(2.9) The power resistors mount on the rear heatsink panels
(2.10) The rear of the case is open
(2.11) The fans suck ambient air in from the front of the case. After cooling the heatsinks, the hot air then exits from the rear of the case.
(2.12) The expelled air will be around 80 Deg C, depending on the current setting and the ambient temperature.

(3) CIRCUIT
(3.1) 16 channels, each comprising two power resistors, one NMOSFET, and one opamp.
(3.2) The load current is set by a multi-turn potentiometer.
(3.3) The load current is accurately controlled by a conventional opamp and MOSFET negative feedback loop.
(3.4) The four fans are powered from the 48V supply line. The fan power dictates the minimum current that the load can take, but the fan power consumption could be adjustable in line with the current set. This would eliminate the error due to the fan current.

(4) COSTS (Budgetary)
(4.1) Case and heatsinks: cost of aluminum sheet only, on the assumption that you do the metal bashing
(4.2) Fans: 4 of, 12V, 140mm square @ £8 each = £32.00 total
https://www.ebay.co.uk/itm/CORSAIR-...ffType=OrderSubTotalOffer&_trksid=p5731.m3795
(4.3) Schottky diode: 60V min, 60A min, stud or T0220 case @ £10.00 UK
(4.4) Opamp: 8 of LM358 @ £0.50 each = £4.00 total
(4.5) Resistor: 16 of 100W 4R @ £0.90 each = £14.40 total
(4.6) Resistor: 16 of 100W 8R @ £0.90 each = £14.40 total
(4.7) NMOSFET: 16 of @ £5 each = £80.00 total
(4.8) Voltage Regulator: £3.00
(4.9) Potentiometer multi-turn: £6.00
(4.10) Miscellaneous: £50.00

 

[ElectronsFlow]

a linear approach has non of these problems and represents a lower risk and shorter development time, in my opinion that is.

I agree with your logic here, Also seems due to the cost of the low inductance resistors for PWM style the cost will significantly be reduced by going with the linear approach?

The expelled air will be around 80 Deg C

Should be just right for toasting marshmallow while waiting for the test wouldn't you say.

on the assumption that you do the metal bashing

I've had plenty of experience bashing my head against metal, should be well qualified for the job

Thanks again for your time Spec, you must have spent some considerable time on this

 

[spec]

Hi EF,

That's good- the more I look at it the more I favor a linear approach.

Toasted marshmallows... Hmm my favorite

Ah, so you are a skilled tin basher- the metal work is often the best part of a project. Can you also handle the walnut side panels and titanium fittings?

No probs- your constant current load generator fits in with something else I am working on. It also takes my mind of the tear-jerkers and other pap that missus has on the TV.

I have done a rough draft of the linear circuit which will be in the next post.

spec

 

[spec]

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

POST ISSUE 05 of 2016_10_17

​

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

 

[ronsimpson]

What happens when the battery is removed?
C4 holds voltage for a while.
FET current goes to zero.
Error amps go to full on. Gate voltage = 24-1.5=22.5V. Max gate voltage should be 20V.

What happens when the battery voltage is low?
Error amps go to full on. Gate voltage = 24-1.5=22.5V. Max gate voltage should be 20V.

If the supply was 15 to 21.5V this problem does not happen.
RS

 

[ChrisP58]

ElectronsFlow,
Will this operate unattended? If so, then I would suggest adding a low voltage cutoff circuit to stop the discharge when the battery falls below x volts. Otherwise, the current will be constant down to a point, then will taper off, but the battery will keep being discharged well below its damage point. And be aware that, as soon as you remove the load current, the battery voltage will rise again. So either give the low voltage trip a fair amount of hysteresis, or make it a latch off function.

Also, the effective system resistance, including interconnections, needs to be chosen to ensure that the constant current range continues down to whatever minimum voltage you need to discharge the battery to.

 

[spec]

What happens when the battery is removed?
C4 holds voltage for a while.
FET current goes to zero.
Error amps go to full on. Gate voltage = 24-1.5=22.5V. Max gate voltage should be 20V.

What happens when the battery voltage is low?
Error amps go to full on. Gate voltage = 24-1.5=22.5V. Max gate voltage should be 20V.

If the supply was 15 to 21.5V this problem does not happen.
RS

Good points Ron.

spec

 

[spec]

Also good points ChrisP58; be interesting to see EF's views on them.

Will this operate unattended? If so, then I would suggest adding a low voltage cutoff circuit to stop the discharge when the battery falls below x volts. Otherwise, the current will be constant down to a point, then will taper off, but the battery will keep being discharged well below its damage point. And be aware that, as soon as you remove the load current, the battery voltage will rise again. So either give the low voltage trip a fair amount of hysteresis, or make it a latch off function.

A low voltage cut-off circuit would be pretty straight forward to implement, except that lead acid battery voltage is not very well defined, compared to LiIon, for example. I am assuming lead acid batteries.

the effective system resistance, including interconnections, needs to be chosen to ensure that the constant current range continues down to whatever minimum voltage you need to discharge the battery to.

Yes, as said previously, the wiring, connectors, etc need to be substantial and high temperature. The layout is also important, although not as important as with a PWM approach for two reasons: no frequency considerations and each channel has lower maximum currents.

I will incorporate two star points on the schematic.

The problem with allowing too larger voltage range, while maintaining a constant current of 50A, is power dissipation in the NMOSFETs. Of course, you can just keep adding channels but this makes the unit bigger, more complex, and more expensive.

spec

 

[spec]

I wonder about the 60A diode to protect the circuit from a battery reversal. Would just a 1N4007 diode in series with the three terminal regulator input suffice, as Ron suggested initially. With the 60A diode omitted the the circuit would conduct around 60A by virtue of the NMOSFET parasitic reverse diodes and the high power resistors, but no damage would be caused (famous last words).

spec

 

[ChrisP58]

Also good points ChrisP58; be interesting to see EF's views on them.
A low voltage cut-off circuit would be pretty straight forward to implement, except that lead acid battery voltage is not very well defined, compared to LiIon, for example. I am assuming lead acid batteries.
spec

Actually the cutoff voltage is well defined. It's just not a single voltage for all discharge conditions.

All batteries from reputable suppliers will have charts and graphs on their data sheets for a given battery, showing the minimum recommended cutoff voltage for different discharge currents.

 

[spec]

Actually the cutoff voltage is well defined. It's just not a single voltage for all discharge conditions.

All batteries from reputable suppliers will have charts and graphs on their data sheets for a given battery, showing the minimum recommended cutoff voltage for different discharge currents.

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

 

[alec_t]

you can just keep adding channels but this makes the unit bigger, more complex, and more expensive.

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 .