Ampere-Hour meter

[spuffock]

The rate of charging the capacitor will not be linear, and will depend on the difference between the capacitor volts and the opamp output. You will need to use a current mirror to ensure that the current into the capacitor is proportional to the current being measured. I can see a bad circuit where the capacitor will never reach the threshold voltage with a low current.

 

[john1]

Hi,

Have you considered using an old electricity meter from a house supply ?
I think they give a reading in Ampere Hours.

Not so easy to zero though,
you may have to make a note of the last reading each time,
so that you know what number to aim for ...

Regards, John

 

[IRQ57]

Ampere Hour meter

Hi John,

Yes, it was a possible solution until I needed to stop automatically the process once a specific quantity of AH has been reached. Thanks for your suggestion.

Now I am working in the Spuffock's concept, but I found some difficulties measuring very low levels of current. He told me about.... Initial current is 15 A at 278 VDC, but temperature correction may force the power supply (220 Vac 3 ph ) to reduce current down to 1 A, and in this condition my meter will not work with precision.

Regards

 

[john1]

Hi IRQ57,

Thank you for your reply,
i wasn't actually sure that i had understood what you wanted.

I found it amusing that you want the supply to stop once a
specific amount of A.H. has gone through,
because thats exactly what a lot of household electricity
meters were designed to do.

They used to do it based on how much money went into the slot,
but these days they do it based on a 'key' that the house-
holder pays to have 'topped up'.

Such meters are quite common, and work well even at very low
current loads. They have to i suppose, people get quite picky
about money. However, i feel that such a meter could easily
be made to respond to your needs.

I assume you had tried the type that don't cut the supply ...?
And i'm not sure that they work on DC anyway ....

It looks like you are now involved in making an electronic
solution to this, and of course i wish you well with it,
and i hope you get it to work how you want it to work.

Could you please give some info about the currents involved,
i may have missed it but all i found was that the system
starts at 15 Amperes, and can be reduced by the temperature
monitoring, to as low as 1 Ampere.

What A.H. are you aiming at, or is this a system variable,
for different units ... what is it usually ?

And i don't quite know what you mean by the meter that doesn't
work properly at one ampere ?

Regards, John

 

[john1]

Hi IRQ57,

I got your PM ok and i have read it carefully. I'm posting
this in the forum as well as sending a PM.

I've been thinking about your AH monitoring quite a bit. I
think that Nigel Goodwin has the right approach here, namely
to run the unit at a constant current, which as you clearly
understand perfectly well means that the Ampere Hour amount
becomes just a function of time elapsed.

However by altering the current, as an over-temperature
control, this makes it awkward to make something simple.

So i suggest that you consider controlling the temperature
in the same way that temperature control is normally done,
that is by the stat simply turning off the supply, when it
gets over temperature, and turning it back on when it gets
under temperature.

I realise that you have gone to the trouble of making a
current control unit, to run this at or near max temps, but
it makes monitoring difficult, and using a standard type of
stat arrangement will in my opinion achieve effectively the
same temperature control.

The advantage to this is clear,
the current can be set.

The AH amount would then still be just a function of time
elapsed, but it would be the 'on' time elapsed, not the
'off' time.

Put simply, the timer would run during the 'on' time and
then the AH amount could be set on the timer.

I puzzled over this to find the simplest way i could,
and i hope you find this helpful.

Regards, John

 

[IRQ57]

Ampere hour meter

Hi all,


This note is to close the case. Thank you very much for all the
attention paid.
Regards.

 

[Klaus]

You could also try this idea: Measure the voltage drop across a small load resistor, you may have to amplify it. Feed this voltage into a VtoF converter chip, giving you a variable frequency output for a varying current through your resistor.
Use this frequency to drive a counter. If you calibrate it right the counter will display amp hours if read (or sampled) at suitable time intervals.
Klaus