current measurement.

[yafch]

hello,
just made a power supply and finnished making of voltmeter and ampere meter using icl7107. There are a lot of examples available on web, how to install a volt meter and ampere meter. but ampere meter works only when a load is connected at the output of power supply.meaning there by it measures the amount of current that the load consumes.right?

my question is that is it possible to install an ampere meter in a power supply so that it could measure the setting of output current without any load at output, while it has to stay in continuos operation in power supply.

i made a power supply with current adjustment. and my ampere meter can measure upto 5A. this meter is based on commonly available design with 0.01 ohm resistor between inputs for 5A measurement. give your suggestions please.
thanks a lot.

 

[duffy]

Yes, it measures the load output. Without a load, there won't be any current for it to measure!

If you want to just indicate the current LIMIT, you may be able to do this by taking a voltage reading off the current limit adjustment potentiometer, but it depends on how your current limiter is designed. Some of them won't read anything until current flows.

 

[Willbe]

so that it could measure the setting of output current without any load at output,

I routinely set the voltage with no load, shorted the output and set up the current limit, and went with it.

 

[Boncuk]

If you short the output of your power supply you do have a current flow. In that case the short is the load - hence the current flow should be indicated on the ameter.

If your readout is zero I suggest to use a separate power supply for current measurements.

Boncuk

 

[yafch]

ok. then if only way to adjust the current limit is by making a short at the output of the supply,then it should be for short period of time, As the output transistors or mosfets may get too hot or even burn out. further this can be done only if the power supply has no short circuit protection.with a short circuit protected power suppy it may shut down, depending on the design of short circuit protection.

after all their must be a better way to do this. or a commercially available meter to do this kind of job. hoping more comments about it.

 

[audioguru]

A current meter in series with the load ruins the voltage regulation of the power supply.
Instead of adding a resistor in series with the load to measure its voltage that is produced by the current, instead connect a voltmeter across the sensing resistor for the current regulating function. If you made the power supply from Electronics-Lab then the voltage across R7 indicates the load current and since R7 is outside the negative feedback then it does not affect the voltage regulation.

Calibrate the current-regulating adjustment pot and mark its dial with the amps.

 

[Willbe]

ok. then if only way to adjust the current limit is by making a short at the output of the supply,then it should be for short period of time, As the output transistors or mosfets may get too hot or even burn out. further this can be done only if the power supply has no short circuit protection.with a short circuit protected power suppy it may shut down, depending on the design of short circuit protection.

after all their must be a better way to do this. or a commercially available meter to do this kind of job. hoping more comments about it.

Foldback reduces power under short circuits, but the supplies I used could probably withstand a short indefinitely; I don't know how they did it.

 

[Boncuk]

I don't see any problem using willbe's method. Starting out a the lowest current value and increasing it to the desired one shouldn't fry any part in the power supply.

If it does it's a poor design anyway and won't require current measurement, but a good handle to dump it in the garbage can.

And no, there isn't such thing like a current measuring device using another method than that already described.

Boncuk

 

[yafch]

by placing the meter in series with the power outputs the regulation of power supply although worked but not very well for me. So as he said to place the meter across the current sense resitor.this is better than before. although i need to check it by applying different loads at outputs. i,ll do it latter. untill now i have checked with a load equal to the half of full capability of current and voltage regulation range of power supply. hope to get every thing working fine. thanks all for your kind replies.