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LDR circuit ( Application of transistor as switch)

vick5821

Jan 22, 2012
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Is it true that by measuring the voltage difference across the shunt resistor, this is not that accurate way ? or I should measure the potential difference at the both point and minus them to get the better accurate reading ?

Thank you
 

(*steve*)

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That one is powered and measures the same voltage.

You are probably looking for a device that has separate power. This will be evident from the specs saying that it can measure down to zero volts (or lower)

edit: measuring voltage across a shunt will give you a measure of the current flowing through it. (as well as the voltage across it)
 

CocaCola

Apr 7, 2012
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As Steve stated, that one requires 7.5V- 20V as the measurement range, not exaclty suitable for you 6V circuit...

Personally I like the look of analog meters in a teaching environment, anyway, at least in instances like this...
 

vick5821

Jan 22, 2012
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so,it is impossible to measure the voltage across shunt resistor and display it in the digital voltmeter? Yea,agree that should use analog panel as in teaching aid
 

(*steve*)

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There's nothing stopping you from measuring a voltage across a shunt resistor.

If you use one of those digital voltmeter modules you have to be sure that you have floating inputs if you're not measuring a ground referenced voltage.
 

vick5821

Jan 22, 2012
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What s floating inputs ? I mean it is impossible to measure the voltage across shunt resistor already since the voltage between the shunt resistor are not enough to power up the digital display
 

KrisBlueNZ

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Re floating inputs, what Steve means is that your digital meter needs to have differential inputs, not a single ground-referenced input.
The meter would have to be powered from the positive and negative supplies of your circuit, and it would have to measure the voltage across the shunt resistor, which is not connected to either supply rail. This means the meter has to have a differential input. Not all voltmeter modules have this feature.
 

vick5821

Jan 22, 2012
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Re floating inputs, what Steve means is that your digital meter needs to have differential inputs, not a single ground-referenced input.
The meter would have to be powered from the positive and negative supplies of your circuit, and it would have to measure the voltage across the shunt resistor, which is not connected to either supply rail. This means the meter has to have a differential input. Not all voltmeter modules have this feature.

Oh mean the meter must have different power supply and cant share the same.okay
 

KrisBlueNZ

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Yes, the meter must have power supply inputs that can be connected to the power rails of your circuit (you may need to increase this voltage if you can't find a meter that will work from 6V).

The inputs to the meter need to be independent of the power supply connections, and they need to respond to the voltage difference between the two input connections, which will be connected to the ends of the shunt. Therefore the meter will respond to the voltage across the shunt. These types of inputs are called differential.

Other points if you're considering a digital display:

It may need to be quite sensitive - ones I've seen have a full scale of 200 mV which corresponds to a full scale of 200 uA base current assuming you're using a 1K shunt resistor. You can of course change the shunt resistor. You should probably show the shunt resistor in the reference schematic of your circuit, since it will have some effect on the circuit's behaviour, especially if its value is high.

Ideally it should be configurable for decimal point position and units displayed. You don't want it to show "1.23 V" when the quantity you're interested in is actually "123 uA". I don't know if you'll be able to find a meter this flexible. Perhaps you should look for a digital meter that is already pre-configured to measure low currents. It would have the shunt resistor built into it, and it would display the correct unit.

If this is a teaching aid, I don't see any good reason why it's so important to build in a meter to show the base current - other parameters, like the base voltage and collector current, are just as important to understanding the circuit. If you're not going to put meters in for them, why have one for the base current? The reason we suggested putting the shunt resistor on the board was just to simplify the connection of an external multimeter to the circuit. Maybe you're taking the idea too far?
 

vick5821

Jan 22, 2012
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Yes, the meter must have power supply inputs that can be connected to the power rails of your circuit (you may need to increase this voltage if you can't find a meter that will work from 6V).

The inputs to the meter need to be independent of the power supply connections, and they need to respond to the voltage difference between the two input connections, which will be connected to the ends of the shunt. Therefore the meter will respond to the voltage across the shunt. These types of inputs are called differential.

Other points if you're considering a digital display:

It may need to be quite sensitive - ones I've seen have a full scale of 200 mV which corresponds to a full scale of 200 uA base current assuming you're using a 1K shunt resistor. You can of course change the shunt resistor. You should probably show the shunt resistor in the reference schematic of your circuit, since it will have some effect on the circuit's behaviour, especially if its value is high.

Ideally it should be configurable for decimal point position and units displayed. You don't want it to show "1.23 V" when the quantity you're interested in is actually "123 uA". I don't know if you'll be able to find a meter this flexible. Good luck!

If this is a teaching aid, I don't see any good reason why it's so important to build in a meter to show the base current - other parameters, like the base voltage and collector current, are just as important to understanding the circuit. If you're not going to put meters in for them, why have one for the base current? The reason we suggested putting the shunt resistor on the board was just to simplify the connection of an external multimeter to the circuit. Maybe you're taking the idea too far?

Ya, my idea is just connect a two terminal out for the shunt resistor for the easy measurement of the voltage across the shunt resistor and then CALCULATE the base current :) This is better right ? So now I have to determine what type of terminal connector to be used :)
 

CDRIVE

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The gain of your transistor is extremely high. A 1K base resistor will have minuscule adverse effect.

Regarding the Pin Jacks that you PM'd me about... Did you search Digikey? I don't like doing leg work.
 

vick5821

Jan 22, 2012
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how do I place the components into the board? Seems like it is not through hole
 

CDRIVE

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http://www.ebay.com/itm/5-pieces-ne...ultDomain_0&hash=item3cc6d245a3#ht_872wt_1037

Bottom is a hole. So I think I need to have another component ? Is it ? Which is bottom and which is top ? I think the hole one is TOP is it correct ?

Those Jacks are intended for through hole mounting. They're not intended for PCB mounting. They're used on panels or on an enclosure box. Holes are drilled in the box or panel with a diameter that provides a tight press fit. Then connection is made via wire soldered to the turret ends.

If you want PCB test points I saw them on ebay too. They're very cheap but won't hold a test probe independently. You must make contact and physically hold the probes. On the other hand, you can buy a set of probe clips that mimic a spring loaded Oscilloscope probe. The DMM probes plug into one end and the other end has a spring loaded hook.
 

vick5821

Jan 22, 2012
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Those Jacks are intended for through hole mounting. They're not intended for PCB mounting. They're used on panels or on an enclosure box. Holes are drilled in the box or panel with a diameter that provides a tight press fit. Then connection is made via wire soldered to the turret ends.

If you want PCB test points I saw them on ebay too. They're very cheap but won't hold a test probe independently. You must make contact and physically hold the probes. On the other hand, you can buy a set of probe clips that mimic a spring loaded Oscilloscope probe. The DMM probes plug into one end and the other end has a spring loaded hook.

which was the one you saw ? Thank you
 
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