...The lamp is rated at 20mA...
...The lamp is rated at 20mA...
FYI - the datasheet on your Jameco link says 200 mA.
You maybe want to buy a couple power resistors for measuring current. A 0.1R 5 watt resistor. And a 1R 5 watt resistor.
I'm puzzled regardless as the DMM manual says 315mA overload protection but the label on the meter itself says 200mA MAX Fused.
You maybe want to buy a couple power resistors for measuring current. A 0.1R 5 watt resistor. And a 1R 5 watt resistor.
You maybe want to buy a couple power resistors for measuring current. A 0.1R 5 watt resistor. And a 1R 5 watt resistor.
By the way, do those go together in series? I was kind of settled on getting a shunt, the type that mounts and is encased in metal or is metal(s). Just haven't ordered one yet.
You maybe want to buy a couple power resistors for measuring current. A 0.1R 5 watt resistor. And a 1R 5 watt resistor.
By the way, do those go together in series? I was kind of settled on getting a shunt, the type that mounts and is encased in metal or is metal(s). Just haven't ordered one yet.You do have the leads in the DMM amps jacks I hope.
Black in common, Red in the highest range just to start with then a lower range if more resolution is needed and only if that lower range is not exposed to excessive current.
Stuartambient: You would put one of the resistors in series with your circuit, then measure the voltage across the resistor. 1 ohm resistor will give you 1 volts per amp ( V = IR, V = Volts, I = current, R = resistance) and the .1 will give you 0.1 volts per amp. Since they're 5 watts capable, you can go up to about 2 amps on the 1 ohm and 20 amps on the 0.1. They will get pretty hot at those currents though.
Stuartambient: You would put one of the resistors in series with your circuit, then measure the voltage across the resistor. 1 ohm resistor will give you 1 volts per amp ( V = IR, V = Volts, I = current, R = resistance) and the .1 will give you 0.1 volts per amp. Since they're 5 watts capable, you can go up to about 2 amps on the 1 ohm and 20 amps on the 0.1. They will get pretty hot at those currents though.
Thanks Paul, trying to decide if I need precision .1% tol and how much of a difference would it make with a power resistor at 5% tol. Big difference but maybe not to me? Could through mA's off by alot or if I can get a good reading off the resistor alone then work the difference into the math?
Stuartambient: You would put one of the resistors in series with your circuit, then measure the voltage across the resistor. 1 ohm resistor will give you 1 volts per amp ( V = IR, V = Volts, I = current, R = resistance) and the .1 will give you 0.1 volts per amp. Since they're 5 watts capable, you can go up to about 2 amps on the 1 ohm and 20 amps on the 0.1. They will get pretty hot at those currents though.
Thanks Paul, trying to decide if I need precision .1% tol and how much of a difference would it make with a power resistor at 5% tol. Big difference but maybe not to me? Could through mA's off by alot or if I can get a good reading off the resistor alone then work the difference into the math?
Didn't we go through some calculations in that other thread, on how much the resistor or shunt tolerance value and the DMM's own accuracy affect the final accuracy of your measurement? I seem to recall posting some example calculations.
The tolerance of the resistor will determine the accuracy of the voltage that appears across it. 5% is quite a big variation in measurement - but if you are happy that an actual 200mA current is showing as something between 190mA and 210mA, then you're good to go.
If you want to be a lot more accurate, you can measure the actual resistance of your shunt and do some V=IR math - but you may need to consider temperature effects on the shunt resistance, especially if it gets noticeably warm.
When you get down to it, you can use ANY resistor as a shunt - providing the burden voltage is acceptable - and use V=IR to calculate the current. Using resistors in the decadic sequence of 0.01, 0.1, 1, 10, etc means you can read values directly from the meter, so long as you pay attention to where the decimal point goes.
How do they get there?
QuoteHow do they get there?
They get there by you magically keeping their case temperature at 25 degrees with your powdered unicorn heatsink. Look on page 2 for the derating curve, and the line where it says 150 degrees max case temperature.
The nice thing is you can use a heatsink pretty easily with those resistors. But you'll have to supply it.
As another solution along with the great ones given.
I use cheap Harbor Freight meters for currents. Always start at the highest range. Since the wires do not change, I do not make the mistake of using the wrong jack. Also the meters are accurate.
Since they are cheap, you can use several in your circuit.
Funny thing is since I started using them (only for current) I have not blown a fuse. The only bad thing is they use 9v batteries.
along with a Data Sheet for some Caddock CVRs. You can order a large handful of these in various different resistances and power handling ranges for less than the cost of the single Shunt you listed in that other thread. The popular component houses stock them and can have them delivered to you in a few days.
An observation that I think nobody mentioned :
Even with a 0.1 ohm resistor, at higher currents you will have some voltage drop. So if you power a product with 3.3v and it uses 1A and you put a 0.1 ohm in series with the circuit, you'll drop 0.1 volts on the resistor and your product will see only 3.2v
May seem like very little, but I've tried this trick on digital cameras with 3v input and was wondering why my camera reset itself while charging the flash, and it took me a while to realize for a few ms the flash charging drew upwards of 3A from the power source, dropping the voltage to less than around 2.7v and causing the camera to turn itself off.
Chips like ACS712 having a fixed 1.2 mOhm (approx) resistance are not affected by this, you really can't push enough current through them for this resistance to overheat the chip.
Another tip
Hall effect sensor chips are awesome.........
An example of such chip would be Allegro ACS712
Such chips work better than some multimeters - some multimeters can measure currents up to 10A but have limitations, like for example they say "measure up to 15 seconds, then wait 5 minutes" or something like that - that's because if the current is high, the internal current shunt heats up and then the value of the current shunt changes and measurements are no longer correct... and if the current shunt heats up too much, it desolders itself from the pcb.
Chips like ACS712 having a fixed 1.2 mOhm (approx) resistance are not affected by this, you really can't push enough current through them for this resistance to overheat the chip.
Be sure to get a HF IR meter, if you do not have one