EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: rjardina on August 29, 2014, 05:02:29 am
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I'm working with a lot of flame sensors/rods. I would break the connection and put my meter in series to take a microamp reading.
there are also a lot of flame rods that are the flame igniters too. So the same wire that taking the flame sense is also the 5-10kv spark, obviously my meter is not rated for that.
I was thinking I could use a 600V to 1kV varistor to short my two probes together before it got to my meter for when it is sparking. Along with two TVS diodes, one on each probe after the varistor and before the meter to prevent any other high voltage from reaching the meter. And once it stops sparking i would take my flame reading. Is there any reason this wouldn't work?
thanks for your thoughts and time.
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If your meter is in series with the circuit and set on a current range then it's not really going to see 10 kV. The meter is going to float with the sparking voltage and as long as there is no ground path the meter will only see a small current flowing through it (just like a bird sitting on a power line). I don't imagine the spark current is more than a few milliamps and this isn't likely to overload the internal current shunt resistor or blow a safety fuse.
Make sure you keep your hands well away from the meter and test leads so you don't introduce a ground path and consider setting the meter and test leads on a rubber mat for double insulation.
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My coworker told me the same thing. it blow the fuse then the rest of the meter followed, well at least I imagine it doing in that order.
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are you perhaps using a low z industrial meter meant to disregard ghost voltages?
some meters are built to have a low input impedance so you don't pick up RFI and coupled signals
i.e. if I take my lab meter to measure a disconnected circuit running in my wall along other things I might pick up like 50V, but it is a useless ghost not capable of harming or doing work
however if i take my cat IV rated "work" meter with a low input impedance it will read close to zero. This meter has a load resistor in it so only powerful signals result in a reading. The higher the input impedance of your meter is the noisier the measurement will be.
Maybe the low Z functionality is part of the ammeter circuit, so in ammeter mode you are actually measuring your low value shunt + your lower but higher in comparison to the shunt value load resistor. I am not sure how these meters are made though.
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My coworker told me the same thing. it blow the fuse then the rest of the meter followed, well at least I imagine it doing in that order.
Did this actually happen, or did you only imagine it happening?
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are you perhaps using a low z industrial meter meant to disregard ghost voltages?
OP wants to make CURRENT measurements. Anything you use to measure currents will have relatively low impedance - couple of ohms for the micro-amps range.
If you blow a 500mA fuse on a 10kV circuit due to transient current when you make/break the loop, you might want to attach the multimeter while leads are shorted to spare the initial surge - that's what I do when I check a car for leakage current since all the electronics may draw a few amps for the first few seconds after reconnecting the battery before dropping to the usual 20-70mA if everything is working normally. Having TVS across terminals in case your fuse blows or you accidentally plug the meter in while setup for volts or ohm measurement would not be a bad idea either.
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Pretty much what Ian says, just keep the meter in series , and mount it on a foam rubber pad .
You may have issues with the fast rise time of the ignitor damaging the meter, in that case I would
(a) Use an external 1kohm 1watt resistor across the meter terminals, and use the 200mV = 200uA range
(b) Put a bidirectional TVS (these have CA in the part number) , e.g. 1.5KE10CA across the terminals (i.e. in parallel with the 1k)
(Anything from 5v to 20v would work, e.g. 1.5KE05CA to 1.5KE20CA)
(c) Put a 100nF to 1uF capacitor across the above two items (so the pulse current passes through the capacitor)
(d) Connect the negative of the meter to the ignitor, and the positive to the electrode (This puts the stray capacitance inside the meter on the ignitor side, otherwise the charging current for this Cstray has to go through the shunt resistor).
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Just clarifying,
my step (a) you have to set your multimeter to 200mV and interpret the value as if it were 200uA full scale.
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If it's an oil burner ignitor then the current can be over 20mA at 15 KV or more.
http://www.duomo.co.uk/TRK.aspx (http://www.duomo.co.uk/TRK.aspx)