Hey forum
I have a low-flow turbine water flow meter, which consists of a Hall Senor,
amplifer and NPN pulsed output. The meter basically give you pulses in proportion to the flow rate. Its a three wire device, +24Vdc, 0Vdc and Pulse Output. The meter is installed in a cabinet with other electronics equipment and works fine in most of the products. However in one particular product the meter was giving more pulse then it should have been.
So began the search as to why. Firstly i installed a new meter and even though it wasn't affected as badly its readings did fluctuate more then i can accept. I went back to the first meter and decided to use this to find the source of the problem. I installed a 470uf capacitor from the meters +V and GND assuming it was some form of conducted emmissions and the cap did the job. So taking that it was conducted noise i removed the capcitor and started shutting of the other electronic devices in the unit and could'nt find any culprit. I then powered the meter from an external benchtop power supply and the meter worked great. At this stage iam thinking its the 2 power supplies that are installed in the product, This is where this product differs from others in that it contains 2 power supplies.
What i dont know is if the power supplies (each Containing AC-DC brick and DC-DC converter) are generating the noise or is it just passing from the mains through the power supplies. The final thing i did was tried a few other capacitors ranging from 1000uf to 4.7nf to see if i could find one that did'nt work thus giving me an idea of the frequency of the noise (not sure if my logic here is correct???). All the capcitors worked!!!!!
Another point of interest is that if i connected my scope leads to the pulse output the meter works great (I know the leeds probably have some capcitance associated with them) , and for the strangest thing after all my messing with capacitors it turns out that all i have to do is touch one leg of a capacitor (any value) to the meters +V and leave the other hanging in mid-air and guess what the meter woks!????? how is this possible???
Also i discovered that re-routing the wire a different way around the cabinet away from the outputs of the power supplies greatly improves the reading!!
My final conclusion is that it is radiated emissions inside our product????
Can someone please enlighten me as to what this is, or give me an idea as to how i should really find the source of this problem??? iam not the kinda person that will just put the capacitor permanently on the board without really understanding why, i need to know!!!!
Thanks
I have a low-flow turbine water flow meter, which consists of a Hall Senor,
amplifer and NPN pulsed output. The meter basically give you pulses in proportion to the flow rate. Its a three wire device, +24Vdc, 0Vdc and Pulse Output. The meter is installed in a cabinet with other electronics equipment and works fine in most of the products. However in one particular product the meter was giving more pulse then it should have been.
So began the search as to why. Firstly i installed a new meter and even though it wasn't affected as badly its readings did fluctuate more then i can accept. I went back to the first meter and decided to use this to find the source of the problem. I installed a 470uf capacitor from the meters +V and GND assuming it was some form of conducted emmissions and the cap did the job. So taking that it was conducted noise i removed the capcitor and started shutting of the other electronic devices in the unit and could'nt find any culprit. I then powered the meter from an external benchtop power supply and the meter worked great. At this stage iam thinking its the 2 power supplies that are installed in the product, This is where this product differs from others in that it contains 2 power supplies.
What i dont know is if the power supplies (each Containing AC-DC brick and DC-DC converter) are generating the noise or is it just passing from the mains through the power supplies. The final thing i did was tried a few other capacitors ranging from 1000uf to 4.7nf to see if i could find one that did'nt work thus giving me an idea of the frequency of the noise (not sure if my logic here is correct???). All the capcitors worked!!!!!
Another point of interest is that if i connected my scope leads to the pulse output the meter works great (I know the leeds probably have some capcitance associated with them) , and for the strangest thing after all my messing with capacitors it turns out that all i have to do is touch one leg of a capacitor (any value) to the meters +V and leave the other hanging in mid-air and guess what the meter woks!????? how is this possible???
Also i discovered that re-routing the wire a different way around the cabinet away from the outputs of the power supplies greatly improves the reading!!
My final conclusion is that it is radiated emissions inside our product????
Can someone please enlighten me as to what this is, or give me an idea as to how i should really find the source of this problem??? iam not the kinda person that will just put the capacitor permanently on the board without really understanding why, i need to know!!!!
Thanks