Dear Board Users,
i do have a question related to the extended usb interface board vm 140.
The card has 8 analoque outputs with 0…5/10 direct current voltage.
My Question is: what is the maximum amperage of each analogue output during using the 5V and during using the 10V as maximum voltage? On several shops i found different information about this topic and i’m no longer sure about my own electronic knwoledge 
Thank you very much for your help!
In regards,
Markus
Based on the output buffer’s specs (a TLV274 I believe), I would say 5-6 mA at 5V out, 9-10 mA at 10V out. These outputs are designed to be sensed, not to directly drive anything…
I do not want to drive anything. I just need 5mA to conduct through a mucous membran for a few seconds.
Do you think, i could do this with the vm140 analoque outputs?
Thanks.
I do not believe the 5mA requirement will present any problems for the VM-140’s outputs, however for that application, and assuming that it is the 5mA flow through the membrane that is the critical parameter, the first thing you need to know is the resistance of the MMb (Mucous Membrane) between your contact points.
This could be directly observed with an Ohmmeter to some degree of accuracy, or better yet determined experimentally using a variable voltage supply (the VM-140 analog out would work) and measuring the current flow at a known voltage–I suspect you may find it is rather low, around 100Ω/cm².
Start out at 0.0V and gradually work upward, I do not know to what extent changes in the galvanic coupling of your contacts and the MMb may affect things, I suppose there is also a potential issue with the moisture content of the membrane.
This latter method would also let you directly observe what voltage will be required to drive 5mA through the MMb. For example if the MMb resistance between your contact points is 100Ω then only 0.5V ( 0.005A * 100Ω) will be needed to create the desired current flow.
BTW–electrically you are “driving” something, driving 5mA through the MMb…
I’m not sure, if i understand you correctly.
If i do have no lineage between the analoque output and the mmb,
i should have the same amperage at the mmb as at the analoque output. Is that correct?
I just will loose current voltage at each resistance!?
Amperage I = 5mA by 5V Output Voltage
Analoque Output
+°----------->---------------° MMB
-°--------------<------------°
Or do i forget something.
My technical electricity lessons at university are a few years ago
Thank you!
[quote=“compizer”]I’m not sure, if i understand you correctly.
If i do have no lineage between the analoque output and the mmb,
i should have the same amperage at the mmb as at the analoque output. Is that correct?
I just will loose current voltage at each resistance!?
Amperage I = 5mA by 5V Output Voltage
Analoque Output
+°----------->---------------° MMB
-°--------------<------------°
Or do i forget something.
My technical electricity lessons at university are a few years ago
Thank you![/quote]
If the VM-140 were a constant current source (controlling its output voltage so as to maintain a set current regardless of load resistance) then what you have put forth would be true, however it is not a constant current source.
What you will need to do is to determine what voltage is required to maintain the desired 5mA current through the MMb, which is dependent mostly upon the resistance of the MMb (which like all other conductors has a characteristic electrical resistance). You will need to apply a voltage source to the MMb, and at the same time measure current that is flowing.
When 5mA is flowing measure the voltage and then divide that voltage by 0.005A to determine the resistance of the MMb. This will likely vary considerably based upon physical properties of the MMb as stated in my earlier post.
so, i try to do a summary:
the amperage offered by the analoque output of the vm differs addicted to the resistance of resistance (of the mmb),
because the analoque output isn’t a constant current source.
so i’ve to do an experiment with the vm140 and the mmb to check out which current voltage at the analogue output offers me the 5mA?
Thank you very much for your help!!
I do have another question:
Do you know another usb based interface card, which can offer me a constant current voltage. The current voltage should be controllable by a pc software?
Thanks.