Documentaion Error?

In the section contact load
linuxcnc.org/docs/devel/html/plasma/qtpl...html#qt-contact-load

It states:

If using a Mesa card, the input resistance could be 5000Ω (7i96) or 10000Ω (7i76E), giving a contact current of 4.8mA or 2.4mA assuming a supply voltage of 24V (I = V/R).

But the 7i76e Manual states:

The 7I76E field inputs have a nominal input resistance of 20K Ohms to field power ground.

Where does 10000 ohm come from?
That should be 20000 ohms and 1.2 mA to me.

I Might add that the calulations do not allow for resistor tolerance. Nore does it allow for any safety margin which would be prudent.
Using Resistor with 5% tolerance and allowing 20% safety margin from the E24 range we get:

Resistance
R = 24 / (0.0208 -0.012 )
R = 1224.5 Ohms

1224.5 +- 5% tolerance
= 1224.5 +- 61.25 Ohms
= 1163.25 to 1185.75 Ohms
+ allow 20% safety margin
20% of 1163.25 = 232.65
Resistance required = 1224.5- -61.25 - 232.65
R = 930.6k
Closest resistor in E24 range = 820 Ohms (next one up = 1200 Ohms, too high)

Current draw
24/820
I = 29.3 mA
Current through resistor
I = 29.3 - 1.2
I = 28.1 mA

Power dissipation:
24^2/820 = 0.702 W
Allowing for 5% tolerance
24^2/820*0.95 = 0.739 W
1 W minimum

Note: the next resistor up in the E24 range is 1200 ohms which is not within the required minimum tolerance value of 1163 ohms 

tommylight wrote:

rodw wrote:

If using a Mesa card, the input resistance could be 5000Ω (7i96) or 10000Ω (7i76E), giving a contact current of 4.8mA or 2.4mA assuming a supply voltage of 24V (I = V/R).
But the 7i76e Manual states:

The 7I76E field inputs have a nominal input resistance of 20K Ohms to field power ground.


Not the same thing, the first is reffering to an External resistance wired in series to the input on the Mesa board, the second refers to actual input resistance in relation to ground, meaning if you measure with a DVM set to Ohm between the input and ground it will show 20KOhm.
The latter also means that if you connect directly 24V the input will draw 1.2mA.
7i96 and 7i76E inputs and outputs are different types with different ways of connecting and use, so referring to them in one sentence should be avoided.

Only referring to the 7i76e. You have to look at resistance to ground on it as thats how the circuit is closed.

phillc54 wrote:

rodw wrote:

Where does 10000 ohm come from?

It seems that this has changed since I last downloaded a copy.

 

Yeh, I went back to my 2017 copy stored in Google Drive and it says 10k too.

I actually revised the maths above based on specs of another similar relay that said minimum current 5v 100 mA, 24 V 30 mA. I do remember PCW mentioning this. I think this makes sense as it would be tied to the contact material. Reworking on 30 mA, I came up with a 620 Ohm Resistor with power dissipation of a fraction under 1 W so a 3 or 5W resistor should be used. This resulted in a current of  39 mA.

using 30mA
24/0.03 = 800 ohms
800* 5% = 40 ohms
therefore need 760 ohm minimum
Add 20% safety margin (152 ohms)
Resistor required 760-152 = 608 ohms
Closest E24 series 620 ohms (reduces safety threshold to 18.4%)

current draw 24/620 = 39.4 mA
Power Dissipation 24^2/620 = 0.929 W
Use 3-5 W resistor for safety

I think its really important to allow for the resistor tolerance in the calculations to be sure it works for everybody reliably. Likewise adding a safety margin beyond the barest minimum is good practice.