# Transceiver alters the value of voltage divider's output?

Hello everyone! The post it’s not for a project, only a question out of curiosity.

I had used before a SN75176a differential bus transceiver in which I had inputted two bus signals to it. One signal was 0V to24V and the other -24V to 0V. The recommended operating conditions of the transceiver say that the voltage at any bus terminal should be min -7V and max 12V. When I used a voltage divider (for example 1Mohm and 400kohms gives +/- 6.86V) and I measured the voltage that would go into the transceiver, was much smaller, for example 1.5V (I don’t remember the exact value). Probably, the transceiver has its own resistor within and alters the value from the voltage divider? Is that normal or a problem and it would be better to use a voltage regulator in these cases? Without Transceiver voltage 6.86V, with Transceiver much less.

The transceiver was not burned, so either I was lucky or it’s normal :).

Hi, Use lower values of resistors in your potential divider. Like values in 10's of K, not 100's of K or M of ohms.

The DMM you are using and the bus transceiver have a loading impedance that is causing a loading effect on your divider when you use very high values of resistance.

Tom.... :)

hi, thanks for your answer! I used so high resistors, because I didn't want to take much current from my bus System... What should I do then?

And in which case would a voltage Regulator be a better solution?

A voltage regulator would not work. It's a power device and not equipped to switch quickly like you need for data.

The datasheet I found for SN75176a says that it has a 12k input impedance. Add this 12k alongside your voltage divider and redo the calculations.

Do you have termination resistors on your bus? If not, go and look it up and see what values you should be using.

I still suck a bit at circuits.. :/

If I read the datasheet right, I should have maximum supply current 40mA and we want the current approximately at 26mA..

Also, voltage at any bus terminal can be -7V to 12V, so I was thinking an input to pins 6 and 7 at -7V to 7V when input from the buses is -24V and 24V respectively.

Bus termination resistors are a bit different as usual, they use 2 of 220ohms in parallel as shown in the picture.

If you are running a 110 ohm differential pair at +/-24V that's 6W to dissipate in the termination! Can you explain more about this bus?

If the bus recommends 220Ohm termination resistors, then R1+R2+R3=220. Then you don't need a separate resistor and the bus is properly terminated. As Mark says, that's an unusual termination. It may be drawn with two 220Ohm resistors next to each other but physically they should be at opposite ends of the wire.

To analyze the voltage divider circuit, replace the transceiver with a 12K resistor. (Where pins 6 and 7 are.) Now you have 12K in parallel with R2 and you can calculate the required value of resistors to give the voltages you want.

Also think about what might go wrong. I imagine that this bus has plugs? It may be possible to plug the plug in a bit crooked so that the high voltage +24V makes contact on R1 just a microsecond before R3 is connected. How much current will flow through R1 and diode D1 into the 5V rail? Is this less than the power consumption of all your 5V devices? If it is more, then the 5V rail will be dragged up, out of control of the 5V voltage regulator.

I have made many posts about this project and I am afraid I’ll get reported :P Problem is that I don’t have much information myself, but you can find all the information that I have gathered here:

https://forum.arduino.cc/index.php?topic=355031.15

I suggest you read posts #16, 18, 24, 26 from page 2 and the whole page 3..

Normally an external company is responsible for this bus system, but because they don’t really help we thought to build something on our own.. The supervisor (I’m just an intern) wants to take measurements from 3-4 possible positions, close to where he thinks an interruption is. He wants to monitor these places and if the voltage in one or more of them is zero for more than 3 seconds to light a LED.

I am not trying to solve the whole problem or question if there the idea of the supervisor is right, as I don’t have enough experience, knowledge or time (two more months), I just want to build something simple, that he asked and even if it doesn’t help them, I will have done what I was asked and learn some things..