Voltage Limiter

How about the use of Zener diodes to clip the voltage to +/- 30V?

Is this the supply or a signal?
Can't just use zenners anyway, you need series resistance else the zenners go pop.
If you have series resistors you need to do the sums to avoid excessive power dissipation, depends on the current draw.
Voltage dependant resistors are ok for the odd transient, but not for this sort of thing.

Del

Del, Its a DC motor drive for a robotic arm. It can give the motors up to 60 v dc at 5 amps. What I am trying to long is the time for PID tuning. In open loop it goes from 0 to 60 in less then 1.5 seconds. In closed it take about .5 seconds longer. I just don't want to hurt the logger if it goes about 30 volts +/-. Does this help? Thanks

Right... so it's a drive voltage which you are monitoring as the input signal to your logger?
Use a potential divider.... two 10k resistors should do it in series across the drive voltage. Take the centre point to the logger...you have now halved the signal, so it won't exceed the logger input limits.
(Using zenners would distort the signal and make the logged signal useless)

Hope this helps.
Del

PS Maybe play with resistor values to give best performance, or stick a pot' on there.

Good answer Del, but I do have a one unlikely caveat in executing this, a minor adjustment and one possible flaw. The input impedance of the data logger should be at least 50k to not significantly load the divider network. The manual or manufacturer's web site will identify the data loggers impedance. I would expect the data logger to have an impedance of 1Meg or more. If your data logger has a lower impedance though this will only effect the scaling factor, not the suitability of the circuit. So you can just scale accordingly. This leads to my minor tweak, if your data logger cannot record anything above 30V and your driver/motor circuit ever produces 61V then this information will be lost for the data logger will record only the 30V. To solve this one just needs to adjust the ratio of the two resistors. In Qqberci's diagram increasing R1 to 15K and leaving R2 at 10K gives now a transfer ratio of 2/5 instead of 1/2.

Now for the possible flaw in this measurement that the OP unknowingly created. If your driver is a PWM circuit the voltage appearing across the motor will become a useless mess of voltages as the driver current gets switched quickly on and off causing the motor inductance and back EMF to generate a complex set of voltages across the terminals of the motor. These voltages may exceed the expected 60V level. To correctly monitor this type of motor control, one should sense the current running into the motor instead. This can be easily accomplished by a shunt or DC current transformer.

The logger has 1m ohm Independence. And the can't confirm just how the PID controller out put is but using both 2 10K resistors or one 15 and one 10 am able get a nice ramp form and the motor runs. And does not seem to be effected have three motors running to lift the Z axis and its going up level. So think that you all have given me the right answer. For the 240 volt will monitor the SSR pulse from the controller as its only 24vdc. Many thanks to all!

Though of that but not being a EE wondered if that would be that simple. During logging would like to be able to see the ramp in voltage but not the true voltage. Guess its worth a try ... Many thanks.

Del, Many thanks. Do remember this circuit from days of ol. If the two resistor are in series with the motor will it mess up the reaction of the motor speed as I need to have the 'normal' out put of the encoder. (2500 per rev). For the 1 or 2 seconds of the test. Can I put the divider in to ground and hook the logger in the center and not effect the motor or is this not going to be even noticed by the motor? Thanks again Del.... Robert

After a quick read only....using a voltage divider (i.e. potentiometer) is a simple way to scale down a voltage exactly - having high resistance means that power dissipation will be minimal and low enough for many applications BUT you sure as heck can't have those resistances in series with anything that needs current - like a motor for example. The voltage divider has to be across the voltage supply or in other words, in parallel with the motor (or load of interest).

Cheers

Sometimes a simple drawing tells more than 100 words... What Del recommended is something like this:

AAARRRGGHHHH
NO!!!
The resistors are across the motor (not in series) so they don't effect it.
You then tap off the centre point to your logger which then records faithfully exactly half of the motor drive voltage.
Like this.

Del

Many Thanks to All! Gave that a try and the parallel hook up is just right. Thanks so much to all,

Del, Qqberci, and TreverM..Thanks!

The suggestion(s) here assume that the logger input does NOT have one side grounded, and likely so. If so, then this will not work.

Sounds to me like the motor driver output swing is +/-60VDC.

Using Zenor diode is bad idea; as your requirement is voltage data logging. Solution is quite simple; use voltage divider. Whether it 0 to +/- 60V or 0 to +/- 230 V use combination of resistors (potential divider) to covert in to 0 to +/- 30V. I ma using 0 to +/- 2.000 V to measure 0 to +/- 24V successfully.