Dropping resistor or amplifier

[rmayer]

Here's the setup:
Multiple variacs ganged on a shaft and controlled by stepper motor. Each variac has its own rectifier and ripple smoothing cap. Ripple is acceptable for app.
Controlling PLC needs to see rectifier output VDC in order to step accordingly. In order to see it I have a signal conditioner in line that is meant to take ripple-laden VDC and convert it to nice 4-20mA analog input. Problem is conditioner only accepts 0-10VDC and operating range for the ripply VDC output is 0-20V. We measured the current draw from the conditioner to be like 0.02mA (measured with variacs set to 10VDC), practically nothing. Tried a 500K dropping resistor hoping to cut the voltage seen at the conditioner in half. It didn't work and practically no drop at the conditioner. Before I start throwing more resistors in series and enjoy the linearity issues, is there a better way via simple op-amp or whatever to do this? Keep in mind my grasp is mechanical so KISS is an appreciated principle.

Thank you in advance for anyone's reply.

 

[Skogsgurra]

Use a voltage divider instead of a dropping resistor. Like this (I keep it very simple and detailed for you):

DC voltage---10 kohm----10 kohm--0 volts.

This is a voltage divider. Anything you input to the left is divided by two and can be read on junction between the two resistors. Twenty volts in will give you 10 volts out.

Linearity issues. This voltage divider has an internal resistance equal to 5 kohms. If your converter needs 0.02 mA, then the error will be around 0.02x5=0.1 V or 1%. If that is too much, then use 1 kohm resistors to get down to 0.01 V, which is 0.1%.

In the latter case, the resistors will develop 10V times 10 mA, which is equal to 100 mW. A safe value for standard 250 mW or 400 mW 1% film resistors. The 10 kohm resistors develop around 10 mW each. No sweat.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[itsmoked]

That'll do it.

If you want to get highly complex you can add a capacitor across the:

----10 kohm--0 volts

This would aid in filtering. Make it a 1uF ceramic.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rmayer]

Thanks for the quick reply.

This is turning out to one of those things you kick yourself because you should have known it, and I did, must have had it repressed back in the depths of the abyss.

Have a great day.