Air gap in high resolution optical encoders

As the resolution of optical encoders increases, the distance from sensor to disk decreases. In the incremental encoder industry, this distance is called the “Air gap”.  In the side view photo of an optical encoder above, the two red lines indicate the air gap between the sensor and disk  in a QD145 incremental rotary encoder.

In the photo below I have added a human hair to show perspective.

For more information on optical encoders, contact Quantum Devices  at (608) 924-3000.

Cable length considerations with Incremental Encoders

The QD145, QD200 and QR12 series of optical encoders have  28 AWG conductors in the standard flying lead cable.

This gauge of cable is excellent for tight bends and fitting in applications where space is a premium.  The conductors can easily handle the 250 mA max current requirement of the encoder.

A smaller gauge conductor means that there will be a limit to the length of the cable. This is due to the DC resistive loss in the conductor that causes a slight voltage drop.  The longer the cable, the greater the voltage drop.

This voltage drop reduces the voltage seen at the encoder.

For an incremental encoder with a 28 AWG cable operating at 5VDC,  this limitation occurs at 17.85 feet.

There are a few ways around this incremental encoder cable length limitation:

1)       Splice the cable and go with a larger wire gauge for longer cable runs.

2)       Increase the power supply voltage to compensate for the voltage drop in the cable.

3)       For Incremental only (Non-commutated Encoders) Quantum Devices offers a 26 AWG cable.  26 AWG conductors bring the cable length limitation to 28.1 feet.

For other options, or help in determining the right wire gauge or incremental encoder for your application, you can reach Jim at (608) 924-300.

QD145 Incremental Encoder IP-66 Sealing Option

Quantum Devices Inc. has a mounting option that allows for IP-66 sealing of the QD145 Incremental Encoder.  There are two o-rings in the clam shell design; One o-ring seals the encoder to the mounting surface (usually a motor), and the other o-ring seals the end bell housing that covers the encoder.  This is a popular, inexpensive, option for customers who may not need the IP-66 sealing,  but want some sort of end bell protection over the encoder.

Cross-referencing RENCO Incremental Encoders

We have always been able to come up with a suitable replacement for Renco encoders, but our customers are making us aware that we need to point that out.

For those who don’t know, Renco, after being absorbed by Heidenhain, made the decision to  eliminate much of their product line.  This move has left quite a few of their customers in the lurch. To help fill this need, we recently started promoting our ability to cross-reference Renco encoder lines on our web site with the rather obvious image you see above.  Clicking on this image will take you to a request form where you can let us know which Renco Encoder you are trying to cross.

Keep in mind that we can cross other encoder manufacturers as well.

As a  general reference, the following Renco incremental encoder cross-reference table can show you which style of Quantum Devices Incremental Encoder will likely work best.

Renco Encoder Quantum Devices Encoder
RHS15 QD145 or QR12
RCM15 QD145 or QR12
RM15 QD145 or QR12
RCH20 QD145 or QR12 or QD200
RHS20 QD145 or QR12 or QD200
RM21 QD145 or QR12 or QD200
RCM21 QD145 or QR12 or QD200
RCH50 QD145 or QD200
R50i QD145 or QR12 or QD200
R35i QR12 or LP12
R22i QR12 or LP12
RA25 QDH20
RS25 QDH20

Calculating Output Frequency for Rotary Encoders


Here are three handy calculations for incremental rotary encoders:

Frequency  = (RPM X Line Count/60)

Use this formula when line count of the encoder and RPM are known.  The answer is in Hertz (Hz) or cycles per second.  Keep in mind that many encoders and drives will have a Maximum frequency limit that you should not exceed.


Line Count = (Frequency X 60/RPM)

Use this calculation when Frequency and RPM are known.  Can be useful for determining the highest resolution encoder you can use without over driving the Maximum output specifications on the encoder or equipment it is being connected to.


RPM = (Frequency X 60/Line Count)

Use this formula when frequency and Line count are known. This is useful when you are trying to determine the speed of rotation.


Quantum Devices Inc. has a calculator for finding RPM, Encoder output Frequency,  or Line count (Resolution) when given the other two variables.

You can check it out here :



Quantum Devices is a manufacturer of rotary encoders.  Incorporating QDI’s patented sensor technology, our QPhaseTM Family of Encoders feature high resolution, increased frequency response and superior reliability over temperature (0 to 120 degrees C). QDI incremental and absolute encoders set a new standard and are backed by a 2 year factory warranty. We invite you to request an evaluation unit, additional information or price quotations.

The Quantum Devices main web site can be found at