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PWM Solenoid
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Frequently Asked Questions

General Operation
Applications Connections / Wiring
Replacement Part Numbers  

FAQ – General

What are the differences between the Model 205, the Model 400 and the Model 402 PWM Drivers / Controllers?
The 3 Models are provided to allow for selection based on application and cost. The Model 205 PWM Controller with its built-in pushbutton / LCD user interface is designed primarily for a benchtop / laboratory environment for development and test. The Model 400 is designed as a cost effective solution for test stand applications, functioning as a signal conditioning module. The Model 402 can be applied anywhere a serial port is available. The Model 205 provides for more features, higher PWM carrier frequencies, and more adjustability than the Model 400 and the Model 402. The Model 400 PWM Controller functions as a signal conditioner for analog to PWM duty cycle output applications and is excellent for PLC PWM control applications. The Model 402 is similar in performance to the Model 400, however, it is only controllable via RS-232, but is our lowest cost PWM driver. See APM, Inc. Technical Bulletin 0001 for a comparison table of Model 205, 400, and 402 specifications and features. Specific questions may be addressed to

What is the difference between analog based and digital based Pulse Width Modulation (PWM) techniques? What are the advantages?
All PWM Controllers from APM, Inc. feature digital PWM generation technology. Digital PWM generation is prevalent in the automotive industry for controlling solenoid valves and for acting as 1 wire transmitters. Digital based PWM generation technology is more stable than analog based techniques. Analog based techniques which use passive component (resistor/capacitor) adjustment circuitry (pots) is subject to frequency and duty cycle value drift and hysteresis due to tolerances and variances in component value due to temperature. Furthermore, digital techniques allow for instantaneous changes in duty cycle value from one value to another with no interim values (e.g. direct change from 20% to 80% possible in one waveform period). This aids in obtaining fast control system response. The Model 205 PWM Controller features the use of a Field Programmable Gate Array (FPGA) device for PWM generation in digital hardware. This allows for very accurate frequency and duty cycle generation.

What is provided with the Model 400 kit?
The Model 400 PWM Driver / Controller requires configuration prior to application. It’s operating frequency and other features must be pre-set prior to use. The intent of the Model 400 PWM Controller kit is to provide the user with all the necessary cabling, as well as a power source, such that the Model 400 may be configured from a standard PC in a desktop setting. It is recommended that the first Model 400 purchase be a kit. Future Model 400’s may then be configured using the original kit. For details on the kit contents, see the data sheet and kit guide on the Model 400 Product page.

What is the purpose of the PWM Controller internal output fuse?
It is difficult to protect the PWM Controller or the applied load under varying customer applications, load specifications, operational voltages and currents. Since most of the PWM Controller applications are industrial or automotive related, the PWM Controller output fuse is set from the factory for 12V DC applications. In general, the output fuse is present to protect the PWM Controller output MOSFET from accidental short circuit conditions. If desired, an in-line fuse can be wired in to your application for load protection.

Can my Model 205 PWM Controller be upgraded to operate at higher frequencies?
The Model 205 PWM Controller has been in production since 1998 and there are 4 versions of hardware. The capability of your unit depends on the hardware version. Contact APM, Inc. support with the serial number from your unit and we will advise regarding the capability of your unit.

Can a PWM Controller be made to control my specific application?
Yes. APM, Inc. has been supplying PWM Driver / Controllers for customer applications since the inception of the unit (1998). We can supply customized versions (hardware and / or software) of any model, tailored to meet your requirements for function, features, size, or cost. We also offer complete electronic circuit design and embedded controller design services. Contact APM, Inc. engineering for assistance with your application.

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FAQ – Operation

Is it possible to disable the pushbutton interface on the Model 205 when the unit is operating in the RS-232 command mode (so that an operator cannot modify its settings)?
Yes. This is a configuration option in the Model 205. Refer to User’s Manual section 4.6 for information on how to configure the Model 205.

Is it possible to configure the Model 205 without using the serial interface?
Yes.  Hold the SELECT pushbutton down and apply power to the unit. A message will appear on the LCD which says to release the pushbutton to begin configuration.  The Model 205 will display all configurable settings, one at a time, and allow for adjustment using the UP/DOWN pushbuttons. At the end, you may either save the settings, discard the settings, or return the controller to the default settings. On the next power-on, the controller will power-on with these newly configured settinsgs.

Can the PWM Controller start-up at some other value other than 0% duty cycle?
Yes. All PWM Driver / Controller Models (205, 400 and 402) have configurable start-up capability to power-on at something other than the factory default setting. Consult the configuration section of the User Manual for the controller.

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FAQ – Applications

Can the Model 205 (400, or 402) PWM Driver / Controller drive more than 4A (at 12V)? How can the unit be applied in higher current application?
Not directly. The PWM Controller can be used as a signal generator to drive a higher current capable device. See APM, Inc. Application Note 0001 above.

When is it necessary to use the DFA-100 (Difference Amp Module) with the Model 205 PWM Controller?
If it is possible that the analog signal source is not on the same electrical circuit with the Model 205 PWM Controller power supply, and there can be a ground loop, then it is likely that a DFA-100 Difference Amp Module will be necessary. When using a PLC analog output, or an isolated analog output, it is also likely that the DFA-100 will be necessary. The DFA-100 provides 2 channels of differential inputs for the analog frequency and duty cycle inputs on the Model 205. The differential inputs provide protection against common mode voltages and ground loops.

Note that version 3.X Model 205 units can be configured with differential analog inputs. Contact APM, Inc. for this option on the controller.

Is the Model 400 PWM Driver / Controller susceptible to problems due to ground loops?
No. The Model 400 analog input is fully differential, designed for industrial PLC applications. The analog ‘-‘ input is not tied to the controller / system ground.

Is it possible to electrically isolate the PWM Controller (Model 205 or 400) when using it in an RS-232 control configuration?
A ground loop may be formed between a control system or PC commanding the PWM Controller using its serial (RS-232) port. The Model 205 or 400 PWM Controller serial port does not provide electrical isolation. Opto-isolated RS-232 port modules are commercially available from many vendors for around $100. An example is the Model 232SPHI4 from B&B Electronics ( If you are using RS-232 controlled PWM, consider the APM Inc. Model 402 PWM Driver / Controller. The Model 402T has electrically isolated RS-232.

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FAQ – Connections / Wiring

How is the PWM Controller (Models 205, 400, or 402) connected to act as a signal generator?
For transmitting applications (applications where the PWM duty cycle commands a module of some type) the PWM Controller can be wired to supply an output signal (as opposed to driving a high current consuming load). Since the PWM Controller has an open drain MOSFET output, a load of some type is required to generate an output signal. A resistor at the output terminals is sufficient. The figures below show how to connect the PWM Controller as a signal generator. There are two configurations shown, depending on the configuration of signal destination (internal pull-up or not). If the signal destination does not have an internal pull-up then a resistor load is required as the PWM Controller output is an open drain configuration.

Connections / Wiring Connections / Wiring

Note that the with respect to the PWM Controller, a 100% duty cycle means that the MOSFET output is on and the voltage at the device input will be 0V. At 0%, the output will be floating, or pulled up to V+ by the internal or external resistor. This ‘inversion’ may be compensated for in the Model 205 by using the Polarity setting function and the analog action function in the Model 400 – see the respective User’s Manuals for using these functions.

Finally, V+ can be any voltage within the range of the PWM Controller input voltage requirements (9 to 24V for the Model 205, 400 or 402R, 9 to 14V for the Model 402T). For 5V signal generation use a dual supply configuration as shown in the respective User’s Manuals.

If the internal diode in the Model 205 (or Model 400 or Model 402) PWM Controller is not used is it necessary to use the Output+ connection?

What kind of cable is required to connect the PWM Controller to a PC?
The Model 205 and Model 402 is wired such that a straight cable can be used from a PC with a DB-9 style serial port.
The Model 400 requires a miniDIN6 to DB-9 adapter cable, and then a straight cable to the PC serial port. The adapter cable is supplied with the Model 400 kit.
Serial port pin assignments and connection tables are provided in the Model 205, 400 and 402 User Manuals.
Cables are available for purchase from APM, Inc. Refer to the Controller Accessories pdf on our website.

Can the Model 400 PWM Controller be connected to an analog input that shares a common ground with the power supply (and the output device)?
Yes. See figure 3.2-4 in the User’s Manual for a sample configuration.

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FAQ - Replacement Part Numbers

What is the part number for the side connector (analog port) for the Model 205 PWM Controller and where may they be purchased?
The Model 205 analog port connector is an 8 pin, 0.1 in. insulation displacement style connector. The recommended part is Amp (Tyco), part number 3-640440-8. It is available from electronic distribution and catalog vendors such as Digi-Key Corporation (, Newark In One (, and Mouser Electronics (

What type of fuse is used in the Model 205, and where can they be purchased?
The Model 205 uses a standard 3AG type, glass fuse. The installed part is a Littelfuse 0312004.HXP.  Any 3AG, slo or fast blo type fuse may be installed.  In addition to electronic component distributors, 3AG fuses are available from your local hardware store.

What type of fuse is used in the Model 400, and where can they be purchased?
Due to its small size, the Model 400 PWM Controller uses a fuse that is only available from APM, Inc. or electronic component distributors.  The fuse is a Littelfuse Nano SMF type R451 series fuse.  Series R451 fuse part numbers are provided below:

Littelfuse part number 0451004.MRL   Rating  4A
Littelfuse part number 0451003.MRL   Rating  3A
Littelfuse part number 0451002.MRL   Rating  2A
Littelfuse part number 0451001.MRL   Rating  1A

What type of fuse is used in the Model 402, and where can they be purchased?
The Model 402 uses a standard mini-blade automotive style fuse and is easily accessible on the end of the unit.  In addition to electronic component distributors, mini-blade fuses are available from your local hardware store, however, Littelfuse part numbers are also provided below:

Littelfuse part number 0297004.WXNV   Rating  4A
Littelfuse part number 0297004.WXNV   Rating  3A
Littelfuse part number 0297004.WXNV   Rating  2A

2 and 4 Amp fuses for all controllers are available for purchase from APM, Inc.

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