Specifing resisters - Page 1
Forum section: electronics / beginners

author: Kannonbball
author_rank: Newbie
timestamp: July 31, 2014, 07:57:50 pm
content: I am developing a product that requires electronics. I am strong in mechanical design, weak in electronic design. The product utilizes a DC gearmotor. I have that sourced; no problem. I need the device to operate at 5 speeds. I have sourced a perfect 6-position rotary switch. I want to use resistors to get the various speeds. The rotary switch will give me the off position, and five on positions. The first four on positions will give me the first four speeds by sending the power from the 18 volt battery through four different resistors on the way to the motor. The fifth switch position will give me the fifth speed (the highest speed) by simply sending the 18 volt power directly from the battery to the motor.I have tested the entire system and it works perfectly. I am using resistors that I sourced from Mouser. Note that I do not want to use a potentiometer to obtain the various speeds. The load is quite high at about 370 miliamps. The product runs for only about 10 seconds, then is at rest for at least 30 minutes. My only problem is that three resistors that I am using are physically rather large and therefore making the size of the PCB a bit more than I want. I would like to know if smaller resistors are available that will do the same job. China source vendors are OK.Here are the resistors I have tested: (all Mouser part numbers)71-CPF328R000FKB14 3 watts 28 Ohms- 1% This one gives me the lowest operation speed of about 757 RPM. Works perfectly but it's too big physically.588-23J 22RE 3 watts 22 ohms 5% This one gives me the 2nd speed of about 810 RPM. Works perfectly but it's too big physically.71-RS2C-16.9 2.5 watts, 16.9 ohms 1% This one gives me the 3rd speed of about 844 RPM. Works perfectly but it's too big physically.594-5083NW8R200J 2 watts, 8.2 ohms 5% This one gives me the 4th. speed of about 900 RPM. Works perfect and is very acceptable physical size.It's therefore the first three resistors that I seek something to do the same job but in a smaller physical size.Help is appreciated.

author: Joule Thief
author_rank: Regular Contributor
timestamp: July 31, 2014, 08:10:42 pm
content: Maybe a DC to DC buck converter and eliminate all the high power resistors.The buck converter control potentiometer can be replaced with a low power version of the selector switch to choose 5 different control VOLTAGES for the gearmotor.

author: jmoreland79
author_rank: Regular Contributor
timestamp: July 31, 2014, 08:16:38 pm
content: The problem is even if you find physically smaller components, it seems you want to pack them together on the PCB so then you will lower the power rating.The 3 W rating is based on a certain amount of space around the component to allow for heat dissipation. Effectively, if you took four 3 W resistors and laid them out side by side on a PCB, you probably de-rate them to round 1.5W each or less.That said, you probably aren't going to find components all that much smaller at those values.

author: AG6QR
author_rank: Frequent Contributor
timestamp: July 31, 2014, 08:24:44 pm
content: Chain your resistors together in series to get the higher values. For example, instead of a 16.9 ohm, add another 8 ohm (approximate) resistor to the first one. A nice thing is that, when multiple resistors are chained together in series, the power load is spread across them, and neither one needs to have as high a rating. That means they can be smaller.There are more sophisticated possibilities. You could use one transistor on a heatsink (MOSFET or BJT) to dissipate your power, and use the switch to switch between various ways of biasing the transistor with very small, low-power circuits. An LM317 regulator could be used in a similar way.

author: IanB
author_rank: Super Contributor
timestamp: July 31, 2014, 08:42:34 pm
content: The resistors will be smaller if you pick a lower power rating (e.g. 1 watt instead of 3 watts).If you only have a very short duty cycle, 10 seconds on, 30 minutes off, then you may be able to support higher transient loads than the resistor's nominal continuous rating.Testing will help.Measure the voltage across the resistor in circuit under load. Power is given by V2/R. E.g. 10 V across a 20 ohm resistor would be 5 W.If you apply 5 W to a 1 W resistor for just a few seconds you could find the temperature rise is within reasonable limits.Measure the temperature reached by a resistor when subjected to the normal load. If it remains below 100 C it should be OK.

author: mariush
author_rank: Super Contributor
timestamp: July 31, 2014, 09:26:20 pm
content: You could use replace your small resistors with a LED driver, since what you're doing is basically the same thing: limiting the current going to the motor.This for example:http://uk.farnell.com/on-semiconductor/nsi50150adt4g/led-driver-pwm-50v-to-252-3/dp/2382343It would allow you to adjust current from 150mA to 350mA, see page 4 on datasheet, about 4 ohm for 350 mA, 1000 ohm for 150mA... and the resistors only have to be rated for up to 1w. Quite fine control.

author: wraper
author_rank: Unknown
timestamp: July 31, 2014, 09:46:31 pm
content: http://www.ebay.com/itm/12V-36V-3A-Pulse-Width-PWM-DC-Motor-Speed-Controller-RC-Regulator-Switch-12V-24V-/310983082372?pt=LH_DefaultDomain_0&hash=item4868094184You can use something like this, just replace potentiometer with a switch with small fixed resistors so you will get discrete steps. With such a thing there will be almost zero heat.