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Servo Dog Relay Control

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I hope somebody can help. Apparently I must be overlooking something. Here is the problem. I am trying to control 4 relays to control a DC motor (forward and reverse) and some 12VDC LED lights. The relay switching is accomplished by use of  transistors and the relays have 12 VDC coils (See the attached diagram). I have done this before but I was using only 1 relay. I am now trying to control 4 relays, when pin 3 goes high (which controls relay 1) all of the relays become energized but all of the other pins are not energized. I have tried this with PN2222 transistors and TIP 120 transistors with the same result. Any help will be greatly appreciated. Thank you.

Servo Dog Relay Control.GIF

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In general terms, your circuit looks OK - probably.  Confirm that you have the outputs setup as digital outputs in the ServoDog Utility?

Have you measured the voltage on both sides of the base resistors.  It's been a while since I used a digital output from a ServoDog, so going by memory.  As I recall, the SD output should switch from about 0 volts to about 5 volts when the channel is turned on.  You should see that on SD output (or the SD side of the base resistor - which may be easier to measure).  The transistor base should go up to about 0.7 volts when the SD channel is on.  You may need to provide some pull down the base lines to ground.  If the voltmeter is showing the base voltage up there a bit, try a few K ohm resistor from base to ground and see if that helps.

Is the 12 volt supply that is powering the relays the same supply that is powering the ServoDog?  If not, is the negative of the two supplies tied together (and the lead to pin 1 does not count)?

 

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Thanks for replying Jim. Yes, the servo dog output channels are set to digital outputs. The output pins do switch from 0 to 5 vdc. When I tried this with the PN2222 transistors I had a base resistor of 2K2 which dropped the base voltage from 5 to approximately 4.4 which limits the current to about 2 mA, when I tried it with the TIP 120 transistors (which are Darlington transistors) the voltage dropped from 5 V to 3.6 with a base current of about 4 mA when using the 820 ohm resistor. The 12 VDC power supply is providing power to both the servo dog and the relays. To me it appears to be some type of feedback which would power all of the relay coils. I just can't imagine from where. I will try to increase the base resistance and see what happens. I'll let you know when I get to work on it later today or tomorrow.. Thanks.

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I found my problem. It was a dumb mistake on my part (I'll blame it on my age - I'm 72😀). I mounted the TIP 120 transistor to a common heat sink but I forgot to insulate each transistor. When one was energized I was getting feedback from the  mounting tabs. Everything is working properly now.  Thanks for the suggestions.

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That will do it.  Glad you got it...

 

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