Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: John_D on January 08, 2012, 05:36:20 pm
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I have just finished building the board for Doug's Stout Reverb with the 5 volt relay switching between the normal channel and a boost gain stage. I wanted to make sure that the relay was working before I mounted and wired everything into the chassis. I applied 5 volts DC to the relay with a 6-volt battery and a dropping resistor. I checked to see if the contacts were switching with a meter but they always remain in the non actuated position.
I have a number of these relays so I tried several of them totally out of the circuit by connecting 5-volts across pins 1 and 16 (1 being + and 16 being-). Still no switching. I'm assuming 3 or 4 relays can't all be defective. The 6 volts are being supplied by 4 D-Cell batteries. I'm assuming that there should be enough current there to kick over a tiny coil but you know what happens when we assume. Any ideas?
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What is the coil resistance of each relay?
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where did you source these relays? Part number? data sheet?
they may very well be polarity sensitive also. have you reversed your polarity?
Remember to also use a snubber diode to prevent flyback spikes.
As you mentioned it's unlikely they are all bad, relays in general aren't too complicated so we should be able to work this out.
***edit**
that dropping resistor, double check and see that you're actually getting 5v when connected to the relay
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The 5-volt relay is the one supplied by Doug the resistance of the coil is 122 ohms.
After playing around with it some more last night I found that I could get the relay to activate (I could feel it and hear it click) when I reversed the polarity of my DC battery leads (+ to pin 16 - to pin 1). I just checked the polarity layout on the relay and also on Doug's diagram (http://www.el34world.com/projects/relay_switch5.htm (http://www.el34world.co/projects/relay_switch5.htm))
There are two diagrams (bottom view & top view) if you'll notice the polarities on pin one and pin sixteen are reversed on each picture). The DC out put of my battery pack is 6.3 volts. I was using a 180 ohm resistor in series which dropped it down to 5 volts but when I measured the voltage with it connected to the relay it was only 2.7 volts. I reduced the dropping resistor from 180 Ohms to 50 ohms which gave me about 4.7 volts across the relay coil and the switch finally kicked in. :worthy1:
Thanks for your suggestions you were a big help!
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> the resistance of the coil is 122 ohms.
> I was using a 180 ohm resistor in series
122 ohm load in a 180+122= 302 ohm circuit should get 122/302= 0.4 of the supply voltage. Taking 6.3V it should be 2.54V.
Not clear how you measured 2.7V.
"5V" relays will not reliably flop at 2.5V/2.7V.
> 6.3 volts. I was using a 180 ohm resistor in series which dropped it down to 5 volts
Resistors don't "drop voltage" unless there is a LOAD. Then the drop is largely affected by load.
It appears you got 6.3V through 180r to drop to 5V with only your meter as a load. For that to work out, your meter must be 692 ohms. That's way-way low. VTVMs and DMMs are more like 10,000,000 ohms. Good VOM on 10V range 200,000 ohms. Cheap VOM on 5V range 5,000 ohms.
There is something unexpected going on, either in your method or your tools. More info might clear it up.
BTW, you sure can test "5V" relays with a 6V battery. They do not die at 5.001V. Most have a long-term rating of 150% of nominal voltage. This means you can tap-test with nearly twice the nominal voltage (don't hold 2X voltage more than a few seconds).
> I reversed the polarity of my DC
This is a good point. Many "logic voltage" relays have a built-in diode. When driving from chips and such, a diode may be needed to protect the chip from coil turn-off kick. Adding the diode in the relay is a useful frill for such users. However it does mean that if you connect the coil backward the diode shorts the coil.