Relay issues/questions

[mlp-mx6]

I'm trying to wire up and test my relays before I jump in and wire up the circuit boards. Something weird is going on, or I've got something REALLY wrong.

The power supply is the 5V taps thru a bridge into 4700uF, thru 100 ohms into a second 4700uF. This yields 6.3V unloaded. So far, so good.

However, with the relay in the circuit the voltage drops to 1.3V or so. The relay does engage. This much of a voltage drop does not seem right. The drop from 6.3V to 1.3V is all the current going thru the 100 ohm resistor. That seems like AWFULLY high current. What am I doing wrong? Anyone been down this road?

[ayan]

I'm trying to wire up and test my relays before I jump in and wire up the circuit boards. Something weird is going on, or I've got something REALLY wrong.

The power supply is the 5V taps thru a bridge into 4700uF, thru 100 ohms into a second 4700uF. This yields 6.3V unloaded. So far, so good.

However, with the relay in the circuit the voltage drops to 1.3V or so. The relay does engage. This much of a voltage drop does not seem right. The drop from 6.3V to 1.3V is all the current going thru the 100 ohm resistor. That seems like AWFULLY high current. What am I doing wrong? Anyone been down this road?

OK, let's see if this helps. The problem is that you're putting a 100 ohm resistor in the circuit, which will be in series with the relay coil. I don't know what type of relay you're using, but pulling some data from an NTE datasheet for 5/6VDC relays, coil resistance is 52 ohms (reference: http://www.nteinc.com/relay_web/pdf/R16.pdf), which I will assume is sort of a typical value.

Your circuit looks like this: 6.3VDC voltage source (unloaded, so it will be less in circuit) + 100 ohm resistor + 52 ohm resistor (relay coil), all in series. The theoretical voltage drop across the relay coil is (52/(52+100)) * 6.3 = 2.16VDC. As you can see, your results are not all that surprising at all.

If you want to lower the voltage a little bit on your power supply, which I take it is just rectified 5 VAC (i.e., 7 VDC), put a diode or two forward biased in series with the power supply. Each 1N4007 diode will drop about 0.7V, without messing with your load, so two of them should give you a nice 5.6VDC supply which your relay will surely be happy with. Test that, and if that voltage holds with the relays energized, you're fine. If it drops to below 5 VDC, I would lose one of the two diodes.

Good luck,

Gil

[mlp-mx6]

Gil,

Thanks. Makes sense. OK - so a follow-up question, if I may.

I want to power 2 separate relays from this power supply. Do I need to isolate them from each other in any way? Or can they just be wired to the same supply? (BTW, 47 ohms is the resistance for these relays)

Sorry for the newbie questions, but this is the first channel-switching build for me.

Thanks again,
Michael

[ayan]

Gil,

Thanks. Makes sense. OK - so a follow-up question, if I may.

I want to power 2 separate relays from this power supply. Do I need to isolate them from each other in any way? Or can they just be wired to the same supply? (BTW, 47 ohms is the resistance for these relays)

Sorry for the newbie questions, but this is the first channel-switching build for me.

Thanks again,
Michael

Hi, Michael. You don't need to isolate them, but you need to make sure the two relay coils are wired in parallel. [ If you wired them in series, the minute the second relay was turned on, you'd get half of the voltage drop across the coil of the relay that was energized first. ]

Gil

[mlp-mx6]

Yeah, I know about keeping them in parallel. Basically that means common supply, separate grounds. Thanks for the guidance.

[BobW]

Don't forget to add a diode across each relay coil (Cathode to the + coil side, and Anode to the - coil side). This reduces switching noise as the magnetic field in the coil collapses during turn off.

[mlp-mx6]

Yep, got that done too. Thanks for the reminder, though.

I tested things out tonight and I'm able to switch both on concurrently, so all seems well. Now to get the footswitch working, then the rest of the circuit boards will go in and I'll have a new amp! (to start tweaking...)

[toneman]

Great info on relays Link wouldn,t work-Google-
Relay Technical Information/RELAY COIL