Relays drawing too much current?

by cubix   Last Updated August 14, 2019 13:25 PM

circuit

I have a problem with my circuit. It's an audio mixer with a PFL module - i've decided to send pre-fade signals via relays.

The transformer in +/- power supply is rated 400 mA for each rail.

There are 3 minature relays (4.5V, coil resistance 180 ohm). I'm powering them with a 7805 regulator placed on the main board. The regulator takes the power from +12V rail.

As you can see one of the relays (relay 1) is driven by a transistor, because i'm using a NOR gate and wanted to be sure that there will be enough current for relay to work.

Now my problem:

When i turn ON the device and NONE of the relays is switched ON i have +/- 12 volts on my rails. Everything is fine.

When i switch ON relay 2 or relay 3 the voltage on positive rail drops from +12V to +7V. When i switch ON relay 2 and relay 3 the positive rail voltage drops to 5V.

When i switch ON only relay 1 nothing bad happens - there is +12V on the positive rail.

I've measured current draw of the whole circuit:

  • When every relay is OFF the circuit draws 80mA of current.
  • When relay 2 or relay 3 is ON the circuit draws 120mA of current.
  • When relay 2 and relay 3 is ON the circuit draws 140 mA of current.
  • When only relay 1 is ON the circuit draws 40mA of current.

The transformer is rated 400mA for each rail - i'm not exceeding the current, but the voltage significantly drops, like there is not enough power.

Another thing that confuses me is when i turn ON relay 1 the current draw decreases.

I've tried to power the relays with a 9V battery - turning on relay 2 and relay 3 causes 1V voltage drop on supply rail.

Are the relays drawing too much current or what?



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