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?