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?
Best Answer
"Best guesstimate" is that the 12V regulator is being overloaded.
You do not tell us
What Vin is for the +12V regulator
What heatsink you are using on the 12V regulator
Bonus unknown: Heatsinking on +5V regulator.
The 5V regulator with 12V in dissipates (12-5) = 7 mW/mA. So at 140 mA power dissipation is 7 mW x 140 mA = 980 mW (if Vin remains at 12V). IF the 7805 was not heatsunk at all then it would get hot at that dissipation - so your assertion that " ...so there is no reason for them to heat up. ." is only true if thinks you have not told us allow it to be true.
Similarly, the 12V regulator dissipation depends on its Vin (unspecified), heatsinking (unspecified), stability (probably OK, but unspecified), which version of the 7812 (unspecified) - as sonme are rated at 1A+ - and some at much less. A regulator will drop its Vout when attempts are made to cause Iout to exceed Ioutmax.
I suggest that you load your 12V supply with a resistive load and see how it behaves across eg the 50 - 200 mA range.
Information on unknowns mentioned above will assist us to assist you.