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?

  • \$\begingroup\$ How hot are those regulators getting while this voltage drop is occurring? We are probably going to need to know more about that powersupply. \$\endgroup\$ Aug 14, 2019 at 16:14
  • 1
    \$\begingroup\$ You can try disconnecting the relay contacts so that you are only powering the relay coils. See if your problem persists. If not, you may be loading something in your other circuitry. You can disconnect the relay coils and short the contacts and see which part of your circuit is the problem. \$\endgroup\$ Aug 14, 2019 at 18:32
  • \$\begingroup\$ Regulators are not getting hot at all, as i said the current draw is maximum 140mA so there is no reason for them to heat up. \$\endgroup\$
    – cubix
    Aug 16, 2019 at 8:26

1 Answer 1


"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.

  • \$\begingroup\$ Vin for the 12V regulator is approx 20V. I'm using a small clip on heatsink for 12V regulator and no heatsinking for 5V regulator. They don't get hot at all. I will try to load the regulators with resistors, thanks for this idea. By the way is powering relays from common power supply fine? \$\endgroup\$
    – cubix
    Aug 21, 2019 at 16:13
  • \$\begingroup\$ (20-12) x 100 mA = 800 mW. More = more. Relays on common rail is OK but you MAY hear them switching. | Diodes across relay coil - which you have. Closer to coil the better. Presumably RELAY 1 reduces supply current by turning something off. | You did not answer re variant of 7812 used - this could be 1.5A rated or 1A or 100 mA or ... ? | These are usually stable BUT if you have a scope chck output. From memory they are stable without caps bt have some recommended values in data sheet. | The loaded down supplies suggests something is outside spec. \$\endgroup\$
    – Russell McMahon
    Aug 21, 2019 at 16:48

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