Crude relay coil power supply design

Thread Starter

Charlie47

Joined Aug 11, 2020
2
Hi All
Would like to power 2 110 volt dc coil 30A relays (T9AS1D12-110) with line voltage separately from my microprocessor power supply in an enclosed device. They draw 9 mA each.

I bread boarded a circuit with a bridge rectifier (MDB10SFSCT-ND), then an electrolytic smoothing capacitor of 1uF 200 volt across the dc output of the bridge. Also placed current limiting resistors of 4.7K each to the coils.

I know better to put my scope on any part of this, but I can measure the correct dc volts to the coils.

Believe it or not it works. The coils of the relays promptly pull in and hold but after a few hours start to heat. Not terribly though and nothing else gets hot.

Could there be anything wrong with this method of running these relays on a circuit configured like this in the long range? Is there anything else I should ad?

Thanks in advance for any opinions you guys could offer.
 
Rectified mains for around 170VDC then the 4.7k 1W resistor seems reasonable for 122VDC to the coil.
The T9A relays are a piggy for coil power at 1W so they naturally run warm, more so at high currents as the contacts generate heat as well.

You can reduce relay (coil) holding power with PWM, to 50% or more for cooler operation. You pull in the relay, then after say 50msec switch to pulsing the coil at 20kHz with 50% duty-cycle.
The other approach I have seen is a 48vdc coil fed from a mains capacitive-dropper. It's harder to find relays with high voltage coils in stock.
 
Last edited:

sghioto

Joined Dec 31, 2017
1,334
Be careful running directly off the mains, an isolation transformer would be better and safer.
Why do you need to use 110vdc relays?
Steve G.
 

MaxHeadRoom

Joined Jul 18, 2013
21,233
If these are 110vdc relays, you do not need the resistor or the capacitor, when ran across N.A. mains.
The relays should work fine or are you in UK, Australia etc.
Max.
 

Thread Starter

Charlie47

Joined Aug 11, 2020
2
Thank you for all for looking at this. The reason for 110 volt coils is in my inexperience I conceived that the dedicated power supply would be simpler to build, ie less components on the bom for multiple builds, however if as you say higher voltage relays can become harder to find at times, that’s an important factor. Perhaps I’ll consider 48 volt and redesign.

The already overtaxed micro will be mounted on a separate board with limited communication wires between the two so a PWM feed to these coils would be difficult.

Max I’m in 60 cycle land. It never occurred to me these would run just off raw rectified AC. Thanks.
 
With the mains DC powered relay coils, you still have an issue with electrical safety and isolation. I'm assuming the MCU must be isolated from hazardous live?
You can use low voltage (i.e.12/24VDC) relays if you have a power transformer, or 5VDC relays if you have a stronger MCU power supply. But 200mA for each relay coil at 5V is a lot.
I see cheap timers from asia using 48VDC relays and a very clever circuit to power everything. Big Clive did a teardown and schematic
 

Dodgydave

Joined Jun 22, 2012
9,316
Thank you for all for looking at this. The reason for 110 volt coils is in my inexperience I conceived that the dedicated power supply would be simpler to build, ie less components on the bom for multiple builds, however if as you say higher voltage relays can become harder to find at times, that’s an important factor. Perhaps I’ll consider 48 volt and redesign.

The already overtaxed micro will be mounted on a separate board with limited communication wires between the two so a PWM feed to these coils would be difficult.

Max I’m in 60 cycle land. It never occurred to me these would run just off raw rectified AC. Thanks.
Instead of a Resistor in series with the coil, use a Capacitor in series with the AC bridge rectifier, like 220 to 330nF this will lower the voltage and not waste heat.
 
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