PIC Power Supply - Modifications

This only takes 21mA average when Relay is On, but >40mA to turn ON
https://www.falstad.com/circuit
click the switch

Increasing C1 to 390 uF from 330uF makes it work at 60Hz @160Vp/√2 = 113 Vavg
while at 50Hz @380Vp/√2 = 269Vavg yields similar good characteristics with < 30mA with relay ON. The 10k Bias depends on final uC peak current of design.

ronsimpson said:

MrAl,
I thought about bring up your idea much earlier but the design requires more thinking.
Most relays will "hold" to 1/2 voltage, but I would not design a product like that with out much testing or getting a guarantee from the relay maker.
I think a 30mA relay would hold at 20mA and 9V reliably. But 15mA and 6V is too close to the edge.
.
.

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Most relay datasheets give you the engineering numbers to do this. An **broken link removed** relay that I just double checked calls it the "must release" voltage, and is typically 10% of the rated voltage. So dropping the applied voltage to 50% is still 40% away from the edge. To understand why this is the case, just play with a small magnet and a piece of iron. It takes a lot more effort to pull them apart when they are touching, that it does to hold them apart by a fraction of an inch. And a relay is nothing more than an electromagnet and a piece of iron acting as the armature. The inverse of this is that it takes less magnetic strength to hold it when the coil core and the armature are touching.

As for how to accomplish this without adding extra hardware, it is quite easy if the relay is driven by a uController. Just PWM the base drive to the transistor. Design the power supply for the nominal coil voltage, but at 25% of the nominal power. But size the capacitor to hold that loaded voltage for 50mS, or ~5 times the operating time of the relay. Run the relay enable line for 100% duty for that 50mS, then drop it to 50%. This is in effect a buck switching regulator that uses the relay coil as both the inductor and the load. At half the effective voltage to the relay, it consumes 75% less power.

Notes:
1) The period of the PWM needs to be significantly shorter than the relay release time.
2) I would suggest changing the relay clamp diode to a schottky type, since it'll be switching of and off rapidly.

Yes Chris, this common characteristic in all magnetic electro-mechanical devices is called Hysteresis and Relays always spec guaranteed levels like
Must Operate threshold, (max) 75% rated voltage
Must Release Voltage (min) 10% rated voltage

But they don't spec may turn off, may turn on.

Only the best Relay companies for quality like OMRON will define this in their test reports,.
As a designer, you must consider Brown Outs, Vibration chatter and other factors and decide what margin you need and Environmental specs you need to test the design .

-10% is safe, -25% is possible, -50% will compromise immunity to vibration and make it faster to open from a brown out , however you define that.,

So it is up to your environmental requirements. to choose the guaranteed HOLD voltage for a given Voltage range and Brownout spec and during operational vibration.

Can you specify this?

Find the link for technical information on this page.
**broken link removed**
Nobody makes better Relays than OMRON in the same price range.

ChrisP58 said:

"must release" voltage, and is typically 10% of the rated voltage. So dropping the applied voltage to 50% is still 40% away from the edge.

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I disagree.
From what I understand all relays must open at 10%. All relays must close at 75%. (based on Chris and Tony above)
If all relays open by 10% then probably many relays will open by 20%. That 50% to 10% area is unpredictable. As Tony said, vibration will open up a relay. For automotive use a relay needs to be driven hard.
What is not given in the data sheets is the must hold voltage. That is very sensitive to temperature and vibration.

The most reliable spec is MUST OPERATE ( OMRON 75%) some others are 80%

If you are in a rugged environment and cannot tolerate contact chatter at resonant frequency, such as in payload design, then you buy expensive mil-std relays or perhaps operate at max rated voltage for more force on the contacts.

If you are a power miser in a benign environment, choose a lower range.

If you want to operate over a wide AC input range. do a worst case analysis and design appropriately.

It all comes done to your requirement specs for environment and reliability of Relay vendor. e.g. increased temperature of ambient also reduces contact force. and avoid the cheapest no name brand without research on MTBF.

Tony Stewart said:

avoid the cheapest no name brand without research on MTBF

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After doing the right design work, China will change all the parts to same money.
Who needs a "power line capacitor"?
My school mate has 100,000 relays he needs to sale soon. Don't ask why.
We have some transistors that have low gain but mostly work.
Our old stock resistors are hard to solder to. But the price is good.