Will this be safe?

Boncuk · Jul 25, 2009

Hi,

this is the first time I have to deal with a relay drawing relatively high coil current (5VDC/180mA)

(Using another type of relay isn't possible due to customer's requirements.)

I'm afraid the relay will cause power fluctuations on the regulator output to cause problems for the MCU supply (VCC).

So I designed the supply like that.

Will this be safe enough for the MCU not to crash?

Boncuk

dougy83 · Jul 25, 2009

You can simulate the power supply with 180mA load to see if it goes below xx volts -- signifying the state you're trying to avoid.

You can also run the relay using a 2 stage drive; a high-current turn on, and a low current (50% is fine, can go lower, but you'd have to check out what the relay drops out at -- datasheet says 10% min but I wouldn't go anywhere near that).

You can either place a capacitor across the series resistor, or use another transistor with RC network or another micro pin. The diodes D1 and D2 are not required in your circuit.

Hero999 · Jul 26, 2009

I'd remove D1 and D2 which aren't needed.

There's no need to regulate the voltage to the relay coil, just use Ohm's law to calculate a value for R3 that will drop about 6.3V at 180mA. With 39R for R3 the voltage across the relay coil will already be below 5.1V so D2 will be doing nothing.

R1 isn't needed, the MCU is push-pull so it will turn the MOSFET off without a pull-down resistor.

Heck, I'd even consider removing R3 and PWM the relay so the coil current stays at about 180mA which could probably be cut to 90mA once it's activated. The trouble is you don't know the coil inductance to getting the right frequency will require some experimentation.

ericgibbs · Jul 26, 2009

hi Hans,
Just run your design thru LTspice.

Can you confirm the VA rating of the transformer.?

You are showing 222mA,????

Hero999 · Jul 26, 2009

C6 and L1 will ring at 160kHz so be careful not to excite it.

ericgibbs · Jul 26, 2009

hi Hans,

Ref to our Chat line figures, this is what I am showing, perhaps other spice user will have some input.

Hero999 · Jul 26, 2009

I don't have the model or symbol for the LM7805, please post it.

C2 is too small, make it at least 2200µF and it should be better.

ericgibbs · Jul 26, 2009

Hero999 said:
I don't have the model or symbol for the LM7805, please post it.

C2 is too small, make it at least 2200µF and it should be better.

hi hero,
I got it as regulator package from LTS. Yahoo.

If you cant find I'll send the lot.

EDIT: Found it!!!!

**broken link removed**

edeca · Jul 26, 2009

Hero999 said:
R1 isn't needed, the MCU is push-pull so it will turn the MOSFET off without a pull-down resistor.

I found I needed pull-downs with a 2n7000 and a PIC, is this a general rule or does it differ? Without the pull-downs the FET 'latched' on until the gate was grounded.

Hero999 · Jul 26, 2009

ericgibbs said:
hi hero,
I got it as regulator package from LTS. Yahoo.

If you cant find I'll send the lot.

EDIT: Found it!!!!

**broken link removed**

Just signed up.

edeca said:
I found I needed pull-downs with a 2n7000 and a PIC, is this a general rule or does it differ? Without the pull-downs the FET 'latched' on until the gate was grounded.

I don't see why you'd need pull downs, the PIC has a push-pull output stage and will connect the gate to 0V anyway when it's turned off.

ericgibbs · Jul 26, 2009

Hero999 said:
Just signed, wouldn't it have been easier just to attach the file though?

hi hero,

I did find and attach the file for you...

Hero999 · Jul 26, 2009

I noticed just after I posted and edited my post but you saw it too soon.

Hero999 · Jul 26, 2009

I still can't simulate your circuit because the relay symbol is missing.

ericgibbs · Jul 26, 2009

Hero999 said:
I still can't simulate your circuit because the relay symbol is missing.

hi,
Just use an inductor/resistor combination.

EDIT: hero, I use these relays.
**broken link removed**

Hero999 · Jul 26, 2009

The resistor value is easy but what inductance value did you use?

ericgibbs · Jul 26, 2009

Hero999 said:
The resistor value is easy but what inductance value did you use?

hi,
I have posted the relays link for you in my EDIT.

Hero999 · Jul 26, 2009

LTspice really needs a compile feature so all non-standard libraries can be bundled with the circuit.

I was right, increasing C2 to 2200µF will get rid of the problem.

2200µF appears to be the minimum value though so go with 2700µF or even 3300µF.

ericgibbs · Jul 26, 2009

Hero999 said:
LTspice really needs a compile feature so all non-standard libraries can be bundled with the circuit.

I was right, increasing C2 to 2200µF will get rid of the problem.

2200µF appears to be the minimum value though so go with 2700µF or even 3300µF.

hi hero,
Hans is on the Chat line at this time.

Hero999 · Jul 26, 2009

ericgibbs,
There's an error in your transformer model. You set the secondary resistance to 28Ω, so no wonder the voltage drop was too high.

I tried setting the primary and secondary resistances to 30Ω and 1Ω respectively but it made the simulation too slow so I've included separate resistors to model it.

To ensure the relay turns on, the series resistor should be 33R or 27R and must be rated to 2W.

It will be fine even with 470µF for the filter capacitor (providing you don't connect any larger loads to it) but there's no harm in it being 1000µF.

ericgibbs · Jul 26, 2009

Hero999 said:
ericgibbs,
There's an error in your transformer model. You set the secondary resistance to 28Ω, so no wonder the voltage drop was too high.

I tried setting the primary and secondary resistances to 30Ω and 1Ω respectively but it made the simulation too slow so I've included separate resistors to model it.

To ensure the relay turns on, the series resistor should be 33R or 27R and must be rated to 2W.

It will be fine even with 470µF for the filter capacitor (providing you don't connect any larger loads to it) but there's no harm in it being 1000µF.

hi,
Since further talking to Hans it appears the transformer secondary is approx 13R.

Will this be safe?

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