Hoping someone can help me out on my first pcb project please. Just got started learning about Arduino and as in lockdown boredom got right into it, learned basic schematic and pcb design on EasyEDA and loads of other things right back to school and Ohms Law again. Been stumped a few times and found the solutions, but I cannot for the life of me find an answer to calculate rc snubber values for arc supressression on my 12v contactor. I think I could work it out if I knew the Henry value for my loads inductance, but all I know is it's a 60 w, 240 mains ac water pump (aquarium type). So I don't know the energy the capacitor needs to absorb. Is it feasible to ballpark this?
Concensus seems to be snubbers are better across the load than the contacts switch. MOV across load is also recommend. "Standard" snubber combos for mains AC are around 100ohm and 0.1mf, not sure if thats just too much for my setup. I'd guess I could do without a snubber but I'd like to learn anyway since I've come this far and operate good practice. Simplified schematic below.
Any advice (other than you don't know what your doing mate give up) appreciated thanks.
Those values sound sensible to me. Some time back I built a snubber for a TRIAC switch, that had a 47 Ohm resistor. I forgot the capacitance, I think it was a bit more but then it was for a 700W, 3-4A output.
Best is to use X rated capacitors. Must be rated 250AC or higher, and they're meant to be used in conjunction with a fuse as they are designed to fail closed. The blown fuse then tells you the cap failed and you have to replace the board.
A regular 1/4W through hole resistor is fine; if using SMD components you need two 1206 resistors in series - they're rated 200V each so 400V between them (for resistors the voltage rating is related to the electricity arcing on the outside of them).
By the way, 0.1 mF = 100 μF (also written uF if you can't type the μ).
0.1 μF = 100nF.
I think the snubber is more effective across the contacts for arc suppression.
There is no rule that you can't have both.
C.C Bates developed a formula for calculating the resistance and capacitance value that is required for the RC network: C = I^2 /10, and Rc= Vo /[10I{1+(50/Vo)}]
V=I*Rc= (Rc/RL) Vo
RC = Resistance of RC Snubber
C = Capacitance of RC Snubber
RL = Load Resistance
V=I*Rc= (Rc/RL)Vo
P = 60W
V = 240Vrms
Vpeak = SQRT(20*VRMS= 1.414213562373 * 240V = 339.41Vpk P = I * V => I = P/V = 60W/240V = 0.250A
C=I^2/10
** = (0.250A)^2/10**
** =** 0.00625 F (6246uF) =6246****uF
The voltage induced at the contact opening can be determined by:
V=I*Rc= (Rc/RL) Vo
V=I*Rc= (Rc/RL)Vo
V= 0.250A*29 = (29/960)*240 = 7.25V Note: freq of line voltage is not given (50Hz or 60Hz) ?
** If given, it is possible to calculate the Capacitive Reactance of the Snubber cap.** Calculating Snubber Values FYI, schematic convention is that the GNDs are shown at the bottom, not the top, so basically, your circuit is upside down, regardless of being electrically correct. (power rails are shown at the top, GNDs at the bottom of the schematic. The relay should be rotated 90 CW, which would be the GND at the bottom. The 12V should be brought out to the left and then 90 CW bend so it is pointing up. The schematic is also inconsistent because the water pump has no connection dots whereas everything else does. Either use them nowhere or everywhere. Also , it is not necessary to draw the water pump as box. It is an electric motor and should be drawn as a motor, not a box. Why use symbols for capacitors, resistors and diodes but not inductors ? Either an inductor symbol or a motor symbol (circle with M insde) is correct.
Snubber across the contacts is a big safety issue, snubbers leak enough current to cause injury or
even death (though not by electrocution, but by the subsequent fall or involuntary reaction).
Snubbers belong to the load, since the component choices scale with the load inductance and current.
They then only waste electricity when the load is on, rather than all the time the load is "off".
Snubber across the contacts is a big safety issue, snubbers leak enough current to cause injury or
even death (though not by electrocution, but by the subsequent fall or involuntary reaction).
I don't understand what you mean. Isn't it necessary to touch something to receive a shock ?
Can you explain to be why anyone (with any common sense) would be touching a snubber on a 240V line ?
I'm having trouble seeing how your comment is relevant unless you are assuming people go around touching
high voltage components .
You point about wasting electricity is a valid one.
For Rc = 29 ohms
and Xc (60Hz) = 0.42 ohms,
IRC Snubber = Vo/29 = 8.27A (when snubber across off contacts)
P = I * V
= 8.27*240
P = 1986W
The voltage induced at the contact opening can be determined by:
V=IRc= (Rc/RL) Vo
V=IRc= (Rc/RL)Vo
V= 0.250A*29 = (29/960)*240 = 7.25V Note: freq of line voltage is not given (50Hz or 60Hz) ?
** If given, it is possible to calculate the Capacitive Reactance of the Snubber cap.** Calculating Snubber Values FYI, schematic convention is that the GNDs are shown at the bottom, not the top, so basically, your circuit is upside down, regardless of being electrically correct. (power rails are shown at the top, GNDs at the bottom of the schematic. The relay should be rotated 90 CW, which would be the GND at the bottom. The 12V should be brought out to the left and then 90 CW bend so it is pointing up. The schematic is also inconsistent because the water pump has no connection dots whereas everything else does. Either use them nowhere or everywhere. Also , it is not necessary to draw the water pump as box. It is an electric motor and should be drawn as a motor, not a box. Why use symbols for capacitors, resistors and diodes but not inductors ? Either an inductor symbol or a motor symbol (circle with M insde) is correct.
Thanks, that's a very detailed answer, you know your stuff. Are you saying I need a 6246uF capacitor and 989ohm (960+29) resistor across the load? Seems a lot different to those "standard" 0.1uF and 120 ohm snubbers. Not questioning you, just saying. In response my Hertz is 50. Thanks for the schematic convention tips...noted. Couldn't find the motor symbol on my software btw.
wvmarle:
Those values sound sensible to me. Some time back I built a snubber for a TRIAC switch, that had a 47 Ohm resistor. I forgot the capacitance, I think it was a bit more but then it was for a 700W, 3-4A output.
Best is to use X rated capacitors. Must be rated 250AC or higher, and they're meant to be used in conjunction with a fuse as they are designed to fail closed. The blown fuse then tells you the cap failed and you have to replace the board.
A regular 1/4W through hole resistor is fine; if using SMD components you need two 1206 resistors in series - they're rated 200V each so 400V between them (for resistors the voltage rating is related to the electricity arcing on the outside of them).
By the way, 0.1 mF = 100 μF (also written uF if you can't type the μ).
0.1 μF = 100nF.
"By the way, 0.1 mF = 100 μF (also written uF if you can't type the μ)."... Yeah sorry about that, just me trying to cause confusion lol. Where do you even find those symbols on a normal keyboard?!
"Thanks, that's a very detailed answer, you know your stuff. Are you saying I need a 6246uF capacitor and 989ohm (960+29) resistor across the load? Seems a lot different to those "standard" 0.1uF and 120 ohm snubbers. Not questioning you, "
NO. READ IT AGAIN.
Rc=RC SNUBBER RESISTOR=29 ohms
R(Load) (your pump) =960 ohms
C can be rounded to 6300 uF
raschemmel:
"Thanks, that's a very detailed answer, you know your stuff. Are you saying I need a 6246uF capacitor and 989ohm (960+29) resistor across the load? Seems a lot different to those "standard" 0.1uF and 120 ohm snubbers. Not questioning you, "
NO. READ IT AGAIN.
Rc=RC SNUBBER RESISTOR=29 ohms
R(Load) (your pump) =960 ohms
C can be rounded to 6300 uF
"In response my Hertz is 50. "
That makes Xc = 0.50 ohms
Xc= 1/(2Pif*C)
I = V/R
= 240V/29.50 ohms
= 8.13A
Psnubber across OPEN contacts= 8.13A * 240V
= 1952W (that seems rather high. I am doing something wrong here ?)
raschemmel:
"Thanks, that's a very detailed answer, you know your stuff. Are you saying I need a 6246uF capacitor and 989ohm (960+29) resistor across the load? Seems a lot different to those "standard" 0.1uF and 120 ohm snubbers. Not questioning you, "
NO. READ IT AGAIN.
Rc=RC SNUBBER RESISTOR=29 ohms
R(Load) (your pump) =960 ohms
C can be rounded to 6300 uF
But 6300 uF capacitor is MASSIVE and all seem to be electrolytic polar ones. This is for ac and a pcb??! Confused.
But 6300 uF capacitor is MASSIVE and all seem to be electrolytic polar ones.
I agree . I think I may be doing something wrong. I know the cap value is related to the load current at contact opening by C=I^2/10
For ILOAD= V/R = 240/960 = 0.25A
C = ((0.250A)^2)/10
= 0.0625/10
= 0.00625 F
raschemmel:
0.mmmuuunnnpppfffaaa
I agree . I think I may be doing something wrong. I know the cap value is related to the load current at contact opening by C=I^2/10
For ILOAD= V/R = 240/960 = 0.25A
C = ((0.250A)^2)/10
= 0.0625/10
= 0.00625 F
I can see your maths is correct. I think he developed that formula for much heavier loads. It seems to be aimed at reed relays rather than standard electromagnetic ones if I interpret it correctly. From other similar examples I've seen I was expecting a value of 0.01 - 0.1 uF (or at least close)
Frequency: DC to 62 Hz 4. Peak Pulse Voltage: 1200 V max.
Try these values. This is an off the shelf packaged snubber.
I'll get back to you on the math but for now just ignore what I said before and go with this.
In the meantime take a look at this
"Standard" snubber combos for mains AC are around 100ohm and 0.1m µf
Since your load is only 60W, there's no reason or benefit to use a snubber with higher capacitance. At 240V/50Hz and 0.1µF capacitance, the steady-state current through the snubber will be about 7.5mA.
Here, the capacitive reactance plus snubber resistor is 31831Ω + 100Ω. (240V/31931Ω = 7.5mA AC)
Oh, the resistor should have a sufficient working voltage rating to handle 240VAC continuous ... usually its sized 1.6W or higher with at least 500V rating because it has to handle the voltage spikes and surge current.
