Problems substituting a relay

[CDRIVE]

I can't find any posts from you that show the effect of varying the input capacitor. The ones I can find relate to changing the smoothing capacitor, C2.

I could have sworn that I did. Might be that senility thing creeping up on me. I'll check again but it's going to be a long journey back.

Chris

 

[CDRIVE]

What happened here is the thread began with replacing the 24V relay with a 12V model. I think these are all my schematics and plots but no post to give them context. As shown, I did plot one with a 2.2uF cap and another with an increased zener voltage. The 12V - 24V issue yucked them all up though.

Chris

https://www.electronicspoint.com/attachments/4867d1342325416-problems-substituting-relay-relay.jpg

https://www.electronicspoint.com/at...7987-problems-substituting-relay-relayckt.jpg

https://www.electronicspoint.com/at...oblems-substituting-relay-mainsdirpwrsup2.jpg

 

[KrisBlueNZ]

I've noticed that similar capacitors to the one originally fitted seem to be rated at 400v AC and 250DV. I guess that accounts for the 250v RMS which is about 400v.

Most capacitors aren't designed to be charged and discharged continuously like this. You need a capacitor that is specifically rated for AC operation.

Like this one - it's a 2.7u too... http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=150795246646

That's 2.7 nF not 2.7 uF. And it's mylar. You need something rated for continuous AC mains operation.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=140818355335

That looks better. 400VAC and 2.5 uF. It's pretty big though and it's not PCB-mount.
You can search on Digikey:
http://www.digikey.com/product-search/en/capacitors/film/131088
Select the capacitance value(s) you want (e.g. 2.2~3.0 uF), AC voltage rating (e.g. 250~400 VAC), and check the In Stock box below the table, and click Apply Filters. With those numbers, I got 32 possibilities to choose from. I would go for one with lots of safety agency approvals such as the more expensive of the Epcos parts, and the Kemet R46 parts.

Edit: This one looks pretty good: http://www.digikey.com/product-detail/en/B32923E3225M/495-3820-ND/1828706

Edit2: That one has a 20% tolerance. These ones might be better:
http://www.digikey.com/product-detail/en/B32924F3225K/495-3823-ND/1828709 - USD 2.72, 2.2 uF, 10%, 310 VAC, 1.24" long, approvals IEC 60384-14, UL 1283, CSA C22.2 No.8
http://www.digikey.com/product-detail/en/MKP1839522634HQ/BC2615-ND/2260513 - 2.2 uF, 5%, 300 VAC, axial part with long leads that might be a good fit, but no safety approvals.

 

[Bassmo]

Whoops - only out by a factor of 1000 - not toooo bad

Thanks heaps to all for helping me get here. This has been a very interesting project.

Next stop, order the parts and see if it works.

I'll be sure to let you all know in due course...

-Paul

 

[gorgon]

Can one put 2 equal value 1W zeners in parallel to allow 2W dissipation?

Not a good idea, but you can put two zeners with half the voltage in series, and gain wattage. If you want 24V 2W, use 2 x 12V 1W in series. You could even use 3 zeners of 8V to expand this. The idea is to divide the load into equal parts to be able to sum the wattage.

TOK

 

[Bassmo]

KrisBlueNZ:

Hi,

I think I need at least a 400V cap since the p-p voltage is about 338V.

I like the look of this:

http://newzealand.rs-online.com/web/p/polypropylene-film-capacitors/6167856/

It's teeny and rated to 700v AC.. as well as cheap! Free shipping too. I'll be buying the relay and zeners at RS too.

When calculating zener power, does one use p-p or RMS. With the relay off, the 27v zener contends with 100mA RMS but up to 323mA p-p. Even at the 100mA * 27v = 2.7W or a massive 8.7W p-p. I think that's not how it's calculated. Also it's half wave so does that halve the wattage to 1.35W RMS so a 2W part will be perfect?

-P

 

[KrisBlueNZ]

I think I need at least a 400V cap since the p-p voltage is about 338V.

No, a cap rated for 300 VAC will be fine. The AC rating is an RMS rating, not a peak voltage rating. Also, you don't need a very large safety margin on the value to allow for disturbances on the mains. Most disturbances are at very high frequencies, and the capacitance is so high that high-frequency signals will not appear ACROSS the capacitor; they will be absorbed by the input series resistor (the fusible one).

http://newzealand.rs-online.com/web/p/polypropylene-film-capacitors/6167856/

That's 2.2 nF, not 2.2 uF. You won't find a suitable capacitor that's much smaller than the ones I found on Digikey.

When calculating zener power, does one use p-p or RMS. With the relay off, the 27v zener contends with 100mA RMS but up to 323mA p-p. Even at the 100mA * 27v = 2.7W or a massive 8.7W p-p. I think that's not how it's calculated. Also it's half wave so does that halve the wattage to 1.35W RMS so a 2W part will be perfect?

I don't know how to calculate the average zener current. That's why I did it using LTSpice. The way I did it was:
Use two zeners in series, and add a normal diode (1N4001 or similar) across the series combination to carry all of the negative current.
Put a small value resistor (I used 100 ohms I think) in series with the zener, at the bottom end, and put a capacitor across it (I used 100 uF I think).

Now the current that flows through the zener in the "reverse" direction (i.e. when the zener is in the zener breakdown state) causes a proportional voltage drop across the resistor and is smoothed by the capacitor, and you can see the average voltage with the simulation. Convert that voltage back into a current and you have the average (mean) current through the zener during the time its cathode is positive. The current through the zener in the other direction is not important because the voltage across it is only ~0.7V so the power dissipation during that part of the mains cycle is negligible.

 

[Bassmo]

Darn it! That's twice I've picked nF instead of uF.

I meant this:

http://newzealand.rs-online.com/web/p/polypropylene-film-capacitors/4974292/

Thanks for spotting this (again!) and for the tips.

-P

 

[KrisBlueNZ]

http://newzealand.rs-online.com/web/p/polypropylene-film-capacitors/4974292/

That one looks good, apart from the tolerance (20%). It might be your best bet though.

FYI here's the schematic and some traces from a simulation to detect the average (mean) zener current.

In the schematic I have replaced the zener with a reverse-connected diode (which takes the "negative" current from the input capacitor), and a 27V zener in series with a diode and a shunt resistor. The voltage across the shunt resistor represents the current flowing through the zener when it is in its zener breakdown state, i.e. when the cathode is positive relative to the anode. This voltage is smoothed (averaged) by the 1K resistor and the 100 uF capacitor and measured on the "Vc" node.

I've temporarily removed the relay coil load by changing the coil resistance to 999,999 ohms, because this forces the zener to absorb more current.

The green trace (using the current scale on the right side of the graph) represents the current through the parallel diode, which peaks at about 160 mA. This current is actually a "negative" current but is shown as positive because LTSpice displays the conduction current as positive when you select a diode to monitor current flow.

The blue trace (again using the current scale on the right) represents the current through the zener. As you can see, it only passes current when the current from the input capacitor is positive. This occurs when Phase is rising relative to Neutral and current flow (conventional current) is forwards through the input capacitor. This current peaks at about 110 mA. (Again the trace is upside down.)

The red trace shows the voltage across Cc, the averaging capacitor, which is the average (mean) voltage across Rc, which represents the zener current. It levels out at around 26 mV, showing that the mean zener current is around 26 mA.

You multiply that current by the zener voltage, 27V in this case, to get the mean dissipation. 0.026 * 27 is 700 mW. With the input capacitor changed from 2.2 uF to 2.7 uF, this current rises to 36 mA which is nearly 1W dissipation. This is why I said that the input capacitor value is quite critical, which was true even though my calculations for the best value were wrong.

The reason I'm not interested in the current that flows when the zener cathode is negative is that during this time, the zener conducts in the forwards direction, like a diode, and its forward voltage is only ~0.7V so it dissipates very little power.

 

[KrisBlueNZ]

Sorry to flog a dead thread, but I've just discovered another mistake in my simulation.

In LTSpice, a sinewave AC source with a "voltage" of 230V actually has a PEAK voltage of 230V, not an RMS voltage of 230V. If you want a 230V RMS voltage source, you have to specify a voltage of 325V. "Go figure", as they say in the USA.

So my previous simulations were using the wrong mains voltage. Current in the input capacitor is therefore 1.414 times the current I calculated. This is good because it means a 2.2 uF capacitor will work with a good safety margin, but it means the power dissipation in the zener will be higher than I calculated.

With a 325V (peak) input voltage, a 2.2 uF input capacitor and no relay coil current, using the same simulation schematic as in my most recent post, the mean zener current is about 47 mA. Assuming a 27V zener this is 1.27W. So you should use a zener rated at 2W or even 3W, especially if you use an input capacitor with 20% tolerance.

 

[CDRIVE]

Tina has the same conventions. It would be nice to have a lazy people button or check box to select Peak or RMS. This way the user can enter an RMS value and have it automatically converted.

Chris

 

[KrisBlueNZ]

I think it's non-intuitive. AC sinewave voltages are pretty much always specified by their RMS value. The least they should do is clearly point out that it's the PEAK value you're entering, not the RMS value.

 

[CDRIVE]

Yes, I've questioned why but dismissed it because the Sims are waveform plots which are instantaneous values. .

 

[Bassmo]

Luckily I ordered a 5W Zener just to be sure...
Also beefing up the 8.2v one slightly too.
Just waiting on that 2.2u cap from England and then the results will be in...

-P

 

[(*steve*)]

I think it's non-intuitive. AC sinewave voltages are pretty much always specified by their RMS value. The least they should do is clearly point out that it's the PEAK value you're entering, not the RMS value.

Actually, there is a reason...

Often you can pick the voltage and the waveform separately.

Since the ratio between peak and RMS varies with waveform, being able to select an RMS voltage for a square wave would be really problematic if you varied the duty cycle whilst the RMS value remained the same.

 

[CDRIVE]

Agreed but it would still be nice to have an RMS option for the Sine wave. It would save the time of having to calculate what your entering. I have done what Kris did but since being burned I don't forget to enter peak values any longer. I just don't want too.

 

[KrisBlueNZ]

Often you can pick the voltage and the waveform separately. Since the ratio between peak and RMS varies with waveform, being able to select an RMS voltage for a square wave would be really problematic if you varied the duty cycle whilst the RMS value remained the same.

That partly explains why the software wants a peak voltage value, but IMO it doesn't excuse the unintuitive dialogue box. This is a shame because generally I've found LTspice IV to be very intuitive and nicely implemented.

The dialogue box allows you to select the "function" (pulse, sine, exp, sffm, pwl), then accepts parameters for "DC offset[V]", "Amplitude[V]", "Frequency[Hz]" and so on. Once the Sine function is selected, the dialogue box could easily ask for the voltage to be specified as an RMS value.

But I don't even think they need to go that far - they just need to be clear about what they mean by "Amplitude[V]". If a sinewave is selected, without any qualifier I would assume the amplitude number should be RMS. I would no more assume it would be a peak voltage than I would assume it would be peak-to-peak.

I'll be sending them feedback sometime anyway.

 

[(*steve*)]

That partly explains why the software wants a peak voltage value, but IMO it doesn't excuse the unintuitive dialogue box.

It would probably be useful to just display the calculated RMS value. That would be enough to prompt you to remember that you're not entering RMS values.

I can't speculate why. If it were not Spice, I would hazard a guess that the programmer who created the dialog box didn't understand what RMS was and why it might be important. (or perhaps he was so good at multiplying be 1/sqrt(2) that he didn't think anyone would need it).

 

[CDRIVE]

Considering how programmatically easy it is to provide the user RMS data entry it's one heck of an oversight. .

Chris

 

[KrisBlueNZ]

It would probably be useful to just display the calculated RMS value. That would be enough to prompt you to remember that you're not entering RMS values.

Yes, that would be a good solution. I will suggest that to them.