[SOLVED] relay for AC 230V circuit check and relay datasheet check help

Hi,

Don't blame it on the zeners. They work like given in the datasheet.
If you are not satisfied with the zeners, then most probably it's because
* you don't treat them correctly
* or your expectation is wrong.
(Both could be avoided if you read the datasheets)
Learn how to use them ... and you will get the desired result.

"Hating" a electronic device is like a mason says he hates bricks or concrete....

Klaus

I would have used a BJT but want to use the logic MOSFET(s) mainly because of this application note by the manufacturer

Click to expand...

That's a bit like asking a Ford dealer which make of car is best.

I understand the MOSFET technology well, but have you considered where exactly the current flows when the relay de-energizes? The back spike appears across the relay coil (obviously) so even if the MOSFETs internal diode conducts, where is the remainder of the spike dissipated - hint: it's through the supply line. In automotive applications the supply is considered an acceptable dumping ground for noise and spikes, the wiring is short, it is all screened by the car body and the battery appears like the ultimate decoupling capacitor. The situation is quite different in your application.

Don't be so harsh to Zener diodes - they are actually very good and a far cheaper alternative to regulator ICs. They work in clamping mode so they have no overhead requirement and work down to virtually zero forward current. You could use a single resistor and a Zener in your design and drop the 'off' consumption from 5mA to less than 1mA without sacrificing anything. Also consider that if your 12V supply drops by more than about 1V, the regulator will cease to function but a Zener would still work fine.

Brian.

Hi,

I forgot, Season's Greetings, gents.

Klaus, you may be right; and, to continue the analogies, an apt saying is: "A bad workman blames his tools", and here I am. I have read the Zener datasheet more than once.

r.e. Zeners I don't "hate" them, I'm just extremely prejudiced against them. I haven't found all the above you both say in practice. e.g. the 5.1V 1N5338B goes from 4.85V to 5.36V and I must have been doing something wrong with it because it needed a large amount of current to regulate within those parameters and a large dropout voltage to operate when I tested it. I'll have another go to check I wasn't doing anything odd and I'll be back...

I can't be bothered looking for it, so maybe I'm making this up, but I am certain there is even at least one thread on this forum that discusses the far from ideal qualities of Zeners.

Very witty, Brian, any old MOSFET or BJT would do ( I bought those logic level MOSFETs and subsequently found their app note). It had occurred to me about the relay back emf but as it's not doing any harm, the relay's tiny I would say... The (transformer) supply is a 15V AC secondary that feeds the regulator.

The venerable 1N5338B is rated at 5W continuous dissipation, that's 5.1V while passing nearly 1 Amp. The sort you need is more like this one:
https://www.onsemi.com/PowerSolutions/product.do?id=MM3Z5V1S although tight tolerance isn't really needed. Note the price and compare it to the 7809!

Remember that the Zener current only has to be the difference between minimum and maximum load of the remaining circuit, all it has to do is 'mop up' any excess voltage as the load on the supply reduces. Your CD4060 probably draws less than 1mA when running and the variation due to internal logic changes is probably only a few uA so even running it at 1mA current is probably overkill.

Brian.

Ahhh... Thanks. I hadn't realised you have to select them for the right Zener load current too. That's why the 5338 seemed so dismal when I was playing with it at relatively low output currents. I bought a pack of five of them about two years ago and beginner's purchasing error struck again...Thanks for clearing that one up.