zener diode help

twhitehousescat

anyone remember simple electronics

I have a simple circuit , low voltage AC , that when live needs to trip a small (12v DC direct current ) relay

I am told the parts I need are a simple diode , a resistor and a zenner


to put in context , the AC is from a motorcycle alternator , that may show say max of 35v AC at full revs , and the relay is to power a h/light (12v dc)

its just the component layout I need , I have tools and other needed parts

there are some pics on google images , but they all tend to differ

the guy that suggested this route , gave me the following cryptic instructions

"this is done on my old zzr600 by tapping into one of the stator wires that feeds the reg rec, fit a diode (stripe away from the stator wire) this will turn the ac into dc, then you need a resistor (10k will do). Other side of the resistor goes to the stripe side of a 12v zener diode, the other side to ground and then finally the stripe side of the zener feeds the coil of a relay that you then use to power your light."

unrecordings

sounds like the zener is protecting the relay so that only the positive portion of the AC signal reaches the relay. Modern relays probably don't need this, but it's a sensible addition

The resistor is (I think) to limit current - I'll have to figure out to post a drawing now...

twhitehousescat

yes there may be 35v AC , and the relay will be 12v DC I thought the zenner was to drop excess voltage to ground , ie a 12v zenner would only allow 12v that has been rectified to dc (via the normal diode) going to the relay

twhitehousescat

this is how I read the guys instructions , gen - diode - resistor - relay , with a zener diode added to the circuit , does the zener go in A or B

Cornucopia

I would say B. The Zener diode will hold the voltage at the relay down to 12v, but the rest needs the resistor to dissipate.

With a difference of 35v to 12v, this is likely to generate an amount of heat, especially if it is intended to be switched on for long periods.

Is there a 12v supply available, in which case you could use an opto-isolator? Or try to find a 36v relay.

twhitehousescat

the idea us that the h/light will not be on when engine is not running , 35v relay , umm no the voltage will be zero when off , rizing to say 20v ac at fast idle and then to max at say 4k plus , so there is not a stable ac voltage to work with , the load would be a micro relay

spec: approx 1.9w , this is type used https://docs-emea.rs-online.com/webdocs/0e1f/0900766b80e1ff89.pdf

techquest

twhitehousescat wrote: »

this is how I read the guys instructions , gen - diode - resistor - relay , with a zener diode added to the circuit , does the zener go in A or B

As Cornucopia I would say B as the resistor is connected in series with the zener diode to limit the current flow through the diode.

twhitehousescat

ok , I will call at CPC tomorrow and collect parts to do a trial build

coffeehound

twhitehousescat wrote: »

spec: approx 1.9w , this is type used https://docs-emea.rs-online.com/webdocs/0e1f/0900766b80e1ff89.pdf

Not sure if this is right but if that relay requires 1.7 watt at 12 volts to pull in the contacts, that would be about 142 mA. Whereas OP’s suggested circuit would be limited to 35 V (minus diode drops) / 10 kohm = 3.5 mA, so not enough current to power the relay?

twhitehousescat

ok based on pic above and info given , so you have any suggestions

smallest relay i can locate is "Coil consumption 540mW"

i have some like this https://www.ebay.co.uk/itm/12V-Relay-coil-10-Amp-contacts-10A-volt-Power-PCB-mnt/301760053489?hash=item46424d00f1:g:9TcAAOSwnOJWE5uj:rk:5:pf:0

coffeehound

Looks better: 540 mW would need 45 mA @ 12 V.

However since your generator volts are variable and will be half-cycle only, I’d *guess* you’d want to reduce assumed volts to 20 V and recalc resistance to get that current:

R = 20 volts / 0.045 A = 444 ohms

Your man said this sort of setup worked for him so hopefully there aren’t issues with the relay dropping out at low revs? You’ll need the diodes to be happy with the current coming through at peak revs. Remember peak volts is peak rms ac voltage / 0.707. So for 40 volts ac rms, peak voltage is 57 volts. Uprate components accordingly