# Connect incandescents in series or parallel?

Discussion in 'The Projects Forum' started by campeck, Sep 20, 2010.

1. ### campeck Thread Starter Active Member

Sep 5, 2009
194
3
http://genet.gelighting.com/LightPr...&ModelSelectionFilter=FT0010:Decorative_Globe

I need to run 7 of these 25W Edison bulbs through a relay that is rated 10A 125VAC
The end effect will be a flasher circuit. I have 28 bulbs going through 4 relays.

I have never really experimented with switching AC power or using light bulbs.
I assume in series the current and brightness will be smaller? But will my 10A relay handle the 7 25W bulbs.
I don't know the resistance of the bulbs so I can't calculate the current...Or am I missing something?

2. ### campeck Thread Starter Active Member

Sep 5, 2009
194
3
Wait...
P/V = 25/125
Is it .2A?

If so then I will connect them in parallel for max brightness.

3. ### prb22786 Member

Sep 19, 2010
15
0
P = E*I = (E*E) / R = I*I*R

R = (E*E) / P

25W @ ~120 V

R ~ 576 Ω

All approximate of course, but for simple circuits works fine.

Side note also. For what you're doing you may want to look into SCRs/TRIACs. With a mechanical relay, you tend to generate a lot of electrical noise, not to mention the contacts wear out over time. It really depends how rapidly you're switching the load. Both devices also use less driving current since they're semiconductors.

4. ### campeck Thread Starter Active Member

Sep 5, 2009
194
3
Well its a 4 relay board that has integrated transistors, on leds, relays, and protection diodes. So All I have to do is hook my Microcontroller ports into the inputs.
Trying to keep it simple! The fastest switching rate I will achieve should be about 20Hz max. can mechanical relays hit that?

5. ### prb22786 Member

Sep 19, 2010
15
0
Do high speed switching relays exist? Sure. You'd have to check the datasheet for your particular relay though to know the max speed it can operate at. Keep in mind with a digital circuit nearby (i.e. your microcontroller) you're going to be creating an awful lot of transients. If you don't already know what relay chatter is you're going to soon find out! Your relay contacts will wear out eventually as well.

Then again, at a switching rate of 20 Hz, you won't even be able to see the light bulbs going on and off. I'm thinking they'll just dim.

Wire those babies up in parallel and let us know how you make out.

6. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
20Hz with a 10Amp rated relay would be impractical. Go solid state.

7. ### campeck Thread Starter Active Member

Sep 5, 2009
194
3
See thats a rough estimate...I may be over thinking the speed I want.
it might even be as low as 5Hz...There are going to be 7 lights on of 28 that chase each other around a square. Starting out at about .5Hz and then ramping up to a speed of lets say some fast Las Vegas signs.

8. ### prb22786 Member

Sep 19, 2010
15
0
1 Hz = 1 cycle per second

So, you've got four separate loads. Lets say your clock signal (the command to switch from load 1 to load 2, load 2 to load 3...) is 1 Hz. Every second you'll switch one set of lights off and the next on. So each individual load will only switch every 4 cycles. Once your clock signal gets above 4-8 Hz the visual effect will suffer.

GetDeviceInfo is right, mechanical relays are very impractical with that kind of switching speed and load. Your relays are going to spark and wear very rapidly. I'd look into Triacs or SSRs. Both can be easily controlled using a digital input.

9. ### campeck Thread Starter Active Member

Sep 5, 2009
194
3
Ok. Any info on Triacs of SSRs? I have never used any of these. This is for a movie so the duration that the circuit will be operating will be not be long. maybe .5Hz ( so each relay activating in 8 seconds) for 45 minutes a day with the ramp up speed of 5hz (one relay turning on/off every .8sec.) only being activated for 10 seconds once a day. So the relays will be activating around 400 times daily max. Is that pushing it? I can even tell them to turn it off when it's not in the shot.

10. ### prb22786 Member

Sep 19, 2010
15
0
It should be fine short-term. Its not the best solution but it will provide the effect you're looking for. Just don't be surprised if you have to replace a relay.

11. ### GetDeviceInfo Senior Member

Jun 7, 2009
1,571
230
I've held a Millwright TQ for 25 years, but I hate mechanics and do away with them whenever possible.