Hi Folks,

I have a couple of questions I hope someone can answer.

Q1) If I connnect a DC supply across a
thermistor in series with a resistor
TAKE TEMP READING AS THE VOLTAGE DROP ACROSS Thermistor

WHICH IS BETTER TO USE ABOVE CIRCUIT OR THERMISTOR
IN A DC BRIDGE CIRCUIT, WHICH IS BETTER AND ????WHY?????

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Q2) I need to switch on a SODIUM HIGH PRESSURE BULB

12hours ON
12hours OFF

If I use a relay, will the relay survive being on for 12hours at 240v @ 4A
Should I use a high power TRIAC??


Your opinion is always welcome!

__________________
Best Regards

 

your first question is not quite clear to me what you are asking?

For question 2 I'd use a relay. Just make sure that the relay is rated for at least 240V 4 amps...a ten amp would be fine.

 

Q1) Which is better depends... If I were making just one of these and I did not need great resolution or accuracy in temperature readings, I would hand pick the resistor and just use the R+RT method since it will be cheapest and easiest to get working. Your temperature accuracy will be dominated by how accurately you can measure the volts.

The bridge method can work also and has advantages like being somewhat immune to TC's of the resistors etc.. The balancing nature of it in effect cancels out some of the non-ideal characteristics of resistors. But it is more parts.

Q2) I'm pretty sure (not 100%- havn't checked) that you can purchase a power relay that can handle this load just fine. If you can, I think using it would be better because it is easier to drive than a TRIAC. The triac will require gate drive circuitry adding complexity and more opportunities for things to go wrong. But the relay will certainly be more expensive than the triac.

In essence both parts can be made to handle the load you are after but you'll have to weigh the tradeoffs like cost versus complexity.

Hope this helps

 

1 Use the bridge circuit ..slap a comparator on the out and two pots and you can set upper and lower limits. ( just 1 if you just need 1 threshold)
2 Check here great source for new and used gear VERY cheap!

http://www.mpja.com/

for your app a suggestion:
http://www.mpja.com/productview.asp?product=12689+RL

 

I'd definitely go for the relay - I'm not sure how well a typical triac circuit behaves with an inductive load, such as a discharge lamp. Additionally, it provides the advantage of electrical isolation.

Relays and contactors are available in any current rating from 100mA to 1000A.