what is the reason behind voltage regulator ic is not functioning correctly

Thread Starter

etech7

Joined Jun 9, 2015
38
I made a project where two 6V relay is supplied from a 9v regulated dc supply. this dc supply is made up with 12V transformer, bridge rectifier, 100uF filtering cap, LM 7809 and a 10nF cap.
when no relays are energized output voltage is nearly 9v. when one relay is energized voltage drops to nearly 7v. and when both relays are energised voltage drops to nearly 5v. each relay is driven by bc547 transistor with 1k base resistor. is this normal? or is regulator damaged? will it be fixed if I replace it with new one? what should I do to get constant 9v output without modification at relay side?
 

bertus

Joined Apr 5, 2008
20,234
Hello,

My first impression is that the filter capacitor is way to small.
Try a 1000 uF capacitor over there.

Bertus
 

Thread Starter

etech7

Joined Jun 9, 2015
38
What are the relay coil resistance values or current ratings? What is the transformer VA rating?
Post a schematic.
coil resistance is 106-107 ohm on my meter check. current rating unknown. it is written on its body that "SRU-06VDC-SL-C"
I don't know the va rating also. all I know its 230/12 v transformer. but I think its va is not much a problem because secondary voltage remains unchanged whatever ralys do. I am not able to upload it's schematic right now.
 

Thread Starter

etech7

Joined Jun 9, 2015
38
What are the resistances of the relay coils?

I'm guessing that the 7809 is miswired, or defective.
Mmmm... Perhaps... 106 or 107 ohm, coil resistance by meter check. another point should note here that when i installed the regulator first time it's output was slightly above 9V. but now it is 8.88V on no load. and around 5V drops on it when both the relays are excited.
 

dl324

Joined Mar 30, 2015
9,620
Measure the voltage at the input and output of the regulator with one relay energized.

What are the current ratings for the regulator and the transformer?
 

bertus

Joined Apr 5, 2008
20,234
Hello,

I have rotated, scaled, cropped and changed the perspective using the gimp:

etech7_schematic.jpg


I do not see any Back-EMF diodes on the relays.

Bertus
 

Thread Starter

etech7

Joined Jun 9, 2015
38
Hello,

I have rotated, scaled, cropped and changed the perspective using the gimp:

View attachment 94641


I do not see any Back-EMF diodes on the relays.

Bertus
Thanks for the correction. mistakenly I forgot to draw back emf diodes. I used one diode for one relay and another is just left alone. what if i don't use those diodes by the way?
 

spinnaker

Joined Oct 29, 2009
7,837
Hello,

I have rotated, scaled, cropped and changed the perspective using the gimp:

View attachment 94641


I do not see any Back-EMF diodes on the relays.

Bertus

I wish people would put a tenth of much effort into posting their schematics the first time as you put into fixing them. This is the second one you had to fix today and neither OP thanked you so I am doing so. Thanks for your efforts Bertus.

Edit: so you did get a thank you on this one while I was posting! Nice to know when you are appreciated. :)
 

spinnaker

Joined Oct 29, 2009
7,837
Thanks for the correction. mistakenly I forgot to draw back emf diodes. I used one diode for one relay and another is just left alone. what if i don't use those diodes by the way?
Do you have a scanner? Just place the drawing on the scanner next time.

Or better download something like DipTrace. It is easy to use. ExpressPCB is probably easier but DipTrace has more features.
 

dl324

Joined Mar 30, 2015
9,620
I used one diode for one relay and another is just left alone. what if i don't use those diodes by the way?
The back EMF could damage the transistors.

If the AND gate you're using is 7408, you're over stressing the part. Max VCC is 5.5V. It will tolerate 7V (non-operating).
 
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