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Relay for powering hair-dryer

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mersenne31

New Member
I am trying to run a hair dryer through a KUHP relay. I have connected a 12V signal to the control lines of the relay which are switched on via a 2N3904 npn transistor. A 5V signal turns on the transistor and hence the relay. However, I have tested this circuit and it is very unreliable.
The relay sometimes turns on after a delay of about a second (you hear a slight hum before you hear the contacts click) and sometimes does not turn on at all. At other times however, it seems to work fine. I am using a USB-1024 HLS Measurement Computing DAQ to send the 5V signal.
Any advice?
 

Mike_2545

Super Moderator
So you have 2 power sources, one 12v and one 5v? Are the grounds common? (are they tied together?)
 

kchriste

New Member
Forum Supporter
Posting the circuit will yield better replies. It's possible that you are not giving the 2N3904 enough base current to turn it on fully. What is the value of the base resistor? Relay part #? The datasheet for the relay that I THINK you are using says the coil current is 100ma. Information like the datasheet for the DAQ which I found here also helps.
 
Last edited:

mersenne31

New Member
Thank you for your quick replies, I appreciate it. The value of the base resistor is 10k ohms. I just checked with a multimeter and the ground does not go to 0.0V when the transistor is turned on. It goes to 0.184V and I think that is the problem.
The relay part# is KUHP-11DT1-12.
I'll try to explain it better, I have a transistor which first switches on an LED. The same connections of this transistor go to another transistor which turns on the relay. I am doing this because the relay requires 12V to turn on and the LED one uses 5V.
Now, for the LED transistor, ground goes to 0.184V and not 0.0V. The circuit has already been milled and therefore difficult to change. Therefore, any changes I have to make will have to be made on the the transistor circuit for the relay. I hope that makes things a bit clearer.
 

kchriste

New Member
Forum Supporter
Can you post a schematic? I can't quite visualize the circuit from your description. Even a crude drawing in MS-paint or a scan of something hand drawn will do.
 

mersenne31

New Member
I will post a circuit later on, but I think you both helped me solve the problem. The base current was too small because of the extremely large resistor I was using. I would like to get deeper into what was wrong but right now am in quite a hurry. Thanks immensely for your help :)
 

Tedfred

New Member
Post you circuit if you can. You can hand draw the parts and wire connection on paper. Do the best that can. If you don’t know the correct symbol just draw a simple picture of the part. Have some one scan your drawing in to the computer as a GIF or JPG file. Then up load to here where with your posting. Also can use information on your power supply, As it might be part or all of the problem. Hearing hum coming from the relay tell me that there may be unwanted AC coming into the circuit from some where. Also some time redrawing the layout might uncover a misstate or misplaced wire. It which case you will have solved the problem.
 

KMoffett

Well-Known Member
I might be too late...but...use a 470Ω base resistor. That relay needs 100mA @12vdc. I usually assume a current gain of 10 to assure full saturation of a drive transistor. (5Vcc-0.6Vbe)/10mA=440Ω Also, don't forget an anti-flyback diode across the relay coil....anode to the transistor's collector and cathode to +12V supply.

Ken
 
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