ULN 2803 transistor array driving solenoids, HOT

I'm trying to control a bunch of small solenoids by powering them with a ULN2803. I'm running a test program that has them on much more often than I will need. The solenoids do get a little bit hot, but what I'm worried about is the the ULN2803 gets really, really hot. I have two questions:

  1. will turning the solenoids on less often prevent it from heating up as much?

  2. i may have too much current or something, but i don't know how to test the current draw of the solenoids. how do i check the current draw? Also (showing my level of knowledge here), is the draw different if you have one on at a time versus having many on at the same time?

thanks in advance

Hi,

I'm trying to control a bunch of small solenoids by powering them with a ULN2803. I'm running a test program that has them on much more often than I will need. The solenoids do get a little bit hot, but what I'm worried about is the the ULN2803 gets really, really hot. I have two questions:

  1. will turning the solenoids on less often prevent it from heating up as much?

Yes.

  1. i may have too much current or something, but i don't know how to test the current draw of the solenoids. how do i check the current draw?

check this out : http://www.scienceshareware.com/how-to-measure-DC-current-with-a-dmm.htm

Could you post the results, there are limits as to how much current an 2803 can take.

Also (showing my level of knowledge here), is the draw different if you have one on at a time versus having many on at the same time?

Yes, if all the selenoids are of the same type they will all draw the same current. Having 2 of them on will double the current. Its like current=NumberOfSelenoids * CurrentDarwnByOneOfThe Selenoids

Eberhard

How much total current can the UNL2803 take? It says 50v/500ma per device but its not clear if all of them can be on at once drawing 4.5A

Hi,

How much total current can the UNL2803 take? It says 50v/500ma per device but its not clear if all of them can be on at once drawing 4.5A

this really depends on the number of outputs you are using. You must check the datasheet, is has some function plots concerning the maximum power dissipation.

If you are using for instance all 8 outputs, and if they are switched on for about 50 % of the time your maximum current should not exceed 250mA per output. But you can use outputs in parallel if you need more current.

Eberhard

check this out :
http://www.scienceshareware.com/how-to-measure-DC-current-with-a-dmm.htm

Could you post the results, there are limits as to how much current an 2803 can take.

The solenoids have almost no resistance, like 3 ohms, so the math (voltage / resistance = current) says at 9.6V it would be 3 Amps. I can’t get a reading with a multimeter, probably because it won’t go that high. i don’t think that much is really necessary, since 9.6V at 200ma adapter runs the solenoids fine. Should I be adding resistance to the circuit?