On May 26, 2006, at 11:11 AM, Olin Lathrop wrote:
>> Small-Signal Current Gain     Ic = 3A, Ie = 4V, f=1MHz
>> (not sure if this is what you mean by gain specs)
>
> That shows the parameters at which they were going to give you the  
> gain, but
> not the gain itself.  That is probably on the next line, which you  
> didn't
> show.  Also, 4V doesn't make sense for Ie.  You probably didn't copy
> something correctly.

Proofreading is always helpful:  Ic = 3A, VCE = 4V, f = 1MHz

At the top of the data sheet there is:

High DC Current Gain: hFE = 2500 typ @ Ic = 4A

> In any case, I was trying to see if your transistor was being given  
> enough
> base drive.  How much current do the relays draw at your maximum  
> supply
> voltage?  For starters until you can report the real current gain,  
> figure
> such a power transistor needs a base current of 1/15 the maximum  
> collector
> current.  If your relays draw 5A, then that means you need 335mA of  
> base
> current.

So after some time with the circuit and a meter here's what I measure:

This time using a 24V, 1.7A power supply.

When engaged the current through the solenoid (Ice, if I understand  
the nomenclature correctly) is 1.2A.  Vce is 5.5V.  The Base to  
Emitter current when engaged is 17mA, which seems very low.   
Grounding the base doesn't change CE voltage or current.

> lso note the power dissipation.  Even at Vce of 500mV that is
> still 2.5W.  A free standing TO-220 isn't going to take that for  
> long, and
> Vce is likely between 1V and 1.5V at maximum current.

For the purposes of the actual project, I'm just hitting things with  
the solenoids, so I'm sending 1ms - 10ms pulses.

thanks for your help,

darren
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