Hi - thanks for all your help so far… I have a slightly different question:
If the circuit below was to be used with 24 volts instead of 12 volts, would the pull down resistance of 1K be enough, or should a different resistor be used?
Regards
Tim
If you are using the transistor TIP142T, it is OK to use 1k resistor.
The base current will be about 23mA.
hFE (DC Current Gain) is 500, so the collector current can be 11.5A.
Here is the datasheet: datasheetcatalog.org/datashe … s/4135.pdf
superb! many thanks once again 
Tim
still worried about frying my k8055… another question im afraid 
with the relay circuit, is there any danger to the Digital output on the K8055 if used for 24V?
This effectively connects directly to the +24V (via the relay electromagnet anyway) without a resistor… does that not mean that the K8055 could get too much power?
I know the outputs are pull down connections, but what is the maximum voltage that could be safely connected to a K8055 digital or analogue output?
Tim
The K8055 outputs are rated for 50V @ 100mA–how much current does your relay draw?
Hi Cliffyk
The 12V one I have is as follows
Nominal voltage: DC12V
Operating range (V): 9.6-13.2
Coil resistance: 160Ω±5%
Current at nominal voltage: 75mA
so that would be within the 50V @ 100mA… assuming I am looking at the right detail! 
I will check out the spec of available 24V DPDT relays.
What would be the solution if the current is beyond 100mA?
Tim
Your 12V relay would work as the indicated 75mA draw when activated, is within the K8055 capacity, however should you find that the relay you wish to use requires more current you can add a PNP transistor to drive the relay coil:
The indicated 2N3906 transistor can comfortably handle 500mA.
BTW: R1 and R2 are “pull-up” resistors…
Cool - thankyou!
Tim
HI,
I would like to know if it is possible to buy this board to add onto my K8055 so that I can use this to drive a 12? Motor using my K8055 board
Hello,
I have a question, is it possible to use a 2N3055 for Q1?
If yes what is the value of R1 knowing that the power supply is 12V 15A?
Regards
Didier.
[quote]is it possible to use a 2N3055 for Q1?[/quote]This is not possible. The DC current gain of the 2N3055 is too low.
To get 10A collector current you need to have 2A base current.
You have to select other transistor having more gain.
I suggest to use a Darlington transistor.
Here is one example 2N6284 (NPN):
onsemi.com/pub_link/Collateral/2N6284-D.PDF
The DC current gain is 750-18000 at 10A collector current.
You need to have base current 10A/750 = 13mA
The pull-up resistor R1 should be R = (12V-2.8V)/13mA = 708 ohm
The nearest standard value 680 ohm is OK.
ok, thanks
and for a 15A collector current R1=68 ohm is ok ?
Didier
[quote=“did037”]ok, thanks
and for a 15A collector current R1=68 ohm is ok ?
Didier[/quote]
For 15A operation the gain will be closer to 500 so you will need to have 15/500 = 30mA base current. This will require a (12V-2.8V)/30mA = 307 ohm resistor. 270 or 330Ω being the nearest standard values.
This chart from the datasheet shows the collector saturation regions of the 2N6284 device, in it I have indicated the mid-range of these regions for both 10A and 15A operation. You can see that on the 10A curve the mid-range is 13mA (green arrow) and on the 15A curve it is at 30mA.
Also note that Vce is going to be 2.0V+ at 15A collector current–this means the device will be dissipating 30W, so make sure you use a proper heat-sink…
ok thanks but my customer is sold out for 2N6284, he offer a MJ11016, is it good ?
Didier
The MJ11016 should work fine…
Thanks it’s good, it’s alive