I have 5x56 ohm 6v relays connected up to different digital channels of the the k8055 (2 of them are sharing the same channel). connected to ground and clamp i have a 6v battery connected.
the relays switch on and off with the labview template provided by ni, however when i turn on approximately more than 3 relays, there appears to be not enough current to switch more relays on. is anyone familiar with this problem or can you point me in the right direction?
The open collector outputs on the K8055 are rated for 50V/100mA,
so one of your 56Ω relays operating at 6.0V gobbles that up (6.0/56 = 107mA)
For DC circuits Intensity of current (in Amperes) = Electromotive force (in Volts) / Resistance (in Ohms) – or I = E/R.
Remember “an Indian saw an Eagle over a Rock”
Two relays are probably just barely operable, with considerable overloading of the K8055 outputs,
each of which cannot sink enough current to drive three relays.
You need to use a single relay actuated by the K8055, to control the multiple relay fanout.
You also need some black wire to help keep things sorted out…
If I understand right you can turn on only about 3 relays. Maybe you can turn on also the two relays sharing the same channel?
The outputs of the ULN2803 transistor array are independent and the current fed to one output does not affect the current capability of other outputs.
I think the battery voltage decreases a little and the rest of the relays can’t be pulled on. The pull-in voltage of the relay is higher than the drop-out voltage. The other relays stay turned on but the rest of the relays can’t be turned on due to too low voltage.
Have you checked the battery voltage and the voltage over the relay coils when all the relays are turned to be on?
Please note: When activated the digital output of the K8055, the collector-emitter saturation voltage remains at the output.
Yup…
I forgot about the drop across the output transistors, so 1 relay @ 56Ω = 91 mA…
thanks for the reply guys. i tried with a 9v battery as well and i still get the same problem. looks like i need to design a circuit with more relays and using single relays to control multiple relays at a time. this is quickly getting more complex than i originally intended. damnit 
i wish there was an easier way, my brain cant handle all the combinations of the arrangement.
What you need to do is use as many 8055 outputs as you need to control one relay each, and then think of those relays as the 8055 outputs.
i think id prefer to run the relays off transistors. are there any circuits on here like that?
You can use the open collector output of the K8055 to drive a PNP transistor of appropriate capacity, like this:
The listed transistor is a medium power TO-220 package, you could probably control at least 5 or 6 of your relays
without a heat sink. But keep an eye on the temperature and use a small heat sink if needed.
You might also be able to use a smaller package TO-5 transistor, like a 2N3906 (NTE159) which is rated for 800mA,
to control 4 or 5 relays–use one of those little clip-on heat sinks.
thanks, that circuit worked a treat!
Cool…
