Under-Rated Load Resistor Worry
Under-Rated Load Resistor Worry
(OP)
I am working on using USB serial to communicate with a high speed spindle driver to automatically run a spindle.
I am using an ATMEGA to flip transistors and manually set each of the required serial pins to either high or low to get the outcome I want on the spindle driver.
The schematic and text in the operation manual require that each pin get 24VDC and at least 100mA.
The schematic shows some resistors in line with the solid state relays. while the device was turned off I measured their resistance to be 4.3k ohms each, but I was planning on putting my own 250ohm 10W resistors in to be certain about there always being a load on the power supply I sourced.
The device isn't that big, and I am worried that the 4.3k ohms of resistance is not rated for the 50W it would need to be for the specified power they called for.
Am I going to completely mess this driver up? Does my plan even make sense and am I reading the schematic correctly?
Please see attached picture or the manual: http://www.nsk-nakanishi.co.jp/industrial-eng/down... Pg.20
Thanks,
-Eric
I am using an ATMEGA to flip transistors and manually set each of the required serial pins to either high or low to get the outcome I want on the spindle driver.
The schematic and text in the operation manual require that each pin get 24VDC and at least 100mA.
The schematic shows some resistors in line with the solid state relays. while the device was turned off I measured their resistance to be 4.3k ohms each, but I was planning on putting my own 250ohm 10W resistors in to be certain about there always being a load on the power supply I sourced.
The device isn't that big, and I am worried that the 4.3k ohms of resistance is not rated for the 50W it would need to be for the specified power they called for.
Am I going to completely mess this driver up? Does my plan even make sense and am I reading the schematic correctly?
Please see attached picture or the manual: http://www.nsk-nakanishi.co.jp/industrial-eng/down... Pg.20
Thanks,
-Eric





RE: Under-Rated Load Resistor Worry
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Under-Rated Load Resistor Worry
http://www.wolframalpha.com/input/?i=100mA*100mA+*...+
I measured with a digital multi-meter between points 1 and 2 and I assumed the item in the box was a resistor, was this not a fair assumption to make?
http://i.imgur.com/7tLD9ti.png
I am just worried their resistors in line with the SS-relay, which you called voltage drops, are not rated for what I thought was the amount of power dissipation they needed. I really don't want to blow anything out on this part.
Thanks for the quick reply!
RE: Under-Rated Load Resistor Worry
your 250ohm resistor, however, will siphon off 96mA, or almost all of the current capacity of the supply.
TTFN

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RE: Under-Rated Load Resistor Worry
If the power supply can only put out 2.4 watts then I wouldn't be able to trigger the relays with this supply which is supplying what they called for. This makes me think I misread the diagram and/or am over thinking this?
Thanks again for the replies.
RE: Under-Rated Load Resistor Worry
RE: Under-Rated Load Resistor Worry
Do I have to either pop it open or talk to the manufacturer?
Is it safe to assume since they called for 24v and 100mA that it is safe to put that in even if my application is not the normal one?
RE: Under-Rated Load Resistor Worry
The assumption is fine, the measurement is not. The resistor is in series with a diode. Your measurement includes the forward bias voltage of the diode, which is almost impossible to discount accurately.
Yes. The resistor is selected by the designers to provide exactly the right current when supplied by a low impedance, 24V DC source. As long as you're only supplying 24V, you wont blow it up. If your power supply is not powerful enough to supply 100mA (ie. at least 2.4W per input), it might not work, but it definitely wont blow it up.
RE: Under-Rated Load Resistor Worry
RE: Under-Rated Load Resistor Worry
So I guess I can use the one power source to activate one pin at a time at 100mA or multiple at lower current and the SSRs should still trigger.
Thank you for all the help and corrections everyone!
RE: Under-Rated Load Resistor Worry
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Under-Rated Load Resistor Worry
If the device is rated for 100mA then it doesnt make sense to over complicate it?
Thanks,
-Eric
http://i.imgur.com/f0JJyVV.png
RE: Under-Rated Load Resistor Worry
No, the internal resistors will limit each pin as necessary.
Take your 250R resistor out. The internal resistors already do what you're trying to do.
That's it. The subtle aspect you're struggling with is that the device is rated for a supply with a maximum of 24V and a minimum of 100mA. As long as your supply doesn't present any more than 24V, and can deliver at least 100mA, it will work. The resistors will take care of restricting the current to the necessary level (something less than 100mA, depending on how many pins are on). We (electrical people) are so used to dealing with voltage sources that this subtlety barely enters the conscious.
Let me give you the tired old water and hose analogy to give you a mechanical perspective. The device is a spinning sprinkler with a configurable number of spouts. It is rated at 24kPa of pressure and 100mL/minute of flow. If you supply it 24kPa and don't turn any of the spouts on, it will draw no flow. If you turn one spout on, it will draw some flow but definitely not 100mL/minute. If you turn on all spouts it will draw up to 100mL/minute. If your supply can't provide the required 100mL/minute, the water will only dribble out of the spouts and it might fail to spin. If your supply can provide 10000mL/minute, the sprinkler will still only draw 100mL/minute provided there's 24kPa of pressure. If there's 50kPa of pressure, you might force 200mL/minute through the spouts, spinning the sprinkler to destruction. So to correct operate the sprinkler, you need to set the pressure tap so you get close to but not more than 24kPa and at least 100mL/minute.