130VDC schematic help needed (mechanical guy warning) 2

[mcgyvr]

Sorry I'm a mechanical guy stuck with an electrical guys work (cause he was layed off a while ago).

The attached schematic is for a simple circuit used in one of our products. This circuit was designed for -/+24 to 48vdc operation. Now we need to adapt this for -/+130vdc operation (140vdc max.)
But I can't change the relay, rectifier bridge or bi-color led.

I can however replace the resistor and add resistors/components anywhere in the circuit.

All resistors wattage needs to be double worst case.
I was hoping this could be accomplished by simply adding a couple resistors somewhere. But since I'm the dumb mechanical guy I don't know where. Help

I have included the electrical specs for the relay and digikey part number for rectifier bridge in the schematic. Let me know if you need anything else..

 

[MacGyverS2000]

A simple resistor divider to ground at the power input would be all that's required, unless I'm missing something. A 1k and a 3k would work fine... maybe lower if you need more current (didn't look at the specs).

Dan - Owner

http://www.Hi-TecDesigns.com

 

[VEBill]

You're looking at several watts of power dissipation since you need to drop more than 100 volts at several tens of milliamps. And given the variation in current with the switch (and relay) open or closed, if you need a less variable voltage versus current then you'll need to have some current going to ground (in other words, a voltage divider as Dan mentioned). This would improve the voltage stability against current, but adds yet more power dissipation.

 

[mcgyvr]

Here are my calculations.
If I used a 2.4K (r1) and 1K (r2) resistor
I would get a Vout of 38VDC.
r1 would dissipate 3.5W
r2 would dissipate 1.46W
With the double wattage resistor requirement I would need a 7W and 3W resistors.

Is that correct?

Anyway to keep to 3w resistors max?

 

[VEBill]

If you use separate series voltage dropping resistors (one for the LEDs, and another for the relay), then this would eliminate current variations in those series resistors. That stability of current would permit simple series resistors for both instances (as opposed to voltage divider). And that in turn would minimise power dissipation.

In other words, add a resistor in series with the relay coil. Change the resistor for the LEDs.

ALSO: Double check power dissipation for all. Also, make sure the switch is safe for high voltages. Make sure the enclosure can handle the extra heat. Check your math at both high and low voltages.

 

[MacGyverS2000]

You may also consider a plug-n-play DC/DC switcher to lower the voltage... more expensive than a few resistors, but very straightforward.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[mcgyvr]

Thanks guys this has helped a lot and finally forced me to learn a little about the electrical side of life .

One last question about calculating the dropping resistor for the relay section.
Am I supposed to subtract the coil resistance from the calculated resistor value to get the real value of the resistor I would place in that circuit?

 

[VEBill]

The total voltage will be across the total resistance (sum), and the voltages across the resistor and the coil will each be in proportion to their resistance.

Total power dissipation will be increased by several times. Major risk of heat problems depending on packaging.

There are dozens of other things to double and triple-check. For example:

The diode bridge is rated at 200v reverse breakdown. At 140 volts applied, it has much less margin than before. It might be okay, but it is something to be aware of in case you start seeing failures. If this was a mass production unit, I'd consider changing the bridge to one with a higher margin. But this decision depends on the application (criticality, risk, costs).

 

[mcgyvr]

And sure enough the customer just called and said they don't even need the 130V operation anymore and that 48V is just fine.

Thanks for all the help. Its a good exercise for me anyways.
I did/learned the calculations, made a prototype on a breadboard, already switched the rectifier before you mentioned it to the 400V version, put thermocouple probes on the relay,etc.. to verify temps. Everything seemed just fine, then they call and say its not necessary anymore.. oh well
thanks