Heated bed voltage question

[bvandiepenbos]

I just purchased a Onyx rev 7 HPB
Printed on it is 12VDC 12A so that is 144wats, correct?

Actual measured resistance is .8 ohms

I will be running this with a PSU that can be turned up to 15 volts
www.ebay.com/itm/400-Watt-Volt-Range-9-5-to-15-VDC-36-Amp-HAM-and-CB-Radio-Power-Supply-13-8V-01-/171631856316?&_trksid=p2056016.l4276

The Rambo board heated bed input is fused 15A

My question is, will the Rambo be OK at 15 volts.?
I do not want to use a SSR
knowing actual bed resistance and input voltage how do I calculate amps it will draw?

btw, this is for a customer machine I am building, the goal is to be able to get up to 100c in a more reasonable amount of time.

[jimustanguitar]

The .8 ohms is the only "true" spec that you have to go off of. Take your voltage divided by .8 and you'll have your current.

[BeeAmaker]

The Rambo will be fine at 15 volts, the question is will the fuse blow?
15 volts / 0.8 ohms = 18.75 amps, my guess is the fuse will probably blow, or at best, blow at random times. But you can always try it and see if it holds. It may only blow at higher temps. When Using PWM like this, the actual amps may not be reached high enough or long enough to blow the fuse.

[bvandiepenbos]

Thanks Glenn, your info helps a lot. I will just give it a try.

It is the automotive blade style fuse, so I could put in a 20 amp ?
I bet even running at 13 volts would be a noticeable improvement.

[jimustanguitar]

The Rev7 is supposed to heat up pretty quickly though, you might just try it first with plain old 12vdc.

[BeeAmaker]

I would NOT put in a 20 amp fuse. The fuse protects the components and traces on the board, not the hot bed so 15 amps might be OK, but 15.1 amps might burn a trace, or otherwise let the smoke out.

[bvandiepenbos]

I updated my Rostock MAX "KITT" to a rev 7 bed a few months ago and it heats very slowly...
14 minutes to reach 75c
21 minutes 85c
30 minutes 92c
60 minutes tops out at 98c
this is with a 1/16" aluminum heat spreader between pcb & glass.

with the aluminum entire bed is within 3c
without aluminum it varies 6c, heats even slower and tops out at 94c
without aluminum or glass, just bare pcb it varies 8c and never gets above 85c

Note: this is with SeeMeCNC wimpy ATX PSU

[sgraber]

I think I'd replace the power supply...

[BeeAmaker]

I agree with sgraber. There is a reason why they are $25

[bvandiepenbos]

Jan 7, 2015 11:31:11 GMT -5 sgraber said:

I think I'd replace the power supply...

yes, I know. I just have not got around to it.


Any recommendations on a decent ATX psu that is not to pricey?

[jimustanguitar]

Without spending some real dough, they're all cheap Chinese crap. The best you can do for a budget one is get one that's rated higher on 12v output than what you need, and stabilize the 5v output with resistor. The specs on things like power supplies are inflated anyway, so you almost have to do this. If you need 15 amps I'd buy one that does at least 25.

After killing 2 ATX supplies in a row, I spent $5 more and got a logysis that says it'll do 25A for about $30. Worked ever since, and has way more miles on it than the supplies before it combined.