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Discussion Starter #1
I've got a 1500W ceramic heater ready to go in, at 144V with a fan blowing across it, used about 10-12 amps. Tested it without a relay, just had the HV wire connected to one side of the 144V and held the other end. When done, it arched pretty good, makign me guess a normal 12v coiled 30 amp relay might not handle it.

How is a relay rated? I have one on order (E-Bay) that is a 12V coil, 30 amp 120V main. Should be good? How about arching, or was it me sliding the wire against the battery lug?
 

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There are a couple or so threads here about switching high voltage DC.

Basically it is difficult as the DC will continue to arc and either cause the contacts to fail to open, by welding them closed, or continue to pass current when the contacts are open, by continuous arcing, or burn away the contacts on only a few breaks rendering the contacts useless when closed.

AC mains rated relays and contactors could be rated at hundreds of amps AC at hundreds of volts but will only be able to break a few tens of DC volts and maybe only milliamps.

The reason for this is that with AC the current passes through 0volts twice on each cycle and that allows the contacts to break and the arcing to stop.
With DC the arc is held at the high voltage and so it will continue to arc.

I think the kilovac relay or other solid state relays are the prefered methods.
 

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learned the same thing hard way - my two 16A 240V AC relays got fried after 1 week of use with heater at 80V DC. Will look for DC relays, they are more bulky and bit more expensive.
But I plan to use simple flip switch. Not sure if it will last, but I see much less arcing there.


I know there was a topic, but can't find it now - Can Solid State Relays be used for DC?
 

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There are DC solid state relays (SSR) that can be used (Crydom etc...). Look for the ones using MOSFETs (be wary of cheap $20 SSRs on Ebay), as they heat up less. They should also be used with heat sinks and conductive paste. They are available up to 400 kV and 40 kA.
 

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The DC relay with magnetic blowout from kta-ev.com works great and comes with the little resistors, capacitors and a diode so the contacts will last a LONG time. I am having a little issue with fusing the power to the heater on my 96v system though as the start-up amps appear to be around 50a for a second or so when the element is cold.
 

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Haven't hooked them up yet, but I'm going to be using a couple of these SSRs:

Magnecraft #6312AXXMDS-DC3

Got them for $20 each at Newark. They are MOSFET. Another $10 for a Crydom heat sink that fits. The relays handle 200VDC, 12 amp load.

SolidStateRelays800.jpg




 

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Hi,

Further to dtbakers comments:
"...little resistors, capacitors and a diode so the contacts will last a LONG time."

Wouldn't this be an easy/cheap solution to upgrade an AC contact to DC? Perhaps just a capacitor will work, like the one that protects the points (I'm dating myself) in an ice. What values would be required?

Don
 

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learned the same thing hard way - my two 16A 240V AC relays got fried after 1 week of use with heater at 80V DC. Will look for DC relays, they are more bulky and bit more expensive.
But I plan to use simple flip switch. Not sure if it will last, but I see much less arcing there.


I know there was a topic, but can't find it now - Can Solid State Relays be used for DC?
So you ignored my advice, huh? :D

The thread you were looking for is here: http://www.diyelectriccar.com/forums/showthread.php?p=151179#post151179


BTW - a toggle switch is going to have the same problem with DC as the relay did (think about it - they are similar in all aspects except one uses a coil to move the contacts and the other uses you).
 

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Wouldn't this be an easy/cheap solution to upgrade an AC contact to DC? Perhaps just a capacitor will work, like the one that protects the points (I'm dating myself) in an ice. What values would be required?

Don
Don - the capacitor placed across the points in distributors worked by absorbing the kickback when the points opened so that the voltage didn't rise much above 12V, then discharging that stored energy the next time the points closed.

Here the problem is not so much from inductive kickback (though there is always some) but that anytime you try to mechanically interrupt a DC voltage greater than, oh, about 30V or so an arc can/will form* between the contacts. Because the pack voltage is pretty much always greater than ~30V, placing a capacitor (er... "condenser") across the relay contacts won't help.

You could use a "commutating switch" to terminate the arc soon after the contacts of the relay open. This is basically another switch in series with a capacitor across the contacts. You fire the commutating switch after the relay contacts open and the capacitor presents a brief "short-circuit" to the arc, quenching it. This is how the old SCR-based motor controllers like the GE EV-1 worked.

* - check out the voltage across the electrodes of an arc welder while it is welding to see what I mean...
 

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Discussion Starter #10
Newark, I gotta get these. I bought the E-bay solid state and am guessing they won't work for long. Did anyone mention how hard it is to remove an S10's dash. It aint easy, but the ceramic heater is mounted to an adapter plate and is installed. Now to put the dash back together...
 

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Hi,

Further to dtbakers comments:
"...little resistors, capacitors and a diode so the contacts will last a LONG time."

Wouldn't this be an easy/cheap solution to upgrade an AC contact to DC? Perhaps just a capacitor will work, like the one that protects the points (I'm dating myself) in an ice. What values would be required?

Don
All I know is the 'kit' from kta costs $57 and seems to work fine (except might need 30a fuses running at pack voltage 96v and lower).
 

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Discussion Starter #12
Wired the 12V coil 40 amp dc relay in the S10 with the ceramic heater, worked 5 times, then arched till I cut main power! Pretty light!! Now waiting for the solid state relay, 200V 40 amps. Should work...?
 

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Wired the 12V coil 40 amp dc relay in the S10 with the ceramic heater, worked 5 times, then arched till I cut main power! Pretty light!! Now waiting for the solid state relay, 200V 40 amps. Should work...?
I went back... looks like your main pack is at 144v. So, I would GUESS that max spike at start would be less than 40amps, but you for sure want to do the resistor/capacitor and diode thing even with a relay rated to carry the voltage in the main. DC is very hard on contactors....
 

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Wired the 12V coil 40 amp dc relay in the S10 with the ceramic heater, worked 5 times, then arched till I cut main power! Pretty light!! Now waiting for the solid state relay, 200V 40 amps. Should work...?
thats 4 more times than my first relay lasted it welded itself the first time

the KTA ones work great,, worth the money for sure I will be trying one at 360 vdc we will see how it does there.

Brian
 

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What is the resistor / diode thing?
In layman’s terms
A diode allows electricity to flow in only one direction.
A resistor, Well it’s like a valve, only allowing a pre determined amount of current to pass.
 

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How about using an inverter, it dont have to be sine wave quality to run a heater. Yes I know there will be some conversion losses, but the a/c availability may come in handy.

Roy
 

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How about using an inverter, it dont have to be sine wave quality to run a heater. Yes I know there will be some conversion losses, but the a/c availability may come in handy.

Roy
This has been discussed before.
An inverter with a 12v input will have to draw huge currents to run a heater but there doesn't seem to be any affordable or available inverters that have an input voltage that matches typical pack voltages.

If there was one that gave a decent output of a few kW from, say, 144vdc then it opens up a lot of possibilities for providing domestic power back up.

However, you would still want a high voltage, high current contactor to switch the inverter on and off under load for safety.
 

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Discussion Starter #20
As the other guy said, it will require alot of current with the low 12VDC voltage. I have mine connected to the 144V pack and when not driving it uses 1.5 amps if the fan is not on. Turn on the fan and the current will go to 12-14 amps. Start driving, the pack voltage goes down and the heater amps go to 16-17A. I had a large spark every time I disconnected it, till I figured this out. So now I turn the fan off first and let it sit till the amps go down, now the spark is minimal! Still waiting for the 200V 40 amp solid state relay to show up.
 
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