# Solid state relay leakage current



## mk4gti (May 6, 2011)

Is it possible that they failed? I hear they usually fail "turned on"


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## SWF (Nov 23, 2007)

mk4gti said:


> Is it possible that they failed? I hear they usually fail "turned on"


It is possible, but they are rated to handle much higher current and voltage (10A/220VDC) than what they are exposed to in my system. They are both new, and I noticed before I installed them that the resistance across the load terminals was not as high as I would have expected (cannot recall exact value), so I was not too surprised to see that they were passing enough current to turn on the circuits they are controlling before activating the relay. From what I have read this is not uncommon with SSRs.


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## cruisin (Jun 3, 2009)

SWF said:


> I am using two Omron G3NAD210B solid state relays in my build, one to turn on the high voltage side of my DC/DC and one to turn on the KSI circuit on my curtis 1238 controller. I was doing some testing this weekend and found out that both of these have enough leakage current to partially or completely power their circuits before the relay is actually turned on.
> 
> In the case of the SSR controlling the DC/DC, the voltage that is passed by the relay starts about 10V but then gradually creeps up until it is high enough to turn on the DCDC. My system voltage is 96V nominal, and at full charge is about 114V.
> 
> ...


You should not use a SSR for either one of the fuctions that you have installed them to control. If you PM me I can steer you to the right product to use. The SSR can fail very easy in the closed position.


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## SWF (Nov 23, 2007)

cruisin said:


> You should not use a SSR for either one of the fuctions that you have installed them to control. If you PM me I can steer you to the right product to use. The SSR can fail very easy in the closed position.


Thanks for the input Cruisin. Would you be able to suggest to me here in this discussion which relay I should use? This may be useful for others who are trying to use SSRs for similar applications. I have seen others use SSRs for the KSI control, so I thought it would be OK. I would have preferred a mechanical relay, but could not find a good one that was rated to handle the voltage and current, other than going all the way up to a tyco contactor, which seemed like overkill.


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## cruisin (Jun 3, 2009)

SWF said:


> Thanks for the input Cruisin. Would you be able to suggest to me here in this discussion which relay I should use? This may be useful for others who are trying to use SSRs for similar applications. I have seen others use SSRs for the KSI control, so I thought it would be OK. I would have preferred a mechanical relay, but could not find a good one that was rated to handle the voltage and current, other than going all the way up to a tyco contactor, which seemed like overkill.


For a starter you should use the relay that came with your HPEV system or Curtis controller. It is not rated for DC but I have not had one fail since using the new one currently available from HPEV dealers.


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## SWF (Nov 23, 2007)

cruisin said:


> For a starter you should use the relay that came with your HPEV system or Curtis controller. It is not rated for DC but I have not had one fail since using the new one currently available from HPEV dealers.


I bought the controller and motor (AC20) used, and they did not come with a relay. Can you just tell me which relay is currently supplied?


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## FWD (Feb 3, 2012)

is the controlller working? on this leakage voltage? or still shutdown.

if not then there is no problem! the voltage maybe high but when the controller really wanted to startup it would not do to because the current wil be 0.


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## SWF (Nov 23, 2007)

FWD said:


> is the controlller working? on this leakage voltage? or still shutdown.
> 
> if not then there is no problem! the voltage maybe high but when the controller really wanted to startup it would not do to because the current wil be 0.


The controller powers up completely. No error codes, spyglass display works, and I can control the motor.


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## Lee Hart (Oct 16, 2009)

SWF said:


> Thanks for the input Cruisin. Would you be able to suggest to me here in this discussion which relay I should use? I would have preferred a mechanical relay, but could not find a good one that was rated to handle the voltage and current.


I agree with Crusin; these are not appropriate applications for an SSR (Solid State Relay). First, because they fail *on*, which is a bad or even dangerous condition. Second, because of the nature of the loads.

The KSI input is very low current; you would need a relay with a very low off-state leakage, and a rated carry current similar to the load you are actually switching, which is 50ma or less.

The DC/DC input has the problem of a huge peak inrush current. Most of them have large input filter capacitors, whose peak charging current can be 100 amps or more. SSRs are not good at handling such high peak currents.

I would consider something like a Schrack PT52A012B. This is a 4PDT relay with a 12vdc coil and contacts rated for 6a at 250vac or 30vdc, 40vdc at 1amp, 100vdc at 0.4a, and 300vdc at 0.25a. One source is www.digikey.com, PB1157-ND, $8.86 each.

The voltage rating of relay contacts in series add. This relay has 4 poles. Each by itself can only break 30vdc at 6 amps; but by wiring all 4 poles in series, you have a relay that can safely switch 30x4=120vdc at 6 amps. For most DC/DC converters, a 6amp rating is adequate.

If your DC/DC draws a huge current spike at turn-on (witnessed by a big spark when the relay contacts close), then you need to add an inrush limiter. Without one, no relay (mechanical or SSR) would last long. Inrush limiters are a little $1 part, also available from vendors like www.digikey.com.

For the KSI input, even a single pole of such a relay will work, because it is rated to switch low currents even at very high voltages. However, use a 2-pole relay anyway (such as a PT22A012B, the 2-pole version of the above). Connect KSI to the Common of both poles. Connect NC of one pole to B-, and NO of the other pole to to a fuse to whatever power source turns on the controller. Done this way, if the NO relay contact fails closed, you get a *short* when the relay turns off and the other pole shorts KSI to B-. This blows the fuse, guaranteeing that the power is off if the relay fails.


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## SWF (Nov 23, 2007)

Many thanks Lee, this is very helpful. Using multiple relays crossed my mind at one point, but I never took the thought further and considered a multiple pole relay.

My DC/DC lists a typical inrush current of 60A, something I did not notice when I was going through the other specs. So I'll go ahead and also use an inrush limiter as you suggest.


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## Lee Hart (Oct 16, 2009)

SWF said:


> Many thanks Lee, this is very helpful. Using multiple relays crossed my mind at one point, but I never took the thought further and considered a multiple pole relay.
> 
> My DC/DC lists a typical inrush current of 60A, something I did not notice when I was going through the other specs. So I'll go ahead and also use an inrush limiter as you suggest.


Yeah, that 60 amp inrush is a fuse and relay killer!

You can search for "inrush limiter" on the usual electronic component supplier websites like Digikey and Mouser. To pick one, find your DC/DC normal operating current. It's best to *measure* it, as the specs probably won't tell you what it draws on DC (most of them are in fact AC power supplies just being sold as DC/DCs).

Example: Let's say your DC/DC draws 5 amps when feeding a discharged 12v battery. Go to www.mouser.com and search for "inrush limiter", then click on "circuit protection". It lists (at the moment) 86 parts, and filter boxes to choose the specs. Select a range, like 5-6 amps (so you'll have some choices). It comes up with 9 candidates, ranging from $0.90 to $1.41 each. Pick one that's 5 amp (or more), in stock, and with the highest cold resistance (to minimize the inrush current stress on your input fuse and relay). At the moment, it looks like Mouser #871-B57238S100M, 5 amps hot, 10 ohms cold, $0.94 each.

These things get HOT! Mount it on a black bakelite screw terminal strip. Bakelite won't burn or melt. Don't mount it close to anything that can burn or melt.

When cold, it has a high resistance (10 ohms in this case). When the relay turns the DC/DC on, the peak current is therefore your pack voltage divided by the resistance. I think you had a 96v pack? The peak current is thus I = 96v / 10ohms = 9.6amps. Size your fuse and relay accordingly. Far better than 60 amps!


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## swoozle (Nov 13, 2011)

Lee Hart said:


> The voltage rating of relay contacts in series add. This relay has 4 poles. Each by itself can only break 30vdc at 6 amps; but by wiring all 4 poles in series, you have a relay that can safely switch 30x4=120vdc at 6 amps. For most DC/DC converters, a 6amp rating is adequate.


Great information Lee, thank you. 
Could you say a few more words about this part? It seems counterintuitive (to someone not knowledgeable) that you'd get any advantage in relays in series. In parallel the current through each would be divided. Isn't that what you want when the relays break the connection?
How do you get voltage splits across what is essentially a zero resistance path (closed relays in series)?


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## SWF (Nov 23, 2007)

Lee Hart said:


> Yeah, that 60 amp inrush is a fuse and relay killer!
> 
> You can search for "inrush limiter" on the usual electronic component supplier websites like Digikey and Mouser. To pick one, find your DC/DC normal operating current. It's best to *measure* it, as the specs probably won't tell you what it draws on DC (most of them are in fact AC power supplies just being sold as DC/DCs).
> 
> ...


Thanks again for the detailed information.

I'm using a Meanwell SD-500H-12, and it lists a typical input current of [email protected] I'll measure it as well as you suggest. This DC/DC actually has a remote on/off function I could also use, but it requires a voltage signal to turn it off. I plan to physically disconnect my traction pack and aux. battery when I am not using my EV, so the only way I could use this function is to set up a battery just for this function. I liked the idea of turning the input off with a relay rather than using this function.

I already have a 10A/125VDC fast blow fuse on the input side, and it did not blow yet. I will go ahead and add the inrush limiter.


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## Lee Hart (Oct 16, 2009)

swoozle said:


> Great information Lee, thank you.


You are most welcome.


swoozle said:


> Could you say a few more words about... relays in series?


Sure! When a switch or relay opens, the contacts move apart. The space between them goes from zero to whatever the maximum is when fully off.

Above about 30 volts, an arc will form between the contacts. The arc stretches out as the contacts move apart. The arc won't stop until:


Something interrupts the current. On AC, this automatically happens 120 times per second (at the next zero-crossing). On DC, this won't happen.


The gap gets too wide for the voltage to sustain the arc. This is the only way to stop it on DC.
You can get enough distance to stop the arc either with two widely spaced contacts, or by having multiple contacts whose total gap adds up to enough. The voltage rating of contacts in series adds, because the total air gap is the sum of the gaps from each contact.

Obviously, this depends on all the contacts being in the same device (same switch or relay), so they all open and close at the same time. Small differences of a millisecond or so won't matter, but big differences do!

There are also other ways, applied in bigger, more expensive situations where you have a *really* tough arc to kill; blowout magnets, arc ladders, contact interrupters, etc.

Note: The current rating of switches or relay contacts in parallel *do not add*. A closed contact's resistance is very low, but not very stable. It changes each time the contact closes, and as the contact wears and the surfaces oxidized. The current in each paralleled contact follows the resistance. If one contact has 0.01 ohm and the other has 0.02 ohms, then there will be a *2:1 difference* in the current between them!

If you assume a worst-case resistance difference of 2:1, then two 10 amp contacts in parallel are only good for 15 amps, not 20 amps.


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## O'Zeeke (Mar 9, 2008)

Lee Hart said:


> Yeah, that 60 amp inrush is a fuse and relay killer!
> 
> You can search for "inrush limiter" on the usual electronic component supplier websites like Digikey and Mouser. To pick one, find your DC/DC normal operating current. It's best to *measure* it, as the specs probably won't tell you what it draws on DC (most of them are in fact AC power supplies just being sold as DC/DCs).
> 
> ...


Thank you Lee for all your help. Please excuse the lack of knowledge on my part for the following, but Is there any polarity requirement with this inrush limiter?, and is this wired across the DC/DC input leads or is it wired in series? 
Thanks, I appreciate all your valuable input.


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## Lee Hart (Oct 16, 2009)

O'Zeeke said:


> Is there any polarity requirement with this inrush limiter? And is this wired across the DC/DC input leads or is it wired in series?


No polarity requirement. An inrush limiter is basically just a resistor. It works on AC or DC, and doesn't care about polarity. You wire it in series with the DC/DC input, just like a fuse.



O'Zeeke said:


> Thanks, I appreciate all your valuable input.


You are most welcome. Success with your EV!


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