55VDC on a 30VDC SSR

Discussion in 'General Electronics Chat' started by ralphg, May 8, 2013.

  1. ralphg

    Thread Starter New Member

    Sep 4, 2011
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    I have 2ea HONGFA SSR's rated at 50A 30VDC The data sheet gives only the 30VDC rating and no max V rating.

    I wish to power a soft start DC motor at 55V at max 7A using one of the relays or two relays in series if needed.

    Would the two SSR's in series see the full 55V across the contacts on each SSR or 27.5V?

    If over voltage fails the SSR (s) what mode would be the most likely condition, open or closed?

    Thank you.
    Ralph
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    What's the model #?

    Voltage will probably be more or less divided. However even 27.5V is very near 30V. I recommend using one with a higher voltage rating.
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    My thinking, (and - disclaimer - there's a good chance I'm wrong) is that the SSR has a voltage drop like a diode - let's say for example that it's .7V; they won't turn on at exactly the same time, so the first one switches and dumps 54.3V (55V-.7V) across the second one. Then, if that doesn't kill the second one, then now we have 53.6V (55V - .7V -.7V) across both SSRs, and that should kill both of them.
     
  4. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    You forgot the load. 0.7V + 0.7V over the SCRs (or much less if MOSFETs) and the rest on the load.

    I agree that they may not have the exact same characteristics and therefore not have the exact same voltage drop all the time.

    IMO it's not a good idea to put them in series.
     
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  5. ralphg

    Thread Starter New Member

    Sep 4, 2011
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    Thank you for the responses.

    The SSR model is HONGFA HFS33 30D50M. They are both about 5 years old and cost $40 each back then, so I would like to use them if I can safely and reliably.

    Each relay is rated at 1500 watts (30V * 50A)

    Yes!, agreed this is not a good idea. However, the way these are loaded they are about either 10% or 20% of rated wattage. Is over voltage going to kill them while running at 1/7 the rated current? The startup is 65V @ 500mA and then the current ramps up from there and the V decreases to 55. The DC pump motor (Grundfos 6 SQF-2) has an internal MPPT controller.

    I did hook up the two relays in series and observed 57V @ 6A across them (348 watts). I measured 50mV across the contacts on each relay and 100mV across both which would lead me to believe they are splitting the V. Both were running cold to the touch. They are on big heat sinks.

    The power source for the DC motor are PV panels and they are shaded now, so more testing tomorrow.

    Thank you
    Ralph
     
  6. crutschow

    Expert

    Mar 14, 2008
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    30VDC is the maximum rating. Since it is a DC SSR it likely uses MOSFETs for the switches. SCR's don't work for DC relays.

    Connecting two series is not a good idea since one will likely turn off slightly before the other and thus the full voltage will momentarily appear across the faster one, which could zap it. You can try them in series, but be prepared for them to possibly short as the faster one fails, followed by the slower one.
     
    Last edited: May 9, 2013
  7. ralphg

    Thread Starter New Member

    Sep 4, 2011
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    Todays observations:

    At Max power 48VDC 7.6A = 360Watts running through two SSR's in series rated at 30V 50A each. The SSR's are MOSFET technology.

    There is a slight loss of 8mV per A across each relay. total = 112mV

    Both relays are about 10 degrees F above ambient.

    Help me out with this one please. If two switches are in series, and you open either switch, the circuit is open and V goes to 0, right. How then could either get zapped if V is 0? Is this a characteristic of a MOSFET?
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Wrong. The V across the load goes to zero but the voltage across the open MOSFET goes to the supply voltage. That's how any switch works and that's what can zap the MOSFET.
     
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