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Old 03-05-2021, 14:39   #16
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Re: Use Of Relays

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Respectfully, that's incorrect.

You've cited a 45-year-old paper about switching AC loads using TRIAC-based SSRs. Yes, they drop 1.5V, which is far less significant in a 110-240VAC systems, especially since the power requirements of today's AC systems are much lower than those of 1976. Consider an 100-watt equivalent LED bulb which only now draws 15W. I digress...

Modern DC SSRs are completely different. They are switched efficiently by low dissipation MOSFETs. This is a distinction with a huge difference. For example, a 10A DC SSR might have a maximum voltage drop of 0.17V.

Modern boat freezers draw as little as 3A. The corresponding voltage drop might be in the area of 0.08V. At 12VDC nominal, that's 99.3% efficiency.

Nonetheless, we still don't know the make/model/power requirements of the OP's freezer. All we know is that it is 12VDC.
Thank you very much xxxx for this more current and better information about DC SSRs.
You have increased my spares situation by about 100% with this info.

I guess I still have a question if the SSR voltage drop is so small - without a heat sink the metal back of the DC SSRs gets surprisingly hot in use - even with the about 4A load of my aft fridge. Using the correct heat sink obviously drops surface temperatures, but is still sensibly warmer than ambient.
This heat generation has to be coming from voltage drop within the SSR.
Is it possible that an 80mV voltage drop create so much heat?
Being an inveterate tinkerer, I buy SSRs from Marlin P. Jones at very reasonable cost. Can they be selling ancient stock that does not compare with more recent manufactured SSRs?
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Old 03-05-2021, 14:51   #17
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Re: Use Of Relays

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Originally Posted by AndyEss View Post
Thank you very much xxxx for this more current and better information about DC SSRs.
You have increased my spares situation by about 100% with this info.

I guess I still have a question if the SSR voltage drop is so small - without a heat sink the metal back of the DC SSRs gets surprisingly hot in use - even with the about 4A load of my aft fridge. Using the correct heat sink obviously drops surface temperatures, but is still sensibly warmer than ambient.
This heat generation has to be coming from voltage drop within the SSR.
Is it possible that an 80mV voltage drop create so much heat?
Being an inveterate tinkerer, I buy SSRs from Marlin P. Jones at very reasonable cost. Can they be selling ancient stock that does not compare with more recent manufactured SSRs?
Use your multimeter to measure the voltage drop from the input to output of the SSR. This voltage times the current is the power in watts dissipated as heat.
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Old 03-05-2021, 15:17   #18
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Re: Use Of Relays

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Can they be selling ancient stock that does not compare with more recent manufactured SSRs?
Not necessarily ancient stock, but just not as efficient components.


I just looked at their DC SSR selection but there isn't much information. Some have limited information, some don't have any datasheet at all.

Looking at the Hoymk SSR-10DD, it's very affordable at $6.95. The datasheet specifies a voltage drop of 1.6V. In your 4A application, that means dissipating about 6.5W of heat and power consumption.

There's nothing wrong with that, it just means your 48W fridge is now uses 54.5W.

Something like the Crydom EL series might cost you $35ish, but that's the price paid for efficiency.


If you're happy with the current one, keep it
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Old 03-05-2021, 15:39   #19
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Re: Use Of Relays

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Use your multimeter to measure the voltage drop from the input to output of the SSR. This voltage times the current is the power in watts dissipated as heat.
I put in a $18 Chinese powermeter, complete with its own shunt to keep track of fridge efficiency. That’s when I noticed the about 1.5Vdc drop.
I see xxxx has came in with confirmation - cheaper SSRs suffer from what is to me at least, an unacceptable 10% drop in voltage to my 12Vdc device.
The higher priced SSR that xxxx references in his post will pay for at least 5 cheap mechanical relays, so my original conclusion still seems to be valid - the correct relay control for this application seems to be mechanical, not SSR. This of course depends on device longevity.

I run almost all my systems off solar and wind, so I try to make every Watt (or invested dollar) count.
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Old 03-05-2021, 19:03   #20
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Re: Use Of Relays

wow, thread drift. How did we end up on SSRs?


SSRs have their applications. I've used them. They require very low drive and don't have the high inrush current (mentioned upthread). They're silent. They switch very quickly. Unlike mechanical switches, there is no wear with each operation. That makes them great for certain applications, like broadcast studios (because they're silent) and PWM modulated heating appliances (because they can switch high current once a second without wearing out), and test labs (because they switch quickly).


Good ones are expensive, compared to mechanical relays.


You can get good ones at Digi-key and places like that, and compare specifications across vendors, and buy one with the voltage drop you want. They all run hot with higher power loads. With higher power loads they have to be bolted to an aluminum chassis or heat sink. Voltage drop will be proportional to maximum switched voltage so you have to choose the right ssr.
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Old 03-05-2021, 20:30   #21
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Re: Use Of Relays

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I run almost all my systems off solar and wind, so I try to make every Watt (or invested dollar) count.
Hehehehehe... OK, I'll bite. Not much else to do in this pandemic

I agree that commercial SSRs are expensive. Of course, if every single last watt is critical, direct switching with a #8 or #10 wire is probably the most efficient. That's assuming a 30ft round trip.

In a 14.4V LiFePo4 system, an inexpensive 50-ohm automotive electromechanical coil relay will cost you around 4.25 Watts. That's about 91% efficient.

You say you're a tinkerer. A simple MOSFET relay module can be made for less than $2 and can be used as a drop-in for your coil relay. With an "on-resistance" of about 0.022 ohms, the power "cost" should be about 0.3W--about 99.4% efficiency. You could probably find even more efficient FETs if you dig. You can also buy a pre-made version for under $5 each.

Check out this video
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Old 04-05-2021, 06:48   #22
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Re: Use Of Relays

Thank you xxxx.
I will finish looking at that video as soon as I find my spool of solder. Since it is nowhere to be seen, and I was last splicing an NMEA2000 cable from a wind transducer to a Raymarine STng cable (to communicate with my RM mfd) inside my engine compartment, I can only assume the solder fell into the bilge under the engine.
Tinkering meets reality.
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Old 04-05-2021, 07:18   #23
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Re: Use Of Relays

If you require a relay that requires power only to switch state, they do exist. They're called magnetic latching relays.
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