40amp @ 24 to 220 VDC SSR $15
 

http://i46.photobucket.com/albums/f1...slot/DCSSR.jpg

Found these on Ebay..
Solid State Relay SSR 5-220V DC, 40A + Heat Sink | eBay

I have one connected to my 115Vdc 7A (800W) solar PV array.
It's control comes from a square wave generator.
When it gets sunny again, I plan test difference frequencies & duty-cycles,
to see if there is a sweet spot frequency that will give max output power.

Testing today with a 3500 uF cap across the PV, at 4-5 amps (overcast)
was pretty good. No excessive heat. :D
I can't wait to see if the output can get up near the full 800w..

Those look to be pretty impressive.
I've never seen Solid State Relays that were rated for that amperage that were DC, reason being, a lot of the AC only SSR's specs seem to say that they turn on or off mid sign wave, when the voltage is at -0-, I even have one that is rated at 80 amps, 240v AC.
The spec's that they list are a little confusing and the fact that they come with a big heat sink I have to wonder how long it really will last, but if it's used in a situation where it's over sized it should be fine, right?

I was amazed to find these (this cheap) at 40 amps. Must be a pretty good sized MOSFET in there.

I think you can find these without the weird heat-sink,
but this one had a good price and was in the right amps range.

I saw some 20A versions a bit cheaper, but I wanted it to be 4X over-rated.. ;)

When it turns on, it's discharging the 3500 uF cap which could be up to about 145 vdc.
145v / 10 ohms =14.5A (a very short pulse).
Depending on the sun.


Since the max power point of the array is 115V, I'm going to set the
frequency so the rapidly firing SSR will keep the cap right around 115v.
Hopefully, that will send an average of 7 amps to the load. (800w)!
It's sorta a PWM heater driver..

It will be interesting to see if this hack can hot-rod my system to near 800w.
Which is it's rated power. It if does a good job in bright sunlight,
I'm wondering what the performance will be during poor sunlight condix.?.
Will the high output freq be useful at all when it's partly cloudy??

Dead as a door nail.
 

http://i46.photobucket.com/albums/f1...0slot/R019.jpg

It never got above 95F (per the IR scan pistol), never felt very warm
while running at about 10A average. But, it died at less than 15A.

It ran as expected at low power, 5 to 10 amps average.
It worked perfectly at 10A, using various drive frequencies (audio range).
I never needed to turn on the fan, since the heat sink was hardly warm to the touch.
(I mounted the heat sink on a larger heat sink, to use as a base).

The source was PV (solar panels) running a maximum of 144 volts, limited to 7 amps. (w/ a Capacitor on the PV side).
But, since the SSR was being turned on & off, the surge into a 10 ohm resistive load,
could have been as much as 14.4 amps. (144v/10 ohms = 14.4A)
With a 10 ohm load and 144 volts supply, no higher amperage is possible.
It never saw anything anywhere near 40 amps!!

My guess, these things aren't really 40 amp switches.. Don't waste your money..

I'm going try using some power MOSFETs for my project, and forget DC SSRs.. :mad:

I'm not understanding what your goal is. (was) The unit is a relay that is controlled on/off with a D.C. voltage between 3-32 and will switch a D.C. voltage on/off between 24 to 220 @ 40 amps to a load. The heat sink will dissipate the heat created by the load on the switched side.

The goal was to:

A. keep the SSR off, until the PV charged the capacitor up to 144 volts. (Vmp of the array).
B. Then turn on the SSR to discharge the cap into a 10 ohm load. (But not discharge it below aprox 100v)
C. Repeat.

Without the SSR, the 800w array will typically only heat up the load to around 400w.
Because, the load resistance is so low, it drags the PV array down to 60v.

By allowing the cap to reach the max power point voltage of the array,
before discharging it into the load, more power is delivered to the load.

So, the goal is to harvest 800w from the 800w array.. :D

Producing heat from any power source is dependant on the amount of constant current available. The amount of current used is calulated by the resistance of the heating device. Pulsing it only reduces the duty cycle, which would reduce the constant current input to the heating device to around half, regardless of frequency.

Lowering the resistance of the heating device will allow the array to achieve a higher operating voltage but you are still restricted to the amount of current that your array can provide. If you reconfigured your array to a lower operating voltage to produce more current, you may have better results. Raw current produces heat, not voltage.

The amount of current is limited by the PV panel ratings.
My CS6P-200 panels will not put out more than 7 amps.
So, that the limit of the amperes, according to Ohms law..
But, the voltage available from 4 panels in series (@7A) is 115.6 VDC.. (abt 800w worth).

This circuit isn't my invention, I'm just copying part of a standard MPPT circuit.
Only, I'm using manual 'tracking', to optimize delivery of power from my PV to the 10 ohm load.

Before the SSR died, I was able to get it working a bit.
I had a 60 watt light bulb(120V) connected in parallel with my 10 ohm load.
And with the right duty cycle (about 30% I think) it would really light up the lamp nicely,
even when the sun wasn't that strong.
Which kinda made me think the bulb was seeing some pretty good voltage (and power).

So, I don't have a problem with the circuit not working, my problem is the burned out SSR..

Not to worry, they are sending me a new one.. :D

If you configure those panels to 24 volts, you'll get 4 times the current. Run ~28 amps into your 10 ohm load and she'll warm up for ya.. ;)

The label says 28.9 volts at max power (Vmp)..

28.9 volts / 10 ohms = 2.89 amps. Fer a grand total of 83.5 watts.. :(