regulating a high power heating element with a pulse driven SSR

jfox

I am regulating a high power heating element with a pulse driven SSR. I am driving it with a prop driving a 2N3904
for 5 volts (direct didn't work) driving a 90 amp ssr.im only switching it about 10 or 15 times a second with something similar to this
http://cgi.ebay.com/SSR-40-DA-Solid-State-Relay-40A-Output-24-380VAC-/310315296107?pt=LH_DefaultDomain_0&hash=item48403ba56b
with this
CON
_xinfreq = 5_000_000
_clkmode = xtal1 + pll16x

PUB LedOnOff
dira[3] := 1
dira[18] := 1
repeat
outa[3] := 1
outa[18] := 1
waitcnt(clkfreq/12 + cnt)
outa[3] := 0
outa[18] := 0
waitcnt(clkfreq/20 + cnt)


this is usually done with a very expensive thyristor controlled power supply
Im running a test element that draws 20 amps on constant 220 measured with a clipon amp gauge. pulsed with this timing it shows 14 amps. system seems to work great.
Questions how efficient is this ?whats the best frequency how fast could I run this ,(device says 10millisecond turn off time)? how to calculated true RMS voltage/current ,I dont have scope or wattmeter. comments?(beside "Dont electrocute yourself!"
Jim
Riverpottery.com

kwinn

If the original circuit used a thyristor then it is intended for 220VAC, and that would be 50/60Hz. The best you could do would be to turn it on /off every half cycle of the power line. Not much point in doing it any faster since the thermal inertia of the heating element is so high. The best method to control that is with a zero crossing triac and duty cycle control (ie x cycles on, y cycles off). It would be pretty efficient. the power loss is the current times the voltage across the ssr when it is conducting, which is usually less than 1 volt.

jfox

Thanks Kwinn
Im upgrading the software so i can adjust it and display the setting on an LCD.Ill report later Jim