diff --git a/src/com/lushprojects/circuitjs1/client/SCRElm.java b/src/com/lushprojects/circuitjs1/client/SCRElm.java
index 62fb41359..1ba7c3824 100644
--- a/src/com/lushprojects/circuitjs1/client/SCRElm.java
+++ b/src/com/lushprojects/circuitjs1/client/SCRElm.java
@@ -76,6 +76,7 @@ void reset() {
 	volts[anode] = volts[cnode] = volts[gnode] = 0;
 	diode.reset();
 	lastvag = lastvac = curcount_a = curcount_c = curcount_g = 0;
+	state = false;
     }
     int getDumpType() { return 177; }
 
@@ -97,6 +98,7 @@ void undumpXml(XMLDeserializer xml) {
     double ia, ic, ig, curcount_a, curcount_c, curcount_g;
     double lastvac, lastvag;
     double gresistance, triggerI, holdingI;
+    boolean state;
 
     final int hs = 8;
     Polygon poly;
@@ -217,6 +219,24 @@ void stamp() {
 	diode.stamp(nodes[inode], nodes[cnode]);
     }
 
+    // Evaluate the on/off state once per timestep, not during Newton-Raphson
+    // iterations.  Previously the state switching was inside doStep(), causing
+    // the anode resistance to flip-flop between on (0.0105) and off (10e5) on
+    // every sub-iteration when an external series resistance was present,
+    // preventing convergence (issue #851).
+    //
+    // Use separate trigger/hold conditions with latching, matching the pattern
+    // used by TriacElm.  Once the gate current exceeds triggerI the SCR latches
+    // on, and stays on until the anode current drops below holdingI.  The gate
+    // only triggers; it does not need to remain driven to keep the SCR on.
+    void startIteration() {
+	if (ia < holdingI)
+	    state = false;
+	if (ig > triggerI)
+	    state = true;
+	aresistance = state ? .0105 : 10e5;
+    }
+
     void doStep() {
 	double vac = volts[anode]-volts[cnode]; // typically negative
 	double vag = volts[anode]-volts[gnode]; // typically positive
@@ -226,11 +246,6 @@ void doStep() {
 	lastvac = vac;
 	lastvag = vag;
 	diode.doStep(volts[inode]-volts[cnode]);
-	double icmult = 1/triggerI;
-	double iamult = 1/holdingI - icmult;
-	//System.out.println(icmult + " " + iamult);
-	aresistance = (-icmult*ic + ia*iamult > 1) ? .0105 : 10e5;
-	//System.out.println(vac + " " + vag + " " + sim.converged + " " + ic + " " + ia + " " + aresistance + " " + volts[inode] + " " + volts[gnode] + " " + volts[anode]);
 	sim.stampResistor(nodes[anode], nodes[inode], aresistance);
     }
     void getInfo(String arr[]) {