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diac troubles in a heater circuit

Thread starter #1
I have a rather simple circuit that controls heating elements. After testing each component, I find them all good except the r20 (47 ohms resistor And the diac.( the diac show a few ohms in each direction when tested with an ohm meter). I have replaced the 47 ohm resistor but don't have an idea of what diac would work in this circuit.
This is were my electronics stop and your help is needed.
This diac is the only component that I can't identify. I will add a circuit diagram I found that is wired exactly the same.
the component values are very close but slightly different. they are:
r20 = 47 ohms
c24 = .224uf
triac = 56026
c23= .104uf
r19 = 1k ohms
vr4 = 0-500k ohms
diac = unkown
the load is a 240 volt heating element.
thanks for any help.
Rex
 

rjenkinsgb

Active Member
#2
Diacs are fairly generic components, pretty much any one you find should work fine.

eg. See below - just get the appropriate case style, either wire ended or surface mount.
https://www.mouser.co.uk/Semiconductors/Discrete-Semiconductors/_/N-awhng?Keyword=diac&FS=True

Any of the DB3 type should be fine in that circuit. The DB4 has a slightly higher threshold voltage which is probably not needed but would make little difference.

[... Just realised it says "DB3" in the drawing...]
 
Thread starter #3
Thanks so much for the help and info. I ordered a db3 this morning.

2 day latter:
received diacs today and added it to circuit. installed circuit in dryer and it works perfectly.
thanks for your help.
 
Last edited:

Willen

Well-Known Member
#4
Once I have found same circuit inside a Fan controller. Load was not so big but it damaged two big TRIACs in TO-220 in short period. Later few engineer here suggested to add a 470 ohms series resistor between DIAC to Gate of TRIC and 4.7k resistor from Gate to Gnd to limit firing voltage/current through the DIAC. Then worked fine for long time. Does this DIAC failure related to same problem?
 
Thread starter #5
I don't know why the circuit failed, but it has worked for many years. with new parts installed, it is working great again. Thanks for the info.
 
Thread starter #6
Once I have found same circuit inside a Fan controller. Load was not so big but it damaged two big TRIACs in TO-220 in short period. Later few engineer here suggested to add a 470 ohms series resistor between DIAC to Gate of TRIC and 4.7k resistor from Gate to Gnd to limit firing voltage/current through the DIAC. Then worked fine for long time. Does this DIAC failure related to same problem?
The diac failed again 4 days latter! I'll try the limit resistors and see how it works. thanks for the tip.
 

rjenkinsgb

Active Member
#7
I'd try a fairly low value resistor. Remember that the diac fires at about 30V, the resistor must be low enough to guarantee the triac fires properly - eg aim for around 500mA minimum. The diac is rated for 2A pulse so that is still well within its working range, but should also be adequate for any general triac.

eg. 15 Ohms or 10 Ohms.

A faulty triac could cause the diac to fail.
 

Reloadron

Well-Known Member
Most Helpful Member
#8
In your posted circuit R20 100 Ohms and C24 0.1 uF are used as a snubber and really only needed for an inductive load. Since you mention a heating element (240 Volt) you should have a resistive load, not that a snubber is bad but for a heater element it is unneeded. You don't mention the heater element load power? Phase fired triac circuits like this are very inexpensive, depending on the load you could replace the entire circuit likely with a simple 300 watt or 600 watt lamp dimmer switch for under $20 USD. If your existing unit keeps failing I would just replace the whole thing.

Ron
 

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