JDW0
Automotive
- Mar 22, 2016
- 15
In terms of relay coil voltage spike suppression, the following thread focuses on using a 1N4007 diode in series with a Zener across the relay coil:
thread248-291225
But there are times when you need to suppress coil spikes but cannot put components across the relay coil. In this case, a single Zener diode across the Collector and Emitter of a common-emitter BJT (which drives the low side of the relay coil), should be adequate to suppress the coil's voltage spike to the Zener voltage. And using a Vz = 2 x Vcc (with Vcc being the coil's positive voltage) would ensure the presence of the Zener does not slow the switching mechanism in any significant way.
My question is how to properly calculate the wattage / power rating of the Zener. While we could just use a multi-Watt Zener "for safety," that is a rather "shot-in-the-dark" method that ignores size and cost. A 0.5W 1N5252B 24V Zener costs much less than a 5W 1N5359B 24V Zener, and there is a large size difference as well. As such, it is advantageous to know if the smaller and lower cost 0.5W Zener would be a long-term reliable choice to suppress relay coil spikes.
Consider this circuit:
The OMRON G8NW-2 relay shown is a single device that houses 2 coils and 2 switches. Here is the datasheet:
In the above circuit we are using an NPN BJT to switch the low side of both coils, with their high side connected together at an automotive +12V. (That 12V is filtered, so don't mind about Load Dumps.) The Zener voltage was chosen to be twice Vcc, which is 24V. The resistance of one relay coil is 225Ω at 20°C, but the datasheet shows a worst case of 180Ω at -40°C. Low resistance means more current flow. At 225Ω we have 0.64W and 53mA current, but at 180Ω we have 0.8W and 64mA current; and again, that is for only 1 of the 2 coils.
I built a test circuit that matches follows the above schematic, first without any suppression and then with the Zener across the collector and emitter. On a scope I measured relay coil spikes at the low side of the relay coil (at the Collector of the BJT) to be from 100V up to about 143V:
The transistor used has a Vcc=50Vmax specification. Vce (voltage across the Collector and Emitter) will obviously be lower than Vcc with the relay in the circuit, low side connected to the Collector. But even if Vce was spec'd at 50V, that's still only half of the measured voltage spikes coming off the relay coils. As such, we need to protect the transistor, and that's where our Zener across the Collector and Emitter comes in.
Assuming a safe maximum Vce of 40V, it takes between 150us and 175us for the spike to decay to that level (varies by Vcc voltage level):
And with the 0.5W 24V Zener connected across the transistor, we see this on the scope:
But what is the minimum reliable wattage rating we should choose for the 24V Zener? That's my question. These are short-term voltage spikes, not continuous high voltage. And again, there are times when you need to reduce both cost and size, making a random selection of a high-wattage Zener impractical. I used a 1N5252B 24V 0.5W Zener across the transistor with the OMRON relay shown in the schematic for my measurements above. I've even used that same 0.5W Zener in a bench-top test circuit with 2 larger automotive relays which draw 122mA each. I only tested for a matter of minutes, not hours or days or weeks. But the 0.5W 1N5252B did not blow. (And actually, even without any suppression at all, for my short-term bench testing, the high voltage did not fry the transistor.) So I am curious if a 0.5W rated 24V Zener across the transistor would be a long-term reliable choice for relay coil spike suppression in this particular application.
I would appreciate hearing your thoughts.
thread248-291225
But there are times when you need to suppress coil spikes but cannot put components across the relay coil. In this case, a single Zener diode across the Collector and Emitter of a common-emitter BJT (which drives the low side of the relay coil), should be adequate to suppress the coil's voltage spike to the Zener voltage. And using a Vz = 2 x Vcc (with Vcc being the coil's positive voltage) would ensure the presence of the Zener does not slow the switching mechanism in any significant way.
My question is how to properly calculate the wattage / power rating of the Zener. While we could just use a multi-Watt Zener "for safety," that is a rather "shot-in-the-dark" method that ignores size and cost. A 0.5W 1N5252B 24V Zener costs much less than a 5W 1N5359B 24V Zener, and there is a large size difference as well. As such, it is advantageous to know if the smaller and lower cost 0.5W Zener would be a long-term reliable choice to suppress relay coil spikes.
Consider this circuit:
The OMRON G8NW-2 relay shown is a single device that houses 2 coils and 2 switches. Here is the datasheet:
In the above circuit we are using an NPN BJT to switch the low side of both coils, with their high side connected together at an automotive +12V. (That 12V is filtered, so don't mind about Load Dumps.) The Zener voltage was chosen to be twice Vcc, which is 24V. The resistance of one relay coil is 225Ω at 20°C, but the datasheet shows a worst case of 180Ω at -40°C. Low resistance means more current flow. At 225Ω we have 0.64W and 53mA current, but at 180Ω we have 0.8W and 64mA current; and again, that is for only 1 of the 2 coils.
I built a test circuit that matches follows the above schematic, first without any suppression and then with the Zener across the collector and emitter. On a scope I measured relay coil spikes at the low side of the relay coil (at the Collector of the BJT) to be from 100V up to about 143V:
The transistor used has a Vcc=50Vmax specification. Vce (voltage across the Collector and Emitter) will obviously be lower than Vcc with the relay in the circuit, low side connected to the Collector. But even if Vce was spec'd at 50V, that's still only half of the measured voltage spikes coming off the relay coils. As such, we need to protect the transistor, and that's where our Zener across the Collector and Emitter comes in.
Assuming a safe maximum Vce of 40V, it takes between 150us and 175us for the spike to decay to that level (varies by Vcc voltage level):
And with the 0.5W 24V Zener connected across the transistor, we see this on the scope:
But what is the minimum reliable wattage rating we should choose for the 24V Zener? That's my question. These are short-term voltage spikes, not continuous high voltage. And again, there are times when you need to reduce both cost and size, making a random selection of a high-wattage Zener impractical. I used a 1N5252B 24V 0.5W Zener across the transistor with the OMRON relay shown in the schematic for my measurements above. I've even used that same 0.5W Zener in a bench-top test circuit with 2 larger automotive relays which draw 122mA each. I only tested for a matter of minutes, not hours or days or weeks. But the 0.5W 1N5252B did not blow. (And actually, even without any suppression at all, for my short-term bench testing, the high voltage did not fry the transistor.) So I am curious if a 0.5W rated 24V Zener across the transistor would be a long-term reliable choice for relay coil spike suppression in this particular application.
I would appreciate hearing your thoughts.
