# Alternatives to relay/contactors in DC circuits

I wanted to switch a 60Vdc line to ON/OFF using a 60Vdc controlling signal for some kind of a feedback system (yes both are 60Vdc). Presently I have two options visible:

1. Solid State 60Vdc input relay
2. A dc/dc buck converter that changes my controlling signal to 12Vdc and then I use a normal 12Vdc coil relay or a solid state relay.

I wanted to know if there are any other methods I am not aware of. Also if not, then which of the above is better.

I am using this to remove the auto-restart feature in a battery protection module(PCM) by providing a feedback that isolates the PCM from the battery bank.

The job of the PCM currently is to cut off the load and charger under fault conditions like over voltage, over current, etc. It has an auto-restart feature which switches ON the load and charger after checking else shuts off again. I wanted to remove this auto-restart feature.

To restart the battery pack then, I will have to provide a voltage from external source or circuitry so that the relay connects and then checks if the system is fit to run or not. Thus the checking part would be in my hand and the battery would be isolated from all possible sources and sinks under fault condition.

P.S. I have added a rough sketch to help explain my case.

## 2 Answers

If you use a solid state relay you can just step down the voltage with a couple of resistor since the input current might be low enough. Just read the spec of your favorite SSR and size the resistors accordingly. If the maximum input current is $I_{max}$ and the minimum on voltage on the input is $V_{IH}$ then you want that even when $I_{max}$ is flowing in the relay there is ate least $V_{IH}$ as its input.

This approach might work but you are dissipating power, that might not be acceptable. The DCDC seems like an overkill to me, a couple of transistor are enough to translate a voltage. The 60Vdc relay might be a good choice but I bet they cost more and they dissipate quite a lot of power too... Search for the SSR that you need based on the contacts specification, i.e. maximum load current/voltage and minimum load voltage, and then see if there's something that consumes less than 50mA on the input, where 50mA is a rough estimate of what you can reasonably obtain with a couple of resistors.

• I can always use that DC/DC converter to power up other 12Vdc components like the ventilation system of the battery box. – Ayush Jun 11 '14 at 12:54
• Well you did not mention that, if you want other low voltage devices kick in when the relay cuts the battery then go with the DC/DC, you will need it anyway. – Vladimir Cravero Jun 11 '14 at 13:08

If the relay coil doesn't take much power you can use dropper resistors to power it. You already have "isolation" because of the relay coil-contact separation so the expense of a DC-DC converter is possibly too much. This assumes that most DC-DC converters are "isolating".

If the power lost in the coil dropper resistor was considered too high then use a non-isolating buck converter to drive the relay coil - this will be cheaper than a (usually isolating) DC-DC converter.