In short. Relay stabilisers switch transformer taps with relays, giving stepped correction at a lower price. Servo stabilisers adjust smoothly with a motor and tighter accuracy. Relay types suit light, noncritical loads; servo types suit motors and sensitive equipment.
How they differ
| Criterion | Relay type | Servo type |
|---|---|---|
| Correction | Stepped, tap by tap | Continuous |
| Output accuracy | Wider band | Tight band |
| Speed | Fast switching | Slower motor travel |
| Contact wear | Relay contacts age with switching | Brush and roller wear |
| Cost | Lower at small ratings | Higher |
| Typical use | Domestic and light commercial | Industrial and sensitive loads |
Where relay types struggle
- High inrush loads can stress relay contacts.
- Voltage steps may be visible in lighting.
- Sensitive equipment may dislike stepped changes.
- Rating headroom is often small.
Where they do fine
- Light loads such as lighting and small appliances.
- Low-cost protection against moderate drift.
- Short-life or temporary installations.
See also servo vs static.
Decision steps
- Identify the load and its tolerance.
- Check inrush against the rated overload.
- Decide the acceptable output band.
- Consider life cost, not only purchase price.
Illustrative example
A small shop with lighting and a freezer uses a relay type at low cost. When the shop adds a large cold-room compressor, the relay unit chatters at start. The owner moves to a servo unit sized for the compressor.
Step size and its effect
A relay stabiliser has a limited number of transformer taps. If each tap changes output by, say, 8 to 10 V, the output moves in steps of that size as input voltage varies. The output band is therefore wider, and sensitive equipment may see jumps. A servo unit moves continuously, so its tolerance can be much tighter.
Contact life and switching
Relay contacts wear with each switching operation, and more so when switching under load. Repeated switching in a fluctuating supply shortens life. Heavy inrush loads, like compressors, stress the contacts further. A servo unit has wear elsewhere, in brushes and rollers, but switching is gentler.
| Factor | Relay type | Servo type |
|---|---|---|
| Output band | Wider | Narrower |
| Typical price | Lower | Higher |
| Wear point | Relay contacts | Brushes, rollers, motor |
| Noise | Audible clicking | Quiet motor sound |
| Best application | Lighting, small appliances | Motors, equipment, process |
When a relay unit is a reasonable choice
- Light loads with wide voltage tolerance.
- Budget-limited, noncritical applications.
- Temporary installations.
When to avoid it
- Frequent compressor or motor starts.
- Sensitive electronics or precision machines.
- Continuous heavy industrial duty.
Evaluating total cost
Add the cost of replacements, downtime and equipment damage to the purchase price. A cheaper unit that is replaced twice may cost more than a servo unit that lasts.
The master engineering question
How much does a wrong or stepped voltage cost the load compared with the price difference?
What to send us
Send load types and supply data. Send your application details.
Questions engineers ask
Is relay type bad?
No, it suits light loads at low cost.
Does it click?
Yes, relay switching is audible.
Can it handle an AC?
Only if rated for compressor inrush.
How long do relays last?
Depends on switching frequency and load.
Values are illustrative. Use manufacturer data.